Isotope record of mineralogical changes in a spectrum of aqueously altered CM chondrites

Abstract

The recent fall of the relatively unaltered CM chondrite Maribo provides a unique opportunity to study the early stages of aqueous alteration on the CM chondrite parent body. We show using transmission electron microscopy of a matrix FIBsection from Maribo that this meteorite mainly appears to consist of tochilinite-cronstedtite intergrowths (TCIs), but also contains regions of amorphous or nanocrystalline silicates, anhydrous silicates and FeNi metal aggregates with thin iron oxide rims, suggesting that it experienced aqueous alteration to a relatively small degree. A comparison of Maribo with increasingly altered CM chondrites such as Jbilet Winselwan and Bells shows that during progressive aqueous alteration (1) the TCIs are replaced by coarser sulfides and increasingly Mg-rich serpentine, and (2) the abundance of 15 N-rich hotspots increases, whereas the magnitude of their 15 N enrichment decreases. We observe that the overall N isotope variability related to aqueous alteration is an order of magnitude lower than the variability observed between different chondrite groups. We suggest these high order differences are the result of heterogeneous accretion of insoluble or soluble organic carriers of 15 N to the different chondrite parent bodies. D/H ratios of matrices from Maribo, Jbilet Winselwan and Bells increase with progressive aqueous alteration, a trend that is opposite to expectations of mixing between D-poor water and D-rich organic matter. We argue that this behaviour cannot be related to Fe oxidation or serpentinization reactions and subsequent loss of D-poor H 2 gas. We offer an alternative hypothesis and suggest that CM chondrites experienced two-stage aqueous alteration. During the first stage occurring at relatively low temperature, mixing of increasing amounts of D-poor water with D-rich organic matter results in a decrease of D/H ratio with increasing degree of alteration. During the second stage of alteration occurring at relatively high temperature (T < 300°C), decomposition of TCIs in CMs of petrologic type <2.7 releases gaseous D-poor water that results in increase of the D/H ratio of the CM matrices. Finally, we report on changes in the organic structure of Maribo, Jbilet Winselwan and Bells using Carbon-K and Nitrogen-K edge electron energy loss spectroscopy. The organic matter initially has higher aromatic/aliphatic ratios (e.g., Maribo) and lower abundances of ketone and carboxyl functional groups, which we suggest are the result of chemical degradation of double bonded carbon from oxidation during hydrothermal

The role of Bells in the continuous accretion between the CM and CR chondrite reservoirs

“…; Van Kooten et al. ). Regardless of these close affinities to CR chondrites, Bells is classified as a CM chondrite, albeit an anomalous one (Mittlefehldt ), since it shares with other CM chondrites a common oxygen isotope composition of magnetite and matrix minerals (Rowe et al.…”

“…; Van Kooten et al. ) and its mineralogy, even though heavily altered, includes a significant amount of FeNi metal (Figs. c–e), which are features typically observed in CR chondrites.…”

“…), and the anomalous Bells (CM2.1/2.2, Van Kooten et al. ). Murchison, Mighei, Murray, Bells (the un‐weathered fall), and Cold Bokkeveld are part of the permanent collections of the Natural History Museum of Denmark and were acquired in 1973, 1906, 1991, 1995, and 1862, respectively.…”

Testing the genetic relationship between fluid alteration and brecciation in CM chondrites

“…, , ; Van Kooten et al. ), and oxygen isotopes are a powerful tool for constraining the physicochemical conditions (T, W / R , pH, etc.) and relative timing of secondary phase precipitation.…”

“…Aqueous alteration has been extensively studied in CM chondrites (e.g., McSween 1979a;Benedix et al 2003;Rubin et al 2007;Lee et al 2014;Pignatelli et al 2016Pignatelli et al , 2017Pignatelli et al , 2018Van Kooten et al 2018), and oxygen isotopes are a powerful tool for constraining the physicochemical conditions (T, W/R, pH, etc.) and relative timing of secondary phase precipitation.…”