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Earth and Planetary Science Letters
Journal Prestige (SJR): 3.166
Citation Impact (citeScore): 5
Number of Followers: 183  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 0012-821X
Published by Elsevier Homepage  [3183 journals]
  • Lithosphere differentiation in the early Earth controls Archean tectonics
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): F.A. Capitanio, O. Nebel, P.A. Cawood, R.F. Weinberg, F. Clos The processes that operated on the early Earth and the tectonic regimes in which it was shaped are poorly constrained, reflecting the highly fragmentary rock record and uncertainty in geodynamic conditions. Most models of early Earth geodynamics invoke a poorly mobile lid regime, involving little or episodic movement of the lithosphere, above a convecting mantle. However, such a regime does not reconcile with the record of Archean tectonics, which displays contrasting environments associated with either non-plate tectonics or plate tectonics. Here, we propose a regime for the early Earth in which progressive melt extraction at sites of divergence led to the formation of large portions of stiffer lithospheric lid, called proto-plates. These proto-plates enabled stress propagation to be focussed at their margins, which were then the locus for extension as oppose to shortening, under-thrusting and thickening to form adjoining proto-cratons. We test this hypothesis embedding lithospheric stiffening during melt extraction in thermo-mechanical models of mantle convection, under conditions that prevailed in the Archean. We demonstrate the emergence of migrating, rigid proto-plates in which regions of prolonged focused compression coexist with remnants of the stagnant lid, thereby reproducing the widespread dichotomy proposed for the Archean tectonic record. These diverse tectonic modes coexist in a single regime that is viable since the Hadean and lasted until the transition to modern plate tectonics.
  • On the role of chemical weathering of continental arcs in long-term
           climate regulation: A case study of the Peninsular Ranges batholith,
           California (USA)
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Hehe Jiang, Cin-Ty A. Lee The long-term stability of atmospheric pCO2 is dominantly determined by the balance between the rate of CO2 input from magmatic/metamorphic degassing and the efficiency of CO2 uptake by silicate weathering via eventual precipitation as marine carbonate and by organic carbon burial. Silicate weathering is thought to represent a negative feedback to changes in atmospheric pCO2. The Late Cretaceous-Early Cenozoic was characterized by elevated atmospheric pCO2 and greenhouse climate, widely thought to be due to increased magmatic flux from mid-ocean ridges, flood basalts and continental arcs. Of interest here is the role of continental arcs in modulating long term atmospheric CO2 contents and climate. Continental arc magmatism is accompanied by rapid uplift and erosion due to magmatic/tectonic thickening of the crust, thus, the development of continental arcs might also enhance the global efficiency of silicate weathering, in turn increasing the efficiency of carbon sequestration. To assess the contribution of continental arcs to global carbon sinks, we conducted a case study in the Cretaceous Peninsular Ranges batholith (PRB) and associated forearc basin in southern California, USA, representing one segment of a Mesozoic circum-Pacific continental arc system. Arc magmatism occurred between 170-85 Ma, peaking at 100 Ma. Erosion occurred during magmatism and continued well into the early Eocene, with forearc sediments representing the products of this protracted unroofing of the arc. By calculating the depletion of Ca in forearc sediments relative to their arc protoliths, we estimate chemical weathering fluxes (in equivalent CO2) to be ∼106 mol km−2yr−1 near the end of magmatism, decreasing to ∼105 mol km−2yr−1 by the Early Eocene. Integrated over the entire magmatic and post-magmatic history of the batholith, the total amount of CO2 consumed through chemical weathering is comparable to that degassed. However, during magmatism, the regional degassing flux exceeds the regional weathering flux even though the efficiency of weathering is increased due to magmatic and tectonic uplift. After magmatism ends, continued erosion from remnant topography causes the arc to transition into a net regional sink, increasing the global efficiency of chemical weathering and amplifying cooling after magmatism ends. Understanding how atmospheric CO2 and climate vary over long timescales requires an understanding of how magmatic orogenies enhance degassing but also increase the strength of the global silicate weathering feedback.
  • Variable modification of continental lithosphere during the Proterozoic
           Grenville orogeny: Evidence from teleseismic P-wave tomography
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Alistair Boyce, Ian D. Bastow, Eva M. Golos, Stéphane Rondenay, Scott Burdick, Robert D. Van der Hilst Cratons, the ancient cores of the continents, have survived thermal and mechanical erosion over multiple Wilson cycles, but the ability of their margins to withstand modification during continental convergence is debated. The Proterozoic Grenville orogeny operated for ≥300 Myr along the eastern edge of the proto-North American continent Laurentia, whose age varied north-to-south from ∼1.5−0.25 Gyr at the time of collision. The preserved Grenville Province, west of the Appalachian terranes, has remained largely tectonically quiescent since its formation. Thick, cool, mantle lithosphere that underlies these Proterozoic regions is typically identified by elevated seismic velocities but lithospheric modification by fluid/melt-derived metasomatic enrichment above a subduction zone, can lead to a reduction in VP with little effect on VS and density. Absolute P-wavespeed constraints are therefore a vital complement to existing S-wave tomographic models of North America to investigate craton edge modification mechanisms in the Grenville orogen.New P-wave tomographic imaging of the North American continent, which benefits from recent developments in arrival-time processing of regional network deployments from the Canadian shield, reveals along strike wavespeed variation in the Grenville orogen. In the north, high seismic wavespeeds (to depths of 250 km) extend eastwards, from the Archean core of North America to beneath the Canadian Grenville Province. In contrast, below the southern U.S., high lithospheric wavespeeds are restricted to west of the Grenville Province, in particular at depths less than 150 km. We argue that subduction-derived metasomatism beneath eastern Laurentia modified the southern Grenville, prior to thermal stabilization and perhaps mantle keel formation. Beneath the northern Grenville, the thick, depleted Laurentian lithosphere resisted extensive metasomatism. Along strike age differences in Grenvillian terranes and their resulting metasomatic modification histories suggests that at least 250 Myr is required for Proterozoic lithosphere to gain resistance to modification.
