Molybdenum-isotope signals and cerium anomalies in Palaeoproterozoic manganese ore survive high-grade metamorphism

Alexandre Raphael Cabral,Armin Zeh,Nívea Cristina Vianna,Jan Pašava,Lukáš Ackerman,Vladislav Chrastný,Bernd Lehmann

doi:10.1038/s41598-019-40998-5

Abstract

Molybdenum (Mo) and its isotopes have been used to retrieve palaeoenvironmental information on the ocean–atmosphere system through geological time. Their application has so far been restricted to rocks least affected by severe metamorphism and deformation, which may erase or alter palaeoenvironmental signals. Environmental Mo-isotope signatures can be retrieved if the more manganese (Mn)-enriched rocks are isotopically depleted and the maximum range of δ98Mo values is close to the ~2.7‰ Mo-isotope fractionation known from Mo sorption onto Mn oxides at low temperature. Here, we show that the Morro da Mina Mn-ore deposit in Minas Gerais, Brazil, contains Mn-silicate–carbonate ore and associated graphitic schist that likely preserve δ98Mo of Palaeoproterozoic seawater, despite a metamorphic overprint of at least 600 °C. The extent of Mo-isotope fractionation between the Mn-silicate–carbonate ore and the graphitic schist is similar to modern Mn-oxide precipitates and seawater. Differences in δ98Mo signals are broadly reflected in cerium (Ce) anomalies, which suggest an oxic–anoxic-stratified Palaeoproterozoic ocean.

Highlights

Assessing when and how oceans became oxygenated is crucial to trace the evolution of early life, and to understand the genesis of metalliferous deposits in marine settings
Www.nature.com/scientificreports whether such a correlation is universal and realistic for rocks metamorphosed at higher grade, such as those found at the Morro da Mina Mn-ore deposit in Minas Gerais, Brazil
At Morro da Mina, amphibolite-facies metamorphism of marine sediments rich in Mn and organic matter resulted in the formation of queluzite, a Mn-silicate–carbonate rock containing graphite–molybdenite (MoS2) intergrowths, and Mo-bearing graphitic schist[15]

Summary

Introduction

Assessing when and how oceans became oxygenated is crucial to trace the evolution of early life, and to understand the genesis of metalliferous deposits in marine settings. Www.nature.com/scientificreports whether such a correlation is universal and realistic for rocks metamorphosed at higher grade, such as those found at the Morro da Mina Mn-ore deposit in Minas Gerais, Brazil. At Morro da Mina, amphibolite-facies metamorphism of marine sediments rich in Mn and organic matter resulted in the formation of queluzite, a Mn-silicate–carbonate rock containing graphite–molybdenite (MoS2) intergrowths, and Mo-bearing graphitic schist[15].

Results

Conclusion