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Contributions to Mineralogy and Petrology
Journal Prestige (SJR): 2.747

Citation Impact (citeScore): 3
Number of Followers: 11

Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1432-0967 - ISSN (Online) 0010-7999
Published by Springer-Verlag

[2469 journals]

Correction to: Partitioning of Fe2O3 in peridotite partial melting
experiments over a range of oxygen fugacities elucidates ferric iron
systematics in mid-ocean ridge basalts and ferric iron content of the
upper mantle
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  PubDate: 2022-06-27
Widespread refertilization of cratonic lithospheric mantle related to
circum-craton plate subduction: evidence from peridotite xenoliths from
the central North China Craton
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  Abstract: Abstract Subduction of the Pacific slab has been widely credited for the destruction of the eastern North China Craton (NCC). However, the nature of the subcontinental lithospheric mantle (SCLM) beneath the central and western NCC is less well constrained, which has hindered the understanding of cratonic responses to repeated Phanerozoic circum-craton subductions. Here, we report the in situ major- and trace-element contents and Sr isotopic compositions of minerals from 27 peridotite xenoliths obtained from Cenozoic basalts from Xiyang‒Pingding, central NCC, to gain new insights into how plate subduction affected the Archean lithospheric keel. On the basis of olivine major-element and clinopyroxene trace-element contents, as well as clinopyroxene Sr isotopic compositions, we divide the xenoliths into four types. The high-Mg# types 1 and 2 peridotites show high olivine forsterite (Fo) contents, convex-upward rare-earth element patterns, and high 87Sr/86Sr (0.7053‒0.7062) in clinopyroxene, and they could be relicts of an old cratonic root. The high-Mg# type 3 peridotites exhibit high (La/Yb)N and low Ti/Eu and 87Sr/86Sr (0.7038‒0.7044), and they are interpreted as reaction products of Archean SCLM and carbonatitic melts from the asthenosphere with a contribution of recycled oceanic components. The low-Mg# type 4 peridotites display low 87Sr/86Sr (0.7029‒0.7033) and represent fragments of modified SCLM that has been recently refertilized by asthenospheric melts. Complex architectures of the SCLM underneath Xiyang‒Pingding reflect multistage overprinting of mantle metasomatism. Recent influxes of asthenospheric melts played a critical role in transforming refractory SCLM to fertile SCLM. Combined with previous studies, our findings suggest that the SCLM beneath the central NCC became progressively more refertilized from the center toward both the northern and southern margins, a spatial pattern that was likely generated by N‒S-trending subductions of the circum-craton plates.
  PubDate: 2022-06-24
Arsenopyrite oxidative dissolution in NaCl solution at high-temperature
and high-pressure conditions: kinetics, pathways, dissolution mechanism
and geological implications
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  Abstract: Abstract Arsenopyrite (FeAsS) is one of the sulfide minerals of seafloor massive sulfide deposits. The presence of sodium chloride and high-temperature and high-pressure (HTHP) geological conditions seriously affect the process of arsenopyrite weathering. However, electrochemical oxidative dissolution has never been considered in the context of seafloors, though it has already been shown to increase dissolution significantly in terrestrial deposits. In this work, in situ electrochemical techniques and surface analysis were used to investigate the behaviors of oxidative arsenopyrite dissolution in different concentrations of NaCl at temperatures ranging from 280 to 360 °C and pressures ranging from 12.0 to 20.0 MPa. In the initial stage, arsenopyrite was oxidized to S0, As(III), and Fe(II). The S0 and As(III) were ultimately converted into SO42− and AsO43− and entered the solution. The Fe(II) was converted into α-FeOOH, γ-FeOOH, and Fe2O3 as a passivation film. The presence of Cl− ions promoted the oxidative dissolution of arsenopyrite without changing its oxidation mechanism. Higher temperatures or greater pressures promoted the oxidative dissolution of arsenopyrite by enhancing charge migration and ion diffusion. Under the experimental HTHP conditions, the oxidative arsenopyrite dissolution rate constant was 8.0 × 10–5 mol∙m−2∙s−1. This work expands the understanding of the geochemical cycles of Fe, As and S and provides an experimental basis for the formation of secondary minerals from arsenopyrite weathering under the hydrothermal solution conditions of the seafloor.
