Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1695g&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13D-1695
Mathematics
Logic
[3612] Mineralogy And Petrology / Reactions And Phase Equilibria, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Recent remote mineralogical data acquired by the Moon Mineralogy Mapper (M3) has led to the identification of Mg-spinel-rich lithologies, likely associated with anorthosites [1,2]. This rock type has three notable petrologic characteristics: 1) a high fraction of spinel (~ 30%) 2) a low fraction of mafic minerals (less than 5% olivine or pyroxene) and 3) an unusual spinel composition (low-Fe (<5%), low-Cr and high Al). In contrast, most lunar spinels are Fe-rich ulvöspinels and chromites, which are the product of low pressure crystallization from Fe-rich lunar picritic basalts [3]. The Mg-rich spinels in the anorthosite are most similar to those found in pink spinel troctolites. The anorthosite differs from the spinel troctolite in having smaller amounts of mafic minerals and lower FeO contents in the spinel (5% versus 10% [4]). The high spinel mode, low Fe content of the spinel and lack of mafic minerals in the spinel anorthosite are not consistent with simple crystal fractionation of or crystal accumulation from basaltic lunar magma compositions [3]. However, Mg-rich spinel has been produced in experiments examining the compositions of lunar basalts which dissolved anorthosite [5]. This suggests a mechanism for forming spinel-bearing anorthosites by melt-wallrock reaction of lunar basaltic magmas with the anorthositic crust. Such reactive processes are well documented in terrestrial magmatic systems [6]. To test whether melt-wallrock reaction could have produced the spinel-anorthosite, we have conducted a set of experiments in which Apollo 15C green glass and pure anorthite glass were juxtaposed. Experiments at 1400-1450oC and 1-8 kbar produced an assemblage of anorthite, spinel (>10%) and minor mafics (<5%) in the reacted anorthite portion of the assembly. The experimental spinels are high-Mg, low-Cr and low-Fe (~10 wt%) spinels. The spinels are very similar to those found in the pink-spinel troctolites, but too high in Fe compared to the inferred composition of spinel in the anorthosites. Thus, melts with lower Fe/Mg ratios than lunar picrites are required to produce the observed Mg-rich spinels. We suggest that Mg-suite parental melts [7] have appropriately low Fe/Mg to form lunar Mg-spinel anorthosites by reaction with the lunar anorthositic crust. Based on the experimental data, a simple model can be derived to predict spinel compositions produced by different melt-wallrock reactions as a function of pressure and temperature. In summary, Mg-spinel could be a reaction product wherever mafic melts come into contact with the lunar anorthositic crust. Thus, Mg-spinel lithologies should be a widespread, though perhaps low-volume, part of the lunar rock record. With the use of compositional models, mapping out the spatial distribution and composition of such spinel-bearing lithologies will shed light on the long history of melt production and melt transport on the Moon.
Cheek L.
Dhingra Deepak
Ganskow G.
Hess Patricia
Jackson Charles
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