Shock-induced formation mechanism of seifertite in shergottites

Bläß, U. W.

doi:10.1007/s00269-013-0580-x

Cited by 15 publications

(26 citation statements)

References 37 publications

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“…, ). Bläß () suggested that seifertite back‐transformation will produce numerous amorphous lamellae in accordance with our petrographic observations. Additionally, the transition pressure of metastable seifertite could be lower than 30 GPa, within the thermodynamic stability field of stishovite.…”

Section: Discussionsupporting

confidence: 88%

Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975

Xiao

Balta

et al. 2015

Meteorit & Planetary Scien

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Abstract-We present a study of the petrology and geochemistry of basaltic shergottite Northwest Africa 2975 (NWA 2975). NWA 2975 is a medium-grained basalt with subophitic to granular texture. Electron microprobe (EMP) analyses show two distinct pyroxene compositional trends and patchy compositional zoning patterns distinct from those observed in other meteorites such as Shergotty or QUE 94201. As no bulk sample was available to us for whole rock measurements, we characterized the fusion crust and its variability by secondary ion mass spectrometer (SIMS) measurements and laser ablation inductively coupled plasma spectroscopy (LA-ICP-MS) analyses as a best-available proxy for the bulk rock composition. The fusion crust major element composition is comparable to the bulk composition of other enriched basaltic shergottites, placing NWA 2975 within that sample group. The CI-normalized REE (rare earth element) patterns are flat and also parallel to those of other enriched basaltic shergottites. Merrillite is the major REE carrier and has a flat REE pattern with slight depletion of Eu, parallel to REE patterns of merrillites from other basaltic shergottites. The oxidation state of NWA 2975 calculated from Fe-Ti oxide pairs is NNO-1.86, close to the QFM buffer. NWA 2975 represents a sample from the oxidized and enriched shergottite group, and our measurements and constraints on its origin are consistent with the hypothesis of two distinct Martian mantle reservoirs: a reduced, LREE-depleted reservoir and an oxidized, LREE-enriched reservoir. Stishovite, possibly seifertite, and dense SiO 2 glass were also identified in the meteorite, allowing us to infer that NWA 2975 experienced a realistic shock pressure of~30 GPa.

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Section: Discussionsupporting

confidence: 88%

Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975

Xiao

Balta

et al. 2015

Meteorit & Planetary Scien

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“…Cristobalite was documented alongside and in close spatial relation with all the natural occurrences of the high pressure, post-stishovite silica polymorph seifertite78 found in heavily shocked Martian or Lunar meteorites. Our experiments show that at non-hydrostatic environment and at ambient temperature conditions cristobalite follows α→II→X-I transformation path; at sufficiently high pressures (∼30–40 GPa)91418 it will eventually transform to quenchable seifertite or seifertite-like phase, bypassing the equilibrium formation of stishovite and CaCl 2 -structured polymorph.…”

Section: Discussionmentioning

confidence: 82%

Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation

et al. 2017

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In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms—a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.

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“…; Chen et al. ; Bläß ) and olivine‐pyroxene‐spinel oxybarometer (Sack and Ghiorso , , , , , ). The olivine‐spinel oxybarometer calculations gave an oxygen fugacity of QFM −1.4 at 855 °C (Table ).…”

Section: Resultsmentioning

confidence: 99%

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

Wang

Zhang

Pang

et al. 2017

Meteorit & Planetary Scien

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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.

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Shock-induced formation mechanism of seifertite in shergottites

Cited by 15 publications

References 37 publications

Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975

Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975

Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation

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

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