  • Stress heterogeneities and failure mechanisms induced by temperature and
           pore-pressure increase in volcanic regions
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): M.E. Belardinelli, M. Bonafede, M. Nespoli We study the strain and stress fields produced by temperature and pore pressure increases within and outside a Thermo-Poro-Elastic (TPE) inclusion (the source region), embedded within a medium (the matrix) in isothermal drained conditions. This model is suitable to describe a crustal region in a volcanic environment pervaded by hot pressurized fluids released by an underlying magma chamber. After introducing the pertinent constitutive relations, a formal solution for the displacement field is provided in terms of the Green's function for an elastic medium with drained isothermal elastic moduli, employing a generalization of Eshelby (1961) procedure. If an unbounded medium is considered, a displacement potential can be introduced, obeying the Laplace equation within the source region and the Poisson equation within the matrix. If a spherically symmetric source region is considered, simple analytical solutions are obtained for the displacement, the strain and the stress fields, showing that thrust faulting mechanisms are promoted within the source region while normal faulting mechanisms prevail in the embedding matrix. Employing reasonable numerical values for the thermo-poro-elastic parameters, suitable to describe highly porous sedimentary rock, strain and stress variations are found to be significant even for moderate changes of temperature and pore pressure. Variations of the Coulomb failure function are high in the TPE region and are strongly dependent on the friction coefficient and pore pressure. Application of these results to the 1982-84 and 2011-13 unrest episodes at Campi Flegrei caldera (Italy) suggests that an oblique dike intrusion across a previously unfaulted TPE region took place with a mixed tensile-thrust dislocation mechanism in both events, as previously inferred from accurate inversion of geodetic data.
  • Millimeter-level ultra-long period multiple Earth-circling surface waves
           retrieved from dense high-rate GPS network
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Aizi Guo, Sidao Ni, Jun Xie, Jeffrey T. Freymueller, Yong Wang, Baolong Zhang, Ziye Yu, Yixin Yao, Wanjun Ma High-rate GPS networks have proven to be capable of continuously recording strong ground motion, which is valuable for studying earthquake rupture processes and seismic hazard mitigation. Previous studies of high-rate GPS usually resolved dynamic ground displacements of some major seismic phases stronger than a few millimeters. In this study, we explore the feasibility of retrieving much weaker ground motions (sub-millimeter) in long duration records by stacking high-rate GPS network. We processed data of the 2011 Tohoku-Oki earthquake recorded by more than 600 high-rate GPS stations of the Plate Boundary Observatory (PBO) and analyzed kinematic displacement results with seismic array methods. From these high-rate GPS seismograms, we identified abundant seismic phases, not only including clear body waves such as multiple S waves (S, SS, SSS, SSSS traveling along minor/major great circle arc, and SSSSSS beyond 360 degrees), but also the R1–R5 Rayleigh waves and G1–G6 Love waves, some of which travel around the Earth multiple times. Many of these seismic phases have not been reported by previous high-rate GPS studies. The group velocity dispersion curves extracted from multiple earth-circling Love waves of high-rate GPS are consistent with those of broadband seismic records in the period from 150 to 600 s, which can sample the mantle structure down to top of the lower mantle, thus important for understanding interaction between upper mantle and lower mantle. So the dense high-rate GPS networks should be useful for deep mantle research as an additional valuable dataset. With the development of multi-system GNSS and the more precise modeling of error sources, high-rate GNSS data should have great potential for accurately detecting more geodynamic processes with much wider bandwidth in geoscience studies.
  • Speleothem biomarker evidence for a negative terrestrial feedback on
           climate during Holocene warm periods
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Canfa Wang, James A. Bendle, Sarah E. Greene, Michael L. Griffiths, Junhua Huang, Heiko Moossen, Hongbin Zhang, Kate Ashley, Shucheng Xie Understanding how terrestrial carbon storage feeds back on warm climate states is critical for improving global warming projections. Soils may act as a positive feedback on climate if warming increases soil carbon decomposition rates. Conversely, if increases in net primary production (NPP) exceed increases in decomposition, the climate feedback will be negative. Here we utilize the first palaeoclimatic application of compound-specific δ13C measurements on n-fatty acid biomarkers (extracted from a stalagmite from central China) to constrain the response of catchment terrestrial carbon cycle feedbacks during warmer phases of the Holocene. We resolve proportional increases in C3 plants in the catchment area during these warmer/wetter intervals. Moreover, we infer that heterotrophic soil respiration was highly substrate selective, indicating that NPP outpaced decomposition and the catchment behaved as a carbon sink (mediated and enhanced by changes in the relative proportion of C3 vs. C4 plants). Thus, we provide palaeoclimate evidence that subtropical soils in a warmer/wetter climate acted as a sink for organic carbon, and thus as a negative climate feedback, during warmer climatic phases.
  • Pb isotope evidence for rapid accretion and differentiation of planetary
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): J.N. Connelly, M. Schiller, M. Bizzarro Group IVA iron and siliate-iron meteorites record a large range of cooling rates attributed to an impact-related disruption of a molten and differentiated ca. 1000 km diameter planetary embryo of chondritic composition before re-accretion of mainly the metallic core with minor silicates. To better understand the timing of primary accretion, disruption, re-accretion and cooling of the Group IVA parent body, we have determined Pb-Pb and Al-Mg ages for the Group IVA silicate-iron Steinbach meteorite. A Pb-Pb age based on multiple fractions of late-phase, slowly-cooled orthopyroxene from Steinbach yields an absolute age of 4565.47 ± 0.30 Ma corresponding to a relative age of 1.83 ± 0.34 Myr after formation of calcium-aluminium-rich inclusions (CAIs). This is the oldest U-corrected Pb-Pb absolute age for a differentiated meteorite. We use the deficit Al-Mg dating method on one whole rock sample and two mineral separates to produce a model age of 1.3−0.3+0.5 Myr after CAI formation corresponding to the depletion age of Al relative to Mg in the source material for Steinbach. Assuming this fractionation event occurred in the pre-impact parent body, this provides a maximum time after CAI formation for the disruption of the original Group IVA parent body. Together, these ages require that the original parent body accreted very early and differentiated prior to the impact-related break up, re-accretion and cooling between 1.3−0.3+0.5 Myr and 1.83 ± 0.34 Myr after CAI formation. These ages are fully consistent with a growing body of evidence from meteorites and astronomical observation supporting the early and efficient growth of planetary embryos and with numerical models of pebble accretion that predict rapid growth of embryos in the presence of chondrules. This time frame for the efficient formation of planetary embryos by chondrule accretion is inconsistent with a proposed ∼1.5 Myr delay in chondrule formation, a contradiction that is resolved by a non-canonical abundance of 26Al in the inner Solar System during at least the first million years of the protoplanetary disk.