  PubDate: 2022-06-23
Slab break-off-related magnesian andesites and dacites with adakitic
affinity from the early Quaternary Keçiboyduran stratovolcano, Cappadocia
province, central Turkey: evidence for slab/sediment melt–mantle
interaction and magma mixing
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  Abstract: Abstract Voluminous moderate- to high-magnesian [Mg# = molar Mg/(Mg + Fe2+) = 44–64] andesitic and dacitic rocks with high silica (mostly 61–66 wt%) adakitic affinity (Y = 13–22, Yb = 1.3–2.1, Sr/Y = 18–44, La/Yb = 10–25) and common mafic magmatic enclaves (MMEs) are first reported in the Keçiboyduran stratovolcano (KSV) from the Cappadocia volcanic province (CVP), Central Anatolia, Turkey. We present comprehensive whole-rock geochemistry and Sr–Nd–Pb isotope data, mineral chemical compositions and 40Ar–39Ar ages for KSV samples. Based on the volcanostratigraphy and 40Ar–39Ar dating results, two successive eruption ages of 2.2–1.6 Ma (stage I: amphibole-rich) and 1.6–1.2 Ma (stage II: pyroxene-rich) were established for the KSV, corresponding to the Gelasian and Calabrian stages of Early Pleistocene, respectively. Textural and geochemical evidence indicates that the KSV magnesian andesites–dacites are products of a hybrid magma formed by mixing between mantle-derived mafic and crust-derived felsic magmas with further fractionation and minor contamination during magma storage and ascent. Our new data, combined with previous geological and geophysical results suggest that parental magnesian mafic melts of the KSV rocks originated from a heterogenous mantle source generated through the metasomatism of mantle wedge material by subducted sediment-derived melts, and then partially melted through asthenospheric upwelling in response to slab break-off. The mafic magma underplated the overlying lower crust, resulting in its partial melting to generate crustal felsic magma. Both magmas mixed at lower crustal levels creating MME-rich hybrid magmas. Subsequently, the hybrid magmas were emplaced at different depths of the crust (c. 4–11 and 11–15 km for the stage I and II, respectively), where they crystallized at moderate temperatures (c. 1180–840 °C) and under relatively high oxygen fugacity (LogƒO2 = − 11.4 to − 9.2), water-rich (H2Omelt = 5.6–3.6 wt%) and polybaric (~ 1.2 to 5.1 kbars) conditions, and underwent fractionation of primarily amphibole ± pyroxene causing adakitic affinity. We propose a new petrogenetic model for the early Quaternary magnesian/adakitic andesites/dacites of the CVP in a post-subduction tectonic setting. Our results provide robust evidence for slab break-off of the eastern Cyprus oceanic lithosphere and put further constraints on the tectonic evolution of the eastern Mediterranean collision zone during the Early Quaternary.
  PubDate: 2022-06-23
Variation of plagioclase shape with size in intermediate magmas: a window
into incipient plagioclase crystallisation
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  Abstract: Abstract Volcanic rocks commonly display complex textures acquired both in the magma reservoir and during ascent to the surface. While variations in mineral compositions, sizes and number densities are routinely analysed to reconstruct pre-eruptive magmatic histories, crystal shapes are often assumed to be constant, despite experimental evidence for the sensitivity of crystal habit to magmatic conditions. Here, we develop a new program (ShapeCalc) to calculate 3D shapes from 2D crystal intersection data and apply it to study variations of crystal shape with size for plagioclase microlites (l < 100 µm) in intermediate volcanic rocks. The smallest crystals tend to exhibit prismatic 3D shapes, whereas larger crystals (l > 5–10 µm) show progressively more tabular habits. Crystal growth modelling and experimental constraints indicate that this trend reflects shape evolution during plagioclase growth, with initial growth as prismatic rods and subsequent preferential overgrowth of the intermediate dimension to form tabular shapes. Because overgrowth of very small crystals can strongly affect the external morphology, plagioclase microlite shapes are dependent on the available growth volume per crystal, which decreases during decompression-driven crystallisation as crystal number density increases. Our proposed growth model suggests that the range of crystal shapes developed in a magma is controlled by the temporal evolution of undercooling and total crystal numbers, i.e., distinct cooling/decompression paths. For example, in cases of slow to moderate magma ascent rates and quasi-continuous nucleation, early-formed crystals grow larger and develop tabular shapes, whereas late-stage nucleation produces smaller, prismatic crystals. In contrast, rapid magma ascent may suppress nucleation entirely or, if stalled at shallow depth, may produce a single nucleation burst associated with tabular crystal shapes. Such variation in crystal shapes have diagnostic value and are also an important factor to consider when constructing CSDs and models involving magma rheology.
