Phase transition processes of olivine in the shocked Martian meteorite Tissint: Clues to origin of ringwoodite-, bridgmanite- and magnesiowüstite-bearing assemblages

Northwest Africa (NWA) 7755 is a newly found enriched lherzolitic shergottite. Here, we report its detailed petrography and mineralogy. NWA 7755 contains both poikilitic and non-poikilitic lithologies. Olivine has different compositional ranges in the poikilitic and non-poikilitic lithologies, Fa 30-39 and Fa 37-40 , respectively. Pyroxene in the non-poikilitic lithology is systematically Fe-richer than that in the poikilitic lithology. The chromite grains in non-poikilitic lithology are highly Ti-richer than those in the poikilitic lithology. The chemical variations of olivine, pyroxene, and chromite between the poikilitic and non-poikilitic lithologies support a two-stage formation model of lherzolitic shergottites. Besides planar fractures and strong mosaicism in olivine and pyroxene, shockinduced melt veins and pockets are observed in NWA 7755. Olivine grains within and adjacent to melt veins and/or pockets have either transformed to ringwoodite, amorphous phase, or dissociated to bridgmanite plus magnesiow€ ustite. Merrillite in melt veins has completely transformed to tuite; however, apatite only has partially transformed to tuite, indicating a relatively sluggish transformation rate. The partial transformation from apatite to tuite resulted in fractional devolatilization of Cl and F in apatite. The fine-grained mineral assemblage in melt veins consists mainly of bridgmanite, minor magnesiow€ ustite, Fe-sulfide, Fe-phosphide, and Ca-phosphate minerals. The coexistence of bridgmanite and magnesiow€ ustite in these veins indicates a shock pressure of >~24 GPa and a temperature of 1800-2000°C. Coesite and seifertite are probably present in NWA 7755. The presence of these high-pressure minerals indicates that NWA 7755 has experienced a more intense shock metamorphism than other enriched lherzolitic shergottites.

Section: Discussionmentioning

confidence: 99%

Petrogenesis and shock metamorphism of the enriched lherzolitic shergottite Northwest Africa 7755

Wang

Zhang

Pang

et al. 2017

“…; Miyahara et al. ), it is timely to revisit the current reasoning and praxis of using the shock classification of chondrites and its implications for the shock pressure barometry of stony meteorites in general.…”

Section: Shock Thermo‐barometry Of Meteoritesmentioning

confidence: 99%

Revising the shock classification of meteorites

Fritz¹,

Greshake

Fernandes

2017

114

152

The current shock classification scheme of meteorites assigns shock levels of S1 (unshocked) to S6 (very strongly shocked) using shock effects in rock-forming minerals such as olivine and plagioclase. The S6 stage (55-90 GPa; 850-1750°C) relies solely on localized effects in or near melt zones, the recrystallization of olivine, or the presence of mafic high-pressure phases such as ringwoodite. However, high whole rock temperatures and the presence of high-pressure phases that are unstable at those temperatures and pressures of zero GPa (e.g., ringwoodite) are two criteria that exclude each other. Each type of high-pressure phase provides a minimum shock pressure during elevated pressure conditions to allow the formation of this phase, and a maximum temperature of the whole rock after decompression to allow the preservation of this phase. Rocks classified as S6 are characterized not by the presence but by the absence of those thermally unstable highpressure phases. High-pressure phases in or attached to shock melt zones form mainly during shock pressure decline. This is because shocked rocks (<60 GPa) experience a shock wave with a broad isobaric pressure plateau only during low velocity (<4.5 km s À1 ) impacts, which rarely occur on small planetary bodies; e.g., the Moon and asteroids. The mineralogy of shock melt zones provides information on the shape and temporal duration of the shock wave but no information on the general maximum shock pressure in the whole rock.

“…The FIB‐supported TEM observations show that equigranular, lamellar, spherulitic, and segmentation textures appear as olivine dissociation reaction texture; the difference depends on the thermal history of the impact events (Miyahara et al. , ).…”

Section: High‐pressure Polymorphs Of Olivinementioning

confidence: 99%

High‐pressure minerals in shocked meteorites

Tomioka

Miyahara

2017

Self Cite

106

101

Heavily shocked meteorites contain various types of high-pressure polymorphs of major minerals (olivine, pyroxene, feldspar, and quartz) and accessory minerals (chromite and Ca phosphate). These high-pressure minerals are micron to submicron sized and occur within and in the vicinity of shock-induced melt veins and melt pockets in chondrites and lunar, howardite-eucrite-diogenite (HED), and Martian meteorites. Their occurrence suggests two types of formation mechanisms (1) solid-state high-pressure transformation of the host-rock minerals into monomineralic polycrystalline aggregates, and (2) crystallization of chondritic or monomineralic melts under high pressure. Based on experimentally determined phase relations, their formation pressures are limited to the pressure range up tõ 25 GPa. Textural, crystallographic, and chemical characteristics of high-pressure minerals provide clues about the impact events of meteorite parent bodies, including their size and mutual collision velocities and about the mineralogy of deep planetary interiors. The aim of this article is to review and summarize the findings on natural high-pressure minerals in shocked meteorites that have been reported over the past 50 years. 2017