  • Glacial-interglacial dust and export production records from the Southern
           Indian Ocean
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Lena M. Thöle, H. Eri Amsler, Simone Moretti, Alexandra Auderset, James Gilgannon, Jörg Lippold, Hendrik Vogel, Xavier Crosta, Alain Mazaud, Elisabeth Michel, Alfredo Martínez-García, Samuel L. Jaccard We present 230Th-normalized dust and export production fluxes for two contrasted marine sediment cores spanning the Antarctic Polar Front, close to the Kerguelen Plateau in the Southern Indian Ocean, covering the last glacial cycle.We report glacial lithogenic fluxes comparable to the South Atlantic and higher than in the South Pacific sectors of the Southern Ocean. Structural and temporal discrepancies with dust reconstructions from Antarctic ice cores and the Atlantic and Pacific sectors of the Subantarctic Zone (SAZ) point towards Southern Africa and/or the Kerguelen Plateau as an additional source of lithogenic material to the Southern Indian Ocean during the last ice age.In the SAZ, export production proxies respond to iron (Fe) fertilization with total organic carbon (TOC) fluxes as high as those previously reported from the Atlantic sector of the Southern Ocean. However, the correlation between export production and dust proxies is weaker than in the other sectors, and shows a muted response of export production during peak glacials. We hypothesize that this muted response may be related to macronutrient (co-)limitation imposed on phytoplankton growth possibly induced by a northward displacement of wind-driven upwelling and/or the polar frontal system during peak glacials.The Antarctic Zone (AZ) record depicts the typical pattern of enhanced export production during interglacials and comparatively low productivity during glacials suggesting a decrease in the supply of macronutrients to the AZ surface during ice ages compared to warm periods, as previously proposed. However, a muted response of opal fluxes during marine isotope stage (MIS) 5e argues for a southward migration of the frontal system during warmer MIS 5e, possibly causing silicic acid (co-)limitation at this site.These results illustrate the important role of Fe fertilization and changes in wind-driven upwelling intensities in modulating the strength and efficiency of the biological carbon pump in the Southern Ocean, but also highlight the potential role of frontal movements in explaining export production variability at sites located close to the Antarctic Circumpolar Current (ACC) fronts. Overall, our new datasets provide new insights on the regional differences in dust and export production patterns that need to be considered in order to evaluate the efficiency of carbon sequestration in the different sectors of the Southern Ocean during ice ages.
  • Lead isotopes as tracers of crude oil migration within deep crustal fluid
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Nadège Fetter, Janne Blichert-Toft, John Ludden, Aivo Lepland, Jorge Sánchez Borque, Erica Greenhalgh, Bruno Garcia, Dianne Edwards, Philippe Télouk, Francis Albarède Although Pb, U, and Th may be fractionated between crude oil and formation waters, Pb isotopes are not. This unique property makes Pb isotopes a particularly useful marker of hydrocarbon generation and migration. Here we show that Pb isotopes offer a new vision of long-range (secondary) oil migration relevant to the formation of oil fields. North Sea oils are largely generated from Jurassic black shales, yet their Pb isotopes are mixtures of Cenozoic to Proterozoic end-members. The same observation is made for crude oils from the Paris Basin, the Barents Sea, Libya, Kuwait, Kazakhstan, and Australia. Bulk Pb in crude oil therefore, for the most part, is foreign to its source rock(s). Our high-precision Pb isotope data on 195 crude oils worldwide, the first such data set in the published literature, and 17 Northern European black shales indicate that deep-seated Pb components originating beneath the source rocks are ubiquitous in crude oil. This implies that oil fields are embedded in basinal convective systems of hydrous fluids heated from below. Plumes of hot fluids rise from the lower thermal boundary layer, which Pb isotopes require douse the basement, into the core of the porous-flow convective cell where they dissolve the newly formed hydrocarbons sequestered in the source rocks. The fluids finally unload unmixed formation waters and crude oil at the base of the upper (conductive) boundary layer where they can be trapped in favorable sites. Based on these new insights we argue that Pb isotopes in crude oil constitute a good tracer of oil migration.
  • Physicochemical models of effusive rhyolitic eruptions constrained with
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Francisco Delgado, Julia Kubanek, Kyle Anderson, Paul Lundgren, Matthew Pritchard The 9 month long 2011-2012 eruption of Cordón Caulle (Southern Andes, Chile) is the best instrumentally recorded rhyolitic eruption to date and the first time that the effusion of a rhyolitic flow has been observed in detail. We use Interferometric Synthetic Aperture Radar (InSAR), with time-lapse digital elevation models (DEMs) and numerical models to study the dynamics of coupled magma reservoir deflation and lava effusion. InSAR recorded 2.2-2.5 m of subsidence after the first three days of the eruption, which can be modeled using a spheroidal magma reservoir at a depth of ∼5 km, ∼20 km long, and with a pressure drop of 20-30 MPa. The source is elongated in the NW-SE direction and its large dimensions imply a large plumbing system active throughout the eruption and spanning neighboring volcanoes, with a slight change in the geometry halfway through the effusive phase. TanDEM-X and Pléiades DEMs record the extrusion of both the rhyolitic lava flow and the intrusion of a shallow laccolith around the eruptive vent after the third day of the eruption, with a total volume of ∼1.45 km3 DRE. The laccolith was emplaced during the first month of the eruption, during both the explosive and effusive stages of the eruption. Both the reservoir pressure drop and the extruded volume time series follow quasi-exponential trends, and can be explained by a model that couples the reservoir pressure decrease, time- and pressure-dependent variations in the magma properties inside of the reservoir, and conduit flow. This model predicts both the temporal evolution and amplitude of both time series during the effusive phase, and a magma compressibility of ∼10−10 Pa−1, half the reported compressibility of the magma of the sub-Plinian explosive phase. Further, we estimate that the reservoir contained 1-3 wt.% dissolved H2O at the onset of lava effusion, with no exsolved CO2 and H2O in the reservoir throughout the effusive phase. This implies that the magma was significantly degassed after the explosive phase. The remaining volatiles in the magma after the explosive stage might have caused magma fragmentation, consistent with the hybrid explosive-effusive style observed during the waning of the eruption.