  PubDate: 2022-06-22
Common assumptions and methods yield overestimated diffusive timescales,
as exemplified in a Yellowstone post-caldera lava
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  Abstract: Abstract To interpret modern-day unrest at Yellowstone Caldera, timescales leading up to its most common type of eruption—effusively emplaced rhyolite—must be quantified. This work takes advantage of the different rates of elemental diffusion in clinopyroxene to calculate the magmatic timescales of events preceding eruption of the ca. 262 ka Scaup Lake rhyolite, which ended ~ 220,000 years of dormancy in this high-silica system. Here, we present diffusion chronometry timescales accounting for various sources of error and using multiple elements from NanoSIMS measurements of clinopyroxene rims. We combine these with previously published timescales from sanidine rims to better understand the relationship between timescales captured by different minerals from the same volcanic event. We show that timescales archived by rims of different types of phenocrysts from the same lava may not be concomitant. The Scaup Lake rhyolite appears to have undergone several rejuvenation events over ~ 5000 years before its eruption, and the last events (< 40 years before eruption) were not recorded by clinopyroxene. This work highlights the importance of using multiple methods to determine a timescale for a given process. Although many studies use Fe–Mg zonation from BSE images to calculate diffusive timescales alone, we show that these are maximums or overestimates if not referenced to the appropriate initial condition. Instead, we demonstrate that diffusion chronometry conducted with multiple elements in multiple mineral phases with rigorous error propagation produces the most robust and accurate temporal results. In addition, we recommend that diffusion chronometry results not be interpreted in isolation, but rather in a holistic petrological approach that includes consideration of the relevant phase equilibria and crystal growth and dissolution rates.
  PubDate: 2022-06-21
Long-lived dacitic magmatic systems and recharge dynamics in the Jemez
Mountains volcanic field, western USA
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  Abstract: Abstract We examine the connected history of dacite-dominant volcanic rocks of the Tschicoma Formation, erupted between 5.5 and 2 Ma from the Jemez Mountains volcanic field, western USA. Zircon samples from two separate eruptions have continuous SHRIMP U–Pb age spectra spanning 0.84–1.08 Myr duration (3.12–3.96 Ma and 3.50–4.58 Ma, respectively), following an episode of zircon crystallization 0.28–0.50 Myr earlier (at 4.46 Ma and 4.86 Ma, respectively). Zircon chemical variations, as well as ubiquitous resorption textures that commonly show large core-rim age differences (up to 720–740 kyr), suggest that they grew in separate melt lenses. Zircons were likely stored at near-solidus or even sub-solidus conditions after crystallization, but may have been reactivated in response to at least four major magma recharge events every 300–400 kyr and smaller events in between. A cycle of zircon dissolution (from heating), recrystallization (during cooling), and storage repeated in different locations in the Tschicoma mush system throughout its lifespan; each recharge-induced heating stage may last for several hundred to more than a thousand years based on calculations of zircon dissolution. We envisage the melt lenses to be distributed in a crystal mush zone, coalescing into a single magma batch as magma recharge occurs shortly before eruption. Once active, increasing magma supply rates may trigger large-scale partial melting of the pre-existing mush and caldera-forming eruptions.
  PubDate: 2022-06-18
Reactive fluid flow guided by grain-scale equilibrium reactions during
eclogitization of dry crustal rocks
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  Abstract: Abstract Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to ‘wet’ eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P–T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for ‘dry’ and ‘wet’ cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains.
