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.
Until the dawn of the high-precision era in modern isotope geochemistry about a decade ago, geologists were convinced that all evidence of the Earth’s youthful exuberance during the first several hundred million years of its history have been erased by various processes acting on our dynamic planet. Recent studies of long- and short-lived isotopic systems demonstrated that this was not the case, and the race to uncover the details has begun. The main goal of this research project was to constrain the origin and temporal evolution of absolute and relative abundances of the so-called highly siderophile (iron-loving) elements (HSE, including rhenium, osmium, iridium, ruthenium, platinum, and palladium) present in the silicate portion of the Earth in the early Archean (3.5 – 3.3 billion years ago) using very high-temperature lavas, called komatiites, from South Africa. The specific objectives of this study were to calculate absolute and relative HSE abundances in the deep sources of these komatiite systems, to synthesize the HSE data obtained in this and our previous NSF-funded research, and combine these data with the other, mostly lithophile (rock-loving) element isotopic data to integrate into a model of the evolution of the HSE in the Archean mantle. Our new high-precision isotopic data are most consistent with formation, followed by long-term isolation, of deep-seated (down to 2,900 km below the Earth’s surface) domains, at ca. 4400 Ma, as a result of crystallization of a primordial terrestrial magma ocean. We also established total platinum and palladium abundances in the sources of early Archean komatiite systems to be between 60% and 70% of those present in estimates for the modern primitive mantle. These are within the range of the total HSE abundances present in the sources of late Archean komatiite systems, indicating little change in HSE abundances in the Archean mantle between 3.5 and 2.7 Ga. Our data require that late accretion of HSE was largely complete by the time of crystallization of a putative final terrestrial magma ocean. The HSE variations observed in early Archean komatiites may reflect sluggish mixing of diverse post-magma ocean domains, characterized by variably fractionated lithophile element and HSE abundances and indicate that the silicate portion of the Earth called mantle may have never been well mixed. The grant enabled the PI to serve as a mentor to a number of graduate and undergraduate students and visiting scientists in the Isotope Geochemistry Laboratory (IGL) at the University of Maryland (UMd). The PI served as a mentor to Brian Connolly, an undergraduate student at UMd. Connolly was originally hired in summer 2010 to work on Weltevreden komatiite samples as a basis for his GEOL393/394H senior thesis. The training he received provided him with the skills essential for his future career as a geoscientist. Connolly’s work was ultimately published in Connolly et al. (2011). Visiting Ph.D. student Christophe Robin-Popieul (University of Grenoble) also conducted HSE and Os isotopic measurements at UMd as part of a project directed towards the origin of the Weltevreden komatiites. He successfully defended his dissertation in 2011. Some of the results he obtained were also incorporated in Connolly et al. (2011). Visiting Ph.D. student Eduardo Rocha-Júnior (Universidade de São Paulo) conducted HSE and Os isotopic measurements on Paraná flood basalts; these results were published in Rocha-Júnior et al. (2012). Rocha-Júnior successfully defended his dissertation in 2011. The PI also served as a mentor to Alex McCoy-West, a graduate student from the ANU, who visited the IGL in 2011 to determine Re-Os isotopic compositions and HSE abundances in mantle xenoliths from Australia and Tasmania. The data he obtained during his visit formed the basis for a major portion of his Ph.D. dissertation and were published in McCoy-West et al. (2013). In late 2011, the PI hosted Dr. Yulia Larionova, a staff scientist from the Institute of Geology of Ore Deposits (IGEM), Russian Academy of Sciences in Moscow. Dr. Larionova came to the IGL to learn Re-Os isotopic techniques with the ultimate goal to set up this method at her home institution. The PI served as a mentor to Dr. Svetlana Tessalina, a Senior Research Fellow from Curtin University of Western Australia. Dr. Tessalina visited the IGL in 2012 to gain hands-on experience with the Re-Pt-Os isotopic methods in order to establish the techniques at her home institution. The results of this study will be reported at the Australian Earth Science Convention in July 2014. The research results from this grant have been presented at the Fall 2010 AGU Meetings, and at the 2010-2013 V.M. Goldschmidt Conferences; eight papers have been published in peer-reviewed journals. The PI was also invited to give two lectures at the IGEM. The komatiite work, coupled with tungsten isotope results from our Lab for Komati and Kostomuksha komatiites (Touboul et al., 2012), has been highlighted in media worldwide.
