Possible melting produced chondrule destruction in NWA 6604 CK4 chondrite

Abstract

In analyzing a thin section of the NWA 6604 CK4 meteorite, only altered chondrules and various components that are probably left behind the destruction of former chondrules can be observed. We suggest that melting, grain size decrease, resorption of the original chondrules, and crystallization of opaque minerals were the main processes that destroyed the chondrules. Four different events could be identified as having occurred during this alteration. First, opaques crystallized along former fractures producing chains of separated grains. Later, opaques and Ca-rich minerals crystallized together in veins and large melt pockets; this was the strongest recrystallization phase involving the largest volume of melt. This occurred along different fractures than the first phase above. During the third phase, only Ca-rich plagioclase crystallized along thin veins, and in a fourth phase, fractures formed again, partly along those formed during the second phases but without substantial mineral infill. Two simple possible case models should be considered for this meteorite: alteration by purely impact-driven processes or mainly by melt-driven processes. Although for CK4 chondrites, the shock-produced alteration driven by impact is the more accepted and widespread approach, melting is also compatible with the observed textural characteristics of chondrule destruction. During melting, recrystallization took place producing iron-rich minerals earlier and Ca-Si-rich ones later. The penetration of melts into veins contributed in the chondrule destruction. The stress directions also changed during these alterations, and minerals that formed later filled differently oriented fractures than the earlier ones. From our observations, we favor a view where heat-driven melting and recrystallization produced the destruction and uniform mineralogy in the sample.

Multiscale and Correlative Analytical Electron Microscopy of Extraterrestrial Minerals

“…The primitive (unaltered) chondrites contain complex mixtures of micrometer sized magnesian olivine and low-Ca pyroxene, amorphous ferromagnesian silicates, Fe, Ni-metals, sulphides and organic phases. These meteorites also contain fractions of material that was probably vaporized and recondensed during high-temperature transient heating events associated with the formation of chondrules and refractory inclusions (Kereszturi et al, 2015;Bizzaro et al, 2017). The differentiated meteorites, on the other hand, are fragments of asteroids that experienced chemical and physical changes that led to a separation of the metallic part from silicates leading to metallic core and silicate mantle.…”