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1. Identifying lithological tracers with first row transition element partitioning in natural pyroxenites

   Lang, Otto ; Lambart, Sarah ( December 2020 , American Geophysical Union - Fall meeting 2020)

   null (Ed.)

   Mantle heterogeneity has a first-order control on the petrological and geochemical differences of erupted mafic lavas across the globe. It is debated whether this heterogeneity reflects only chemical variability or also lithological differences in source regions. Because of their various partitioning behaviors between mantle minerals, First Row Transition Elements (FRTEs) have been identified as potential lithological tracers. Here, we investigate the various parameters that control FRTE partitioning between common mantle phases through a comparison of partition coefficients calculated from natural pyroxenites obtained from the Earthchem database with previous partitioning experiments and new electron microprobe analyses. Using naturally occurring pyroxenites from alpine massifs and xenoliths provides the opportunity to explore the behavior of FRTEs on a much larger range of compositions and temperatures than covered by experimental studies. Our preliminary results show that natural partition coefficients for Fe and Mn depend on temperature and vary distinctly between lithologies. The effect of composition, however, is difficult to resolve and will require further inspection. Natural exchange coefficients, or Kd’s (mineral/mineral) for Mn/Fe, largely match previous experimental data across peridotite and pyroxenite compositions for garnet/clinopyroxene(cpx), orthopyroxene/cpx, and olivine/cpx. However, natural samples often present evidence of chemical disequilibrium and/or secondary alteration which can significantly increase the scatter in analyses. Importantly, despite the larger uncertainty on the natural Kd’s than on experimental ones, natural exchange coefficients show distinct values between the various pairs of minerals. These distinctions, and the fact that Kd’s do not seem to be influenced by temperature, make the bulk Mn/Fe ratio in lavas a good lithological tracer, supporting previous claims. Hence, we show that natural compositions can be used to expand trends in FRTE distribution behavior across a wider range of temperatures (500-1500°C) and compositions than determined previously by experiments alone. 

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2. Origin of Compositional Gradients with Temperature in the High-SiO2 Rhyolite Portion of the Bishop Tuff: Constraints on Mineral–Melt–Fluid Reactions in the Parental Mush

   https://doi.org/10.1093/petrology/egab087  

   Jolles, Jameson S ; Lange, Rebecca A ( December 2021 , Journal of Petrology)

   Abstract The Bishop Tuff (BT), erupted from the Long Valley caldera in California, displays two types of geochemical gradients with temperature: one is related to magma mixing, whereas the other is found in the high-SiO2 rhyolite portion of the Bishop Tuff and is characterized by twofold or lower concentration variations in minor and trace elements that are strongly correlated with temperature. It is proposed that the latter zonation, which preceded phenocryst growth, developed as a result of mineral–melt partitioning between interstitial melt and surrounding crystals in a parental mush, from which variable melt fractions were segregated. To test this hypothesis, trends of increasing vs decreasing element concentrations with temperature (as a proxy for melt fraction), obtained from published data on single-clast pumice samples from the high-SiO2 rhyolite portion of the Bishop Tuff, were used to infer their relative degrees of incompatibility vs compatibility between crystals and melt in the parental mush. Relative compatibility values (RCVi) for all elements i, defined as the concentration slope with temperature divided by average concentration, are shown to be linearly correlated with their respective bulk partition coefficients (bulk Di). Mineral–melt partition coefficients from the literature were used to constrain the average stoichiometry of the crystallization/melting reaction in the parental mush: 32 % quartz + 34 % plagioclase + 31 % K-feldspar + 1·60 % biotite + 0·42 % titanomagnetite + 0·34 % ilmenite + 0·093 % allanite + 0·024 % zircon + 0·025 % apatite = 100 % liquid. The proportions of tectosilicates in the reaction (i.e. location of eutectic) are consistent with depths of melt segregation of ~400–550 MPa and an activity of H2O of ~0·4–0·6. Temperatures of <770–780 °C are constrained by allanite in the reaction. Evidence that a fluid phase was present in the parental mush is seen in the decreasing versus increasing H2O and CO2 contents with temperature in the segregated interstitial melt that formed the high-SiO2 rhyolite portion of the Bishop Tuff. The presence of an excess fluid phase, which strongly partitions CO2 relative to the melt, is required to explain the compatible behavior of CO2, whereas the fluid abundance must have been low to explain the incompatible behavior of H2O. Calculated degassing paths for interstitial melts, which segregated from the parental mush and ascended to shallower depths to grow phenocrysts, match published volatile analyses in quartz-hosted melt inclusions and constrain fluid abundances in the mush to be ≤1 wt%. The source of volatiles in the parental mush, irrespective of whether it formed by crystallization or partial melting, must have been primarily from associated basalts, as granitoid crust is too volatile poor. Approximately twice as much basalt as rhyolite is needed to provide the requisite volatiles. The determination of bulk Di for several elements gives the bulk composition of the parental leucogranitic mush and shows that it is distinct from Mesozoic Sierran arc granitoids, as expected. Collectively, the results from this study provide new constraints for models of the complex, multi-stage processes throughout the Plio-Quaternary, involving both mantle-derived basalt and pre-existing crust, that led to the origin of the parental body to the Bishop Tuff. 

