Back transformation and oxidation of (Mg, Fe)2SiO4 spinels at high temperatures

Li, Ming; Kim, Y. H.; Manghnani, Murli H.; Ushadevi, S; Ito, Eiji; Xie, Hui

doi:10.1007/bf00234000

Cited by 31 publications

(21 citation statements)

References 19 publications

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“…) starts to decompose at >130 °C (beginning near 130 °C and completed at 480 °C; Durben and Wolf ), (2) akimotoite at >730 °C (Reynard and Rubie ), and (3) ringwoodite at >900 °C (Ming et al. ; Kimura et al. ).…”

Section: Generation Of Shock Metamorphic Effectsmentioning

confidence: 99%

Revising the shock classification of meteorites

Fritz¹,

Greshake

Fernandes

2017

Meteorit & Planetary Scien

115

152

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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.

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Section: Generation Of Shock Metamorphic Effectsmentioning

confidence: 99%

Revising the shock classification of meteorites

Fritz¹,

Greshake

Fernandes

2017

Meteorit & Planetary Scien

115

152

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“…It is important to note that once formed, the transformation of olivine to ringwoodite is a reversible process, with back-reaction to the a-(Mg,Fe) 2 SiO 4 structure (orthorhombic olivine) under ambient pressure conditions and temperatures >1173 K (Ming et al 1991;Kimura et al 2003). For a shock pressure of 20-25 GPa the shock temperature of the host rock is approximately 420 K .…”

Section: Mechanisms Of Olivine Transformationmentioning

confidence: 99%

Mechanisms of ringwoodite formation in shocked meteorites: Evidence from L5 chondrite Dhofar 1970

Walton

McCarthy

2017

Meteorit & Planetary Scien

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The formation of the high‐pressure compositional equivalents of olivine and pyroxene has been well‐documented within and surrounding shock‐induced veins in chondritic meteorites, formed by crystallization from a liquid‐ or solid‐state phase transformation. Typically polycrystalline ringwoodite grains have a narrow range of compositions that overlap with those of their olivine precursors, whereas the formation of iron‐enriched ringwoodite has been documented from only a handful of meteorites. Here, we report backscattered electron images, quantitative wavelength‐dispersive spectrometry (WDS) analyses, qualitative WDS elemental X‐ray maps, and micro‐Raman spectra that reveal the presence of Fe‐rich ringwoodite (Fa44‐63) as fine‐grained (500 nm), polycrystalline rims on olivine (Fa24‐25) wall rock and as clasts engulfed by shock melt in a previously unstudied L5 chondrite, Dhofar 1970. Crystallization of majorite + magnesiowüstite in the vein interior and metastable mineral assemblages within 35 μm of the vein margin attest to rapid crystallization of a superheated shock melt (>2300 K) from 20─25 GPa to ambient pressure and temperature. The texture and composition of bright polycrystalline ringwoodite rims (Fa44‐63; MnO 0.01─0.08 wt%) surrounding dark polycrystalline olivine (Fa8‐14; MnO 0.56─0.65 wt%) implies a solid‐state transformation mechanism in which Fe was preferentially partitioned to ringwoodite. The spatial association between ringwoodite and shock melt suggests that the rapidly fluctuating thermal regimes experienced by chondritic minerals in contact with shock melt are necessary to both drive phase transformation but also to prevent back‐transformation.

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“…Yagi et al 1987;Will and Lauterjung 1987;Ming et al 1991;Wu et al 1993). At room temperature, the kinetics of these transformations is so slow that olivines can be studied metastably to a few tens of GPa.…”

Section: Introductionmentioning

confidence: 99%

Compression and amorphization of (Mg,Fe)2SiO4 olivines: An X-ray diffraction study up to 70 GPa

et al. 1995

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Abstract. The effect of (Mg,Fe) substitution on the compression and pressure-induced amorphization of olivines has been investigated up to more than 50 GPa in a diamond anvil cell through energy-dispersive X-ray diffraction experiments with synchrotron radiation. For the four (Mgl _~, Fex)2SiO4 olivines studied, the compressibility is the highest along the b axis and the smallest along the a axis. For compositions with x=0, 0.17, 0.66, and 1, the slope of the volume-pressure curves shows a rapid decrease at pressures of around 42, 34, 20 and 10 GPa, respectively. Assuming K; = 4, we obtained at lower pressures with a Birch-Murnaghan equation of state essentially the same room-pressure bulk modulus for all olivines, namely Ko=131_+6GPa, in agreement with previous single-crystal compression and ultrasonic measurements. At higher pressures, the compression becomes nearly isotropic and the materials very stiff. These changes could precede partial transformation of olivines to a high-pressure polymorph related to the spinel structure. Only a small fraction of olivines seems to transform actually to this phase, however, because most of the material undergoes instead pressure-induced amorphization which take place at considerably higher pressures for Mg-than for Fe-rich olivines.

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Back transformation and oxidation of (Mg, Fe)2SiO4 spinels at high temperatures

Cited by 31 publications

References 19 publications

Revising the shock classification of meteorites

Revising the shock classification of meteorites

Mechanisms of ringwoodite formation in shocked meteorites: Evidence from L5 chondrite Dhofar 1970

Compression and amorphization of (Mg,Fe)2SiO4 olivines: An X-ray diffraction study up to 70 GPa

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