Microstructural investigations on strongly stained olivines of the chassignite NWA 2737 and implications for its shock history

Abstract-The Tissint meteorite is a geochemically depleted, olivine-phyric shergottite. Olivine megacrysts contain 300-600 lm cores with uniform Mg# (~80 AE 1) followed by concentric zones of Fe-enrichment toward the rims. We applied a number of tests to distinguish the relationship of these megacrysts to the host rock. Major and trace element compositions of the Mg-rich core in olivine are in equilibrium with the bulk rock, within uncertainty, and rare earth element abundances of melt inclusions in Mg-rich olivines reported in the literature are similar to those of the bulk rock. Moreover, the P Ka intensity maps of two large olivine grains show no resorption between the uniform core and the rim. Taken together, these lines of evidence suggest the olivine megacrysts are phenocrysts. Among depleted olivine-phyric shergottites, Tissint is the first one that acts mostly as a closed system with olivine megacrysts being the phenocrysts. The texture and mineral chemistry of Tissint indicate a crystallization sequence of: olivine (Mg# 80 AE 1) ? olivine (Mg# 76) + chromite ? olivine (Mg# 74) + Ti-chromite ? olivine (Mg# 74-63) + pyroxene (Mg# 76-65) + Cr-ulv€ ospinel ? olivine (Mg# 63-35) + pyroxene (Mg# 65-60) + plagioclase, followed by late-stage ilmenite and phosphate. The crystallization of the Tissint meteorite likely occurred in two stages: uniform olivine cores likely crystallized under equilibrium conditions; and a fractional crystallization sequence that formed the rest of the rock. The two-stage crystallization without crystal settling is simulated using MELTS and the Tissint bulk composition, and can broadly reproduce the crystallization sequence and mineral chemistry measured in the Tissint samples. The transition between equilibrium and fractional crystallization is associated with a dramatic increase in cooling rate and might have been driven by an acceleration in the ascent rate or by encounter with a steep thermal gradient in the Martian crust.

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“…; Bläß et al. ). Individual euhedral grains reach ~1.5 mm in length, and sometimes display fragmented or skeletal textures (Figs.…”

Section: Resultsmentioning

confidence: 97%

Mineral chemistry of the Tissint meteorite: Indications of two‐stage crystallization in a closed system

Liu

Baziotis

Asimow

et al. 2016

Meteorit & Planetary Scien

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“…(2007a) and Bläß et al. () considered that iron metal nanoparticles were formed by a disproportionation reaction (3Fe 2+ → Fe 0 + 2Fe 3+ ) because they detected Fe 3+ in brown olivine. Furthermore, Van de Moortèle et al.…”

Section: Introductionmentioning

confidence: 99%

Mineralogical study of brown olivine in Northwest Africa 1950 shergottite and implications for the formation mechanism of iron nanoparticles

Takenouchi

Mikouchi

Kogure

2017

Meteorit & Planetary Scien

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Martian meteorites, in particular shergottites, contain darkened olivine (so‐called “brown olivine”) whose color is induced by iron nanoparticles formed in olivine during a shock event. The formation process and conditions of brown olivine have been discussed in the Northwest Africa 2737 (NWA 2737) chassignite. However, formation conditions of brown olivine in NWA 2737 cannot be applied to shergottites because NWA 2737 has a different shock history from that of shergottites. Therefore, this study observed brown olivine in the NWA 1950 shergottite and discusses the general formation process and conditions of brown olivine in shergottites. Our observation of NWA 1950 revealed that olivine is heterogeneously darkened between and within grains different from brown olivine in NWA 2737. XANES analysis showed that brown olivine contains small amounts of Fe3+ and TEM/STEM observation revealed that there is no SiO‐rich phase around iron metal nanoparticles. These observations indicate that iron nanoparticles were formed by a disproportionation reaction of olivine (3Fe2+olivine → Fe0metal + 2Fe3+olivine + Volivine, where Volivine means a vacancy in olivine). Some parts of brown olivine show lamellar textures in SEM observation and Raman peaks in addition to those expected for olivine, implying that brown olivine experienced a phase transition (to e.g., ringwoodite). In order to induce heterogeneous darkening, heterogeneous high temperature of about 1500–1700 K and shock duration of at least ~90 ms are required. This heterogeneous high temperature resulted in high postshock temperature (>900 K) inducing back‐transformation of most high‐pressure phases. Therefore, in spite of lack of high‐pressure phases, NWA 1950 (= Martian meteorites with brown olivine) experienced higher pressure and temperature compared to other highly shocked meteorite groups.

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“…A recent microstructural investigation of these strongly stained olivines supports the two‐stage shock history (Bläß et al. ). The brown staining of olivine is consistent with dispersed metallic nanoparticles, an extreme high density and a high abundance of Fe 3+ (Bläß et al.…”

Section: Resultsmentioning

confidence: 59%

“…The brown staining of olivine is consistent with dispersed metallic nanoparticles, an extreme high density and a high abundance of Fe 3+ (Bläß et al. ). All these features are shock‐induced.…”

Section: Resultsmentioning

confidence: 77%

The water content and parental magma of the second chassignite NWA 2737: Clues from trapped melt inclusions in olivine

Xiao

Hsu

et al. 2013

Meteorit & Planetary Scien

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Abstract-NWA 2737, the second known chassignite, mainly consists of cumulate olivine crystals of homogeneous composition (Fo = 78.7 AE 0.9). These brown colored olivine grains exhibit two sets of perpendicular planar defects due to shock. Two forms of trapped liquids, interstitial melts and magmatic inclusions, have been examined. Mineral assemblages within the olivine-hosted magmatic inclusions include low-Ca pyroxene, augite, kaersutite, fluorapatite, biotite, chromite, sulfide, and feldspathic glass. The reconstructed parental magma composition (A # ) of the NWA 2737 is basaltic and resembles both the experimentally constrained parental melt composition of chassiginites and the Gusev basalt Humphrey, albeit with lower Al contents. A # also broadly resembles the average of shergottite parent magmas or LAR 06319. However, we suggest that the mantle source for the chassignite parental magmas was distinct from that of the shergottite meteorites, particularly in CaO/Al 2 O 3 ratio. In addition, based on the analysis of the volatile contents of kaersutite, we derived a water content of 0.48-0.67 wt% for the parental melt. Finally, our MELTS calculations suggest that moderate pressure (approximately 6.8 kb) came closest to reproducing the crystallized melt-inclusion assemblages.

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Microstructural investigations on strongly stained olivines of the chassignite NWA 2737 and implications for its shock history

Cited by 26 publications

References 33 publications

Mineral chemistry of the Tissint meteorite: Indications of two‐stage crystallization in a closed system

Mineral chemistry of the Tissint meteorite: Indications of two‐stage crystallization in a closed system

Mineralogical study of brown olivine in Northwest Africa 1950 shergottite and implications for the formation mechanism of iron nanoparticles

The water content and parental magma of the second chassignite NWA 2737: Clues from trapped melt inclusions in olivine

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