Geochemistry of Ocean Island Basalts: A Deep Mantle Reservoir or Deep Upper Mantle Melting?
Abstract
Geochemical observations of ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) indicate that the mantle is heterogeneous on scales much smaller than the sizes of the mantle melting zones. Materials with higher incompatible-element and volatile contents will begin melting at greater depths than depleted materials, and thus will tend to be expressed more heavily in OIBs compared to MORBs for two main reasons. First, melting beneath hotspots, which generates most OIBs, is likely to occur at greater depths, owing to the presence of thick lithosphere (intraplate) and/or thick crust (near-ridge). Second, if hotspots are caused by narrow, buoyant mantle upwellings (e.g., mantle plumes), calculations predict a larger flux of mantle will pass through the deepest portion of the melting zone than through the shallowest portion. Such "plume flow" contrasts with flow beneath mid-ocean ridges, where mantle flux is likely to be more uniform through all depths of the melting zone. Assuming the isotopic end-member DMM begins melting shallowest and that other isotopic components, broadly similar to EM1, EM1, HIMU and C (or FOZO), begin melting deeper, calculations of flow and melting of a heterogeneous but well-mixed mantle can explain many differences between OIBs and MORBs in terms of Sr, Nd, and Pb isotope ratios. Models can also explain the relatively high values and large variability of 3He/4He of OIBs relative to MORBs if C is the source of highest 3He/4He. One test of the importance of melting depth is provided by the Hawaiian and Hawaiian-Emperor volcano chain, in which the Hawaiian hotspot erupted lavas onto seafloor of progressively increasing age. He, Sr, Nd, and Pb isotope ratios become increasingly DMM-like towards the oldest parts of the chain [Regelous et al. 2002; Keller et al., 2004], which can be explained by melting of a heterogeneous mantle plume beneath lithosphere of decreasing thickness. The main requirement is that an EM-like source begin melting deepest, a high 3He/4He, C-like source begin melting at intermediate depths, and a refractory, DMM-like source begin melting shallowest. Thus, rather than reflecting a deep reservoir in a compositionally layered mantle, the isotopic characteristics of OIBs could reflect deep melting of a heterogeneous, possibly non-layered mantle.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2005
- Bibcode:
- 2005AGUSM.V42A..03I
- Keywords:
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- 1025 Composition of the mantle;
- 8121 Dynamics;
- convection currents and mantle plumes;
- 8124 Earth's interior: composition and state (old 8105)