Sprecher
Beschreibung
Empirical Constraints on Habitable Early Earth Environments
Elizabeth A. Bell, UCLA
Earth’s rock record grows increasingly sparse with age, and the most ancient existing rocks have mostly undergone metamorphism to varying degrees, altering some aspects of their original mineralogy, morphology, or composition. This is a major complicating factor in the search for the earliest evidence of life, as well as the earliest evidence for Earth’s environmental conditions. Although the widely agreed upon oldest extant rock is 4.03 billion years old (‘Ga’), the chemically and physically robust mineral zircon (ZrSiO4) can provide U-Pb geochronological and other information about this early crust. Zircon older than 4.03 Ga has been discovered at 15 locations around the planet, although the vast majority studied so far come from a handful of sites. They extend the mineral record back to nearly 4.4 Ga and, despite appearing to largely derive from magmas, provide constraints on a number of environmental variables. Evidence for liquid water derives from both stable isotope and trace element compositions. Oxygen isotope evidence for low-temperature water-rock reactions is found as early as 4.3 Ga, with a small minority of the zircon population suggestive of origins via higher-temperature, hydrothermal reactions. Further evidence from silicon isotopes and several trace elements suggests some of the low-temperature water-rock reactions were likely related to subaerial weathering, suggesting the existence of a sediment cycle at the surface. Further examination of trace elements and ‘inclusions’ of other minerals in the zircon can help to further distinguish their source rocks, which may have been a mixture of mantle-like and continental-like sources and magmas melted from both earlier igneous rocks and sediments. Potential environments for early life that are suggested by the zircon data could range from low-temperature surface or near-surface aqueous settings (on either continental or oceanic crust) to hydrothermal systems, which host abundant life on the modern Earth. Future work on the early crust and early environments could involve both an expansion of the sites yielding >4 Ga zircon – which may turn up evidence for more diverse source rocks and environmental settings – and greater scrutiny of the mineral inclusions trapped in many of the >4 Ga zircons, which may yield their own isotopic and geochemical evidence for early Earth conditions and biotic remains.