Melt inclusions in augite of the Nakhla martian meteorite: Evidence for basaltic parental melt

Abstract

Melt inclusions in augite of the Nakhla martian meteorite:Evidence for basaltic parental melt Abstract-Nakhla contains crystallized melt inclusions that were trapped in augite and olivine when these phases originally formed on Mars. Our study involved rehomogenization (slow-heating and fast-heating) experiments on multiphase melt inclusions in Nakhla augite. We studied melt inclusions trapped in augite because this phase re-equilibrated with the external melt to a lesser extent than olivine and results could be directly compared with previous Nakhla melt inclusion studies. Following heating and homogenization of encapsulated melt inclusions, single mineral grains were mounted and polished to expose inclusions. Major element chemistry was determined by electron microprobe. The most primitive melt inclusion analyzed in Nakhla NA03 is basaltic and closely matches previously reported nakhlite parent melt compositions. MELTS equilibrium and fractional crystallization models calculated for NA03 and previous Nakhla parent melt estimates at QFM and QFM-1 produced phase assemblages and compositions that can be compared to Nakhla. Of these models, equilibrium crystallization of NA03 at QFM-1 produced the best match to mineral phases and compositions in Nakhla. In all models, olivine and augite co-crystallize, consistent with the hypothesis that olivine is not xenocrystic but has undergone subsolidus re-equilibration. In addition, measured melt inclusion compositions plot along the MELTS-calculated liquid line of descent and may represent pockets of melt trapped at various stages during crystallization. We attempt to resolve discrepancies between previous estimates of the Nakhla parental melt composition and to reinterpret the results of a previous study of rehomogenized melt inclusions in Nakhla. Melt inclusions demonstrate that Nakhla is an igneous rock whose parent melt composition and crystallization history reflect planetary igneous processes.

Petrology of Martian meteorite Northwest Africa 998

“…Longhi and Pan (1989) found these reactions to be cotectic, not peritectic, for compositions relevant to nakhlite genesis. More recently, Stockstill et al (2005) modeled the crystallization of nakhlite parental magma compositions in MELTS (Ghiorso and Sack 1995) and report no sign of reaction to produce pigeonite from augite. However, some relevant phase diagrams from Longhi (1991) show that the augite-pigeonite liquidus can be peritectic in augite.…”

Expanding the application of the Eu‐oxybarometer to the lherzolitic shergottites and nakhlites: Implications for the oxidation state heterogeneity of the Martian interior

“…Equilibrium crystallization was modeled in 10° temperature steps, stopping at 800 °C. For a presumed nakhlite parent melt (NA03 from Stockstill et al, 2005), closed system crystallization at an initial fO 2 of IW+1 increases the Fe 3+ of the melt by ~49% prior to the onset of spinel crystallization (Fig. 6A).…”

“…However, melt inclusions in these meteorites are sparse and in most cases have experienced significant post-entrapment crystallization and reequilibration (Johnson et al 1991;Ikeda 1998;Varela et al 2001;Goodrich and Harvey 2002;Stockstill et al 2005). Therefore, to measure the REE composition of the melt, the inclusions must first be rehomogenized (e.g., Gaetani and Watson 2000;Varela et al 2001;Danyushevsky et al 2002;Stockstill et al 2005).…”

Oxygen fugacity in the Martian mantle controlled by carbon: New constraints from the nakhlite MIL 03346

“…The liquidus can be calculated utilizing either all iron as FeO, or with Fe 2 O 3 calculated as appropriate for the FMQ buffer (using Kress and Carmichael 1991), but the results are nearly independent of Fe 3+ /Fe 2+ , as the differences are on the order of ~20 °C up to 0.5 GPa. And third, using nakhlite parent melt compositions (Treiman and Goodrich 2001;Stockstill et al 2005), the P and T at which olivine and clinopyroxene of similar composition to that in the nakhlites can be calculated, again using the MELTS algorithm. The results are very similar to the previous assessment, and all three approaches combined indicate that nakhlites equilibrated at low pressures (1 bar) and temperatures near 1100 °C.…”