  • The relationship between zinc, its isotopes, and the major nutrients in
           the North-East Pacific
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Derek Vance, Gregory F. de Souza, Ye Zhao, Jay T. Cullen, Maeve C. Lohan Trace metal micronutrients play key roles in photosynthesis by oceanic phytoplankton. Though they are required in much smaller amounts than the major nutrients (P, N, Si), their bioavailable forms are also present in the seawater solution at much lower levels. Relationships between the dissolved chemistry of the nutrient-type trace metals, their stable isotope variations, as well those of the major nutrients, have highlighted the importance of biological and physical processes in the Southern Ocean in controlling their oceanic biogeochemistry. However, the first-order Southern Ocean processes are overprinted by vertical cycling in other parts of the ocean, particularly upwelling regions remote from the Southern Ocean, with the North Pacific standing out in particular. Here we present new zinc (Zn) concentration and isotope, as well as major nutrient data for the NE Pacific, and couple these new data with a compilation of published data from across the region, with the objective of better understanding the impact of this important region on oceanic biogeochemical cycles.The new Zn isotope data for two stations along Line P (P04 and P26) show a large range in δ66Zn in the upper ocean (−0.4‰ up to>1‰), associated with a very small isotope fractionation but extreme depletion of the dissolved pool during photic zone biological uptake, and the regeneration of this cellular Zn at very shallow depths (50 m). Beneath this, the two profiles approach the δ66Zn value of +0.5‰, seen throughout the deep ocean, by about 500 m. The minimum δ66Zn resulting from regeneration is associated with very high Zn concentrations, particularly at the marginal P04 station where diatoms dominate the phytoplankton ecology. Combining the new data with published Zn and major nutrient concentrations from across the North Pacific emphasises the role of vertical biological cycling in controlling regional biogeochemistry in the North Pacific, resulting in the partial overprinting of biogeochemical signatures transported out of the Southern Ocean by the ocean circulation. Zinc isotope data document the uptake of this metal into diatoms and the co-regeneration of Zn with phosphate in the upper water column. Silica in contrast is regenerated at greater depth, resulting in a decoupling of the Zn–Si correlation that is set in the Southern Ocean and that dominates the Atlantic. Previous work has suggested that the decoupling of Zn and Si in the subarctic North Pacific results from removal of Zn (and other metals) to water column particulate sulphide. In our dataset, and in the compilation of data documenting relationships between Zn and the major nutrients across the North Pacific, this decoupling is clearly due to the different lengthscales of regeneration for organic matter (Zn and P) and diatom opal (Si).
  • Hydroclimatic variability in Southeast Asia over the past two millennia
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Jessica K. Wang, Kathleen R. Johnson, Andrea Borsato, Dillon J. Amaya, Michael L. Griffiths, Gideon M. Henderson, Silvia Frisia, Andrew Mason The spatiotemporal variability of the Asian Monsoon (AM) over the last two millennia has been attributed to a combination of external solar and volcanic forcing and/or internal coupled atmosphere-ocean dynamics, but the relative importance of these mechanisms remains unresolved. The present knowledge of multidecadal to centennial-scale AM variability over Mainland Southeast Asia is not well-constrained, despite substantial progress in understanding seasonal to decadal variability from tree ring records. Here we present the first high-resolution stable isotope (δ13C and δ18O) speleothem record from northern Laos spanning the Common Era (∼50 BCE to 1880 CE). The δ13C record reveals substantial centennial-scale fluctuations primarily driven by local water balance. Notably, the driest period at our site occurred from ∼1280 to 1430 CE, during the time of the Angkor droughts, supporting previous findings that this megadrought likely impacted much of Mainland Southeast Asia. In contrast, variations in stalagmite δ18O reflect changes in rainfall upstream from our study site. Interestingly, the δ18O record exhibits a positive correlation with solar activity that persists after 1200 CE, contrary to the findings in previous studies. Solar-forced climate model simulations reveal that these δ18O variations may be driven by solar-forced changes in upstream rainout over the tropical Indian Ocean, which modify the δ18O of moisture transported to our study site without necessarily affecting local rainfall amount. We conclude that future rainfall changes in Mainland Southeast Asia are likely to be superimposed on multidecadal to centennial-scale variations in background climate driven primarily by internal climate variability, whereas solar forcing may impact upstream rainout over the Indian Ocean.
  • Theoretical studies on the hydrous lower mantle and D″ layer
    • Abstract: Publication date: 1 November 2019Source: Earth and Planetary Science Letters, Volume 525Author(s): Jiajun Jiang, Feiwu Zhang The incorporation mechanisms and elastic properties of hydrous Bridgmanite (Brg) and post-Perovskite (PPv) are investigated by using first principles calculations at the lower mantle pressures. The elastic properties calculations in both (Al, Fe)-free and (Al, Fe)-bearing systems give evidences that hydrogen has less effect on the elastic wave velocities and moduli in (Al, Fe)-free and Al-bearing system. However, the elastic wave velocities and moduli, especially the shear velocity VS and the shear modulus G, are remarkably sensitive to the presence of hydrogen in the Fe-bearing system. The calculated shear velocity anomaly (dVS) are −2.9% for Fe3+SiH-Brg and −3.1% for Fe3+SiH-PPv, which are very close to the average anomaly value of LLSVPs. This result may imply the Ferric-bearing hydrous MgSiO3 is likely a dominated mineral in LLSVPs. Density functional perturbation theory (DFPT) and QHA calculation have also been performed to determine the P–T phase diagram for different hydrous systems. The phase transition boundary between Brg and PPv shifts to higher or lower pressures when the hydrogen atom substitutes in the Mg or Si sites in the lattice, respectively. The existence of volatiles such as hydrogen may account for the strong lateral chemical heterogeneity in lowermost mantle.
