Yamato 980459: Mineralogy and petrology of a new shergottite-related rock from Antarctica

Y980459, a new Martian meteorite from Antarctica, is composed of coarse porphyritic olivine grains (up to 2mm) set in the groundmass of olivine and pyroxene with abundant glassy mesostasis containing dendritic olivine and pyroxene. The overall petrography of Y980459 is similar to those of olivine-phyric shergottites, but the absence of plagioclase and Ca phosphates makes Y980459 unique. Because of the absence of maskelynite, Y980459 is not a shergottite if we employ the classic definition of shergottite. Both olivine and pyroxenes are extensively zoned. The most magnesian olivine composition is Fo86 and the olivine compositions are related to three different occurrence types of olivine (large phenocrysts, groundmass, and mesostasis). Pyroxenes have orthopyroxene cores (En81Fs17Wo2) mantled by pigeonite with the rims of augite. The mineralogy of Y980459 suggests that rapid crystallization of the parent magma caused significant undercooling and plagioclase did not nucleate. Probably, rapid transport of the Y980459 parent magma from the depth to the Martian surface crystallized olivine and pyroxene at first and eruption onto the surface quenched the magma producing the glassy mesostasis. Because olivine and pyroxene compositions of Y980459 are the most magnesian among Martian meteorites, Y980459 would represent one of the most primitive Martian magmas and derive from a highly reduced mantle. It seems that Y980459 contains no cumulus component, suggesting that Y980459 is a melt. In this sense, Y980459 is similar to QUE94201. The similarity in mineralogy and chemistry between Y980459 and olivine-phyric shergottites suggests derivation from a similar highly reduced mantle. However, Y980459 was the only sample that directly erupted onto the Martian surface without any accumulation processes

Signatures of the post-hydration heating of highly aqueously altered CM carbonaceous chondrites and implications for interpreting asteroid sample returns

The CM carbonaceous chondrites have all been aqueously altered, and some of them were subsequently heated in a parent body environment. Here we have sought to understand the impact of short duration heating on a highly aqueously altered CM through laboratory experiments on Allan Hills (ALH) 83100. Unheated ALH 83100 contains 83 volume per cent serpentine within the fine-grained matrix and altered chondrules. The matrix also hosts grains of calcite and dolomite, which are often intergrown with tochilinite, Fe(Ni) sulphides (pyrrhotite, pentlandite), magnetite and organic matter. Some of the magnetite formed by replacement of Fe(Ni) sulphides that were accreted from the nebula. Laboratory heating to 400 °C has caused partial dehydroxylation of serpentine and loss of isotopically light oxygen leading to an increase in bulk δ18O and fall in Δ17O. Tochilinite has decomposed to magnetite, whereas carbonates have remained unaltered. With regards to infrared spectroscopy (4000–400 cm-1; 2.5–25 µm), heating to 400 °C has resulted in decreased emissivity (increased reflectance), a sharper and more symmetric OH band at 3684 cm-1 (2.71 µm), a broadening of the Si—O stretching band together with movement of its minimum to longer wavenumbers, and a decreasing depth of the Mg—OH band (625 cm-1; 16 µm). The Si—O bending band is unmodified by mild heating. With heating to 800 °C the serpentine has fully dehydroxylated and recrystallized to ∼Fo60/70 olivine. Bulk δ18O has further increased and Δ17O decreased. Troilite and pyrrhotite have formed, and recrystallization of pentlandite has produced Fe,Ni metal. Calcite and dolomite were calcined at ∼700 °C and in their place is an un-named Ca-Fe oxysulphide. Heating changes the structural order of organic matter so that Raman spectroscopy of carbon in the 800 °C sample shows an increased (D1 + D4) proportional area parameter. The infrared spectrum of the 800 °C sample confirms the abundance of Fe-bearing olivine and is very similar to the spectrum of naturally heated stage IV CM Pecora Escarpment 02010. The temperature-related mineralogical, chemical, isotopic and spectroscopic signatures defined in ALH 83100 will help to track the post-hydration thermal histories of carbonaceous chondrite meteorites, and samples returned from the primitive asteroids Ryugu and Bennu