My primary interest is the differentiation of the Earth, particularly the formation of the continental crust and global geochemical recycling. I pursue this through the study of island and back arc magmatism, and the geochemical changes accompanying subduction of crustal materials. More recently, I have been concentrating on volatile losses from magmas in arc settings and other environments, and studying the formation of oceanic plateaus.
Research currently is focussed on several main strands: 1. processes of island arc inception represented by the proto-intraoceanic arc of the Izu-Bonin-Mariana system involving data and samples recovered during Expedition 351 of the International Ocean Discovery Program (IODP) in mid-2014; 2. understanding redox processes associated with the development and evolution of island arc magmas and their sources compared with those in other tectonic settings; 3. Identification of the multiple mantle sources involved in the development of magmas emplaced in backarc basins. Activities involve field-based, laboratory, and modelling studies. The research team at the RSES comprises Richard Arculus, postdoctoral fellows Antoine Benard, Philipp Brandl, and Oliver Nebel, and PhD students Sarlae McAlpine and Clare Connolly. Numerous other international colleagues are collaborators in these research activities.
Magmatic and structural responses accompanying initiation of tectonic plate subduction are not well known as rocks that are formed are typically obscured by younger deposits and strata. Induced and spontaneous are two main initiation models. Induction resulting from externally-driven ridge push or slab pull produces compression and uplift. Rifting and magmatism in an extensional setting accompanies spontaneous initiation attributable to subsidence of dense lithosphere along faults and fractures adjacent to relatively buoyant lithosphere. IODP Expedition 351 targeted the Amami Sankaku Basin in the northwestern Philippine Sea at Site U1438, adjacent to the oldest stratovolcano chain of the intraoceanic Izu-Bonin-Mariana arc. The Expedition recovered igneous basement and overlying sediments recording early stages of arc initiation and evolution. The uppermost Amami Sankaku basement comprises basaltic lavas similar to 52-48Ma “fore-arc basalts” that were likely first products of subduction initiation, recovered from the Izu-Bonin-Mariana trench slope. Sedimentary biostratigraphy indicates the Amami Sankaku basement is broadly coeval with the forearc basalts. All these basalts are unequivocally subduction-related, derived from mantle sources more prior melt-depleted than tapped at mid-ocean ridges, and require subducted lithosphere-derived components in their genesis. Trench slope and drilling observations confirm crust-forming, areally widespread, dyke-supplied magmatism accompanied arc inception consistent with spontaneous subduction initiation.
Figure 1. Whole thin section and detailed photomicrographs of basement basalts recovered from the Amami Sankaku Basin at IODP Site U1438, showing different textural types: A) fine-grained basalt with ophitic texture (351-U1438E-78R-2); B) aphyric microcrystalline basalt (351-U1438E-77R-3); For A and B, Views on the left are in plane polarized light and on the right are cross-polarized light. Horizontal dimension of the fields in all images except in bottom right panel is shown by the scale bar in B (left). C) fine- to medium-grained basalt with ophitic texture, clinopyroxene (cpx) and altered olivine phenocrysts (ol) (351-U1438E-80R-1) on left, and details of same thin section on right with cross-polarized (left) and plane polarized (right). Scale bar is for both left and right of this image pair.
The redox state of subduction-associated magmas compared with those erupted at mid-ocean ridge and ocean island (mantle plume-related) locations is controversial, and has significance inter alia for global geochemical recycling, and the transport of sulfur in magmas and hence potential associated ore-forming processes. We have been pursuing these research problems on a variety of fronts including studies of arc-associated peridotites from Kamchatka and Papua New Guinea, and Fe isotope studies of a spectrum of primitive (i.e., high Mg/Fe) arc magmas. The former show unequivocal evidence of oxidation of refractory, prior melt-depleted sources is involved in the production of boninite, a uniquely arc-associated magma type.
Among a variety of analytical studies of backarc magma types, Nebel and Arculus have combined Nd-Hf isotopic studies with previously published noble gas systematics to show that selective ingress of the mantle plume supplying the Samoan hotspot chain of islands is occurring southwards into the Lau backarc basin. The most plausible explanation of the combination of particular 3He/4He and 143Nd/144Nd characteristics of Lau samples is the entrainment solely of the uprising “Focus Zone (FOZO)” sheath of the Samoan plume, and none of the other mantle components (e.g., depleted MORB mantle (DMM), enriched mantle 2 (EM2)) manifest in Samoa.