  PubDate: 2022-06-15
Mineralogical and fluid inclusion constraints on the formation of the
Karakorum Migmatite: implications for H2O-fluxed melting and exhumation of
the South Tibetan Crust
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  Abstract: Abstract This study presents new petrological and fluid inclusion datasets of migmatites from the Karakorum Shear Zone, Ladakh, India, to know the P–T–fluid evolution of mineral assemblages and the associated tectonic history. The presence of plagioclase, quartz, and biotite inclusions in the coarse-grained poikiloblastic pargasite (amphibole) is indicative of the hydration reaction \(bt+pl+\mathrm{qtz}+\mathrm{H}2\mathrm{O} = \mathrm{prg}+\mathrm{melt}\) , which is consistent with diffusive H2O-fluxed melting. Phase equilibria calculations are consistent with migmatization at 0.85–1.02 GPa and 640–670 °C in water-saturated conditions (i.e., 0.7 wt% H2O). The monophase primary and secondary carbonic fluid inclusions present in quartz display eutectic temperatures between − 56.9 and − 56.6 °C, suggesting pure CO2 composition. The isochores of primary CO2 inclusions reveal that the post-peak migmatization event took place between 0.59–0.55 GPa and 550–670 °C, which occurred due to density reversal during the re-equilibration. The fluid inclusion microtextures preserved the signature of isothermal decompression, which is well corroborated with mineralogical P–T calculations. Primary inclusions were preserved initially as carbonic-aqueous fluids; however, the H2O phase diffused out subsequently and dissolved with the melt such that the inclusions became pure carbonic. Fluid infiltration along the Karakorum Shear Zone played a critical role in forming migmatites. The P–T path derived from thermodynamic modeling and fluid inclusion data are consistent with isothermal decompression during exhumation following crustal thickening of the Asian continent (or South Tibaten Crust) between 18 and 15 Ma.
  PubDate: 2022-06-12
A journey towards the forbidden zone: a new, cold, UHP unit in the
Dora-Maira Massif (Western Alps)
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  Abstract: Abstract The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira Massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit. It is only a few tens of metres thick and consists of graphite-rich, garnet–chloritoid micaschists, whose protoliths may be black shales of Silurian age. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P–T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25–27 kbar 470–500 °C) (stage 1), up to the coesite stability field (27–28 kbar 510–530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500–515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite–ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P–T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P–T conditions are comparable to the ones of the Tian Shan coesite–chloritoid-bearing rocks.
  PubDate: 2022-06-08
The formation of three-grain junctions during solidification. Part II:
theory
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  Abstract: Abstract We provide a simple geometric theory of crystal growth which predicts the shape and final dihedral angle of three-grain junctions of an augite crystal with two plagioclase grains. The predicted dihedral angle \(\Delta \) depends on the initial impingement angle \(\psi \) formed by the plagioclase grains, and also on the relative growth rates of the augite and the plagioclase, and shows reasonable agreement with data obtained from natural samples. We show that the two augite-plagioclase grain boundaries will normally curve towards each other, which is consistent with the first two types of junction described in the companion paper. However, the third type, the eagle’s beak, is formed by the meeting of grain boundaries which curve in the same direction. Although it is possible to account for this type of junction by invoking the localised dissolution of one of the plagioclase grains, this is unlikely to occur. A more plausible explanation involves the late impingement of the two plagioclase grains, consistent with the observation that eagles’ beaks are common in gabbros and strongly orthocumulate troctolites, in which the plagioclase framework has not been established by the time augite is growing in substantial quantities. An observed flattening of the curve of \(\Delta \) values at high values of \(\psi \) can be explained by taking into account the importance of interfacial energy in late-stage crystallisation.
