Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
5783093 | Geochimica et Cosmochimica Acta | 2017 | 14 Pages |
Abstract
An impact event recorded in the Northwest Africa (NWA) 8275 LL7 ordinary chondrite was investigated based on high-pressure mineralogy of pervasive shock-melt veins present in the rock. NWA 8275 consists of olivine, low-Ca pyroxene, plagioclase (albite-oligoclase composition), and minor high-Ca pyroxene, K-feldspar, phosphate minerals, metallic Fe-Ni and iron sulfide. Plagioclase and K-feldspar grains near the shock-melt veins have become amorphous, although no high-pressure polymorphs of olivine and pyroxene were identified in or adjacent the shock-melt veins. Raman spectroscopy and focused ion beam (FIB)-assisted transmission electron microscopy (TEM) observations reveal that plagioclase entrained around the center portion of the shock-melt veins has dissociated into a jadeite + coesite assemblage. Alternately stacked jadeite and coesite crystals occur in the original plagioclase. On approaching the host rock/shock-melt vein, only jadeite is present. Based on the high-pressure polymorph assemblage, the shock pressure and temperature conditions recorded in the shock-melt veins are â¼3-12 GPa and â¼1973-2373 K, respectively. Following a Rankine-Hugoniot relationship, the impact velocity was at least â¼0.45-1.54 km/s. The duration of high-pressure and high-temperature (HPHT) conditions required for the albite dissociation reaction is estimated a maximum of â¼4-5 s using the phase transition rate of albite, implying that a body of up to â¼9-12 km across collided with the parent body of NWA 8275. The coexistence of jadeite and coesite, the latter of which rarely accompanies jadeite in shocked ordinary chondrites, as a dissociation product of albite requires relatively long duration HPHT conditions. Thus, the impact event recorded in NWA 8275 was likely caused by a larger-than-typical projectile.
Keywords
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
Masaaki Miyahara, Eiji Ohtani, Akira Yamaguchi,