Preliminary Mo isotope data of Phanerozoic clastic sediments from the northern margin of the Yangtze block and its implication for paleoenvironmental conditions

Abstract

Mo isotopes along with Nd isotopes and other geochemical characteristics of the Phanerozoic clastic sediments from the northern margin of the Yangtze block were analyzed by MC-ICP-MS and some other techniques, spanning the period from Cambrian to Jurassic. The δ98Mo values and eNd(t) in these sedimets were observed to exhibit a large range of variation (−0.65‰2+1.87‰, −1.46–−10.90, respectively). Specifically, the sample from Late Permian Maokou Formation has relatively positive values in both δ98Mo and eNd(t) (+0.97‰, −1.46, respectively). These elevated values are proposed to relate to the input of a large quantity of H2S and HS− into the contemporaneous seawater by the volcanic eruption of the Emeishan mantle plume. Introduction of these sulfide gases would cause MoO4 2− to be transferred into MoS4 2−, favoring the preferential deposition of the heavy Mo isotope as MoS4 2− and thus leaving a positive Mo isotope value. The sample from Late Permian Changxing Formation has a significantly elevated δ98Mo value of +1.87‰, in association with the relatively negative eNd(t) (−10.90) and remain the relatively higher content of both TOC and total sulfur (TS) (1.52%, 2.02%, respectively). This infers the presence of the enhanced primary productivity and high organic burial during the Changxing period. The production of a huge quantity of HS− and H2S by the sulfate-reducing reaction led to the existence of the elevated seawater δ98Mo values during the Changxing period. The preliminary Phanerozoic data reported here allow us to propose that the Mo isotope signature preserved in reductive sediments is related to the regional tectonic settings and the associated environmental conditions, and thus the measurement of δ98Mo promotes our understanding on the evolution of the paleoenvironmental event in the Earth history.