U-Th-Ba Elemental Fractionation during Partial Melting of Crustal Xenoliths and its Implications for U-series Disequilibria in Continental Arc Rocks

Understanding U-series isotopic disequilibria of partially melted crust is integral for determining the effect that crustal assimilation has on the U-series signature of magmas. In this work, U, Th and Ba (as a proxy for Ra) elemental abundances were gathered on the quenched glass in partially melted crustal xenoliths of granitic composition using microbeam techniques. The crustal xenoliths, which are found in basaltic lava, from Mirador Volcano in Chile, are old, and can be assumed to be at secular equilibrium, whereas melting occurred during eruption of Mirador in 1979. A comparison of the ratios Ba/Th and U/Th in the partial melts with those of the whole rock reveal how much fractionation has occurred during partial melting. Different ratios of U, Th and Ba compared to the whole rock substantiate fractionation via partial melting. Thus, assimilation of partial melts of crust can play a role in U-series isotopic disequilibria

Geodynamic implications for zonal and meridional isotopic patterns across the northern Lau and North Fiji Basins

We present new Sr-Nd-Pb-Hf-He isotopic data for sixty-five volcanic samples from the northern Lau and North Fiji Basin. This includes forty-seven lavas obtained from forty dredge sites spanning an east-west transect across the Lau and North Fiji basins, ten ocean island basalt (OIB)-type lavas collected from seven Fijian islands, and eight OIB lavas sampled on Rotuma. For the first time we are able to map clear north-south and east-west geochemical gradients in 87Sr/86Sr across the northern Lau and North Fiji Basins: lavas with the most geochemically enriched radiogenic isotopic signatures are located in the northeast Lau Basin, while signatures of geochemical enrichment are diminished to the south and west away from the Samoan hotspot. Based on these geochemical patterns and plate reconstructions of the region, these observations are best explained by the addition of Samoa, Rurutu, and Rarotonga hotspot material over the past 4 Ma. We suggest that underplated Samoan material has been advected into the Lau Basin over the past ∼4 Ma. As the slab migrated west (and toward the Samoan plume) via rollback over time, younger and hotter (and therefore less viscous) underplated Samoan plume material was entrained. Thus, entrainment efficiency of underplated plume material was enhanced, and Samoan plume signatures in the Lau Basin became stronger as the trench approached the Samoan hotspot. The addition of subducted volcanoes to the Cook-Austral Volcanic Lineament material, first from the Rarotonga hotspot, then followed by the Rurutu hotspot, contributes to the extreme geochemical signatures observed in the northeast Lau Basin

Geodynamic implications for zonal and meridional isotopic patterns across the northern Lau and North Fiji Basins

We present new Sr-Nd-Pb-Hf-He isotopic data for 65 volcanic samples from the northern Lau and North Fiji Basins. This includes 47 lavas obtained from 40 dredge sites spanning an east-west transect across the Lau and North Fiji basins, 10 ocean island basalt (OIB)-type lavas collected from seven Fijian islands, and eight OIB lavas sampled on Rotuma. For the first time, we are able to map clear north-south and east-west geochemical gradients in 87Sr/86Sr across the northern Lau and North Fiji Basins: lavas with the most geochemically enriched radiogenic isotopic signatures are located in the northeast Lau Basin, while signatures of geochemical enrichment are diminished to the south and west away from the Samoan hot spot. Based on these geochemical patterns and plate reconstructions of the region, these observations are best explained by the addition of Samoa, Rurutu, and Rarotonga hot spot material over the past 4 Ma. We suggest that underplated Samoan material has been advected into the Lau Basin over the past ∼4 Ma. As the slab migrated west (and toward the Samoan plume) via rollback over time, younger and hotter (and therefore less viscous) underplated Samoan plume material was entrained. Thus, entrainment efficiency of underplated plume material was enhanced, and Samoan plume signatures in the Lau Basin became stronger as the trench approached the Samoan hot spot. The addition of subducted volcanoes from the Cook-Austral Volcanic Lineament first from the Rarotonga hot spot, then followed by the Rurutu hot spot, contributes to the extreme geochemical signatures observed in the northeast Lau BasinMGJ acknowledges NSF grants EAR-1624840, EAR-1348082, and EAR-1347377. JBT acknowledges financial support from the French Agence Nationale de la Recherche through grant ANR-10-BLAN-0603 (M&Ms—Mantle Melting—Measurements, Models, Mechanisms). MDK acknowledges NSF support for the noble gas laboratory at WHOI (via OCE-1232985 and OCE −1259218) and the noble gas analytical efforts of Joshua Curtice. FEJ acknowledges funding from the Deep Carbon Observatory (DCO). We also thank Paul Hall for insightful discussions and two unnamed reviewers for their helpful comments

Elemental recycling of the Tonga-Kermadec island arc system and the associated Lau and North Fiji basins

Thesis by publication.Running title: Elemental recycling of the Tonga-Kermadec island arc system and the associated Lau and North Fiji basins.Includes bibliographical references1. Introduction -- 2. Lithium isoptopic composition of the Tonga-Kermadec arc and constraints on subduction recycling -- 3. Volatile contents reveal mid-ocean basalt tapping an ocean island magma source -- 4. Geodynamic implications for zonal and meridional isotopic patterns across the northern Lau and North Fiji basins -- 5. An experimental approach to understanding the genesis of silicic arc magmas : a case study from the late volcano in the Tonga-Kermadec arc -- Chapter 6. Conclusion -- 7. Appendices.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.Mode of access: World wide web1 online resource (xl, 295 pages) illustrations (some colour), colour map

Lithium isotopic composition of the Tonga-Kermadec Arc and its constraints on subduction recycling

1 page(s

Experimental constraints on the differentiation of low-alkali magmas beneath the Tonga arc: Implications for the origin of arc tholeiites

Hydrous melting experiments on a basaltic andesite from Late Volcano in the Tonga Arc were conducted at 900– 1250 °C and 1 atm to 2.5 GPa, with a range of added H2O concentrations (0–9 wt%). These were used to constrain conditions of phenocryst precipitation in the basaltic andesite and to better understand the processes of magmatic differentiation beneath Tongan volcanoes. Comparison between the products of experiments and Tongan lavas indicates that basaltic andesites from Late crystallised plagioclase + augite + pigeonite ± orthopyroxene while water-saturated at comparatively low pressures (≤0.2 GPa) but high temperatures (≥1000 °C). Glasses produced at 950 °C and 0.2 GPa are similar in composition to dacites from neighbouring volcanoes, including Fonualei. This confirms previous interpretations of dacite magma genesis by crystal fractionation of plagioclase and clinopyroxene from basaltic andesite parent magmas, rather than by partial melting of lower crust. This produces decreases in Dy/Yb with increasing SiO2 without a role for amphibole, probably due to the high temperatures and low alkali concentrations involved. Tongan magmas are primarily tholeiitic, and their high water contents bring into question previous hypotheses that attribute the tholeiitic trend to low concentrations of magmatic H2O. Attributes inherited from the Tongan parent magmas, including low alkali concentrations, appear to drive tholeiitic differentiation, regardless of water content.This research was supported by the Australian Research Council and specifically grant DP110103284 to Tracy Rushmer and Simon Turner