Re-Os isotope systematics in black shales from the Lesser Himalaya: Their chronology and role in the ~(187)Os/~(188)Os evolution of seawater

摘要

Re and Os abundances and Os isotope systematics have been measured in a number of black shales sampled from outcrops and two underground phosphorite mines, Maldeota and Durmala, in the Lesser Himalaya. The Re and Os concentrations in these samples exhibit wide range, 0.2 to 264 ng/g and 0.02 to 13.5 ng/g, respectively, with a mean Re/Os of approx 25. The ~(187)Os/~(188)Os of these samples also show a broad range 1.02 to 11.6, with many of them far more radiogenic than the currently eroding continental crust. The black shales from the Maldeota and Durmala mines, collected a few meters above the Krol-Tal (Pc-C) boundary and occurring in the same stratigraphic horizon, yield ~(187)Re/~(187)Os isochron ages of 554+-16 and 552+-22 Ma, respectively, consistent with fossil evidences and those assigned for the Pc-C boundary at various other locations. These results indicate closed system behaviour of Re and Os in these mine samples and reaffirm the validity of ~(187)Re/~(187)Os pair to date organic-rich sediments. The age of these mine samples from the outer belt seems to be a few hundred millions of years younger than the preliminary Re-Os age of 839+-138 Ma for black shales from Theog, collected from the inner belt. These initial results lean toward the hypothesis that the inner belt sediments were deposited earlier than the outer be however, more analyses of black shales are needed to confirm this. The role of weathering black shales from the Himalaya in contributing to the Os isotope evolution of seawater over the past approx 25 Ma was assessed using a simple budget model for seawater Os and assuming values for Os fluxes and ~(187)Os/~(188)Os based on the available limited data on Os concentration and its ~(187)Os/~(188)Os in rivers and oceans. These calculations indicate that the required contributions from black shale weathering to account for the entire increase in oceanic ~(187)Os/~(188)Os over the past approx 16 Ma is difficult to satisfy by varying only the Os flux from HTP rivers (~(187)Os/~(188)Os kept constant at 1.324) or their ~(187)Os/~(188)Os (~(188)Os flux constant at today's value, approx 131 moles/yr). The requirements to balance Os isotope budget in ocean through weathering of black shales, however, is easier to accommodate if both the Os flux from HTP (Himalayan-Tibetan Plateau) rivers and their ~(187)Os/~(188)Os are allowed to vary. Measurements of Os abundance in HTP rivers and its isotopic composition are needed to test the validity of the model results.