  • Seismological constraints on the density, thickness and temperature of the
           lithospheric mantle in southwestern Tibet
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Harry Matchette-Downes, Robert D. van der Hilst, Amy Gilligan, Keith Priestley We constrain the lithospheric mantle in southwest Tibet to be cold, thick, and dense by considering seismological observations, isostasy, and gravity anomalies. First, virtual deep seismic sounding (VDSS) indicates that the thickness of the crust increases from 50 ± 4 km beneath the Himalaya to 70 ± 4 km in the Lhasa terrane. This implies a ‘residual topography’ (difference between isostatic elevation of crust and true elevation) of −2.4 ± 1.5 km. Taking into account deviations from isostasy, the lithospheric mantle must be dense enough to depress the surface by 0.9 to 4.5 km. Our joint inversion of fundamental-mode Rayleigh wave dispersion and receiver functions suggests that the vertically-polarised shear-wave speed (Vsv) is 4.6 ± 0.1 km s−1 at depths of 120 to 300 km. From the shear-wave speed profile, we estimate the geotherm, which is on average 200 °C below the 1350 °C adiabat, and suggest that the base of the lithosphere is at a depth of 290 ± 30 km. To match the negative buoyancy, the lithospheric must be denser, on average, than ‘normal’ fertile adiabatic mantle, which rules out a depleted (harzburgite) composition. The density excess can be explained solely by thermal contraction, but we cannot rule out additional density increases due to composition. Our observations are not consistent with a depleted Indian slab underthrusting Tibet in this region, which would result in a lower average density and lower temperatures.Graphical abstractGraphical abstract for this article
  • Volatile behaviour in the 1995-2010 eruption of the Soufrière Hills
           Volcano, Montserrat recorded by U-series disequilibria in mafic enclaves
           and andesite host
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Lucy McGee, Mark Reagan, Heather Handley, Simon Turner, R. Stephen Sparks, Kim Berlo, Jenni Barclay, Michael Turner Mafic enclaves brought to the surface in volcanic eruptions offer insight into the deeper sections of a volcanic plumbing system, where the degree of interaction between deeper, mafic magma and the magma reservoir directly feeding a volcanic edifice is not well-constrained. At Soufrière Hills Volcano (SHV) on the island of Montserrat, Lesser Antilles, mafic enclaves have been a ubiquitous feature of the andesitic eruptions during the five phases of the 1995-2010 eruption. We use the short-lived isotopes 210Pb and 226Ra within the Uranium series decay chain, which are sensitive to volatile transfer and loss (due to the loss or gain of the intermediary daughter 222Rn), in both the enclaves and the host andesite to reveal significant time-related information. The sequentially erupted andesites are almost entirely in equilibrium or have deficits of 210Pb with the deficits becoming more pronounced over several eruptive phases. We model that the andesitic reservoirs involved were subject to continuous volatile loss both before and during the eruption. The majority of enclaves, however, have excesses of 210Pb, showing volatile enrichment that lasted over a decade. The highest (210Pb/226Ra)0 ratios are from enclaves in Phase II, indicating that the deeper mafic system was closed to fresh gas influx from Phase III onwards. Enclaves are modelled as being sporadically entrained in the andesite prior to eruption, suggesting that enclave formation is not a triggering mechanism for each eruptive phase.
  • Gas mobility in rheologically-layered volcanic conduits: The role of
           decompression rate and crystal content on the ascent dynamics of magmas
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): L. Spina, D. Morgavi, A. Costa, B. Scheu, D.B. Dingwell, D. Perugini Unravelling the rheological behaviour of magmas is fundamental for hazard assessment. At shallow depth the combined effects of degassing, vesiculation and crystallization are likely to produce dramatic changes in the rheology, hence modulating flow dynamics and eruptive style. The rheological evolution from a low viscosity crystal-poor, bubble-free, water-rich melt to a highly viscous crystal-rich, vesicular magma containing a water-poor melt often occurs in the conduit. To clarify the viscous flow dynamics of rheologically-layered volcanic conduits, we performed decompression experiments using a magma analogue system characterized by a low-viscous Layer L (10 Pa s) at the bottom and a high-viscous particle-bearing Layer H (≥1000 Pa s) at the top. Silicone oils and spherical glass beads are employed as magma and crystal analogues, respectively. Three sets of experiments address the effects of: 1) decompression rate (ca. 10−2 and 104 MPa/s); 2) crystal content in the high viscosity magma (0, 10, 30 and 70 vol.%); and 3) volume ratio of the two rheological layers (0.6 or 0.3). Our results indicate that decompression rate exerts the most dramatic role, yielding changes in time-scale of outgassing up to two orders of magnitude, and affecting the style of decompression response (permeable outgassing or fragmentation). The solid fraction 1) strongly modulates gas mobility, 2) influences the pervasiveness of fragmentation and 3) affects the extent of mingling in the experimental conduit. These results demonstrate that the properties of a shallow, partially-crystallized portion of the magmatic column and its response to varying ascent rate are primary controls on eruptive style.
  • Evolution of Fault-Interface Rayleigh Wave speed over simulated earthquake
           cycles in the lab: Observations, interpretations, and implications
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Shiqing Xu, Eiichi Fukuyama, Futoshi Yamashita, Shigeru Takizawa To image fault zone elastic and mechanical properties is of great importance for the understanding of fault mechanics and earthquake physics. In this study we show how a previously less-known wave—Fault-Interface Rayleigh Wave (FIRW), can be used to probe the feedback between fault zone properties and earthquake ruptures. We conducted meter-scale rock friction experiments to simulate earthquake cycles in the lab. Three tests were performed, under a fixed normal stress of −6.7 MPa (negative for compression), and a constant loading rate sequentially at 0.01, 0.1, and 1 mm/s. We installed a strain gauge array near the fault to monitor the local dynamics, focusing on mode-II rupture breaking the free edge of the fault and the breakout-induced FIRW propagating backward along the fault. By applying a waveform correlation method, we systematically measured the FIRW speed over an 800-mm-long interval in the central fault portion. The results show that a relative reduction of FIRW speed by 1.4% can be detected near the end of the test loaded at 1 mm/s. Such wave speed reduction can be explained by two mechanisms: rupture-induced brittle damage that softens fault zone rocks, and some dissipative processes (related to fault re-rupturing, viscous damping, etc.) that take energy away from FIRW. Both mechanisms are also supported by other independent observations (e.g. distribution of fault surface wear, behavior of macroscopic friction), implying that they may be coupled. These observations and interpretations reveal an application potential of FIRW for constraining the elastic and mechanical properties of fault zones. We suggest several directions for future research aimed at turning that application potential into reality.
  • Vegetation and environmental changes in tropical South America from the
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Valdir F. Novello, Francisco W. Cruz, Michael M. McGlue, Corinne I. Wong, Brittany M. Ward, Mathias Vuille, Rudney A. Santos, Plinio Jaqueto, Luiz C.R. Pessenda, Tiago Atorre, Ligia M.A.L. Ribeiro, Ivo Karmann, Eline S. Barreto, Hai Cheng, R. Lawrence Edwards, Marcos S. Paula, Denis Scholz δ18O values in speleothems have been utilized to document past changes in South American monsoon intensity. However, changes in regional vegetation and ecosystems have not been part of this discussion, and other cave proxies such as speleothem δ13C values, a useful proxy for vegetation reconstruction, have been neglected due to interpretive complexities. Here we report δ13C values and 87Sr/86Sr ratios in stalagmites, together with XRF-derived elemental chemistry, δ13Corg values and carbon content from a sedimentary profile from the same cave where the stalagmites were collected. In combination with a previously published δ18O record, this enables us to clarify climate and environmental shifts that occurred between the Last Glacial Maximum and the Holocene in central South America. We show that vegetation was sparse during the last glacial period in spite of a previously inferred strong monsoon, and that changes in atmospheric pCO2 combined with local hydrological and temperature feedbacks may have determined vegetation development during this time.
