Evolution of the Archean mantle

: evidence from ¹⁸²W systematics of the Pilbara Craton, Western Australia

Abstract

The ¹⁸²Hf-¹⁸²W decay system is an important tool in understanding the evolutionary history of the early Earth. The positive ¹⁸²W isotope anomalies widely found in Hadean-Archean rocks could provide evidence for the manner in which a late veneer is stirred into the mantle or yield insight into core-mantle interactions. This study focused on the variation and evolution of the tungsten isotope ratios in mantle-derived rocks from the Pilbara Craton, Western Australia. The Δ¹⁸²W values were measured for a series of rocks from the Pilbara region that were komatiitic to basaltic composition, with ages ranging from 3.58-2.77 Ga. Most of the studied rocks have relatively uniform positive ¹⁸²W anomalies with an average Δ¹⁸²W of +9.7 ± 2.2 ppm (2 s.d.). There were no clear differences in the tungsten isotopic ratios of samples with canonical W/Th (0.04-0.24) and non-canonical W/Th ratios. Through studies of tungsten-rich rutiles in the ~3.5 Ga Mount Webber gabbros, a secondary tungsten enrichment event has been dated for the first time. The U-Pb analyses of rutiles from these samples gave ²⁰⁷Pb/²⁰⁶Pb apparent ages of 3.0–2.9 Ga, revealing the addition of a tungsten-rich fluid during metamorphism at this time. A bulk rock Δ¹⁸²W value of +14.5 ± 3.8 ppm for one of these tungsten-rich Mount Webber samples indicates a more radiogenic composition for the tungsten-rich fluid than previously proposed. Furthermore, in contrast to previous work, no systematic decrease in Δ¹⁸²W values with time is observed in the Pilbara Craton samples. This implies that the events or processes that resulted in the decrease of global Δ182W values from typically ~+10 ppm in the Archean to zero or negative at the present day occurred after 2.77 Ga.

Date of Award	2 Dec 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Tim Elliott (Supervisor) & Ian J Parkinson (Supervisor)