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Constraints on petrogenesis and elemental recycling of the Tonga-Kermadec island arc system and the associated Lau and North Fiji basins
thesisposted on 2022-03-28, 17:21 authored by Raul Brens
Subduction zones are sites where subducted materials achieve one of two opposing outcomes, either recycling back to the crust or being transferred to the deep mantle. The composition of arc magmas, and to a lesser extent the associated extensional back-arc basin, reflects the reservoirs that influence the chemical composition of arc lavas and subducted materials: the altered oceanic crust, the mantle, the overlying crust, and occasionally hotspots. To better understand the complex processes occurring in subduction zones, a series of studies investigate the process in which subducting elements recycle and interact with various upper mantle components and ultimately form into igneous rocks. This investigation takes a multifaceted approach to further understand the history and source of magmatism in intra-oceanic arcs, using as an example the Tongan arc from inception (subduction) to completion (petrogenesis and eruption). Constraining petrogenesis and elemental recycling within the Tonga-Kermadec Island arc and Lau back-arc basin system is integral to understanding crust mantle exchange. The Tonga-Kermadec arc setting is ideal to test the chemical influences imparted on the lavas from a range of dynamic forces because these magmas are not contaminated with continental crust. This research focuses on magmas associated with the plate tectonic cycle at both convergent and divergent plate margins within the Tonga-Kermadec volcanic arc, Lau basin, and North Fiji basin system. In order to untangle the intricacies associated with constraining the contributions to intra-oceanic arc magmas (Tonga-Kermadec arc) and to identify the sources affecting mantle evolution over time in the accompanying back arc basins (Lau basin and North Fiji basin), a range of geochemical, geothermobarometric, and high and low pressure experimental techniques have been applied. This study consists of 1) analyzing and quantifying how elements recycle within a subduction zone through the use of Li isotopes; 2) experimentally constraining pressure, temperature and water conditions that drive melting and magma generation at Late volcano, located on the active volcanic front; and 3) by using the major, trace, volatile elements along with radiogenic isotope data to create an intricate and cohesive dataset from back-arc samples collected during the 2012 Northern Lau Transit Expedition (NoLauTe) of the R/V Southern Surveyor (cruise name: ss2012_v02). This study concludes that addition of up to 3.5% sediment is present in the arc front lavas and that back-arc lavas equilibrate with the mantle wedge, which may reflect a longer slab-to-surface path traversed by the magmas. This study also questions the utility of lithium as a possible tracer of recycled material. The (mid-ocean ridge basalt) MORB-like lithium isotopic composition of the Tonga-Kermadec lavas suggests that the lithium elemental and isotopic characteristics reflect either, or a combination of, equilibration with the mantle wedge or sequestration and removal of the heavy Li in the system, possibly linked to slab-convergence rate. Pyroxene thermobarometry for the subaerial volcano studied (Late) suggests fractional crystallization at 1020-1070 °C at 0.8-1.8 kbar. A lack of hydrous mineral phases in all of the experiments as well as in the natural rock sample negates the influence of an amphibolite melt. The experimental results support shallow crustal (2-6 km) crystal fractionation of basalt that produces an evolved magmatic composition. Whereas the lavas from the Lau basin and connecting North Fiji basin indicate a primary influence from a wet plume source (Samoa) and additional extreme geochemical contributions (high 3He/⁴He, high ⁸⁷Sr/⁸⁶Sr, HIMU:high-μ = 23⁸U/2⁰⁴Pb) from the Samoa, Rarotonga, and Rurutu hotspots. For the first time, it is possible to map a clear north-south and east-west geochemical gradient in⁸⁷Sr/⁸⁶Sr across the northern Lau and North Fiji basin: lavas with the mostgeochemically enriched radiogenic isotopic signatures are located to the northeasternLau Basin, while signatures of geochemical enrichment are diminished to the south andwest away from the Samoan hotspot.