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3. Trace element partitioning between olivine and melt in lunar basalts

   https://doi.org/10.2138/am-2022-7971  

   Chen, Sha ; Ni, Peng ; Zhang, Youxue ; Gagnon, Joel ( August 2022 , American Mineralogist)

   Abstract Mineral/melt partition coefficients have been widely used to provide insights into magmatic processes. Olivine is one of the most abundant and important minerals in the lunar mantle and mare basalts. Yet, no systematic olivine/melt partitioning data are available for lunar conditions. We report trace element partition data between host mineral olivine and its melt inclusions in lunar basalts. Equilibrium is evaluated using the Fe-Mg exchange coefficient, leading to the choice of melt inclusion-host olivine pairs in lunar basalts 12040, 12009, 15016, 15647, and 74235. Partition coefficients of 21 elements (Li, Mg, Al, Ca, Ti, V, Cr, Mn, Fe, Co, Y, Zr, Nb, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were measured. Except for Li, V, and Cr, these elements show no significant difference in olivine-melt partitioning compared to the data for terrestrial samples. The partition coefficient of Li between olivine and melt in some lunar basalts with low Mg# (Mg# < 0.75 in olivine, or < ~0.5 in melt) is higher than published data for terrestrial samples, which is attributed to the dependence of DLi on Mg# and the lack of literature DLi data with low Mg#. The partition coefficient of V in lunar basalts is measured to be 0.17 to 0.74, significantly higher than that in terrestrial basalts (0.003 to 0.21), which can be explained by the lower oxygen fugacity in lunar basalts. The significantly higher DV can explain why V is less enriched in evolved lunar basalts than terrestrial basalts. The partition coefficient of Cr between olivine and basalt melt in the Moon is 0.11 to 0.62, which is lower than those in terrestrial settings by a factor of ~2. This is surprising because previous authors showed that Cr partition coefficient is independent of fO2. A quasi-thermodynamically based model is developed to correlate Cr partition coefficient to olivine and melt composition and fO2. The lower Cr partition coefficient between olivine and basalt in the Moon can lead to more Cr enrichment in the lunar magma ocean, as well as more Cr enrichment in mantle-derived basalts in the Moon. Hence, even though Cr is typically a compatible element in terrestrial basalts, it is moderately incompatible in primitive lunar basalts, with a similar degree of incompatibility as V based on partition coefficients in this work, as also evidenced by the relatively constant V/Cr ratio of 0.039 ± 0.011 in lunar basalts. The confirmation of constant V/Cr ratio is important for constraining concentrations of Cr (slightly volatile and siderophile) and V (slightly siderophile) in the bulk silicate Moon. 