  PubDate: 2022-06-07
Atoll garnet: insights from LA-ICP-MS trace element mapping
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  Abstract: Abstract Atoll garnets are uncommon features that have been recognized in contrasting metamorphic environments worldwide, but their origin remains largely debated. Several models have been proposed to explain their formation, including preferential dissolution of garnet cores by fluid infiltration, polymetamorphism, and the coalescence of subgrains. We report atoll-shaped garnets in an amphibolite facies schist from the Palaeoproterozoic New Quebec Orogen, Canada, and investigate their textural and chemical zoning through petrography, electron probe microanalysis (EPMA) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) maps. Textural evidence indicates a subhedral poikiloblastic core, an inclusion ring composed of matrix minerals, and a euhedral rim. Major element distribution maps show flat zoning, whereas trace elements show concentric growth zoning. Such characteristics are consistent with rapid, post-kinematic growth involving Rayleigh fractionation of trace elements and coeval with accessory phase breakdown. Our observations rule out the preferential dissolution, polymetamorphism and coalescence models, and support that the formation of atoll garnet in these rocks is best explained by a kinetic control and rapid growth. Our study concludes that the term “atoll” is more descriptive than genetic, and that the physio-chemical mechanisms leading to its formation should be assessed on a case-by-case basis using complementary tools, primarily including trace element mapping.
  PubDate: 2022-05-29
Exhumation of a migmatitic unit through self-enhanced magmatic weakening
enabled by tectonic contact metamorphism (Gruf complex, Central European
Alps)
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  Abstract: Abstract The Central Alpine lower crustal migmatitic Gruf complex was exhumed in contact to the greenschist-grade Chiavenna ophiolite and gneissic Tambo nappe leading to a lateral gradient of ~ 70 °C/km within the ophiolite. The 14 km long, E-W striking subvertical contact now bridges metamorphic conditions of ~ 730 °C, 6.6 kbar in the migmatitic gneisses and ~ 500 °C, 4.2 kbar in the serpentinites and Tambo schists 2–4 km north of the contact. An obvious fault, mylonite or highly sheared rock that could accommodate the ~ 8.5 km vertical displacement is not present. Instead, more than half of the movement was accommodated in a 0.2–1.2 km thick orthogneiss of the Gruf complex that was heterogeneously molten. Discrete bands with high melt fractions (45–65%) now contain variably stretched enclaves of the adjacent MOR-derived amphibolite. In turn, the adjacent amphibolites exhibit tonalitic in-situ leucosomes and dikes i.e., were partially molten. The H2O necessary for fluid-assisted melting of the orthogneiss and amphibolites was likely derived from the tectonic contact metamorphism of the Chiavenna serpentinites, at the contact now in enstatite + olivine-grade. U–Pb dating of zircons shows that partial melting and diking occurred at 29.0–31.5 Ma, concomitant with the calc-alkaline Bergell batholith that intruded the Gruf. The major driving forces of exhumation were hence the strong regional North–South shortening in the Alpine collisional belt and the buoyancy provided by the Bergell magma. The fluids available through tectonic contact metamorphism led to self-enhanced magmatic weakening and concentration of movement in an orthogneiss, where melt-rich bands provided a low friction environment. Continuous heating of the originally greenschist Chiavenna ophiolite and Tambo gneisses + schists by the migmatitic Gruf complex during differential uplift explains the skewed temperature profile, with intensive contact heating in the ophiolite but little cooling in the portion of the now-exposed Gruf complex.
  PubDate: 2022-05-17
The formation of three-grain junctions during solidification. Part I:
observations
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  Abstract: Abstract The thermodynamic equilibrium dihedral angle at grain junctions in crystalline rocks is set by the grain boundary interfacial surface energies, but the long times required to attain equilibrium mean that the observed dihedral angles in igneous rocks are generally set by the kinetics of crystallisation. We distinguish three types of augite–plagioclase–plagioclase dihedral angle in mafic igneous rocks. In the first, augite grows in the pores of a pre-existing plagioclase framework accompanied by little to no inwards-growth of the plagioclase pore walls. In the second, the plagioclase pore walls grow inwards simultaneously with the augite, and the dihedral angle is generally larger than the original angle at which the two plagioclase grains impinged except when the impingement angle itself is large. The first type is seen in rapidly crystallised rocks, whereas the second is observed in slowly cooled rocks. The third type is highly asymmetric and resembles (and so we call) an eagle’s beak: it is only seen in slowly cooled rocks. It is common in gabbroic cumulates, and is also present in strongly orthocumulate troctolites. Using the mode of interstitial phases to calculate the amount of interstitial liquid present in a series of mafic cumulates from the Rum and Skaergaard layered intrusions, we show that the asymmetry of three-grain junctions in troctolites increases as the rocks progress from adcumulate to orthocumulate (i.e. as the olivine–plagioclase crystal mush becomes more liquid-rich), with eagles’ beaks becoming the dominant three-grain junction geometry for troctolitic mushes containing ∼ 12 vol.% interstitial material (corresponding to ∼ 30 vol.% liquid in the mush). The geometry of three-grain junctions in mafic rocks is thus a function not only of cooling rate, but also of the progression along the liquid line of descent during fractionation. The first two types of junction are formed in relatively primitive liquids, during which the crystal mushes on the margins of the solidifying magma body are formed predominantly of plagioclase and olivine, whereas the eagle’s beak geometry occurs once augite forms an important component of the crystal framework in the accumulating mush, either because it is a framework-forming primocryst phase or because it grows from highly abundant interstitial liquid.
