In 2015 a geothermal exploration well was drilled on the island of Tutuila, American Samoa. The sample suite from the drill core provides 645 m of volcanic stratigraphy from a Samoan volcano, spanning 1.45 million years of volcanic history. In the Tutuila drill core, shield lavas with an EM2 (enriched mantle 2) signature are observed at depth, spanning 1.46 to 1.44 Ma. These are overlain by younger (1.35 to 1.17 Ma) shield lavas with a primordial “common” (focus zone) component interlayered with lavas that sample a depleted mantle component. Following ~1.15 Myr of volcanic quiescence, rejuvenated volcanism initiated at 24.3 ka and samples an EM1 (enriched mantle 1) component. The timing of the initiation of rejuvenated volcanism on Tutuila suggests that rejuvenated volcanism may be tectonically driven, as Samoan hotspot volcanoes approach the northern terminus of the Tonga Trench. This is consistent with a model where the timing of rejuvenated volcanism at Tutuila and at other Samoan volcanoes relates to their distance from the Tonga Trench. Notably, the Samoan rejuvenated lavas have EM1 isotopic compositions distinct from shield lavas that are geochemically similar to “petit spot” lavas erupted outboard of the Japan Trench and late stage lavas erupted at Christmas Island located outboard of the Sunda Trench. Therefore, like the Samoan rejuvenated lavas, petit spot volcanism in general appears to be related to tectonic uplift outboard of subduction zones, and existing geochemical data suggest that petit spots share similar EM1 isotopic signatures.

ReuseThis article is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs (CC BY-NC-ND) licence. This licence only allows you to download this work and share it with others as long as you credit the authors, but you can't change the article in any way or use it commercially. More information and the full terms of the licence here: https://creativecommons.org/licenses/ TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. ABSTRACTGeochemical investigations of mantle heterogeneity as sampled by ocean island basalts (OIB) have long relied on isotopic analyses of whole rock lavas. However, recent work has shown that significant isotopic disequilibrium can exist between the phases (groundmass and phenocrysts) of a single OIB lava. In this study, we target individual olivine hosted melt inclusions from two lavas-one Samoan and one Hawaiian-with melt inclusion 87 Sr/ 86 Sr heterogeneity previously observed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We report 87 Sr/ 86 Sr and 143 Nd/ 144 Nd in individual melt inclusions using thermal ionization mass spectrometry (TIMS). In melt inclusions from Samoan sample AVON3-71-2, we find highly heterogeneous (935 ppm) 87 Sr/ 86 Sr (0.705193 -0.705853, N=6), consistent with previously identified 87 Sr/ 86 Sr heterogeneity (~2030 ppm) by laser ablation multicollector ICP-MS (0.70459 -0.70602, N=12). In contrast, we find very little (251 ppm) 87 Sr/ 86 Sr Cabral et al., 2014;Rose-Koga et al., 2012 and Sr (Jackson and Hart, 2006;Harlou et al., 2009;Sobolev et al., 2011) isotopic compositions that span much of the variability observed in oceanic lavas globally. Despite their volumetrically minor contribution to the whole rock lava, the isotopic diversity preserved in melt inclusions provides important information about mantle melting and the processes that operate in magma chambers. The origin of the chemical and isotopic diversity in melt inclusions remains the source of debate, but has most often been attributed to either assimilation of oceanic crust or mixing of isotopically heterogeneous pristine mantle melts (e.Only a handful of studies have examined Sr isotopic compositions of olivine hosted melt inclusions from ocean island basalt (OIB) lavas. Jackson and Hart (2006) were the first to target individual olivine-hosted melt inclusions for Sr isotopic analysis, utilizing laser ablation multicollector inductively coupled plasma mass spectrometry (ICP-MS) to analyze melt inclusions from Samoan picrites. Jackson and Hart (2006) found 87 Sr/ 86 Sr variability within olivine-hosted melt inclusions from a single basalt as large as 3400 ppm, which is 13% of the total range observed in the lavas from the ocean basins (i.e., 0.7021 [Schilling et al., 1994;Fontignie and Schilling, 1996] to 0.7205 [Jackson et al., 2007]). This result was reinforced by the work of ...

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Rationale Emerging research in the geological and nuclear forensics fields demands increasing analytical precision of isotope ratio measurements with decreasing sample sizes. Here we demonstrate the capability of a newly developed amplification technology to make precise neodymium (Nd) isotopic measurements on 100‐pg standard loads. Methods The reference materials were analyzed as NdO+ to increase the ionization efficiency of the small analyte loads. The Nd isotopic measurements were made using an IsotopX Isoprobe‐T thermal ionization mass spectrometer upgraded with the ATONA™ amplifier system. The ATONA™ amplifier system uses capacitance‐based amplification as opposed to traditional impedance‐based amplification. Results The long‐term gains of the ATONA™ amplifiers are shown to have less than 1 ppm variability. Repeat measurements of the JNdi‐1 reference material demonstrate the ability of the ATONA™ amplification technology to make measurements of 143Nd/144Nd ratios with 23 ppm external reproducibility on 100‐pg loads. The effect of increasing integration time on analytical reproducibility is also displayed as increasing integration time from 10 to 30 s reduced the external measurement uncertainty from 37 to 23 ppm. Conclusions These measurements represent an improvement of more than a factor of 3 in external measurement reproducibility relative to previously published 143Nd/144Nd measurements of 100‐pg loads. This new technology will allow for the measurement of smaller samples for precise isotope ratios and open new avenues of research in the geological and nuclear forensic communities.