Geological and trace element evidence for a marine sedimentary environment of deposition and biogenicity of 3.45 Ga stromatolitic carbonates in the Pilbara Craton, and support for a reducing Archaean ocean

Bedded carbonate rocks from the 3.45 Ga Warrawoona Group, Pilbara Craton, contain structures that have been regarded either as the oldest known stromatolites or as abiotic hydrothermal deposits. We present new field and petrological observations and high-precision REE + Y data from the carbonates in order to test the origin of the deposits. Trace element geochemistry from a number of laminated stromatolitic dolomite samples of the c. 3.40 Ga Strelley Pool Chert conclusively shows that they precipitated from anoxic seawater, probably in a very shallow environment consistent with previous sedimentological observations. Edge-wise conglomerates in troughs between stromatolites and widespread cross-stratification provide additional evidence of stromatolite construction, at least partly, from layers of particulate sediment, rather than solely from rigid crusts. Accumulation of particulate sediment on steep stromatolite sides in a high-energy environment suggests organic binding of the surface. Relative and absolute REE + Y contents are exactly comparable with Late Archaean microbial carbonates of widely agreed biological origin. Ankerite from a unit of bedded ankerite–chert couplets from near the top of the stratigraphically older (3.49 Ga) Dresser Formation, which immediately underlies wrinkly stromatolites with small, broad, low-amplitude domes, also precipitated from anoxic seawater. The REE + Y data of carbonates from the Strelley Pool Chert and Dresser Formation contrast strongly with those from siderite layers in a jasper–siderite–Fe-chlorite banded iron-formation from the base of the Panorama Formation (3.45 Ga), which is clearly hydrothermal in origin. The geochemical results, together with sedimentological data, strongly support: (1) deposition of Dresser Formation and Strelley Pool Chert carbonates from Archaean seawater, in part as particulate carbonate sediment; (2) biogenicity of the stromatolitic carbonates; (3) a reducing Archaean atmosphere; (4) ongoing extensive terrestrial erosion prior to ∼3.45 Ga.

A trace element study of siderite-jasper banded iron formation in the 3.45 Ga Warrawoona Group, Pilbara Craton - Formation from hydrothermal fluids and shallow seawater

Shale-normalised rare earth element and yttrium (REE + Y) patterns for siderite-jasper couples in a banded iron formation of the 3.45 Ga Panorama Formation, Warrawoona Group, eastern Pilbara Craton, display distinct positive Y and Eu anomalies and weak positive La and Gd anomalies, combined with depleted light REE relative to middle and heavy REE. Ambient seawater and hydrothermal fluids are identified as major sources of REE + Y for the BIF. In the case of siderites, strong correlations between incompatible trace elements and trace element ratios diagnostic of seawater indicate variable input from a terrigenous source (e.g. volcanic ash). We propose a volcanic caldera setting as a likely depositional environment where jasper and siderite precipitated as alternating bands in response to episodic changes in ambient water chemistry. The episodicity was either driven by fluctuations in the intensity of hydrothermal activity or changes in magma chamber activity, which in turn controlled relative sea level. In this context, precipitation of jasper probably reflects background conditions during which seawater was saturated in silica due to evaporative conditions, while siderites were deposited most likely during intermittent periods of enhanced volcanic activity when seawater was more acidic due to the release of exhalative phases (e.g. CO2). © 2005 Elsevier B.V. All rights reserved.