  PubDate: 2022-05-11
Garnet EoS: a critical review and synthesis
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  Abstract: Abstract All available volume and elasticity data for the garnet end-members grossular, pyrope, almandine and spessartine have been re-evaluated for both internal consistency and for consistency with experimentally measured heat capacities. The consistent data were then used to determine the parameters of third-order Birch–Murnaghan EoS to describe the isothermal compression at 298 K and a Mie–Grüneisen–Debye thermal-pressure EoS to describe the PVT behaviour. In a full Mie–Grüneisen–Debye EoS, the variation of the thermal Grüneisen parameter with volume is defined as \(\gamma = {\gamma }_{0}{\left(\frac{V}{{V}_{0}}\right)}^{q}\) . For grossular and pyrope garnets, there is sufficient data to refine q which has a value of q = 0.8(2) for both garnets. For other garnets, the data do not constrain the value of q and we therefore refined a q-compromise version of the Mie–Grüneisen–Debye EoS in which both γ/V and the Debye temperature θ D are held constant at all P and T, leading to \(\left( {{\raise0.7ex\hbox{\({\partial C_{{\text{V}}} }\)} \!\mathord{\left/ {\vphantom {{\partial C_{{\text{V}}} } {\partial P}}}\right.\kern-0em} \!\lower0.7ex\hbox{\({\partial P}\)}}} \right)_{{\text{T}}} = 0\) , parallel isochors and constant isothermal bulk modulus along an isochor. Final refined parameters for the q-compromise Mie–Grüneisen–Debye EoS are: Pyrope Almandine Spessartine Grossular V0 (cm3/mol)a 113.13 115.25 117.92 125.35 K0T (GPa) 169.3 (3) 174.6 (4) 177.57 (6) 167.0 (2) \(K^{\prime}_{{0{\text{T}}}}\) 4.55 (5) 5.41 (13) 4.6 (3) 5.07 (8) θ D0
  PubDate: 2022-05-06
Links between continental subduction and generation of Cenozoic
potassic–ultrapotassic rocks revealed by olivine oxygen isotopes: A case
study from NW Tibet
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  Abstract: Abstract Tibetan Cenozoic syn-collisional potassic–ultrapotassic igneous rocks provide unique insights into the processes and origins of metasomatism in the upper mantle, as well as continental subduction and plateau uplift. Crustal recycling in the magma source of the Tibetan potassic–ultrapotassic rocks has been well-documented. However, the nature of the metasomatic agents and the timing of mantle metasomatism are still disputed. Oxygen isotopes are a powerful tracer for identifying any recycled crustal material in the mantle due to the significant fractionation caused by surface water–rock interaction. Here we present an integrated in-situ study of oxygen isotopes and the major/trace elements of olivine in the Cenozoic potassic–ultrapotassic rocks and mantle xenoliths from the western Kunlun area of the northwestern Tibet. Olivines from mantle xenoliths have oxygen isotope compositions that range from elevated δ18O values (5.83 ± 0.78–5.97 ± 0.40‰) to values (5.09 ± 0.56–5.10 ± 0.46‰) that are indistinguishable from typical mantle olivine values of 5.18 ± 0.28‰ (Mattey et al., Earth Planet Sci Lett 128:231–241, 1994). Elevated olivine δ18O values reflect mantle metasomatic processes by an 18O-rich agent, although a few olivine rims show higher δ18O values caused by reactions with host lavas. In contrast, olivine oxygen isotope compositions of potassic–ultrapotassic rocks are higher than typical “mantle” values and those of the mantle xenoliths (6.51 ± 0.74–7.52 ± 0.24‰). There is no sign of crustal contamination, and fractional crystallization did not change the isotopic compositions of the studied potassic–ultrapotassic igneous rocks, thus their high-δ18O signature reflects the nature of the primary melts and their source region. Enrichments in olivine 18O and clinopyroxene Sr isotopic compositions, as well as the enriched trace element compositions of clinopyroxene and orthopyroxene in the mantle xenoliths, indicate that they have been highly metasomatized by silicate materials with a minor carbonate component from recycled Indian continental crustal component, and the enriched lithospheric mantle is the magma source of potassic–ultrapotassic rocks. High olivine δ18O values are a common feature of Tibetan Cenozoic potassic–ultrapotassic rocks, consistent with the mantle metasomatic agents that were derived directly from recycled continental crust material via Indian continental subduction. Our study reinforces the profound link between continental subduction, mantle processes and generation of Tibetan Cenozoic potassic–ultrapotassic rocks.
