Precious metal, Lu-Hf, and Re-Os geochemistry of the metasomatized lithospheric mantle: implications for subcrustal precious metal mobilization and MARID petrogenesis
Creators: Hinde, Jason
Publisher: name: University of Alberta Library
Year: 2022

<p>The connection between metasomatism - the change in chemical composition of a rock via interaction with a fluid or melt - and the formation of metalliferous ore deposits in the Earth’s crust is well established. Similarly, it is now accepted that the Earth’s lithospheric mantle has experienced significant metasomatism following large-scale melt depletion events. Despite the recognition of the importance of this process in the transfer of metals, the ability of mantle metasomatism to mobilize precious metals (the PGE and other HSE) and the extent to which they are concentrated in mantle metasomes is poorly constrained. A better understanding of the distribution and abundance of precious metals in areas of the mantle that have experienced metasomatism as well as the agents of this alteration is required to more critically evaluate models that link metal lode in crustal ore deposits to materials derived from the SCLM. In addition, unravelling the petrogenesis of exotic metasomes (MARID xenoliths) interpreted as crystallization products of mantle-derived melts provides a clearer picture of their original precious metal characteristics and new insights into enriched source regions within the lithospheric mantle. <br>
Here we present new major, trace, and precious metal element geochemistry at both the mineral (silicate, oxide, and BMS phases; via EPMA and LA-ICP-MS) and whole rock (via XRF and ID-ICP-MS) scales from a series of modally metasomatized mantle xenoliths from the Kaapvaal craton in Southern Africa and the Lherz Massif in the French Pyrenees in order to better characterize the effects of metasomatism on precious metal distribution in the mantle. We also provide the first published set of whole rock Re-Os isotope data (via ID-ICP-MS and N-TIMS) for a series of MARID xenoliths, which combined with in situ zircon U-Pb, Lu-Hf isotope results (via LASS-ICP-MS), provide new constraints on MARID petrogenesis and the source regions of their parental magmas. <br>
Lithophile trace element mass balance calculations show that strongly metasomatized mantle xenoliths display significant deficiencies (up to 80%) in LILE that can be accounted for by the presence of trace phases and pervasive kimberlite metasomatism. Additionally, mass balance modelling indicates that mantle BMS fail to completely account for the whole rock precious metal budget, especially in more intensely metasomatized samples, and a combination of BMS and micro-scale PGE-rich alloys is required to completely characterize precious metal abundance. Several of our peridotite samples show PPGE enrichment (6.25 ppb Pd in the GPP xenolith and 29.0 ppb Pt in the PP xenolith) similar to primary magmas associated with the formation of basalt-hosted PGE deposits. However, conflicting evidence from BMS and whole rock data make determining the source of enrichment challenging. <br>
MARID xenoliths display differing Os isotope signatures (γOs = -12 to 116; n  = 4). Two-component mixing between a “pure” MARID composition and depleted lithospheric peridotite show that these contrasting signatures are a result of variable interaction with wall rock peridotite during MARID formation. The highly radiogenic γOs composition of pure MARID samples indicate that their parental magmas are derived from an ancient, subduction-related source region. <br>
In situ zircon U-Pb data from MARID sample AJE-2422 yields 238U/206Pb ages ranging from 86.2 to 125.9 Ma (n = 18) with prominent modes at 90.6 Ma, 94.6 Ma, and 125.8 Ma indicating that MARID zircon growth in the Kaapvaal craton lithosphere occurred over a temporally extended period that coincides with major occurrences of Mesozoic intraplate magmatism. Lu-Hf isotopes from the same sample revealed a consistently enriched signature (average εHfi = -17.3 ± 0.5; n = 18) across the wide temporal span recorded in U-Pb ages. This signature is isotopically distinct from archetypal kimberlites and shows good agreement with published results for South African orangeites and lamproites. We interpret this as strong evidence for the presence of an orangeite-like magma during the crystallization of a MARID assemblage. We envisage a simplified geological model for the formation of MARID-veined lithosphere where isotopically enriched slabs subducted during the Namaqua-Natal orogeny (~1.1 Ga), or potentially mantle pyroxenites associated with this event, were selectively melted during Mesozoic plume activity leading to a protracted period of orangeite-like magmatism that variably stalled and crystallized in the lithospheric mantle under open-system conditions.</p>