Widespread oxidized and hydrated amorphous silicates in CR chondrites matrices: Implications for alteration conditions and H2 degassing of asteroids

Guillou, C. Le; Changela, Hitesh; Brearley, A. J.

doi:10.1016/j.epsl.2015.02.031

Cited by 112 publications

(131 citation statements)

References 78 publications

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“…16 is likely unique to CM chondrites, since these meteorites contain abundant cronstedtite as alteration product relative to other chondrites. This is likely owed to variations in bulk chemical composition due to heterogeneous accretion of chondritic components such as chondrules, matrix, CAIs and ices as well as conditions of alteration on the chondrite parent bodies (LeGuillou et al, 2015). Lastly, we note that the H isotope composition of Jbilet is not homogeneous for both matrix regions analyzed.…”

Section: Decomposition Of Tcismentioning

confidence: 82%

“…The initial mineralogy of chondrite matrices mainly consists of anhydrous silicates (crystalline olivine and pyroxene and amorphous ferromagnesian silicates) and FeNi metal (Abreu and Brearley, 2010;LeGuillou et al, 2015). During the onset of aqueous alteration, the first secondary phases in CM chondrites are tochilinite and cronstedtite (Pignatelli et al, 2017), either as intergrowths (TCIs) or as separate phases (e.g., Maribo, Fig.…”

Section: Decomposition Of Tcismentioning

confidence: 99%

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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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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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Section: Decomposition Of Tcismentioning

confidence: 82%

Section: Decomposition Of Tcismentioning

confidence: 99%

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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“…Furthermore, the high Fe 3+ /ΣFe ratio observed in amorphous silicates in weakly altered CRs, CO, and CMs show that anhydrous amorphous silicates could have been significantly hydrated and oxidized without being altered into phyllosilicates (Le Guillou and Brearley ; Le Guillou et al. ). In conclusion, although nebular or pre‐accretionary alteration may have played a role in generating the characteristics of CM chondrites, its effects have certainly been overprinted by later alteration within the final CM parent body.…”

Section: Discussionmentioning

confidence: 99%

Multiple precursors of secondary mineralogical assemblages in CM chondrites

Pignatelli

Marrocchi

Vacher

et al. 2016

Meteorit & Planetary Scien

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We report a petrographic and mineralogical survey of tochilinite/cronstedtite intergrowths (TCIs) in Paris, a new CM chondrite considered to be the least altered CM identified to date. Our results indicate that type‐I TCIs consist of compact tochilinite/cronstedtite rims surrounding Fe‐Ni metal beads, thus confirming kamacite as the precursor of type‐I TCIs. In contrast, type‐II TCIs are characterized by complex compositional zoning composed of three different Fe‐bearing secondary minerals: from the outside inwards, tochilinite, cronstedtite, and amakinite. Type‐II TCIs present well‐developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors. The results demonstrate that type‐II TCIs formed by pseudomorphism of the anhydrous silicates, olivine, and pyroxene. Hence, there is no apparent genetic relationship between type‐I and type‐II TCIs. In addition, the complex chemical zoning observed within type‐II TCIs suggests that the alteration conditions evolved dramatically over time. At least three stages of alteration can be proposed, characterized by alteration fluids with varying compositions (1) Fe‐ and S‐rich fluids; (2) S‐poor and Fe‐ and Si‐rich fluids; and (3) S‐ and Si‐poor, Fe‐rich fluids. The presence of unaltered silicates in close association with euhedral type‐II TCIs suggests the existence of microenvironments during the first alteration stages of CM chondrites. In addition, the absence of Mg‐bearing secondary minerals in Paris TCIs suggests that the Mg content increases during the course of alteration.

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“…; Le Guillou et al. ). Although goethite is commonly described as a terrestrial weathering product (e.g., Rubin ; Wasson and Rubin ), iron oxide and sulfide layers around metal grains in highly altered CR chondrites (e.g., Renazzo, Al Rais) were likely formed during parent body alteration (e.g., Morlok and Libourel ; Harju et al.…”

Section: Discussionmentioning

confidence: 96%

Cooling rates of type I chondrules from Renazzo: Implications for chondrule formation

Chaumard

Humayun

Zanda

et al. 2018

Meteorit & Planetary Scien

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Cooling rates are one of the few fundamental constraints on models of chondrule formation. In this study, we used Cu and Ga diffusion profiles in metal grains to determine the cooling rates of type I chondrules in the Renazzo CR2 chondrite. To improve previous estimations of cooling rates obtained using this method, we used CT scanning and serial polishing of our sections to analyze equatorial sections of large metal grains. Through the cores of these metal grains situated at the surface of chondrules, the cooling rates calculated range from 21 to 86 K h−1 for a peak temperature Tp ~ 1623–1673 K. A metal grain embedded in the core of a chondrule exhibits a cooling rate of 1.2 K h−1 at a Tp ~ 1573 K. We also measured Cu‐Ga diffusion profiles from nonequatorial sections of metal grains and calculated a lower range of cooling rates of 15–69 K h−1 for Tp ~ 1473–1603 K compared to our results from equatorial sections. The high cooling rates inferred from the lightning model (several thousand K h−1) are clearly at odds with the values obtained in this work. The X‐wind model predicts cooling rates (~6–10 K h−1) lower than most of our results. The cooling rates calculated here are in close agreement with those inferred from shock wave models, in particular for temperatures at which olivine crystallizes (from ~10 to several hundreds K h−1 between 1900 and 1500 K). However, the chemical compositions of metal grains in Renazzo are consistent with the splashing model, in which a spray of metal droplets originated from a partially molten planetesimal. Volatile siderophile element depletion is explained by evaporation before metal was engulfed within silicate droplets. Liquid metal isolated from the liquid silicate crystallized during cooling, reacted with the ambient gas, and then re‐accreted within partially molten chondrules.

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Widespread oxidized and hydrated amorphous silicates in CR chondrites matrices: Implications for alteration conditions and H2 degassing of asteroids

Cited by 112 publications

References 78 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

Multiple precursors of secondary mineralogical assemblages in CM chondrites

Cooling rates of type I chondrules from Renazzo: Implications for chondrule formation

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