  PubDate: 2022-05-03
An empirical H2O solubility model for peralkaline rhyolitic melts
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  Abstract: Abstract The H2O solubility in peralkaline haplogranitic melts has been experimentally determined as a function of pressure (27–200 MPa) and temperature (1123–1523 K). The compositions were based on Ab38Or34Qz28 (AOQ) with 4 and 8 wt% Na2O in excess. H2O solubility experiments were performed in an internally heated pressure vessel and quenched to glasses for analysis. For quantification of H2O contents in the glasses using FTIR analysis, the linear molar absorption coefficients as a function of Na2O excess with respect to AOQ composition were determined, as well as the glass densities as a function of H2O concentration. The H2O solubility increases with increasing pressure, decreasing temperature, and with increasing peralkalinity. A linear dependence between Na2O excess (wt%) and H2O solubility (wt%) was found. It has been previously shown that on a molar basis the different alkalis contribute similarly to the H2O solubility increase so that H2O solubility increases linearly with excess alkali (difference between mole fractions of alkalis and that of alumina). Thus, the dependence of H2O solubility on pressure, temperature and excess alkali obtained from the new data of this study allow a simple prediction of H2O solubility for peralkaline rhyolitic melts based on the excess alkali content. This new empirical model was tested with H2O solubility data from literature for peralkaline haplogranitic and natural peralkaline rhyolitic melt compositions, yielding good agreement (< 10% deviation) between predicted and observed H2O solubility, which is an improvement compared to previous models. The model can be applied to natural peralkaline rhyolitic melts that occur, e.g. on Pantelleria, Gran Canaria, or the East African Rift.
  PubDate: 2022-04-24
Mantle sources and melting processes beneath East Antarctica: geochemical
and isotopic (Sr, Nd, Pb, O) characteristics of alkaline and tholeiite
basalt from the Earth’s southernmost (87° S) volcanoes
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  Abstract: Abstract Mount Early and Sheridan Bluff (87° S) are the above-ice expression of Earth’s southernmost volcanic field and are isolated by > 1000 km from any other exposed Cenozoic volcano in Antarctica. These monogenetic, Early Miocene volcanoes consist of olivine-phyric basaltic pillow lavas and breccias (Mount Early) and pāhoehoe lavas (Sheridan Bluff) whose differentiation is controlled by the fractional crystallization of olivine with lesser quantities of clinopyroxene, plagioclase and magnetite. Fractional crystallization or contamination by crust cannot account for the coexistence of olivine tholeiite and alkaline compositions but their relationship can be explained by change from higher (5–6%) to lower (1.5–2%) degrees of partial melting concurrent with a decrease in peridotite‒melt reaction in a mantle that is heterogeneous on a small-scale. Both magma types have geochemical and isotopic signatures that differentiate them from most of the volcanism found within the West Antarctic rift system. Data trends in Sr–Nd–Pb isotope space indicate mixing of at least two-distinct mantle sources: (1) a relatively depleted component similar to sources for mid-ocean ridge basalt from the extinct Antarctic–Phoenix spreading center, and (2) an enriched component similar to sources for mafic magmas of the Jurassic Karoo‒Ferrar large igneous provinces. The availability of these mantle source types was facilitated by the detachment, sinking and heating of metasomatized continental lithosphere (enriched source) that released volatiles into the surrounding asthenosphere (depleted source) to promote flux melting. Volcanism triggered by lithospheric detachment is, therefore, explicitly applied to Mount Early and Sheridan Bluff to explain their isolation and enigmatic tectonic setting but also to account for source heterogeneity and the ephemeral change in degree of mantle partial melting recorded in their mafic compositions.
