Isotopic and chemical variation of organic nanoglobules in primitive meteorites

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

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“…9). These observations are in agreement with previous nanoSIMS analyses of Bells De Gregorio et al, 2013) …”

Section: N and C Isotope Compositionssupporting

confidence: 83%

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

Kooten

Nagashima

Kasama

et al. 2018

Geochimica et Cosmochimica Acta

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“…In addition, recent studies showed variety of globule morphologies in CR, CM (Changela et al, 2012(Changela et al, , 2013De Gregorio et al, 2013) and CI, CO carbonaceous chondrites , and suggested modification of the morphology of organic nanoglobule during aqueous alteration on the parent body. Therefore, the morphology of organic matter (aggregated globules and/or particles and non-aggregated globules) in Murchison and NWA 801 might record the effect of aqueous alteration.…”

Section: Morphology Of the Isotopically Anomalous Organic Mattermentioning

confidence: 99%

Deuterium- and <sup>15</sup>N-signatures of organic globules in Murchison and Northwest Africa 801 meteorites

2015

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mentioning

confidence: 99%

Towards Automated Segmentation Methods for 3D Tomography Studies of the Morphology of Carbon Nanoglobules in Chondritic Meteorites

et al. 2015

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The presence of nano-scale, roughly spherical carbonaceous features has been shown in several samples of chondritic meteorites [1][2][3][4]. There have been a few proposed mechanisms by which these carbon nanoglobules may form [2,5]. Quantification of nanoglobule prevalence, morphology, and relationship to other meteorite components can provide important evidence to elucidate how the nanoglobules form, and how the carbon contained therein may be altered by interactions with fluids and nearby minerals present on the asteroid from which the meteorite originated.It has recently been shown that 3D FIB-SEM tomography can be utilized to collect reconstruction volumes of chondritic meteorite grains, as was done on a grain from the Tagish Lake 5b sample [6]. A representative FIB-SEM cross-section is shown in Figure 1, showing the complex nature of the sample, as well as highlighting the selected nanoglobule regions of interest. Segmentation of the nanoglobules for the data set in [6] was achieved manually, which poses a significant challenge to further study of this and other samples, as it is very time consuming and subject to variation in the resultant output based on what individual is carrying out the segmentation.Several challenges arise when trying to formulate an efficient and effective automated method for segmentation of the nanoglobules and other constituent phases present in a chondritic meteorite particles. The porous nature and irregular geometries of the particles make segmentation of the entire volume challenging, as back-contrast from the far wall of pores makes automated identification difficult, and regions of which with similar contrast may be misidentified as the nanoglobules. Carbon epoxy infiltration, a common method for filling pores to even contrast, is not a viable method, as the nanoglobules are also carbonaceous, and would not be easily distinguishable from the background epoxy. Additionally, there are other carbon phases, large carbonaceous veins called clasts, which have similar contrast but need to be segmented as a separate phase. While not related to the segmentation of the nanoglobules per se, the sheer number of present phases is also daunting, which is reflected in the complex image histogram given in Figure 2. Identification of nearby phases may be important in determining how the nanoglobules may interact with the parent body [1,7].Despite the complex nature of the problem, an automated, or pseudo-automated, method for segmentation is desired, and will be attempted for the Tagish Lake sample. This method should also be portable for use with other meteorite samples or otherwise. An automated segmentation scheme would allow for much higher throughput, and a more regimented and less subjective definition for each of the phases desired for segmentation. This too would allow for further analysis of the Tagish Lake sample reported earlier, as only ~20% of the total reconstruction volume was analyzed due to the slow manual segmentation.

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Isotopic and chemical variation of organic nanoglobules in primitive meteorites

Cited by 97 publications

References 90 publications

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

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

Deuterium- and <sup>15</sup>N-signatures of organic globules in Murchison and Northwest Africa 801 meteorites

Towards Automated Segmentation Methods for 3D Tomography Studies of the Morphology of Carbon Nanoglobules in Chondritic Meteorites

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