The effects of terrestrial weathering on samarium‑neodymium isotopic composition of ordinary chondrites
Introduction
The systematics of rare earth elements (REE) and the Sm-Nd isotopic chronometer are essential in petrogenetic and radiometric studies of terrestrial and extraterrestrial rocks (e.g., DePaolo, 1988). As a result of their slightly different nuclear and chemical properties, REE respond to common petrological processes, such as partial melting and partial evaporation, by developing fractionated light-REE (from La to Sm) or heavy-REE (from Eu to Lu) elemental patterns (e.g., Hanson, 1980; Davis and Richter, 2014). Radioactive decay of 147Sm (t1/2 = 106 Gyr) to 143Nd is an important tracer for chemical differentiation processes affecting the REE during planetary evolution. In addition, the Sm-Nd isotopic system is one of the most precise and useful dating methods in geology. These properties make REE and Sm-Nd isotopes powerful tools to study the petrogenesis and origin of different magmatic rocks.
Except for returned samples from a few objects such as the Moon and asteroids 25143 Itokawa and 162173 Ryugu and comet Wild 2, excluding small objects such as interplanetary dust particles, meteorites are the only samples available from other solar system bodies. Meteorites allow studying the formation, evolution, and structure of the solar system. Ongoing developments in analytical chemistry, improved elemental and isotopic measurements, and accessibility of more meteoritic material has led us to have a comprehensive though still incomplete vision about the early stages of solar system evolution. These data enable us to see relationships between different meteorite groups, their formation ages, petrogenesis, etc.
The vast majority (~98%) of meteorites available for study are collected in hot and cold deserts. This is particularly the case for some rare meteorite types such as Martian and Lunar meteorites, angrites, etc. These meteorites collected in deserts are referred to as finds, and unlike observed falls, have had relatively long residence times on Earth. Their terrestrial ages range from tens of thousands years (kyr) for most hot deserts finds (Jull et al., 2013) to hundreds of kyr for Antarctica and Atacama finds (Drouard et al., 2019; Welten et al., 1997). The exposure of meteorites to terrestrial environments during this residence time alters their mineralogy, chemistry, and isotopic properties (e.g., Bland et al., 2006). Meteorite weathering is a complex process controlled by different factors including terrestrial residence time, climate, soil composition at the recovery site, meteorite type, size, and shape (e.g., Pourkhorsandi et al., 2017a; Hofmann et al., 2018). As observed in samples collected from different regions of the Atacama desert, meteorite weathering can be variable at sub-regional scale (Munayco et al., 2013). Understanding meteorite weathering processes is critical to avoid any data misinterpretation while working on meteorite finds. Terrestrial weathering of extraterrestrial materials is not only limited to “old” finds, but can occur during the short time span between meteorite falls and their recovery (Bischoff et al., 2011, Pourkhorsandi, 2018, Walker et al., 2018), and even during laboratory storage of samples returned by space missions (Velbel, 2014). All of this evidence points to a potential “threat” from terrestrial weathering on the integrity of cosmochemical data obtained from finds and show the importance of a detailed documentation of meteorite weathering processes.
Most of the studies of meteorite weathering have dealt with mineralogy and major element geochemistry (Bland et al., 2006; Golden et al., 1995; Gooding, 1982; King et al., 2020; Velbel et al., 1991). A few studies also have focused on particular trace elements such as Ba, Sr, and REE (Al-Kathiri et al., 2005; Crozaz et al., 2003; Pourkhorsandi et al., 2017a; Shimizu et al., 1983; Stelzner et al., 1999; Zurfluh et al., 2011). The number of such studies is smaller when it comes to isotopic investigations. However, paucity of information on this topic does not correlate with its importance. In their work on the observed CM chondrite fall Sutter's Mill, Walker et al. (2018) showed disturbances in 187Re-187Os and Re/Os ratios in the fragments which had experienced only one post-fall rain event. Observing these effects on a fall meteorite emphasizes the importance of this matter. Isotopic changes caused by meteorite weathering are also reported in H (Stephant et al., 2018), O (Stelzner and Heide, 1996), noble gases (Cartwright et al., 2010; Kaneoka, 1983; Schultz et al., 2005), Sr (Borg et al., 2016; Borg et al., 2003; Brandon et al., 2004; Elardo et al., 2014; Shih et al., 2007), Hf (Sokol et al., 2008; Tatsumoto et al., 1981), Os (Borg et al., 2003; Brandon et al., 2012), Fe (Saunier et al., 2010), U- and Th- series as well as Cs (Weber et al., 2017), and Pb (Tatsumoto et al., 1981) isotopes.
The terrestrial weathering effects on meteorites' Sm-Nd isotopic system, the focus of this work, are contradictory. For example, (Brandon et al., 2004; Debaille et al., 2007; Elardo et al., 2014; Haloda et al., 2009) report effects of hot desert weathering in their measurements on Martian meteorites to be insignificant. Meanwhile, most of the studies such as (Borg et al., 2016; Edmunson et al., 2005; Shih et al., 2007) and most of the studies on desert meteorites do not report any Sm-Nd isotopic disturbances in the weathered meteorites with already disturbed Rb-Sr systematics.
Despite their importance in cosmochemical studies, REE and especially the Sm-Nd isotopic system have been rarely studied in a systematic manner to track meteorite weathering effects on their composition. In this study we evaluate REE composition of ordinary chondrites from Antarctica, Atacama and Lut hot deserts. We also analyzed the Sm-Nd isotopic systematics of meteorites with heavily affected REE compositions.
