Volcanic degassing, hydrothermal circulation and the flourishing of early life on Earth: A review of the evidence from c. 3490-3240 Ma rocks of the Pilbara Supergroup, Pilbara Craton, Western Australia

Abstract

New data gathered during mapping of c. 3490–3240 Ma rocks of the Pilbara Supergroup in the Pilbara Craton show that most bedded chert units originated as epiclastic and evaporative sedimentary rocks that were silicified by repeated pulses of hydrothermal fluids that circulated through the footwall basalts during hiatuses in volcanism. For most cherts, fossil hydrothermal fluid pathways are preserved as silica ± barite ± Fe-bearing veins that cut through the footwall and up to the level of individual bedded chert units, but not above, indicating the contemporaneity of hydrothermal silica veining and bedded chert deposition at the end of volcanic eruptive events. Silica ± barite ± Fe-bearing vein swarms are accompanied by extensive hydrothermal alteration of the footwall to the bedded chert units, and occurred under alternating high-sulphidation and low-sulphidation conditions. These veins provided pathways to the surface for elements leached from the footwall (e.g., Si, Ba, Fe) and volcanogenic emissions from underlying felsic magma chambers (e.g., CO₂, H₂S/HS⁻, SO₂).

Stratigraphic evidence of shallowing upward and subsequent deepening associated with the deposition of Warrawoona Group cherts is interpreted to relate to the emplacement of subvolcanic laccoliths and subsequent eruption and/or degassing of these magmas. Heat from these intrusions drove episodes of hydrothermal circulation. Listric normal faulting during caldera collapse produced basins with restricted circulation of seawater. Eruption of volcanogenic emissions into these restricted basins formed brine pools with concentration of the volcanogenic components, thereby providing habitats suitable for early life forms.

Fossil stromatolites from two distinct stratigraphic units in the North Pole Dome grew in shallow water conditions, but in two very different geological settings with different morphologies. Stratiform and domical stromatolites in the stratigraphically lower, c. 3490 Ma, Dresser Formation of the Warrawoona Group are intimately associated with barite and chert precipitates from hydrothermal vents, suggesting that component microbes may have been chemoautotrophic hyperthermophiles. Evidence of shallow water to periodically exposed conditions, active growth faulting and soft sediment deformation indicates that the volcanogenic emissions were erupted into a shallow water, tectonically active caldera and concentrated therein to produce an extreme habitat for early life.

Widespread conical and pseudocolumnar stromatolites in the c. 3400 Ma, Strelley Pool Chert at the base of the unconformably overlying Kelly Group occur in shallow marine platform carbonates. Silicification was the result of later hydrothermal circulation driven by heat from the overlying, newly erupted Euro Basalt. The markedly different morphology and geological setting of these only slightly younger stromatolites, compared with the Dresser Formation, suggests a diversity of microbial life on early Earth.

The biogenicity of putative microfossils from this and younger hydrothermal silica veins in the Warrawoona Group remains controversial and requires further detailed study.

Introduction

Of interest to Earth scientists, and of fundamental importance to planned missions to search for possible life on Mars, is knowledge concerning the types of habitats colonised by the earliest life on Earth (Walter and Desmarais, 1993). The oldest known fossil structures are putative microfossils and macroscopic stromatolites that occur in several distinct units within c. 3490–3240 Ma rocks of the Pilbara Supergroup in the East Pilbara Granite–Greenstone Terrane of the Pilbara Craton, Western Australia (Dunlop et al., 1978, Lowe, 1980, Walter et al., 1980, Awramik et al., 1983, Walter, 1983, Schopf and Walter, 1983, Schopf and Packer, 1987, Schopf, 1992, Schopf, 1993, Hoffman et al., 1999, Van Kranendonk et al., 2003). Previous interpretations suggested that deposition of the fossiliferous host rocks was in shallow sedimentary sequences during periodic hiatuses in — or synchronous with — volcanism (Dunlop et al., 1978, Lambert et al., 1978, Groves et al., 1981, Lowe, 1983, Buick and Dunlop, 1990). The occurrence of carbonate and sulphate beds in many of the fossiliferous units led to the interpretation that they were largely deposited through evaporative precipitation from seawater in shallow marine basins (Lambert et al., 1978, Buick et al., 1981, Groves et al., 1981, Lowe, 1983, Buick and Dunlop, 1990).

However, an apparent problem with the simple evaporative model of carbonate/sulphate deposition is the documented relationship between sulphate deposition, growth faulting and volcanogenic hydrothermal exhalations in the stromatolitic Dresser Formation (Nijman et al., 1998, Van Kranendonk, 2000, Van Kranendonk and Pirajno, 2004) and Apex chert (Brasier et al., 2002, Brasier et al., 2004), suggesting that a volcanic input to the habitats of early life must have played a significant role. A further problem is that the abundance of jaspilitic cherts and sulphate beds in Pilbara cherts has been used to argue in favour of an oxygenated Archaean atmosphere and hydrosphere (Ohmoto, 2003), although this is at odds with a wealth of contrary evidence that indicates the hydrosphere and atmosphere were essentially devoid of molecular oxygen at this time (e.g., Chang et al., 1983, Walker et al., 1983, Collerson and Kamber, 1999, Canfield et al., 2000, Farquhar et al., 2001, Holland, 2002).

In this paper, supporting geological evidence is presented, based on field mapping and petrography, of a predominantly hydrothermal component in the deposition of fossiliferous chert strata in the Warrawoona Group, supporting the contention from theoretical considerations and evidence from the rRNA Phylogenitic Tree of Life that hydrothermal processes may have been of critical importance in the development of early life on Earth (Stetter, 1996, Russell and Hall, 1997, Glasby, 1998, Pace, 2001, Reysenbach and Shock, 2002).