  • Corrigendum to “Iron isotopic fractionation in mineral phases from
           Earth's lower mantle: Did terrestrial magma ocean crystallization
           fractionate iron isotopes'” [Earth Planet. Sci. Lett. 506 (2019)
    • Abstract: Publication date: Available online 6 August 2019Source: Earth and Planetary Science LettersAuthor(s): Hong Yang, Jung-Fu Lin, Michael Y. Hu, Mathieu Roskosz, Wenli Bi, Jiyong Zhao, Esen E. Alp, Jin Liu, Jiachao Liu, Renata M. Wentzcovitch, Takuo Okuchi, Nicolas Dauphas
  • Corrigendum to “Isotopic evidence for complex biogeochemical cycling of
           Cd in the eastern tropical South Pacific” [Earth Planet. Sci. Lett. 512
           (2019) 134–146]
    • Abstract: Publication date: Available online 6 August 2019Source: Earth and Planetary Science LettersAuthor(s): Ruifang C. Xie, Mark Rehkämper, Patricia Grasse, Tina van de Flierdt, Martin Frank, Zichen Xue
  • Importance of the advection scheme for the simulation of water isotopes
           over Antarctica by atmospheric general circulation models: A case study
           for present-day and Last Glacial Maximum with LMDZ-iso
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Alexandre Cauquoin, Camille Risi, Étienne Vignon Atmospheric general circulation models (AGCMs) are known to have a warm and isotopically enriched bias over Antarctica. We test here the hypothesis that these biases are partly consequences of a too diffusive advection. Exploiting the LMDZ-iso model, we show that a less diffusive representation of the advection, especially on the horizontal, is very important to reduce the bias in the isotopic contents of precipitation above this area. The choice of an appropriate representation of the advection is thus essential when using GCMs for paleoclimate applications based on polar water isotopes. Too much diffusive mixing along the poleward transport leads to overestimated isotopic contents in water vapor because dehydration by mixing follows a more enriched path than dehydration by Rayleigh distillation. The near-air surface temperature is also influenced, to a lesser extent, by the diffusive properties of the advection scheme directly via the advection of the air and indirectly via the radiative effects of changes in high cloud fraction and water vapor. A too diffusive horizontal advection increases the temperature and so also contributes to enrich the isotopic contents of water vapor over Antarctica through a reduction of the distillation. The temporal relationship, from Last Glacial Maximum (LGM) to present-day conditions, between the mean annual near-air surface temperature and the water isotopic contents of precipitation for a specific location can also be impacted, with significant consequences on the paleo-temperature reconstruction from observed changes in water isotopes.
  • Variation of melting processes and magma sources of the early Deccan flood
           basalts, Malwa Plateau, India
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): K.M. Haase, M. Regelous, S. Schöbel, T. Günther, H. de Wall The Deccan flood basalts cover large parts of India and represent one of the largest volcanic provinces on Earth. We present geochemical and isotopic data on a magnetically defined stratigraphic section of lavas from the northern portion of the Deccan, the Malwa Plateau, which are dated at ∼67.5 Ma. These are therefore some of the oldest Deccan lavas, erupted before the Main Deccan units further to the south. Although the younger Malwa Plateau lavas have undergone significant crustal contamination by at least two distinct crustal components, the older, more alkalic lavas are less contaminated and have similar isotope and trace element compositions to Réunion oceanic intraplate basalts. Our data therefore provide geochemical evidence for a contribution of the Réunion hotspot to the generation of the early Deccan flood basalts. Most lavas reflect fractional crystallization and assimilation of less than 10% of a crustal melt but samples with the highest SiO2 and Sr isotope compositions underwent more extreme crustal contamination. The composition of the least contaminated basalts from the Malwa Plateau suggests that the degree of partial melting increased significantly from the base to the top of the section. Most Malwa Plateau basaltic magmas formed by
  • In situ evidence of earthquakes near the crust mantle boundary initiated
           by mantle CO2 fluxing and reaction-driven strain softening
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Bjørn Eske Sørensen, Thomas Grant, Eric James Ryan, Rune B. Larsen This study aims to understand the process behind the worldwide connection between deep crustal/upper mantle earthquakes and CO2 emissions along faults in rift zones. We do this by studying CO2-induced mineral reactions that facilitate strain localization in peridotites from an ancient rift zone in the Seiland Igneous Province (SIP), North Norway.Strain localization in association with hydration processes is well documented in all types of tectonic settings and has major implications for rheological behavior in active plate margin processes. The implications of CO2-bearing fluids are less studied, though experiments have shown how CO2 can influence the flow laws of olivine by imposing a brittle and more localized type of deformation.This study documents narrow shear zones observed within ultramafic rocks from the Seiland Igneous Province (SIP) comprising large volumes (>20,000 km3) of mafic, ultramafic, silicic and alkaline melts that were emplaced into the lower continental crust (25–30 km) between 570 and 560 Ma under an extensional regime. The extensional shear zones are mm cm-scale and contain extremely fine-grained material with a distinct shape preferred orientation (SPO), but weak to absent crystallographic preferred orientation. The shear zones offset dykes across numerous micro-faults that are documented in areas close to a major fault zone cutting through the area. Within the shear zones, olivine and clinopyroxene react to form orthopyroxene and dolomite at approximately 11 kb and 850 °C according to the reaction:2 Olivine + Clinopyroxene + 2 CO2 = Dolomite + 2 OrthopyroxeneThis reaction formed coronas of orthopyroxene and dolomite between olivine and clinopyroxene in the shear zones. In addition, large olivine grains proximal to the shear zones show a microfabric with subgrain walls decorated by rounded grains of dolomite and more irregular and elongated grains of orthopyroxene. Clinopyroxene grains are separated from the enstatite and dolomite by at least hundreds of microns, suggesting material transport within the shear zone. The shear zones thus provide a unique insight into the interplay between CO2-metasomatism and reaction accommodated strain softening. Carbonation-driven cracking and mineral reaction also serves to reduce grain size, making grain boundary sliding an efficient process, further enhancing the rheological contrast between the shear zone and the host rock. The sudden decrease in rock strength could lead to rapid deformation and triggered pseudotachylite formation during earthquake events in the near proximity of the micro-shear zones. Our observations match the relations between CO2 emissions and earthquakes observed in present rift environments such as the East African rift and in New Zealand, and underline the importance of active shear zones as fluid conduits in the lower crust and upper mantle.
