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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. 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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3. Lithological Heterogeneities in the Mantle: Origins and Contributions to Magma Genesis (Keynote 3a)

   https://doi.org/10.46427/gold2020.1405  

   Lambart, Sarah; Lang, Otto (January 2020, Goldschmidt 2020)

   null (Ed.)

   The quantification of the mantle heterogeneity and its contribution to magma genesis are crucial for our understanding of the nature of Earth’s geochemical reservoirs and recycling processes. Today, the source of basalts is often envisioned as a heterogeneous mantle that comprises a range of lithological heterogeneities, especially pyroxenites, introduced into the mantle by various geodynamic and magmatic processes. Different chemical parameters have been proposed to trace evidence for the contribution of lithological heterogeneities during magma genesis (e.g., major elements1-2, major element ratios1,3 or logratios4, first row transition element concentrations5-6, iron isotopes7) and several empirical models for partial melting of a heterogeneous mantle have been developed in an attempt to quantify the proportion of pyroxenites in the mantle source of magmas8-11. However, the large range of compositions covered by the potential lithologies present in the mantle makes it challenging to determine a unique proxy for their contributions in the magmas2,12. Additionally, these contributions are controlled by various factors, such as the abundances of the different lithologies in the mantle, their respective melting behaviors (solidus temperatures and melt productivities) and the thermal and melting regimes of the mantle. Finally, interpretations from the magma compositions are complicated by the potential presence of volatiles in the mantle source and/or by modifications experienced by the magmas during their journey through the mantle8,12. This keynote presentation will present examples of challenges that can rise when we try to quantify the nature and abundance of the mantle heterogeneity, the efforts that have been published by various authors, and a few potential research directions that could bring prospects for success. 1- Hauri (1996), Nature 382, 2- Lambart et al. (2013), Lithos 160-161, 3- Hirschmann et al. (2003), Geology 31, 4- Yang et al. (2019), JGR-Solid Earth 124, 5- Sobolev et al (2007), Science 316, 6- Sobolev et al. (2008), Science 321, 7- Williams and Bizimis, EPSL 404, 8- Mallik and Dasgupta (2014), GGG 15, 9- Kimura and Kawabata (2015), GGG 16, 10- Lambart et al. (2016), JGR-Solid Earth 121, 11- Brown and Lesher (2016), GGG 17, 12- Mallik et al. (in press), in: Konter J., Ballmer M, Cottaar S, & Marquardt H. (Eds. ), AGU monograph. 

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4. Distribution of REE between amphibole and pyroxenes in the lithospheric mantle: An assessment from the lattice strain model

   https://doi.org/10.2138/am-2022-8831  

   Wang, Chunguang; Liang, Yan; Xu, Wenliang; Sun, Chenguang; Shimizu, Kei (November 2024, American Mineralogist)

   Abstract Amphibole and pyroxenes are the main reservoirs of rare earth elements (REEs) in the lithospheric mantle that has been affected by hydrous metasomatism. In this study, we developed semi-empirical models for REE partitioning between orthopyroxene and amphibole and between clinopyroxene and amphibole. These models were formulated on the basis of parameterized lattice strain models of mineral-melt REE partitioning for orthopyroxene, clinopyroxene, and amphibole, and they were calibrated using major element and REE data of amphibole and pyroxenes in natural mantle samples from intraplate settings. The mineral-melt REE partitioning models suggest that amphibole is not in equilibrium with coexisting pyroxenes in the mantle samples and that the amphibole crystallized at a lower temperature than that of the pyroxenes. We estimated the apparent amphibole crystallization temperature using major element compositions of the amphibole and established temperature- and composition-dependent models that can be used to predict apparent pyroxene-amphibole REE partition coefficients for amphibole-bearing peridotite and pyroxenite from intraplate lithospheric mantle. Apparent pyroxene-amphibole REE partition coefficients predicted by the models can be used to infer REE contents of amphibole from REE contents of coexisting pyroxenes. This is especially useful when the grain size of amphibole is too small for trace element analysis. 

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5. Post-spreading Basalts from the Nanyue Seamount: Implications for the Involvement of Crustal- and Plume-Type Components in the Genesis of the South China Sea Mantle

   https://doi.org/10.3390/min9060378  

   Zheng, Hao; Zhong, Li-Feng; Kapsiotis, Argyrios; Cai, Guan-Qiang; Wan, Zhi-Feng; Xia, Bin (June 2019, Minerals)

   Fresh samples of basalts were collected by dredging from the Nanyue intraplate seamount in the Southwest sub-basin of the South China Sea (SCS). These are alkali basalts displaying right-sloping, chondrite-normalized rare earth element (REE) profiles. The investigated basalts are characterized by low Os content (60.37–85.13 ppt) and radiogenic 187Os/188Os ratios (~0.19 to 0.21). Furthermore, 40Ar/39Ar dating of the Nanyue basalts showed they formed during the Tortonian (~8.3 Ma) and, thus, are products of (Late Cenozoic) post-spreading volcanism. The Sr–Nd–Pb–Hf isotopic compositions of the Nanyue basalts indicate that their parental melts were derived from an upper mantle reservoir possessing the so-called Dupal isotopic anomaly. Semiquantitative isotopic modeling demonstrates that the isotopic compositions of the Nanyue basalts can be reproduced by mixing three components: the average Pacific midocean ridge basalt (MORB), the lower continental crust (LCC), and the average Hainan ocean island basalt (OIB). Our preferred hypothesis for the genesis of the Nanyue basalts is that their parental magmas were produced from an originally depleted mantle (DM) source that was much affected by the activity of the Hainan plume. Initially, the Hainan diapir caused a thermal perturbation in the upper mantle under the present-day Southwest sub-basin of the SCS that led to erosion of the overlying LCC. Eventually, the resultant suboceanic lithospheric mantle (SOLM) interacted with OIB-type components derived from the nearby Hainan plume. Collectively, these processes contributed crustal- and plume-type components to the upper mantle underlying the Southwest sub-basin of the SCS. This implies that the Dupal isotopic signature in the upper mantle beneath the SCS was an artifact of in situ geological processes rather than a feature inherited from a Southern Hemispheric, upper mantle source. 

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