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

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Highlights

  • We measured the Fe3+ content of matrix silicate in a suite of CR chondrites.

  • Nanoscale quantitative analyses are performed (STXM, TEM).

  • Silicates are highly oxidized (Fe3+/Fe ratio 5070%) in all CR chondrites.

  • Kinetically-controlled reactions in system open to H2 loss best explain the observations.

Abstract

The CR chondrites carry one of the most pristine records of the solar nebula materials that accreted to form planetesimals. They have experienced very variable degrees of aqueous alteration, ranging from incipient alteration in their matrices to the complete hydration of all of their components. In order to constrain their chemical alteration pathways and the conditions of alteration, we have investigated the mineralogy and Fe oxidation state of silicates in the matrices of 8 CR chondrites, from type 3 to type 1. Fe-L edge X-ray Absorption Near Edge Structure (XANES) was performed on matrix FIB sections using synchrotron-based scanning transmission X-ray microscopy (STXM). The Fe3+/Fe ratio of submicron silicate particles was obtained and coordinated with TEM observations.

In all the least altered CR chondrites (QUE 99177, EET 87770, EET 92042, LAP 02342, GRA 95229 and Renazzo), we find that the matrices consist of abundant submicron Fe-rich hydrated amorphous silicate grains, mixed with nanometer-sized phyllosilicates. The Fe3+/Fe ratios of both amorphous and nanocrystalline regions are very high with values ranging from 68 to 78%. In the most altered samples (Al Rais and GRO 95577), fine-grained phyllosilicates also have a high Fe3+/Fe ratio (around 70%), whereas the coarse, micrometer-sized phyllosilicates are less oxidized (down to 55%) and have a lower iron content.

These observations suggest the following sequence: submicron Fe2+-amorphous silicate particles were the building blocks of CR matrices; after accretion they were quickly hydrated and oxidized, leading to a metastable, amorphous gel-like phase. Nucleation and growth of crystalline phyllosilicates was kinetically-limited in most type 3 and 2 CRs, but increased as alteration became more extensive in Al Rais and GRO 95577. The decreasing Fe3+/Fe ratio is interpreted as a result of the transfer of Fe3+ from silicates to oxides during growth, while aqueous alteration progressed (higher temperature, longer duration, change of fluid composition). In a fully closed system, equilibrium thermodynamics suggest that the water to rock ratios, typically assumed to be low (<1) for chondrites, should primarily control the iron valency of the silicates and predict a lower Fe3+/Fe ratio. Such a high Fe3+/Fe value could be accounted for, however, if the system was partially open, at least with respect to H2 (and other gases as well). Rapid degassing of the fluid would have favored more oxidizing fluid conditions. Recently proposed scenarios involving some degree of water D/H increase through Rayleigh isotopic fractionation are supported by these results.

Introduction

Carbonaceous chondrites (CC) are among the best-preserved early Solar System materials and allow the investigation of the origins of materials in the proto-solar disk and their modification after accretion within their asteroidal parent bodies. Anhydrous silicates accreted together with water ice. As a result of radiogenic heating, the ice melted leading to the formation of the phyllosilicates (Clayton and Mayeda, 1984, Tomeoka and Buseck, 1988, Brearley, 1995, Rubin et al., 2007). Investigating alteration reactions caused by interactions with aqueous fluids is therefore crucial to constrain the nature of the precursor materials and the conditions of aqueous alteration on the chondrite parent bodies (Zolensky et al., 1989, Young et al., 2003, Brearley, 2006, Bland et al., 2009).

The CR chondrites have been less studied than the CMs and CIs because of their relative scarcity; it is only recently that detailed studies of the matrices of CR chondrites were initiated (Abreu and Brearley, 2010, Schrader et al., 2011, Bonal et al., 2013, Morlok and Libourel, 2013, Jilly et al., 2014, Le Guillou and Brearley, 2014, Le Guillou et al., 2014a, Schrader et al., 2014). The CR chondrites are of special interest because they generally exhibit a lower degree of aqueous alteration and thermal metamorphism than the other chondrite groups. Consequently, their matrices are pristine and preserve fine-grained materials, such as presolar grains, labile deuterium-rich organics and amorphous silicates (Weisberg et al., 1993, Busemann et al., 2006, Alexander et al., 2010, Alexander et al., 2013, Floss and Stadermann, 2009, Abreu and Brearley, 2010, Le Guillou et al., 2012, Le Guillou and Brearley, 2014, Davidson et al., 2014, Floss et al., 2014, Harju et al., 2014, Schrader et al., 2014, Howard et al., 2015). The CR chondrites therefore provide unique insights into the precursor materials of the matrices of CCs, as well the earliest stages of aqueous alteration. In CR3 and most CR2 chondrites, aqueous alteration has not been pervasive, and remained mostly confined to the matrices, which are more susceptible to alteration (Burger and Brearley, 2005, Abreu and Brearley, 2010, Le Guillou and Brearley, 2014). Their matrices consist mostly of a groundmass of iron-bearing amorphous silicate, mixed with nano-scale phyllosilicates. In contrast, most CM and CI chondrites matrices are made of crystalline phyllosilicates, such as serpentine, cronstedtite and saponite (Tomeoka and Buseck, 1988, Rubin et al., 2007). In a few cases, comparatively weakly altered CMs (Paris, Yamato 791198), ordinary chondrites (Bishunpur), CO chondrites (Allan Hill 77307) and ungrouped chondrites (Acfer 094) carry similar amorphous silicates (Brearley, 1993, Greshake, 1997, Chizmadia and Brearley, 2008, Hewins et al., 2013). The ubiquity of this amorphous silicate in weakly altered chondrites has led to the suggestion that it could have been the precursor that accreted to form matrices (Brearley, 1993, Nuth et al., 2005, Pontoppidan and Brearley, 2010). This highly reactive, fine-grained material is the first to be hydrated, and is then progressively transformed into short-range order serpentine/saponite (Chizmadia and Brearley, 2008, Le Guillou and Brearley, 2014). The alteration reaction pathways of these amorphous silicates are therefore a key to understand the early stages of aqueous alteration in chondrites. Serpentinization reactions are extensively studied in the context of the oceanic crust alteration, because of their potential for H2 generation, and for their role in the synthesis of abiotic compounds that provide links to the origin of life on Earth (Benzerara et al., 2007, McCollom and Bach, 2009, Marcaillou et al., 2011, Malvoisin et al., 2012b). These terrestrial studies provide a context allowing comparison to chondrites.