  • Constraints on the volume and rate of Deccan Traps flood basalt eruptions
           using a combination of high-resolution terrestrial mercury records and
           geochemical box models
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Isabel M. Fendley, Tushar Mittal, Courtney J. Sprain, Mark Marvin-DiPasquale, Thomas S. Tobin, Paul R. Renne Deccan Traps continental flood basalt eruptions spanned the Cretaceous-Paleogene mass extinction, erupting over a million cubic kilometers of basalt over a total duration of approximately a million years. The environmental consequences of flood basalt eruptions depend on the timing and amount of volatile release; eruption rates are thus needed to evaluate their potential to cause climate change. Radioisotopic dates are not currently sufficient to resolve sub-ten thousand year eruptive tempos, necessary for constraining the effects of short-lifetime volatiles including sulfur dioxide. Recent studies have demonstrated that increases in mercury concentration in sedimentary records correlate with flood basalt eruptions under some circumstances. However, mercury concentrations have primarily been used to show the presence or absence of flood basalt eruptions. We show that this proxy can be used to quantitatively estimate eruptive rates using a mercury geochemical cycle framework. We illustrate this using new mercury chemostratigraphic records from terrestrial Cretaceous-Paleogene boundary sections in eastern Montana, USA, with multiple high-resolution chronologic constraints. We estimate that Deccan eruptions lasted on the order of centuries and released 500–3000 megagrams (Mg) of mercury per year, corresponding to ∼50–250 km3/a of lava. The box model framework highlights the importance of carefully accounting for differences in sedimentation rate and sampling resolution when comparing mercury records from different locations and depositional environments. While there are uncertainties in the box model estimates due to possible variation in flood basalt mercury emissions and sedimentation rates, they provide a useful framework to quantitatively evaluate the global mercury budget change indicated by changing concentration in sedimentary records. Eruptions of the estimated size would have released enough SO2, if it reached the stratosphere, to cause significant cooling for the duration of the eruption. However, given our constraints on the duration of individual eruptions, these colder periods are likely too brief to be clearly visible in most existing paleoclimate records.
  • Frozen fringe explains sediment freeze-on during Heinrich events
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Colin R. Meyer, Alexander A. Robel, Alan W. Rempel Anomalous coarse-grained sediment layers beneath the North Atlantic likely originated from sediment freeze-on to the base of ice sheets during the last glacial period. These layers represent periods of extreme ice discharge, called Heinrich events, and are variously attributed to ice stream flow instability, ice shelf collapse, or enhanced terminus melting due to ocean warming. In this paper, we study the processes controlling how sediment freezes on to the base of ice streams and predict the volume of sediment conveyed by icebergs during a Heinrich event. The local thickness of frozen sediment is sensitive to the heat flux at the ice-bed interface and the water pressure, both of which also contribute to the controls on basal friction; as the basal water pressure increases, both the frozen sediment thickness and the basal friction decrease. The sediment discharged during a Heinrich event must have frozen on to the ice during the inter-Heinrich period. As the Heinrich event proceeds, the frozen sediment melts off the base of the ice stream, indicating that the thickness of sediments deposits in the North Atlantic may not reliably constrain Heinrich event duration. Choosing reasonable parameters corresponding to the Hudson Strait Ice Stream, our model of sediment freeze-on and discharge is consistent with observational estimates of Heinrich event sediment discharge volume.
  • Linking continental erosion to marine sediment transport and deposition: A
           new implicit and O(N) method for inverse analysis
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): X.P. Yuan, J. Braun, L. Guerit, B. Simon, B. Bovy, D. Rouby, C. Robin, R. Jiao The marine sedimentary record contains unique information about the history of erosion, uplift and climate of the adjacent continent. Inverting this record has been the purpose of many numerical studies. However, limited attention has been given to linking continental erosion to marine sediment transport and deposition in large-scale surface process evolution models. Here we present a new numerical method for marine sediment transport and deposition that is directly coupled to a landscape evolution algorithm solving for the continental fluvial and hillslope erosion equations using implicit and O(N) algorithms. The new method takes into account the sorting of grain sizes (e.g., silt and sand) in the marine domain using a non-linear multiple grain-size diffusion equation and assumes that the sediment flux exported from the continental domain is proportional to the bathymetric slope. Specific transport coefficients and compaction factors are assumed for the two different grain sizes to simulate the stratigraphic architecture. The resulting set of equations is solved using an efficient (O(N) and implicit) algorithm. It can thus be used to invert stratigraphic geometries using a Bayesian approach that requires a large number of simulations. This new method is used to invert the sedimentary geometry of a natural example, the Ogooué Delta (Gabon), over the last ∼5 Myr. The objective is to unravel the set of erosional histories of the adjacent continental domain compatible with the observed geometry of the offshore delta. For this, we use a Bayesian inversion scheme in which the misfit function is constructed by comparing four geometrical parameters between the natural and the simulated delta: the volume of sediments stored in the delta, the surface slope, the initial and the final shelf lengths. We find that the best-fit values of the transport coefficients for silt in the marine domain are in the range of 300−500 m2/yr, in agreement with previous studies on offshore diffusion. We also show that, in order to fit the sedimentary geometry, erosion rate on the continental domain must have increased by a factor of 6 to 8 since 5.3 Ma.
  • Stability of Fe-bearing hydrous phases and element partitioning in the
           system MgO–Al2O3–Fe2O3–SiO2–H2O in Earth's lowermost mantle
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Hongsheng Yuan, Li Zhang, Eiji Ohtani, Yue Meng, Eran Greenberg, Vitali B. Prakapenka We performed high pressure–temperature (P-T) experiments on a model composition of hydrous subducted slabs in the MgO–Al2O3–Fe2O3–SiO2–H2O system using laser-heated diamond anvil cells. The phase assemblages were characterized combining in-situ synchrotron X-ray diffraction and ex-situ transmission electron microscope techniques. The hydrous δ-phase AlOOH–FeOOH–MgSiO2(OH)2–SiO2 coexists with bridgmanite (Bdg), post-perovskite (pPv), or both in a broad P-T range of 104–126 GPa and 1750–2500 K. The hydrous pyrite-type FeOOHx phase was observed in-situ over a P-T range of 112–123 GPa and 1750–2300 K, coexisting with the pPv phase. Chemical analysis on recovered samples showed that considerable amount of Fe2O2(OH)2 (8–13 mol%) and SiO2 (9–13 mol%) in the δ-phase does not reduce its thermal stability compared to the Al-endmember, indicating that the Fe-bearing δ-phase can transport water to the lowermost mantle along the mantle geotherm. In this hydrous system, we observed that Al depletion in both the Bdg and pPv phases can significantly reduce the width of the Bdg to pPv transition in contrast to a wide two-phase coexistence region in a dry Al-rich system. Meanwhile, the Fe enrichment in the pPv phase relative to the coexisting Bdg phase lowers the transition pressure to the depth of the D″ discontinuity. Accordingly, the depth and thickness of the Bdg to pPv transition in subducted basaltic crustal materials can explain the seismically detected D″ discontinuity. Partial melting could be triggered by dehydration of the Fe-bearing hydrous phases due to a steep temperature gradient at the core mantle boundary (CMB), and therefore the ultralow-velocity zones (ULVZs) might be the regions where partial melting occurs at the lowermost mantle.
