Evidence for extreme SmNd fractionation during chemical weathering

Abstract

During subaerial chemical weathering, submarine weathering and other intracrustal processes, SmNd chemical fractionation remains uncertain, while a little is known about the conditions, scale and magnitude, specific processes and actual mechanism for possible SmNd fractionation. Here, an in situ terra rossa weathering profile developed on dolostone is chosen for the study. Based on sequential extraction experiments and measurements of the mineral and geochemical compositions including SrNd isotopic compositions, we suggest that subaerial chemical weathering not only leads to considerable SmNd mobilization, redistribution and secondary enrichment, but also gives rise to extreme SmNd fractionation, apparent ¹⁴³Nd isotope homogenization, nearly complete resetting of the ¹⁴⁷Sm¹⁴³Nd isotope system, anomalous initial ¹⁴³Nd/¹⁴⁴Nd ratios, and meaningless ¹⁴³Nd model age (T_DM) for weathering products. We found that the lanthanide tetrad effect, rather than mineralogical control, is responsible for the observed substantial SmNd fractionation and for the other rare earth element fractionation. Under domination of the lanthanide tetrad effect, the mobility of Sm is preferential to that of Nd during their migration in the form of dissolved organic complexes. Our results also illustrate that resetting of the ¹⁴⁷Sm¹⁴³Nd isotope system in weathering products is controlled by the combined effect of the extraneous dissolved SmNd input and additional SmNd fractionation. Finally, we argue that similar scenarios may occur in submarine weathering, diagenesis and other geological processes.

Introduction

Samarium (Sm) and neodymium (Nd) are rare earth elements (REEs) and behave coherently in most geological processes, while both elements are joined in nuclide parent/daughter pairs by the α-decay, i.e., ¹⁴⁷Sm (half-life t_1/2 = 106 Gyr) and ¹⁴⁶Sm (t_1/2 = 103 Myr; now extinct nuclide) decay to ¹⁴³Nd and ¹⁴²Nd, respectively. It is widely believed that major SmNd fractionation in terrestrial planets occurs during mantle-crust differentiation through fractional crystallization or partial melting (McCulloch and Wasserburg, 1978; DePaolo, 1988; White, 2015). Thereafter, Sm and Nd, as both refractory lithophile elements, should not be significantly fractionated by either chemical weathering or other common intracrustal processes (McCulloch and Wasserburg, 1978; Faure, 1986; Goldstein, 1988; Bennett et al., 1993; Bowring and Housh, 1995; Allègre, 2008; Tessalina et al., 2010; White, 2015). Based on this assumption, long-lived ¹⁴⁷Sm¹⁴³Nd and short-lived ¹⁴⁶Sm¹⁴²Nd isotope systems have been widely used in Earth and planetary science (Faure, 1986; DePaolo, 1988; Allègre, 2008; Jellinek and Jackson, 2015; White, 2015). This critical assumption, however, was challenged by many studies (Ludden and Thompson, 1978; McCulloch and Black, 1984; Whitehouse, 1988; Bridgwater et al., 1989; Rosing, 1990; Manning et al., 1991; Milodowski and Zalasiewicz, 1991; Bros et al., 1992; Munz et al., 1994; Frost and Frost, 1995; Poitrasson et al., 1995; Gruau et al., 1996; Vervoort et al., 1996; Moorbath et al., 1997; Polat et al., 2003; Bock et al., 2004). For example, the fractionation of SmNd and/or the resetting of the ¹⁴⁷Sm¹⁴³Nd isotope system had been suggested in many systems, such as hydrothermal systems (Bridgwater et al., 1989; Poitrasson et al., 1995; Jahn et al., 2001; Monecke et al., 2002, Monecke et al., 2007, Monecke et al., 2011; René, 2008) and metamorphic processes (McCulloch and Black, 1984; Munz et al., 1994; Gruau et al., 1996; Vervoort et al., 1996; Moorbath et al., 1997; Fisher et al., 2020). Nevertheless, the research on these issues is far from conclusive, particularly in subaerial weathering processes (Roaldset and Rosenqvist, 1971; McDaniel et al., 1994; MacFarlane et al., 1994; Feng, 2010; Feng et al., 2011; Babechuk et al., 2014).

In a pioneering work, Goldschmidt (1954) proposed that all REEs are again nearly completely assembled in hydrolysate minerals during the processes of weathering, soil formation, and sedimentation. Later works revealed that substantial REE mobilization could occur during chemical weathering (Balashov et al., 1964; Ronov et al., 1967; Roaldset and Rosenqvist, 1971; Nesbitt, 1979; Duddy, 1980; Lottermoser, 1990; Braun et al., 1993; Viers and Wasserburg, 2004). Several subsequent studies suggested that SmNd were mobile but moved together, so solid weathering products retained the relative abundances of SmNd, and no notable change in the Sm/Nd concentration ratio (hereafter called the ‘ratio’) occurred during chemical weathering (Faure, 1986; Goldstein, 1988; Nesbitt and Markovics, 1997; Viers and Wasserburg, 2004; Allègre, 2008; White, 2015). In contrast, others emphasized that the Sm/Nd ratio does vary during chemical weathering (Roaldset and Rosenqvist, 1971; Nesbitt, 1979; Duddy, 1980; Öhlander et al., 2000, Öhlander et al., 2014; Babechuk et al., 2014). Nevertheless, they related these changes to the incongruent weathering of rocks (i.e., mineralogical control effect), in which the minerals (especially accessory minerals) had quite varied Sm/Nd ratios and SmNd concentrations (Nesbitt, 1979; Duddy, 1980; Öhlander et al., 2000, Öhlander et al., 2014; Hannigan and Sholkovitz, 2001; Viers and Wasserburg, 2004; Ji and Wang, 2008). On the other hand, a few researchers believe that the real chemical fractionation of SmNd may occur during chemical weathering, but still lack convincing evidence (Feng, 2010; Feng et al., 2011; Babechuk et al., 2014).

In this study, our aims are (1) to confirm the SmNd fractionation during chemical weathering, (2) to clarify the actual mechanism for SmNd fractionation, and (3) recognize the resetting of the ¹⁴⁷Sm¹⁴³Nd isotope system. This work is in favor of understanding of the REE fractionation and the REE global cycles, the REE transport with fluid movement, and provides insights into the open behavior of the ¹⁴⁷Sm¹⁴³Nd isotope system.