Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p33d..04e&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P33D-04
Other
[1026] Geochemistry / Composition Of The Moon, [1060] Geochemistry / Planetary Geochemistry, [3630] Mineralogy And Petrology / Experimental Mineralogy And Petrology, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The highlands Mg-suite (HMS) is represented by a series of ancient plutonic highlands rocks which contain Fe-Mg silicates with high Mg* (up to 94) and are generally enriched in incompatible elements (i.e. REE, Th). These contrasting chemical signatures have long puzzled lunar petrologists and their origin has remained enigmatic. One type of model for the origin of the HMS is the mixing of early- and late-stage lunar magma ocean (LMO) cumulates. In some of these mixing models, early olivine-orthopyroxene LMO cumulates are transported to shallower depths by cumulate overturn, where they are mixed with very late-stage incompatible element enriched cumulates (KREEP). However, this model is currently insufficient to explain several observations in the most primitive HMS (i.e. troctolites), including the low Ni and Co contents of the olivine relative to mare basalts and the apparent co-crystallization of highly magnesian olivine and calcium-rich plagioclase. Despite these shortcomings, such a model better explains the Mg-suite’s unique features than do other proposed formation mechanisms (i.e. impact melting, direct crystallization from the LMO, remobilization of KREEP). In order to assess whether early LMO cumulates could have acted as the source lithologies for the HMS, equilibrium crystallization experiments were conducted on the Taylor Whole Moon composition containing Ni and Co. Various authors have proposed that LMO crystallization occurred in a (roughly) two step process: equilibrium crystallization followed by fractional crystallization of the residual liquid. Experiments at 2 GPa, 1700 and 1600°C produced olivine of similar composition (Fo94 and 92, respectively) to the most primitive olivine found in Mg-suite lithologies. The crystal fractions in these experiments were 2% and 29%, respectively. An experiment at 1 GPa and 1450°C, with a crystal fraction of 44%, produced olivine of the composition Fo90. These initial experiments indicate that olivine sufficiently rich in Mg to potentially act as a source for the Mg-suite is produced over a range of crystal fractions by equilibrium LMO crystallization. Olivine settling in this LMO model will form a cumulate pile from which highly magnesian partial melts could have been extracted. Equilibrium crystallization experiments are ongoing to determine the distribution of Ni and Co among the crystallized phases. Furthermore, these experiments determine the amount of crystallization that occurs before olivine with an appropriate Mg* (matching that of the HMS) no longer crystallizes. As the magma ocean continued to crystallize olivine and orthopyroxene, the residual LMO eventually became enriched in Al and Ca. Although fractional crystallization will very likely become the dominant mode of crystallization before plagioclase begins to form, the Al and Ca contents of the liquid and the Mg* of the olivine crystallizing at that time will be pivotal in assessing what direct relationship, if any, the plagioclase in primitive HMS rocks has with the LMO liquid.
Draper David S.
Elardo Stephen M.
Shearer Charles K.
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