  • Crustal maturation through chemical weathering and crustal recycling
           revealed by Hf–O–B isotopes
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Gong-Jian Tang, Qiang Wang, Derek A. Wyman, Wei Dan Juvenile continental crust is dominantly formed at intra-oceanic arcs via subduction zone magmatism. However, it remains unclear how basaltic oceanic arcs convert to granodioritic continental compositions. Here we present zircon U–Pb and Hf–O isotope data, whole-rock B isotope compositions, combined with a synthesis of over 1100 geochemical analyses from magmatic rocks that span a wide range of emplacement ages (∼490–270 Ma), from the Junggar intra-oceanic arc of the southern Central Asian Orogenic Belt (CAOB). Geochemical data show that the Junggar evolved from a juvenile oceanic arc composition (low SiO2, K2O and Rb, and high MgO contents) to one closer to continental crust (high SiO2, K2O and Rb, and low MgO contents) at ca. 300 Ma. All samples show very high and uniform Hf isotope ratios with εHf(t) values from +10.6 to +14.4. Zircon δ18O values and whole-rock δ11B values, however, are highly variable. The Silurian – Carboniferous (pre-300 Ma) rocks have distinctly lower zircon δ18O values ranging from 5.16 to 6.72‰ with an average of 5.78‰, and high δ11B values (−7.5–+12.2‰), suggesting that they were derived from the asthenospheric mantle wedge, which supports the genesis of primitive intra-oceanic crust during that time interval. In contrast, the Early Permian (post-300 Ma) rocks display much higher zircon δ18O values (8.24 to 10.29‰) and lower δ11B values (−9.0 to −12.2‰), combined with the presence of Carboniferous inherited zircons, which requires a source component comprising young, weathered, volcanogenic sediments from the Carboniferous Junggar intra-oceanic arc. The Early Permian rocks were produced by low degrees of partial melting of all sources (ca. 10%) and element and Hf–O–B isotope mixing calculations indicate a contribution of more than ∼50% from weathered sediments in those sources. Thus, the evolution of the Junggar segment from a primitive basaltic intra-oceanic arc toward a young continent was related to the recycling of crustal residues of chemical weathering and the younger magmatism was generated by crustal melting of these weathering products after they were buried to lower crustal depths. Our study highlights that an intra-oceanic arc's own chemical weathering history promotes its transformation into continental crust during collisional events and clarifies the relationship between continental crust formation and intra-oceanic arcs through time.
  • Geochemistry constrains global hydrology on Early Mars
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Edwin S. Kite, Mohit Melwani Daswani Ancient hydrology is recorded by sedimentary rocks on Mars. The most voluminous sedimentary rocks that formed during Mars' Hesperian period are sulfate-rich rocks, explored by the Opportunity rover from 2004–2012 and soon to be investigated by the Curiosity rover at Gale crater. A leading hypothesis for the origin of these sulfates is that the cations were derived from evaporation of deep-sourced groundwater, as part of a global circulation of groundwater. Global groundwater circulation would imply sustained warm Earthlike conditions on Early Mars. Global circulation of groundwater including infiltration of water initially in equilibrium with Mars' CO2 atmosphere implies subsurface formation of carbonate. We find that the CO2 sequestration implied by the global groundwater hypothesis for the origin of sulfate-rich rocks on Mars is 30–5000 bars if the Opportunity data are representative of Hesperian sulfate-rich rocks, which is so large that (even accounting for volcanic outgassing) it would bury the atmosphere. This disfavors the hypothesis that the cations for Mars' Hesperian sulfates were derived from upwelling of deep-sourced groundwater. If, instead, Hesperian sulfate-rich rocks are approximated as pure Mg-sulfate (no Fe), then the CO2 sequestration is 0.3–400 bars. The low end of this range is consistent with the hypothesis that the cations for Mars' Hesperian sulfates were derived from upwelling of deep-sourced groundwater. In both cases, carbon sequestration by global groundwater circulation actively works to terminate surface habitability, rather than being a passive marker of warm Earthlike conditions. Curiosity will soon be in a position to discriminate between these two hypotheses. Our work links Mars sulfate cation composition, carbon isotopes, and climate change.
  • Electrical resistivity imaging of the inter-plate coupling transition at
           the Hikurangi subduction margin, New Zealand
    • Abstract: Publication date: 15 October 2019Source: Earth and Planetary Science Letters, Volume 524Author(s): Wiebke Heise, Yasuo Ogawa, Edward A. Bertrand, T. Grant Caldwell, Ryokei Yoshimura, Hiroshi Ichihara, Stewart L. Bennie, Kaori Seki, Zenshiro Saito, Yasuo Matsunaga, Atsushi Suzuki, Takahiro Kishita, Yusuke Kinoshita Inter-plate coupling on the Hikurangi subduction margin along the east coast of New Zealand's North Island changes from weakly coupled in the north to locked in the south. 3-D inverse modeling of magnetotelluric (MT) data across the coupling transition shows that the electrical resistivity structure is correlated with the areal strain rate, which provides a measure of plate coupling. The correlation between strain rate and resistivity is seen parallel to the strike and dip directions of the subduction. In the region where the strain is extensional, the upper plate is more conductive than in the south where contraction is occurring, and the plates are locked. The increased mid-crustal resistivity in the south can be interpreted to be a consequence of reduced fluid interconnectivity due to contraction or reduced fluid content due to decreased upward transport from sources beneath the subduction interface. The increased mid-crustal conductivity in the extensional area is interpreted to be a consequence of fluid released by subducted sediment that are also the cause of the extension. Seismicity in the upper 5 km of the subducting plate shows similar changes, with a greater concentration of near-interface seismicity beneath the conductive (extensional) region. We interpret the concentration of seismicity below the subduction interface in the extensional region to show fluid sourced from dehydration reactions within the subducting plate or from the de-watering of subducted sediments rising into the upper plate and decreasing the resistivity of the middle crust. Decreased near-interface seismicity beneath the contractional region can be interpreted to be a consequence of decreased upper plate permeability preventing upward fluid escape or reduced lower plate fluid availability.
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