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Wed 4th December
12 – 1 pm
Rm 312.222
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Oliver Nebel
Research School of Earth Sciences, Australian National University
Isotopic mantle reservoirs in space and time
Abstract It is complicated. In the early- to mid-1980’s, geochemists noticed that intra-plate oceanic island basalts (OIB) are isotopically distinct from their mantle-derived cousins born at divergent plate margins, the mid-ocean ridge basalts (MORB). The range of proxies employed to study the origin and nature of this oddity includes radiogenic Sr-Nd-Pb-Hf-Os-He isotopes and an ever-growing number of stable and short-lived isotope systems. From these studies, a picture has emerged within the geochemical community that most (but not all!) researchers nowadays adopt and further develop.
The similarity of some OIB with the chemistry of the crust has led to the idea that once surface-exposed crustal rocks were subducted to the mantle where they subsequently heated to become buoyant and return to the surface in mantle plumes. These then burn through the oceanic plates where they erupt as OIB. However, the range in OIB isotope composition requires multiple, isotopically distinct components, commonly referred to as mantle endmembers, namely enriched mantle 1 (EM1-lithospheric mantle?), enriched mantle 2 (EM2-subducted sediment?), high-µ (HIMU-oceanic crust?), the focal zone component (FOZO, subdivided into FOZO-A (australis) and FOZO-B (borealis)), or the primitive He-mantle (PRHEMA) or the SCHEM mantle (sub-chondritic Earth model mantle) or simply the primitive mantle (PM). Modes, loci and timescales of mantle storage remain unknown. In addition, the mantle reservoirs seem to be present to various proportions in different OIB locations around the globe, yet only some of them mix with the depleted MORB mantle (DMM) sourcing mid-ocean ridges. The latter is also isotopically heterogeneous, with MORB from the Indian Ocean (Indian-type mantle, ITM) being distinct from Atlantic or Pacific MORB (ATM, PTM), all of which depict a spectrum of compositions rather than being sourced from one homogeneous DMM. Illuminating the true nature of these mantle reservoirs is further complicated by the unknown age of the subducted endmembers and the time at which they experienced additional parent-daughter fractionation, either during subduction or within the mantle. I will give an overview on the status-quo of mantle heterogeneity from a geochemical perspective with a focus on HIMU OIB. |
Curtin Applied Geology Seminar: 4th December, Oliver Nebel on Isotopic Mantle Reservoirs in Space and Time
By Katy Evans
3 December 2013
Applied Geology
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