CV and CM chondrite impact melts

Abstract

28 Phone: 1 (865) 974-9805 29 30 Travis J. Tenner 31 Abstract 49 50 Volatile-rich and typically oxidized carbonaceous chondrites, such as CV and CM 51 chondrites, potentially respond to impacts differently than do other chondritic materials. 52 Understanding impact melting of carbonaceous chondrites has been hampered by the dearth 53 of recognized impact melt samples. In this study we identify five carbonaceous chondrite 54 impact melt clasts in three host meteorites: a CV3 red chondrite, a CV3 oxA chondrite, and a 55 regolithic howardite. The impact melt clasts in these meteorites respectively formed from 56 CV3 red chondrite, CV3 oxA chondrite, and CM chondrite protoliths. We identified these impact 57 melt clasts and interpreted their precursors based on their texture, mineral chemistry, silicate 58 bulk elemental composition, and in the case of the CM chondrite impact melt clast, in situ 59 measurement of oxygen three-isotope signatures in olivine. These impact melts typically 60 contain euhedral-subhedral olivine microphenocrysts, sometimes with relict cores, in glassy 61 groundmasses. Based on petrography and Raman spectroscopy, four of the impact melt clasts 62 exhibit evidence for volatile loss: these melt clasts either contain vesicles or are depleted in 63 H 2 O relative to their precursors. Volatile loss (i.e., H 2 O) may have reduced the redox state of 64 the CM chondrite impact melt clast. The clasts that formed from the more oxidized 65 precursors (CV3 oxA and CM chondrites) exhibit phase and bulk silicate elemental 66 compositions consistent with higher intrinsic oxygen fugacities relative to the clast that 67 formed from a more reduced precursor (CV3 red chondrite). The mineral chemistries and 68 assemblages of the CV and CM chondrite impact melt clasts identified here provide a 69 template for recognizing carbonaceous chondrite impact melts on the surfaces of asteroids. 70 71 1. INTRODUCTION 72 The effects of impacts onto most carbonaceous chondrite targets are not well 73 understood. Only a few impact melts have been reported in carbonaceous chondrites, an 74 observation which led Scott et al. (1992) to speculate that volatile components (e.g., H 2 O, 75 CO 2 , S 2 ) in carbonaceous chondrites would respond to impact shocks explosively and prevent 76 the formation of impact melt. CK chondrites were the exception in that at least three samples 77 contain opaque shock veins (Rubin 1992). CK chondrites are notably different from other 78 carbonaceous chondrite groups, as they have typically experienced a greater extent of thermal 79metamorphism, after which some were exposed to impacts that shock-blackened silicates 80 (Kallemeyn et al. 1991). It has been suggested that CV and CK chondrites come from the 81 same parent body based on similarities in their bulk isotopic and element compositions 82 (Greenwood et al. 2010;Wasson et al. 2013). However, differences in the mineral 83 chemistries and calculated intrinsic fO 2 between the CV and CK chondrites cannot 84 parsimoniously reconcile these two car...

The alteration history of the Jbilet Winselwan CM carbonaceous chondrite: An analog for C‐type asteroid sample return

“…Alternative options include shock heating from impacts (Lunning et al. ) or solar radiation (Chaumard et al. ).…”

“…2014b), but otherwise the effects are rarely seen in CM chondrites (Lunning et al. ), possibly because water‐rich asteroids are expected to disrupt in large collisions and not preserve an impact record (Scott et al. ).…”

Shock stage distribution of 2280 ordinary chondrites—Can bulk chondrites with a shock stage of S6 exist as individual rocks?

“…Impact melts or clasts of impact melt breccias are also known from C and R chondrites (e.g., Lunning et al. , ).…”

Shock metamorphism of planetary silicate rocks and sediments: Proposal for an updated classification system

“…An important fact is that among asteroidal meteorites, we know unshocked samples as well as very strongly shocked samples, as represented by impact melt rocks (although mostly as clasts within polymict breccias, e.g., Lunning et al. ). This requires a consideration of the potential collisional scenarios of asteroidal bodies (Hartmann ; Stöffler et al ; see Fig.…”