Day: July 7, 2016

Formation of the lunar highlands Mg-suite as told by spinel

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1Tabb C. Prissel, 1Stephen W. Parman, 1James W. Head
American Mineralogist 101, 1624-1635, Link to Article [doi:10.2138/am-2016-5581]
1Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, Rhode Island 02912, U.S.A.
Copyright: The Mineralogical Society of America

Two competing hypotheses suggest lunar Mg-suite parental melts formed: (1) by shallow-level partial melting of a hybridized source region (containing ultramafic cumulates, plagioclase-bearing rocks, and KREEP), producing a plagioclase-saturated, MgO-rich melt, or (2) when plagioclase-undersaturated, MgO-rich melts were brought to plagioclase saturation during magma-wallrock interactions within the anorthositic crust. To further constrain the existing models, phase equilibria experiments have been performed on a range of Mg-suite parental melt compositions to investigate which composition can best reproduce two distinct spinel populations found within the Mg-suite troctolites—chromite-bearing (FeCr2O4) troctolites and the more rare pink spinel (MgAl2O4 or Mg-spinel) troctolites (PST).

Phase equilibria experiments at 1 atm pressure were conducted under reducing conditions Embedded Image and magmatic temperatures (1225–1400 °C) to explore the spinel compositions produced from melts predicted by the models above. Additionally, the experimental data are used to calculate a Sp-Ol, Fe-Mg equilibrium exchange coe to cient to correct natural spinel for sub-solidus re-equilibration with olivine in planetary samples: Sp-Ol Embedded Image (R2 = 0.956). Melts from each model (≥50% normative anorthite) produce olivine, plagioclase, and Mg-spinel compositionally consistent with PST samples. However, chromite was not produced in any of the experiments testing current Mg-suite parental melt compositions. The lack of chromite in the experiments indicates that current estimates of Mg-suite parental melts can produce Mg-spinel bearing PST, but not chromite-bearing troctolites and dunites. Instead, model calculations using the MAGPOX equilibrium crystallization program predict chromite production from plagioclase-undersaturated melts (<20% normative anorthite). If so, experimental and model results suggest chromite in Mg-suite crystallized from plagioclase-undersaturated parental melts, whereas Mg-spinel in the PST is an indicator of magma-wallrock interactions within the lunar crust (a mechanism that increases the normative anorthite contents of initially plagioclase-undersaturated Mg-suite parental melts, eventually producing Mg-spinel). The constraints for magmatic chromite crystallization suggest Mg-suite parental melts were initially plagioclase-undersaturated. In turn, a plagioclase-undersaturated Mg-suite parent is consistent with mantle overturn models that predict Mg-suite parent magmas resulted from decompression melting of early ultramafic cumulates produced during the differentiation of a global lunar magma ocean.

D-poor hydrogen in lunar mare basalts assimilated from lunar regolith

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1Allan H. Treiman, 2,3Jeremy W. Boyce, 4James P. Greenwood, 2John M. Eiler, 5Juliane Gross, 2Yunbin Guan, 2Chi Ma, 2Edward M. Stolper
American Mineralogist 101, 1596-1603    Link to Article [doi:10.2138/am-2016-5582]
1Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A.
2Division of Geological & Planetary Sciences, Caltech, 1200 East California Boulevard, Pasadena, California 91125, U.S.A.
3Department of Earth, Planetary, and Space Sciences, UCLA, California 90095, U.S.A.
4Department of Earth & Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, U.S.A.
5Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, U.S.A.
Copyright: The Mineralogical Society of America

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Apatite grains in lunar mare basalts contain hydrogen that ranges in D/H ratio by more than a factor of two. For most of these basalts, the D/H ratios in their apatite grains decrease with measures of the host basalts’ time spent at elevated temperature, specifically the Fe-Mg homogenization of their pyroxenes. Most basalts with homogeneous pyroxenes (i.e., with constant Fe/Mg ratio) have apatite grains with low D/H (δD ≈ −100‰), whereas most basalts with heterogeneous pyroxenes (i.e., varying or zoned Fe/Mg) have apatite with high D/H (δD up to ~ +1100‰). This relationship suggests that low D/H values were acquired during thermal processing, i.e., during Fe-Mg chemical equilibration, during or after emplacement. This light hydrogen is likely derived from solar wind implanted into the lunar regolith (with δD from −125‰ to −800‰), and could enter basalts either by assimilation of regolith or by vapor transport from regolith heated by the flow. If a basalt could not interact with regolith rich in solar wind (e.g., it was emplaced onto other fresh basalts), its apatite could retain a magmatic D/H signature. The high D/H component (in the apatites of unequilibrated basalts) is most reasonably that indigenous magmatic hydrogen, i.e., representing hydrogen in the basalt’s source mantles, or magmatic hydrogen that was residual after partial degassing of H2.