A petrologic and trace element study of Dar al Gani 476 and Dar al Gani 489: Twin meteorites with affinities to basaltic and lherzolitic shergottites

Abstract

Abstract-We present the results of a combined mineralogic-petrologic and ion microprobe study of two martian meteorites recently recovered in the Lybian Sahara, Dar a1 Gani 476 (DaG 476) and Dar a1 Gani 489 (DaG 489). Having resided in a hot desert environment for an extended time, DaG 476 and DaG 489 were subjected to terrestrial weathering that significantly altered their chemical composition. In particular, analyses of some of the silicates show light rare earth element (LREE)-enrichment resulting from terrestrial alteration. In situ measurement of trace element abundances in minerals allows us to identify areas unaffected by this contamination and, thereby, to infer the petrogenesis of these meteorites. No significant compositional differences between DaG 476 and DaG 489 were found, supporting the hypothesis that they belong to the same fall. These meteorites have characteristics in common with both basaltic and lherzolitic shergottites, possibly suggesting spatial and petrogenetic associations of these two types of lithologies on Mars. However, the compositions of Fe-Ti oxides and the size of Eu anomalies in the earliest-formed pyroxenes indicate that the two Saharan meteorites probably experienced more reducing crystallization conditions than other shergottites (with the exception of Queen Alexandra Range (QUE) 94201). As is the case for other shergottites, trace element microdistributions in minerals of the DaG martian meteorites indicate that closed-system crystal fractionation from a LREE-depleted parent magma dominated their crystallization history. Furthermore, rare earth element abundances in the orthopyroxene megacrysts are consistent with their origin as xenocrysts rather than phenocrysts.

Defining the mechanisms that disturb the Sm-Nd isotopic systematics of the Martian meteorites: Examples from Dar al Gani 476 and Allan Hills 77005

“…Goodrich (2002) classified DaG 476 and its paired samples (Dar al Gani 489,670,734, and 876) as olivine-phyric shergottites because of the porphyritic texture of the olivine, oxide composition, and modal augite abundance. The modal mineralogy ranges from 44-53% pigeonite, 12-17% maskelynite, 10-24% olivine, 3-13% augite, 4-7% impact melt, 2-4% orthopyroxene, 1-3% carbonates, 1-4% opaques (pyrrhotite, chromite, and ilmenite), and 1-2% merrillite (Mikouchi 1999;Zipfel et al 2000;Folco et al 2000;Wadhwa et al 2001). The olivine contains small melt inclusions, which contain microlites of olivine, augite, and chromite, surrounded by a glass rich in Si and Al (Zipfel et al 2000).…”

Melting of the Martian mantle from 1.0 to 4.5 GPa

“…However, several researchers (e.g., McSween and Jarosewich, 1983;Wadhwa et al, 2001;Barrat et al, 2002;Filiberto et al, 2010) have interpreted the olivine megacrysts in some olivine -phyric shergottites as exotic xenocrysts or cognate xenocrysts mixed into the olivine -free (or olivine -poor) basaltic magma by other processes. Despite the considerable number of studies conducted regarding the igneous origin of basaltic shergottites, the primitive (parent) magma composition of the Martian mantle remains controversial (e.g., Goodrich, 2002;Greshake et al, 2004;Shearer et al, 2008;Usui et al, 2008;Sarbadhikari et al, 2009;Peslier et al, 2010;Filiberto and Dasgupta, 2011).…”

Oxygen Isotope Thermometry of DaG 476 and SaU 008 Martian Meteorites: Implications for Their Origin

“…The results of the fractional crystallization modeling by MELTS algorithm (i.e., a software for thermodynamic modeling of phase equilibria in magmatic systems) [15] using Y980459 as parent composition reveal that the change in temperature (1170-1270 • C) during sequential crystallization of minerals (e.g., opx to pigeonite) from a liquid is associated with a proportional change in mean En content (e.g., pigeonite = 56, opx = 78; [15]). Mean En content in the pigeonite of DaG 489 (paired with DaG 476) is 64 (En [57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] ; [90]), which is within the range shown by the mean En content obtained from fractional crystallization model for the crystallization sequence of opx to pigeonite (i.e., 56-78; [15]). The temperature estimated for DaG 476 from oxygen isotope thermometry (1200 • C) is consistent with the temperature range (1170-1270 • C) obtained from MELTS fractional crystallization modelling [15].…”

“…On the other hand, DaG 476 is the first Martian meteorite found from hot desert in Dar al Gani, Sahara Desert, Libya (1 May 1998), having porphyritic texture; it is composed of olivine megacrysts in a fine-grained groundmass of pigeonite and maskelynite. Average olivine and pyroxene modal abundances are, respectively, 17.2 vol % and 58.5 vol % (i.e., ranging from 10.4 to 24 vol % and 54 to 64.6 vol %, respectively) [33,67,68]. Both SaU 008 and DaG 476 meteorites are grouped as olivine-phyric shergottites given their highly depleted light rare earth elements (LREE) abundances.…”