Our purpose is to characterize the amorphous silicates and secondary hydrated phases in CR matrices, focusing particularly on the valency of iron. No data are available at the scale of the matrix individual grains. However, bulk matrix XANES was performed on CR, CM and CI chondrites (Sutton et al., 2013, Beck et al., 2012). They have shown that increasing aqueous alteration induces increasing bulk Fe3+/Fe ratios. Oxidation of iron occurs during serpentinization processes, and the resulting Fe3+ can be included either into magnetite or serpentine, depending on conditions (Klein et al., 2009). It is coupled to the reduction of water which in turn controls the amount of H2 produced during alteration. Therefore, we have studied CR from weakly altered petrologic type 3 to fully altered type 1 to investigate the effect of increasing alteration on the iron valency in matrices. It should provide constraints on the alteration conditions (temperature, fluid oxygen fugacity, water to rock ratio, open vs. closed system) and help elucidate reaction pathways that involve oxidation. The size of the silicates in matrices is smaller than a micron and measuring their iron valency requires a technique with nanometer-scale resolution, offered by synchrotron-based scanning transmission X-ray microscopy (STXM). At the iron L2,3-edge, calibrations allow the Fe3+/Fe ratio of silicates to be quantitatified with good precision (Bourdelle et al., 2013). Coordinated with TEM, STXM provides the first comprehensive and quantitative insights into the oxidation and mineralogical variation across a CC group.

Section snippets

Samples

Eight samples covering the complete aqueous alteration range of the CR chondrites were investigated (Weisberg et al., 1993, Weisberg and Huber, 2007): Queen Alexandra Range 99177 (QUE 99177, CR3.0; Abreu and Brearley, 2010), Elephant Moraine 87770 (EET 87770, CR2), Elephant Moraine 92042 (EET 92042, CR2), La Paz Icefield 02342 (LAP 02342, CR2), Graves Nunataks 95229 (GRA 95229, CR2), Renazzo (CR2), Al Rais (CR2), Grosvenor Mountain 95577 (GRO 95577, CR1). Samples of meteorite were gently

TEM

Matrix observations of the type 3 and 2 CR chondrites show a dominant groundmass of intermixed, amorphous silicates and very fine-grained phyllosilicates, together with nanosulfides and organic material (Fig. 2; see also Abreu and Brearley, 2010, Le Guillou and Brearley, 2014, Le Guillou et al., 2014a). In addition, less abundant carbonates, tochilinite, iron oxide, enstatite and forsterite aggregates are also present. No systematic textural evolution is observed with increasing aqueous

Effects of Antarctic alteration or FIB preparation

Six of the eight meteorites are Antarctic finds that have experienced terrestrial weathering which could have perturbed the Fe3+/Fe ratios. However, as discussed elsewhere (Abreu and Brearley, 2010, Le Guillou and Brearley, 2014), in QUE 99177, the effects of Antarctic alteration are localized to fractures and cracks that crosscut the matrix, but are not pervasive. The matrix, and in particular the silicate groundmass, was not affected to any significant degree. This conclusion is also

Acknowledgments

We would like to acknowledge the Museum National d'Histoire Naturelle, Paris, France, for providing the Renazzo sample, the Johnson Space Center, Houston, TX, USA, and the Antarctic Meteorite Working Group for all other samples from the Antarctic Meteorite Collection. David Kilcoyne at the advanced light source in Berkeley, USA and Chithra Karunakaran and Jian Wang at the Canadian Light Source, Saskatoon, Canada, are thanked for their precious help on the STXM beamlines. Sylvain Bernard and

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