Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p51a1186u&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P51A-1186
Other
3672 Planetary Mineralogy And Petrology (5410), 5480 Volcanism (6063, 8148, 8450), 6225 Mars
Scientific paper
Shergottite-nakhlite-chassigny (SNC) meteorites yield the most informative data set to constrain igneous process on Mars, even though they may not represent average martian crust. Among the SNC meteorites, olivine-phyric shergottites have been recently studied intensively because some meteorites in this group reflect melt compositions in contrast to other shergottites which contain cumulus crystals. In this study, to infer the melting process of the martian mantle, we focus on the olivine-phyric shergottite Yamato 980459 (Y-98), because it is likely to contain a phenocryst assemblage with the highest Mg-numbers among all martian meteorites and was quenched rapidly after eruption. Y-98 is characterized by magnesian olivine megacrysts (up to 2 mm) and pyroxenes having orthopyroxene cores mantled by pigeonite and augite. These petrographic features have been explained by simple crystallization from a single primitive magma sources; the most magnesian olivine core (Fo86) was thought to have crystallized from a parent magma having the Y-98 bulk composition. However, two new observations are inconsistent with this simple crystallization model. One is that some pyroxenes have pigeonite/augite cores mantled by orthopyroxene, contrary to other pyroxenes. The other is that the cores of all olivine megacrysts (Fo80-85) have a relatively constant CaO content of ~0.2 wt%. These cores are not in equilibrium with the whole-rock composition of Y-98 at any pressure ranging from 1 atm to 20 kbar based on the Ca partitioning model of Libourel (1999) (e.g. equilibrated CaO content is 0.12+/-0.02 wt% in Fo86 olivine at 10 kbar). Although Ca partitioning behavior is affected by melt compositions, such high-Ca and -Fo olivine cannot be explained by assuming any crystallization processes from a primary melt represented by the Y-98 bulk composition. Therefore, the cores of olivine megacrysts are xenocrysts, derived from melting a compositionally different source mantle for the Y-98 main magma. This implies that the olivine-phyric shergottite source mantle could be heterogeneous in terms of major element compositions.
McSween Harry Y.
Usui Takuya
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