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4. Multiple Episodes of Rock-Melt Reaction at the Slab-Mantle Interface: Formation of High Silica Primary Magmas in Intermediate to Hot Subduction Zones

   https://doi.org/10.1093/petrology/egad011  

   Rebaza, Anna M ; Mallik, Ananya ; Straub, Susanne M ( March 2023 , Journal of Petrology)

   Siliceous slab-derived partial melts infiltrate the sub-arc mantle and cause rock-melt reactions, which govern the formation of diverse primary arc magmas and lithological heterogeneities. The effect of bulk water content, composition of reactants, and nature of melt infiltration (porous versus channelized) on the rock-melt reactions at sub-arc conditions have been investigated by previous studies. However, the effect of multiple episodes of rock-melt reactions in such scenarios has not been investigated before. Here, we explore mantle wedge modifications through serial additions of hydrous-silicic slab partial melts and whether such a process may ultimately explain the origin of high-Mg# andesites found in arcs worldwide. A series of piston-cylinder experiments simulate a serial addition of silicic slab melts in up to three stages (I through III) at 3 GPa and 800–1050°C, using rock-melt proportions of 75–25 and 50–50. A synthetic KLB-1 and a natural rhyolite (JR-1) represented the mantle and the slab components, respectively. Right from the first rock-melt interaction, the peridotite mantle transforms into olivine-free mica-rich pyroxenites ± amphibole ± quartz/coesite in equilibrium with rhyolitic-hydrous melts (72–80 wt% SiO2 and 40–90 Mg#). The formation of olivine-free pyroxenite seems to be controlled by complex functions of T, P, rock-melt ratio, wedge composition, and silica activity of the slab-melt. Remarkably, the pyroxenites approach a melt-buffered state with progressive stages of rock-melt reactions, where those rhyolitic melts inherit and preserve the major (alkalis, Fe, Mg, Ca) and trace element slab-signature. Our results demonstrate that lithological heterogeneities such as pyroxenites formed as products of rock-melt reactions in the sub-arc mantle may function as melt ‘enablers,’ implying that they may act as pathways that enable the infiltrating melt to retain their slab signature without undergoing modification. Moreover, the density contrast between the products of rock-melt reaction (melts and residues) and the average mantle wedge (~150 to 400 kg/m3) may help forming instabilities and diapiric rise of the slab components into the mantle wedge. However, the fate of the primitive slab-melts seems to be associated with the length of the pathway of mantle interaction which explains the evident wide magma spectrum as well as their degree of slab garnet-signature dilution. This work and the existence of high-Mg# Mexican-trondhjemites indicates that almost pristine slab-melts can make their way up to crustal levels and contribute to the arc magma diversity.

    

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5. Eu speciation in apatite at 1 bar: An experimental study of valence-state partitioning by XANES, lattice strain, and Eu/Eu* in basaltic systems

   https://doi.org/10.2138/am-2022-8388  

   Tailby, Nicholas D. ; Trail, Dustin ; Watson, Bruce ; Lanzirotti, Antonio ; Newville, Matthew ; Wang, Yanling ( May 2023 , American Mineralogist: Journal of Earth and Planetary Materials)

   Abstract Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity (fO2; iron-wüstite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 °C. Apatite-melt partitioning data for REE3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing fO2. Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: ( E u 3 + ∑ E u ) Apatite  = 1 1 + 10 - 0.10 ± 0.01 × l o g ⁡ ( f o 2 ) - 1.63 ± 0.16 . The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: ( Eu E u * ) D Sm × Gd = 1 1 + 10 - 0.15 ± 0.03 × l o g ⁡ ( f o 2 ) - 2.46 ± 0.41 . The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by ( Eu E u * ) D lattice strain = 1 1 + 10 - 0.20 ± 0.03 × l o g ⁡ ( f o 2 ) - 3.03 ± 0.42 . The Eu valence-state partitioning techniques based on (Sm×Gd) and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or Sm×Gd techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or Sm×Gd techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum DEu3+ = 1.67 ± 0.25 (as determined by lattice strain) to DLu3+ = 0.69 ± 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu3+) is also the polyvalent element used to monitor fO2. These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts. 

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