Radiogenic Sr-Nd-Pb isotope behavior in different grain-sized fine lithic materials during basalt weathering

Shivam Sahu, Satinder Pal Singh, Kumar Batuk Joshi, Yogita Kadlag

Radiogenic isotopes are widely used as sediment provenance tracers; however, their weathering response has not been fully established. This study investigates the weathering sensitivity of Sr-Nd-Pb isotopes in fine lithic fractions (FLF; grain size <2 μm, <5 μm, and <20 μm) capable of long-range transport. Geochemical and isotope compositions have been measured in the physicochemically separated FLF from soil (n = 5) and saprolite (n = 10) samples, along with the bedrock (n = 5) and dike (n = 1) outcrops, all belonging to a small basaltic watershed (~5 km2) in the western Deccan Trap, India. The CIA (chemical index of alteration) varies from 75 to 97 (89 ± 13 (2 SD), n = 15) and 55 to 97 (84 ± 20, n = 30), while MIA(O) (mafic index of alteration under oxidizing condition) ranges from 64 to 86 (75 ± 14, n = 15) and 48 to 79 (64 ± 16, n = 30) in the soil FLF and saprolite FLF, respectively. The 87Sr/86Sr generally increases with grain-size reduction, while Nd and Pb isotopes show an insignificant grain-size effect. The bedrock normalized fractionation factors (α) of 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb obtained for the soil FLF (except those influenced by dike) are 1.015 ± 0.027 (n = 12), 1.0000 ± 0.0001 (n = 12), 1.031 ± 0.024 (n = 5), 1.008 ± 0.007 (n = 5), and 1.010 ± 0.008 (n = 5), while these factors for the saprolite FLF are 1.004 ± 0.007 (n = 30), 1.0000 ± 0.0001 (n = 29), 1.012 ± 0.008 (n = 14), 1.003 ± 0.002 (n = 14), and 1.004 ± 0.003 (n = 14), respectively. The average α values obtained for the saprolite FLF suggest insignificant fractionation of ~1 % for 87Sr/86Sr (Sr/Al2O3 –70 % to –98 %), ~1 unit Nd (Nd/Al2O3 +46 % to –94 %), and ~1–2 % for Pb isotope ratios (Pb/Al2O3 +11 % to –84 %) during moderate to intense weathering of the Deccan Basalt. In contrast, the soil FLF samples show much higher α ranges for 87Sr/86Sr (up to 1.042 for Sr/Al2O3 –98 %) and 206Pb/204Pb (up to 1.045 for Pb/Al2O3 +52 %), but the α ranges for Nd isotopes are still similar to those of the saprolite FLF. The significant positive correlations of 87Sr/86Sr with CIA (R2 = 0.43, p <0.01), MIA(O) (R2 = 0.51, p <0.01) and MnO/Al2O3 (R2 = 0.80, p <0.0001) for the soil FLF reveals a preferential incorporation of 87Sr from unknown source(s) into the residual Mn-oxides, which leads to the grain size-cum-weathering dependency of Sr isotopes. This study highlights the Nd and Pb isotope-based reliable provenance tracing of fine lithic materials; a maximum felsic dust contribution of <10 % to the soil FLF is quantified here. Meanwhile, the radiogenic Sr incorporation in secondary minerals poses a risk of overestimating the contributions from older granitic terranes using 87Sr/86Sr-based mixing calculations.