Variable initial zoning profiles and Fe-Mg diffusion coefficients for olivine: Effects on cooling rates calculated by diffusion modeling in a pallasite

Because chemical zoning of minerals contains information about the thermal history of the parent body, the cooling rate or burial depth can be calculated by solving the diffusion equation. Several factors influence the results of these calculations. We evaluated the cooling rate (and burial depth) calculated by using different initial (starting) zoning profiles and different diffusion coefficients on the basis of the Fe-Mg chemical zoning profile observed for pallasite (Esquel) olivine. Uncertainties in initial compositional profile and Fe-Mg diffusion coefficient in olivine both lead to error in model cooling rates. Discrepancies between different experimentally determined diffusion coefficients lead to uncertainties in model cooling rate that exceed those due to initial zoning profiles by one order of magnitude. These results highlight the need for accurate determination of diffusion coefficients in olivine

Origin of olivine megacrysts and the groundmass crystallization of the Dar al Gani 476 shergottite

The DaG 476 martian meteorite shows a porphyritic texture with megacrysts of olivine and orthopyroxene set in a groundmass of pyroxene and maskelynite. Previous studies on major and trace elements and isotopes of this meteorite implied a relationship to other martian meteorites. However, the origin of the olivine and orthopyroxene megacrysts is still under dispute, and therefore the formation of DaG 476 is unclear, although this sample is one of the most important martian meteorites. We performed crystallization experiments, a MELTS calculation and a cooling rate calculation to investigate the formation of DaG 476. The experimental and calculated results suggest that the parent melt of the DaG groundmass was more Fe- and Al-rich than the actual groundmass bulk composition, suggesting that the groundmass of DaG 476 contains a mafic cumulus component, alternatively fractionated liquid has escaped at the last crystallization stage. We evaluated three models for the origin of the olivine megacrysts (1) phenocryst origin, (2) xenocryst origin: homogeneous olivine was modified by exchange with the host magma and diffusion, and (3) xenocryst origin: chemical zoning of olivine was produced by the fractional crystallization. The mineralogy of DaG 476 and calculation results showed that all models were theoretically possible. However, models (1) and (2) need complex processes to produce observed natures of DaG 476. Hence, model (3) seems the most plausible, although this model also leaves some open questions. The fragment-like texture of olivine and the results of cooling rate calculation suggest that the formation of the DaG shergottites occurred in a rapid cooling condition in any of the formation models (1-3). Therefore, DaG seems to have crystallized near the martian surface

Comparative mineralogy of magmatic inclusions in olivine from the Chassigny and Nakhla martian meteorites

Chassigny and Nakhla frequently contain magmatic inclusions in olivine. Some Al-, Ti-rich augites in Chassigny magmatic inclusions show reverse zoning from the Mg-poor core to the Mg-rich rim. This unique chemical zoning in Chassigny magmatic inclusion gives important implications for the crystallization of the magmatic inclusions. The crystallization stage of the Mg-poor core is different from that of the Mg-rich rim because the zoned augite has a sharp boundary between the Mg-rich and Mg-poor regions. The presence of such Al-, Ti-augite with reverse zoning implies that the bulk composition of the magmatic inclusion changed into Mg-rich. Such a compositional change of the magmatic inclusion can be produced by the melting of the surrounding olivine. There is a possibility that the melting of surrounding olivine has occurred during a shock event. In order to examine the possibility of the melting by a shock event, this study compared texture and constituent minerals of Chassigny magmatic inclusions with those of Nakhla magmatic inclusions. Although Chassigny magmatic inclusions have remarkable radial cracks around them, Nakhla magmatic inclusions only have ambiguous radial cracks. The presence of radial cracks in the Chassigny magmatic inclusion shows that the inclusions have been affected by a heavy impact event because cracks around magmatic inclusions formed by the fracturing of a phenocryst induced by rapid compression and decompression

Mineralogy and petrology of Yamato 000593: Comparison with other Martian nakhlite meteorites

Yamato (Y) 000593 is a new nakhlite recovered from Antarctica and is composed of roughly 80% augite, 10% olivine and 10% mesostasis. Augite is chemically homogeneous except for Fe-rich rims adjacent to the mesostasis. Olivine has more extensive chemical zoning, but the most Fe-rich part is also near the mesostasis. These observations suggest that chemical zoning of both augite and olivine was produced by interaction with the mesostasis. The crystallization history of Y000593 as deduced from this study is as follows. (1) Crystallization of cumulus augite and olivine and formation of symplectites in olivine. (2) Accumulation of augite and olivine. (3) Mesostasis crystallization and interaction of the augite and olivine rims with the intercumulus melt. (4) Aqueous alteration. The petrography and mineralogy of Y000593 is generally similar to other nakhlites, but minor mineralogical differences are observed. These differences resulted from different thermal histories due to different locations (burial depths) in the same cooling cumulate pile. Y000593 is most similar to Nakhla and both samples experienced similar formation histories. However, later mesostasis crystallization of Y000593 was more rapid than Nakhla due to its faster cooling rate. The burial depth of Y000593 would be shallower than 3 m from the surface, and is intermediate between NWA817 and Nakhla. The abundance and mineralogy of the mesostasis as well as augite and olivine rim compositions are related to the burial depths of nakhlites

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