Sedimentary talc in Neoproterozoic carbonate successions

Nicholas J Tosca,Francis A Macdonald,Justin V Strauss,David T Johnston,Andrew H Knoll

doi:10.1016/j.epsl.2011.03.041

Abstract

Mineralogical, petrographic and sedimentological observations document early diagenetic talc in carbonate-dominated successions deposited on two early Neoproterozoic (~ 800–700 million years old) platform margins. In the Akademikerbreen Group, Svalbard, talc occurs as nodules that pre-date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mountains, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick. To gain insight into the chemistry required to form early diagenetic talc, we conducted precipitation experiments at 25 °C with low-SO 4 synthetic seawater solutions at varying pH, Mg 2+ and SiO 2(aq). Our experiments reveal a sharp and reproducible pH boundary (at ~ 8.7) only above which does poorly crystalline Mg-silicate precipitate; increasing Mg 2+ and/or SiO 2(aq) alone is insufficient to produce the material. The strong pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above ~ 8.6–8.7. FT-IR, TEM and XRD of the synthetic precipitates reveal a talc-like 2:1 trioctahedral structure with short-range stacking order. Hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite (hydrated talc) and eventually to talc. This formation pathway imparts extensive layer stacking disorder to the synthetic talc end-product that is identical to Neoproterozoic occurrences. Early diagenetic talc in Neoproterozoic carbonate platform successions appears to reflect a unique combination of low Al concentrations (and, by inference, low siliciclastic input), near modern marine salinity and Mg 2+, elevated SiO 2(aq), and pH > ~ 8.7. Because the talc occurs in close association with microbially influenced sediments, we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry, specifically pH and alkalinity increases driven by anaerobic Fe respiration.

Highlights

Talc, Mg3Si4O10(OH)2, is typically interpreted as a high-temperature mineral that forms from hydrothermal alteration or metamorphism of Mg-rich and ultrabasic rocks (Evans and Guggenheim, 1988; Marumo and Hattori, 1999)
We document the unusual occurrence of early diagenetic talc associated with Neoproterozoic (~800-700 Ma) carbonates deposited on two separate platform margins: the Akademikerbreen Group in Svalbard and the Fifteenmile Group in the Ogilvie Mountains of northwestern Canada
The formation of sedimentary talc and its mineralogical precursors requires a specific set of chemical conditions; its presence places tight quantitative constraints on Neoproterozoic ocean chemistry and provides additional insight into the biogeochemistry of marine sediments at that time

Summary

Introduction

Mg3Si4O10(OH), is typically interpreted as a high-temperature mineral that forms from hydrothermal alteration or metamorphism of Mg-rich and ultrabasic rocks (Evans and Guggenheim, 1988; Marumo and Hattori, 1999). We document the unusual occurrence of early diagenetic talc associated with Neoproterozoic (~800-700 Ma) carbonates deposited on two separate platform margins: the Akademikerbreen Group in Svalbard and the Fifteenmile Group in the Ogilvie Mountains of northwestern Canada. At low temperatures (i.e., less than ~30oC), the Mg-silicate system is controlled mainly by kinetic phenomena (Evans and Guggenheim, 1988; Jones, 1986; Wollast et al, 1968), and so constraints on early diagenetic chemistry are difficult to derive based on thermodynamics alone

Methods

Discussion

Conclusion