Ordinary chondrites are the most abundant meteorite types among falls and finds. Meteorites with different weathering degrees, as determined by their mineralogy, were analyzed in order to pinpoint the relationships between REE composition in meteorites with the region where they were found and weathering degree. The second goal of this work is to investigate the effects of weathering on Sm-Nd isotopic composition of meteorites from hot deserts.
Section snippets
Studied meteorites
Table 1 shows the list of the ordinary chondrites used in this study. Hot desert and Antarctic samples are from the CEREGE (Aix-en-Provence, France) and Royal Belgium Institute of Natural Sciences (Brussels, Belgium) collections, respectively. Ferromagnesian silicates (olivine and pyroxene), (Fe,Ni) metal, and troilite (FeS) are the main primary components of this type of meteorites (Weisberg et al., 2006). During terrestrial weathering they alter to Fe oxyhydroxides (e.g., Pourkhorsandi et
Trace elements
Whole-rock trace element composition of the analyzed meteorites are reported in Table 3. The CI-normalized spider diagrams of Antarctica and hot desert samples (Atacama and Lut) along with those of average composition of different groups of ordinary chondrites (falls) are shown in Fig. 1. For El Médano (EM) 091, EM 120, and Kerman 026 average values of two cuts are presented.
Analyzed samples from Antarctica show relatively unfractionated REE patterns (average LaN/LuN ratio is 1.15 ± 0.18) (Fig.
Discussion
Our data confirms that terrestrial weathering of meteorites changes their trace element (Sr, Ba, REE, Hf, Th, and U) concentrations. In addition, Sm-Nd isotopic measurements of ordinary chondrites from the Atacama and Lut hot deserts show significant effects of terrestrial weathering as manifested by their non-CHUR 147Sm/144Nd and 143Nd/144Nd ratios.
Conclusions
Terrestrial weathering modifies trace element composition of ordinary chondrites, in particular REE. The degree of chemical modification in the Antarctic meteorites collected from the Asuka DCA is lower than that for hot desert meteorites. Antarctic meteorites show REE patterns close or only slightly below that of average falls, but without any noticeable fractionation. This also applies mostly for Sr, Hf, and Th. Uranium contents in Antarctic samples are close to falls value with some
Declaration of Competing Interest
The authors declare than they have no known competing interests or personal relationships than could have appeared to influence the work reported in this paper.
Acknowledgments
H. Pourkhorsandi, V. Debaille, P. Rochette, and J. Gattacceca thank the PHC Tournesol program. H. Pourkhorsandi, R. M. G. Armytage, and V. Debaille acknowledge funding from the ERC StG “ISoSyC”, and V. Debaille also thanks the FRS-FNRS for present support. We thank Dr. D. Porcelli, Dr. A. King, and an anonymous reviewer for their constructive comments and suggestions. Dr. G. Hublet and R. Maeda are thanked for insightful discussions. We thank W. Debouge and S. Cauchies for keeping the clean
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2023, Geochimica et Cosmochimica ActaCitation Excerpt :Apart from Tissint, the meteorites studied here are ‘finds’ that have spent an unknown amount of time in terrestrial hot desert weathering conditions and may therefore have experienced terrestrial alteration. Meteorites in hot desert environments are exposed to an oxidising atmosphere and variable temperatures, affected during burial by desert soils and formation of caliche, and subject to rare but significant rainfall events (Crozaz and Wadhwa, 2001; Crozaz et al., 2003; Al-Kathiri et al. 2005; Pourkhorsandi et al., 2017, 2021). Hot desert weathering is known to affect trace element concentrations such as Sr, Ba, and U (Barrat et al., 2001; Crozaz and Wadhwa, 2001; Crozaz et al., 2003; Pourkhorsandi et al., 2017), mostly due to the formation of carbonate veinlets that fill fractures or shock melt vesicles within meteorites (e.g., Borg et al., 2003; Howarth et al., 2018).
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2022, Chemical GeologyCitation Excerpt :Terrestrial weathering effects are regarded as the alteration of the original component phases of the meteorites to phases more stable at the Earth's surface (Bland et al., 2006). Terrestrial weathering could not only impact the mineralogy and major element geochemistry of meteorites (e.g. Bland et al., 2006), but also the contents of trace elements such as S, Sr, Ba, Mn, LREE (e.g. Crozaz et al., 2003; Lee and Bland, 2003; Pourkhorsandi et al., 2017; Saunier et al., 2010) and isotope systematics of elements such as H (Stephant et al., 2018), Fe (Saunier et al., 2010), Rb-Sr (Borg et al., 2003), and Sm-Nd (Pourkhorsandi et al., 2021) and potentially Ba. Although terrestrial alteration can affect the mineralogy of observed carbonaceous chondrite falls since they are fragile, porous and volatile element rich, the trace element (including Ba) contents of these meteorites are not obviously changed (Bland et al., 2006; King et al., 2020).
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2021, Geochimica et Cosmochimica ActaCitation Excerpt :Finds are generally characterized by a much longer terrestrial residence time than falls (roughly of the order of tens of kyr: e.g., Al-Kathiri et al., 2005). Many finds become weathered during this long residence time, and this process can take place relatively rapidly, particularly in temperate and tropical zones, as well as in hot deserts (Bland et al., 2006; Pourkhorsandi et al., 2021). On average, Antarctic meteorites exhibit old terrestrial ages, on the order of hundreds of kyr (e.g., Nishiizumi et al., 1989), and these are better protected from terrestrial weathering while buried in the ice compared to finds collected from hot deserts (Whillans and Cassidy, 1983; Cassidy and Whillans, 1990).
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Present address: Laboratoire G-Time, Université Libre de Bruxelles, CP 160/02, 50, Av. F.D. Roosevelt, 1050 Brussels, Belgium