[PPS10-P13] Aqueous alteration in the nakhlites Y 000802
Keywords:Mars, Nakhlites, Aqueous alteration
Y 000802 consisted of olivine, clinopyroxene, and mesostasis. The mesostasis consisted of plagioclase, K-feldspar, tridymite, pyrrhotite, phosphate mineral, and titanomagnetite. The alteration texture named iddingsite was distinctive along with the fractures and grain-boundaries of the olivine grains. Laihunite, magnetite, opal, and jarosite occurred in the iddingsite. We found alteration minerals and Mn-concentrated portions in the mesostasis. The alteration occurred in veinlets crossing the mesostasis. Ferroan saponite (+ opal) replaced the plagioclase and K-feldspar grains with maintaining their original crystal habits. Pyrrhotite and phosphate mineral grains showed a dissolution texture. Jarosite, iron oxides, and iron hydroxides also occurred in the veinlets.
There are two different types of Mn-concentrated portions between the feldspar grains replaced with ferroan saponite (+ opal). One of them is an ultra-high-Mn-concentrated portion and the other is a high-Mn-concentrated portion. Mn-concentrated portions accompanied iron. The Mn/Fe atomic ratios of the ultra-high-Mn- and high-Mn-concentration portions are ~3 and ~1, respectively. Mn L-edge XANES spectra showed the existence of Mn2+, Mn3+, and Mn4+ in the Mn-concentrated portions. Mn3+ along with Mn2+ is dominant in the ultra-high-Mn-concentrated portions, whereas Mn2+ (and trace Mn4+) is dominant in the high-Mn concentrated portions. The weak peak of O K-edge XANES at 534 eV and C K-edge XANES at 290 eV also appeared in the Mn-concentrated portions, implying the existence of CO32-. Based on TEM and STXM analysis, the Mn-concentrated portions consisted of hausmannite (Mn2+Mn3+2O4), and trace amounts of manganite (γ-MnOOH), rhodochrosite (MnCO3), and pyrolusite (MnO2). The Mn-concentrated portions included trace amounts of sulfur.
We expect that high-temperature fluid (400–700 °C) ascending in the nakhlite body induced the alteration from olivine to laihunite (+ opal and iron oxides/hydroxides). Mn ion dissolved from olivine (and probably pyroxene) through the formation of laihunite and precipitated as hausmannite + rhodochrosite assemblage in the mesostasis. Considering the pH-Eh diagram in the C-S-O-H system, the Eh and pH of the fluid that precipitated the assemblage are ~0 and 9.5–11.5, respectively. Simultaneously, plagioclase was replaced with ferroan saponite by the fluid.
Subsequently, an acidic fluid, which is due to the dissolution of pyrrhotite, induced the alteration of the hausmannite + rhodochrosite assemblage, thus leading to form manganite and pyrolusite. SO42- and Fe2+ released by the dissolution of pyrrhotite and K released from K-feldspar by the acidic fluid precipitated as jarosite. We expect that these alterations occurred mainly on Earth as a terrestrial contaminant because pyrrhotite can dissolve easily.
Reference  Shiraishi et al. (2018) Aqueous alteration of Yamato 000749 based on multi-probe microscopic observation. The Ninth Symposium on Polar Science, abstract.