Abstract
The ancient geological history of Earth is remarkably preserved in the sedimentary rock record through mineral tracers, including zircon. These grains serve as reliable chronometers and capture crucial information about the early Earth processes and habitability. Rare detrital zircon grains found in Archean terranes globally provide an important - and largely the only - record of crust formation and evolution in Earth's early history. Ancient (4.5 to 3.5 Ga) zircon data have featured prominently in debates on the presence of evolved crust in the early Earth, the role of surface water and conditions of crustal melting. Such information can elucidate timescales of mantle differentiation and concomitant crust formation, reworking of hydrated mafic crust, impact-related melt formation and the onset of modern-style plate tectonics.
Two case studies are presented from the Zimbabwe and Tanzania cratons on the African continent to provide insights into magmatic processes and possible geodynamic settings.
Zircons from Tanzania with ages of 3.6- 4.0 Ga represent some of the oldest preserved mineral records in Africa. Melting in the presence of significant water and/or reworking of sedimentary material appears unimportant. Instead, the data suggest crystallization from crustal melts that had been extracted from an undepleted mantle. The melts originated from interaction of magnesium-rich melts with pre-existing proto-continental crust – a process known as hybridization.
The Zimbabwe zircon record (3.8–3.3 Ga) suggests reworking of a mafic reservoir that separated from an undepleted mantle at 3.9 Ga. After 3.6 Ga reworking of pre-existing crust starts to become more prevalent. Prior to 3.35 Ga, early crust production did not cause complementary enriched and depleted reservoirs. Evidence for recycling and lateral accretion of arc-related crustal blocks until 3.35 Ga is limited.
Biography: Robert Bolhar is a geochemist, with 22 years of work experience in academia (5 years in the resource industry). He obtained his PhD at the University of Melbourne and continued as a post-doctoral research fellow at the Universities of Queensland and Canterbury. His areas of expertise and interest encompass trace element and isotope geochemistry as archives of Archaean surface conditions, crust formation and evolution, geochronology and petrochronology using LA-ICPMS, and geochemical and isotopic fingerprinting of stone artifacts. He established and directs the Mass Spectrometry Facility at Wits. He is an NRF-rated B2 scientist, a Fellow of the GSSA, a SACNASP-registered Professional Natural Scientist and member of the International Association of Geo-Analysts. He also serves in the editorial team of Chemical Geology.