Influence of chemical weathering on the composition of the continental crust: Insights from Li and Nd isotopes in bauxite profiles developed on Columbia River Basalts

Abstract

Glacial diamictites, with ages ranging from $2900 to 0.01 Ma, record the changing composition of the upper continental crust through time (Gaschnig et al., 2014). Li concentrations and isotopic compositions, combined with Pb isotopic compositions, chemical index of alteration (CIA) values and relative Sr concentrations are used here to assess the degree of chemical weathering recorded in these deposits and the origin of this signature. The d Li values of most of the diamictites (ranging from À3.9 to +3.5) are lower than those of mantle-derived basalts (+3.7 ± 2, 2r), and the low d7 Li values are generally accompanied by high CIA and low Sr/Sr * values (or Sr depletion factor, Sr/Sr * = Sr/(Ce * Nd) 0.5 ), reflecting a weathering signature that may have derived from pre-depositional, syn-depositional, and/or post-depositional weathering processes. Profiles through three glacial diamictites with relatively high CIA (a fresh road cut of the Neoproterozoic Nantuo Formation (CIA = 62-69), and drill cores through the Paleoproterozoic Timeball Hill (CIA = 66-75) and Duitschland Formations (CIA = 84-91)) do not show evidence of significant post-depositional weathering. High Th/U, reflecting loss of uranium during oxidative weathering, is seen in all Paleozoic and Neoproterozoic diamictites and a few Paleoproterozoic deposits. Pb isotopic systematics suggest that this signature was largely inherited from preexisting crust, although a subset of samples (the Neoproterozoic Konnarock, Paleozoic Dwyka, and several of the Paleoproterozoic Duitschland samples) appears to have experienced postdepositional U loss. Modern glaciomarine sediments record little weathering (CIA = 47, Sr/Sr * = 0.7, d7 Li = +1.8), consistent with the cold temperatures accompanying glacial periods, and suggesting that limited syn-depositional weathering has occurred. Thus, the chemical weathering signature observed in ancient glacial diamictites appears to be largely inherited from the upper continental crust (UCC) over which the glaciers traversed. The strength of this weathering signature, based on the CIA, is greatest in the Mesoarchean and some of the Paleoproterozoic diamictites and is weaker in the Neoproterozoic and Phanerozoic glacial diamictites. Combining these data with data for Archean shales and other types of post-Paleoproterozoic sedimentary rocks (i.e., shales, mudstones, etc.), it appears that post-Paleoproterozoic upper continental crust experienced less intense chemical weathering, on average, than Archean and Paleoproterozoic upper continental crust.

Variation of lithium isotope geochemistry during basalt weathering and secondary mineral transformations in Hawaii

“…This suggests a negligible influence of atmospheric deposits (both dust and rainwater) on the Li isotope composition of regolith. The result is significantly different from previous results showing that there is a significant influence of atmospheric deposits in Hawaii (Pistiner and Henderson, 2003;Huh et al, 2004), or in other basaltic environments (Kisakü rek et al, 2004;Pogge von Strandmann et al, 2012;Liu et al, 2013). The correlation between d 7 Li value and molar Nb/Li exhibited by the Hawaiian soils studied here supports these calculations (Fig.…”

“…7 Li present is close to d 7 Li of the parent rock, despite $30% Li loss by leaching, indicating that during the initial stage of basalt dissolution Li loss is not associated with significant Li isotope fractionation, in good agreement with experimental and other field studies (Pistiner and Henderson, 2003;Huh et al, 2004;Wimpenny et al, 2010a;Verney-Carron et al, 2011;Pogge von Strandmann et al, 2012;Liu et al, 2013). (2) The larger Li losses at the 20 ka and 150 ka sites (s int = À58% and À49%, respectively; Fig Li associated with Li incorporation into secondary phases.…”

“…Experimental investigations have shown that basalt dissolution is not associated with significant isotope fractionation (Pistiner and Henderson, 2003;Wimpenny et al, 2010a;Verney-Carron et al, 2011) and that the formation of secondary phases, such as smectite and Fe oxides, leads to preferential enrichment of 6 Li into the weathering solids (Williams and Hervig, 2005;Vigier et al, 2008;Wimpenny et al, 2010b). The few existing studies of basaltic soils show that lithium isotopes hold great promise for tracing terrestrial weathering processes, but that the soil isotope signatures can be rapidly buffered by atmospheric deposits such as dust, rain or marine aerosols (Pistiner and Henderson, 2003;Huh et al, 2004;Kisakü rek et al, 2004;Pogge von Strandmann et al, 2012;Liu et al, 2013). Here, we examine the processes responsible for changes in the Li isotope composition during progressive weathering and development of basaltic soils along a humid-environment chronosequence in the Hawaiian Islands.…”

Multi-mode Li diffusion in natural zircons: Evidence for diffusion in the presence of step-function concentration boundaries

“…Later, Lundstrom et al (2005) reported diffusive fractionation of Li isotopes in the Trinity Ophiolite. Following this, a series of studies reported Li diffusion within the country rocks adjacent to Li-rich intrusions (Teng et al, 2006;Marks et al, 2007;Liu et al, 2013), finding large and systematic Li isotopic variation over tens of meters and extremely low δ 7 Li (down to −20❤) values, which they interpreted to be the result of diffusion-driven fractionation of Li isotopes. Jeffcoate et al (2007) and Parkinson et al (2007) observed diffusive fractionation of Li isotopes on a grain scale in peridotites and phenocrysts in basalt.…”

“…On one hand, Li isotopes can be a powerful tracer of chemical weathering (Chan et al, 1992;Pistiner and Henderson, 2003;Rudnick et al, 2004;Liu et al, 2013;Dellinger et al, 2017). On the other hand, the rapid diffusivity of Li and the associated large kinetic isotopic fractionations make Li a promising geospeedometer for determining rates of rapid geologic processes that can be difficult to determine using other perimental studies (Cherniak and Watson, 2010), which showed that Li can diffuse rapidly in zircon under crustal P-T-X conditions.…”