Results of Basement Drilling on Oceanic LIPs

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  • Sano Takashi
    Department of Geology and Paleontology, National Museum of Nature and Science

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  • 巨大海台をつくる基盤溶岩の掘削成果
  • キョダイカイダイ オ ツクル キバン ヨウガン ノ クッサク セイカ

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Abstract

<p>In the first decade of the 21st century, two oceanic large igneous provinces (LIPs), the Ontong Java Plateau (OJP) and the Shatsky Rise, were drilled to recover basement lavas. Coring data show that basement rocks comprise massive sheet flows, similar to continental flood basalts. The high effusion rate to generate the massive sheet flows cannot be explained by normal plate tectonic processes. Because the two LIPs are considered to have formed at plate boundaries, previous workers have suggested that thermal plume heads underlie plate boundaries, generating voluminous magmas. 40Ar-39Ar data show that the duration of LIP magmatism exceeds that of the predictive model for plumes. Minor volcanic episodes between 90 and 44 Ma occurred in the OJP, with the major event occurring at ~122 Ma. For the Shatsky Rise, a ~10 Myr hiatus separates the final massive flows from main volcanism. Petrological data suggest the temperature of the magma source (potential temperature) was ~300°C, higher than the ambient mantle of the OJP, ~150-200°C higher than the Shatsky Rise, and lower than that of the plume model (>400°C). Geochemical data show that lavas from the two LIPs are variably enriched relative to normal upper mantle, suggesting they were derived from a plume source. However, unequivocal plume signatures (e.g., lower-mantle geochemical signatures with high 3He/4He) have not been identified in the volcanic products of the LIPs. Similarly, some lava compositions suggest the contribution of subducted slab materials to their magmatic source. Furthermore, the amount of subsidence following emplacement of the LIPs is less than that predicted by the thermal plume model. Thus, our study shows that a simple thermal plume model does not account for the genesis of the LIPs, indicating that more complex (e.g., thermochemical) models should be developed. Recent oceanic crust drilling programs have improved our understanding of LIP magmatism, but further work is required to constrain its characteristics and source.</p>

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