  PubDate: 2022-04-17
  DOI: 10.1007/s00410-022-01914-9
Hydrous mafic–ultramafic intrusives at the roots of a proto-arc:
implications for crust building and mantle source heterogeneity in young
forearc regions
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  Abstract: Abstract The New Caledonia ophiolite represents a rare example of proto-arc section originated during subduction infancy. The sequence is dominated by refractory harzburgites overlain by ultramafic (dunites and wehrlites) and mafic (gabbronorites) lithologies. In this contribution, we report the first occurrence of amphibole-bearing intrusives in the New Caledonia forearc sequence. This study deals with a petrological and geochemical investigation of a pyroxenite intrusion cut by mafic–ultramafic dikes. The intrusion consists of medium grain websterites, composed of orthopyroxene (30–75 vol %), clinopyroxene (20–50 vol %) and amphibole (2–30 vol %), which occurs as interstitial or poikilitic phase. Whole rocks display moderate Mg# (71–82) and concave downward REE patterns, bearing depleted to flat LREE (LaN/NdN = 0.5–1) and flat HREE segments (DyN/LuN = 0.8–1.1). Bulk rocks mirror clinopyroxene at higher absolute values. Fluid mobile element (FME) enrichments, coupled to Zr–Hf depletion, are observed for both clinopyroxene and bulk rock. Mineral major element variations and textural relationships indicate that the investigated lithotypes derived from hydrous magmas, which underwent extensive fractional crystallisation and post-cumulus processes. Geochemical modelling shows that the parental melts in equilibrium with the pyroxenites share remarkable similarities with the New Caledonia CE-boninite. However, they significantly differ from the equilibrium melts previously reported for the other intrusive rocks of the sequence. As a whole, our new results highlight a greater compositional variability for the liquids ascending into the Moho transition zone and lower crust. This may be also related to the involvement of a highly heterogeneous mantle source during subduction initiation.
  PubDate: 2022-04-16
  DOI: 10.1007/s00410-022-01912-x
Constraining the isotopic endmembers contributing to 1.1 Ga Keweenawan
large igneous province magmatism
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  Abstract: Abstract Continental flood basalt lavas often contain deeply-sourced, thermo-chemically anomalous material that can provide a potential probe of inaccessible reservoirs. However, continental flood basalts interact with geochemically diverse domains within the continental lithosphere, which may complicate interpretations of deep mantle signatures. We examine the role of continental lithospheric mantle in continental flood basalts erupted as part of the 1.1 Ga Keweenawan large igneous province, centered on the Lake Superior region of North America. We show that flood basalts at Mamainse Point exhibit a range of εHf 1100 from −14.1 to +6, plotting along the global εHf—εNd mantle array. Lithospheric mantle melts represented by alkaline rocks from the Coldwell and Seabrook Lake Complexes yield positive εNd 1100 (+0.7 to +4.3) and εHf 1100 from −6.9 to +2.4, placing them below the mantle array. Mamainse Point lavas are interpreted to be variably crustally contaminated melts of the Keweenawan plume and ambient upper mantle; there is no clear evidence for contributions from an enriched lithospheric mantle.
  PubDate: 2022-04-13
  DOI: 10.1007/s00410-022-01907-8