The effects of terrestrial weathering on samarium‑neodymium isotopic composition of ordinary chondrites

Abstract

Following their fall to Earth, meteorites experience weathering. In this systematic study, we evaluate the trace element composition of ordinary chondrites from the Antarctic cold desert, and Atacama (Chile) and Lut (Iran) hot deserts, with an emphasis on rare earth elements (REE). Our data confirms that terrestrial weathering of meteorites in hot deserts changes their trace element (Sr, Ba, REE, Hf, Th, and U) concentrations. However, weathering effects in majority of Antarctic samples are limited to slight Ba, REE, Hf, and Th depletions and in some case to U enrichment. In comparison to the Antarctic meteorites, hot desert samples show greater disturbances and REE fractionation relative to the average fall values. We measured the Sm-Nd isotopic composition of the hot desert meteorites that have heavily affected REE compositions. Our Sm-Nd isotopic data show a significant effect of terrestrial weathering evidenced by non-CHUR ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd ratios. Measurements show a higher variation and lower values of ¹⁴⁷Sm/¹⁴⁴Nd for the Atacama samples than those from the Lut Desert. Deviations from CHUR ¹⁴⁷Sm/¹⁴⁴Nd value are in positive accordance with the degree of La/Lu fractionation caused by weathering. The ɛNd values of Atacama and Lut deserts meteorites range from −2.20 to +1.61, which is wider than the −1.07 to +0.64 range for falls. We suggest that disturbance of primary Sm/Nd ratios resulting from mixing with terrestrial components originating from soil during weathering is responsible for lower ¹⁴⁷Sm/¹⁴⁴Nd ratio in these meteorites. The majority of the Atacama meteorites regardless of their weathering degrees have their REE compositions and ɛNd affected by terrestrial contamination. Both ¹⁴⁷Sm/¹⁴⁴Nd ratio and ɛNd values show no straightforward relationship with weathering degree. However, in both cases the samples with the highest negative isotopic disturbances are H chondrites from the Atacama and Lut deserts. In addition, Ba concentration shows a negative correlation with ¹⁴⁷Sm/¹⁴⁴Nd ratio. Care must be taken into account while dealing with samples collected from hot deserts, even fresh-looking ones.

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 (t_1/2 = 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.