Aqueous Alteration of Olivine in Martian Meteorites at Microscopic Scales

Olivine is a common mineral in martian meteorites and in materials measured in-situ on the surface of Mars. The occurrence and persistence of olivine in rocks that appear to have been subjected to past acid-sulfate alteration is significant and may be a sensitive indicator of the degree and conditions of aqueous alteration. Here we report on coupled analyses by laser Raman spectroscopy and electron probe microanalysis to determine compositional and mineralogic variations during the aqueous alteration of olivine in three martian meteorites (ALHA77005, Lafayette, and MIL03346) and two terrestrial analogues (Lunar Crater, NV, and Mauna Kea, HI).

Alteration products include “iddingsite” in the terrestrial basalts and a variety of alteration phases (jarosite, trioctahedral phyllosilicates, stilpnomelane) in the martian meteorites. Although iddingsite, sensu stricto, is a mixture of goethite and polymerized silicates, Raman spectra only indicate goethite in the Lunar Crater alteration. We compare the patchy alteration of ALHA77005 to the iddingsite of the terrestrial basalts, and contrast these with the alteration products of Lafayette and MIL03346. Iddingsite occurs in olivine throughout the Lunar Crater sample but only in olivine from the oxidized rind of Mauna Kea, and patchy alteration only occurs in a few olivine grains poikilitically enclosed by orthopyroxene in ALHA77005. Iddingsite (in Lunar Crater, Mauna Kea, and ALHA77005) and jarosite (in MIL03346 and ALHA77005) invade the olivine crystal structure with diffuse reaction boundaries whereas the stilpnomelane (MIL03346) and phyllosilicates (Lafayette) occur in veins having sharp contacts with the host olivine. Jarosite is present in olivine of all three of our MIL03346 subsamples but is only present in a few ALHA77005 subsamples (veins of jarosite cross-cut all phases and overprint the patchy alteration). Considering the youth of the Nakhlites (1.3 Ga), the jarosite in MIL03346 suggests that minor acid-sulfate alteration persisted in ground or pore waters into the Amazonian.