Origin and Evolution of the Absolute and Relative Highly Siderophile Element Abundances Present in the Early Earth

University Of Maryland, College Park, College Park MD

Investigators

Abstract

Intellectual Merit. This project is designed to constrain the origin and temporal evolution of the absolute and relative abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd) in the early Archean mantle. This task is crucial for understanding fundamental planetary processes including Earth's primary differentiation, late accretion, and subsequent chemical evolution. The study focuses on ultramafic magmas (komatiites) from the Barberton Greenstone Belt (BGB), which preserved relicts of the very early stages of the Earth's evolution at a time when its mantle was less modified by processes of melt extraction, re-fertilization via crustal recycling, and core-mantle interaction. The BGB komatiites have unique chemical compositions, including extremely high MgO contents and a variety of LREE- and HREE-depleted/enriched signatures that clearly distinguish them from younger komatiites and imply very specific conditions of their magma generation. A total of ca. 150 chemically well characterized komatiite whole rock samples and olivine and chromite separates from the 3.48 Ga Komati, 3.30 Ga Weltevreden, and 3.30 Ga Mendon Formations of the BGB will be examined for high-precision absolute HSE abundances and 186,187Os/188Os isotope systematics. Well-preserved samples, ranging from Al-depleted to Al-enriched compositions, either are already in hand, or will be provided by collaborators; additional samples will become available through the ICDP Barberton Drilling Project, of which the PI is an active participant. Absolute and relative HSE abundances will be estimated for the mantle sources of the three respective komatiite systems at UMd. Together with lithophile trace element and isotopic data, these HSE data will be compared with high precision HSE data for chondritic meteorites, primitive upper mantle, lunar impact melts, and Archean komatiite systems to evaluate secular variations in the composition of the Archean komatiite mantle sources, and to develop models regarding the origin and evolution of HSE in the mantle. Extant hypotheses include [1] additions of HSE as a result of continuous accretion of planetesimals, [2] high pressure metal-silicate equilibration under the magma ocean conditions, and [3] subsequent modifications resulting from mass transfer in subduction zone environments and recycling of oceanic lithosphere. Broader Impacts. Outreach efforts of this project will foster collaborations between scientists from four continents, including the PI and Christophe Robin, a Ph.D. student of Nick Arndt (Univ. Grenoble), and members of the ICDP Barberton Drilling Project. It also will involve a UMd undergraduate student. Support for this research will help sustain the IGL's mission to share and collaborate world-wide, especially with those lacking access to necessary analytical facilities. Due to the unique nature of the Barberton region, it is currently being nominated as a UNESCO World Heritage Site. This project will provide added scientific support for this motion to protect and preserve the natural heritage of the area.