Tungsten isotope signatures of Large Igneous Provinces

Questions on the formation and evolution of planet Earth have preoccupied scientists for centuries. Since its birth, processes have continuously affected and shaped the Earths interior and exterior. Metal-silicate segregation processes in the first few million years have led to a differentiated planet with a metallic core and a silicate mantle. Various disciplines of Earth Sciences, including geochemistry, geodynamic modelling and geophysics, have studied processes within the Earths silicate mantle for decades. Despite the extensive research, many questions still remain on processes taking place within the earths interior. Geochemical clues on early Earth processes are mainly taken from the research of the Earths oldest rocks. Studies of Archean rock samples (>2.5 Ga) have shown the existence of ancient geochemical signatures that must have been formed within the first 60 Ma of solar system formation. Ongoing mantle convection and mixing have been thought to have homogenized the Earths silicate mantle and thus erased these ancient signatures. However, recent investigations have led to the surprising discovery of geochemical signatures that resulted from a long-extinct isotopic system in modern rock samples that erupted onto Earths surface within only the last century. In addition to the still remaining mystery on their formation, these findings raise questions on the mechanisms that preserve these very early formed mantle domains throughout the Earths 4.5 Ga existence. The proposed project aims to build a complete geochemical dataset on samples from Large Igneous Provinces (LIP) to contribute to other research fields to further the understanding on deciphering the physical and chemical conditions under which the Earth formed and evolved. Rocks from LIP, which represent the most extensive volcanic eruptions on Earth, have been interpreted to originate from the deepest parts of the silicate mantle. Consequently, these volcanoes may tap source reservoirs that have remained isolated from mantle mixing processes throughout their lifetime. Recent advances in high-precision analyses of isotopic compositions now permit the detection of small-scale isotopic heterogeneities. Characterization of LIP rock samples using the extinct hafnium-tungsten isotopic system (182Hf182W, t=8.9 Ma) in combination with determined highly siderophile element concentrations will provide further information on the Earths early history, as well as its geological evolution through time. The combination of these two geochemical systems will lead to a better understanding on the formation of source reservoirs that must have formed within the first 60 Ma of solar system history and will therefore lead to advances in other Earth Science disciplines.

The project "Tungsten isotope composition of Large Igneous Provinces (LIP)" focussed on characterizing mantle plume source characteristics using mainly the short-lived 182Hf-182W isotope system in combination with long-lived radiogenic isotopes (Os, Sr, Nd), highly siderophile element systematics and trace element concentrations. The goal was to decipher processes that influence the composition of these volcanic products in continental and oceanic settings. Several LIP were studied and the main findings were a decoupling of He-W systematics, where unlike in ocean island basalts, flood basalts from some continental LIPs, do not reveal anomalous W isotope signatures in samples with elevated 3He/4He ratios. This decoupling could be ascribed to (1) crustal assimilation effects that easily alter the initial W isotope composition while having lesser effects on He isotope systematics and/or (2) primary characteristics of mantle sources that are incorporated during the early stages of plume formation. Hence, providing important information that can be used as boundary conditions for other geoscience disciplines, such as geodynamic modelling or seismic studies. Because of COVID-19 related travel restrictions and laboratory shut-downs, not all proposed LIP sample locations could be studied. Instead, an alternative research focus arose from the collaboration with colleagues from the Department of Lithospheric Research where this project was carried out. This alternate study focussed on W isotope analyses of individual layers of a 2.7 Ga old banded iron formation. The findings from the study of these ancient seawater precipitates revealed distinct differences in silica- and iron-rich bands. Using this data, we were able to introduce a new geochemical tool to simultaneously study the evolution of crust and mantle through time.