Mechanisms of ringwoodite formation in shocked meteorites: Evidence from L5 chondrite Dhofar 1970.
Walton, E. L. and McCarthy, S.
Meteoritics & Planetary Science.
doi: 10.1111/maps.12829
Version of Record online: 14 FEB 2017
“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.”
