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© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth and Planetary Science Letters 453 (2016): 55-63, doi:10.1016/j.epsl.2015.12.008., In order to link the geochemical signature of hot spot basalts to Earth’s deep interior, it is first necessary to understand how plumes sample different regions of the mantle. Here, we investigate the relative amounts of deep and shallow mantle material that are entrained by an ascending plume and constrain its source region. The plumes are generated in a viscous syrup using an isolated heater for a range of Rayleigh numbers. The velocity fields are measured using stereoscopic Particle-Image Velocimetry, and the concept of the ‘vortex ring bubble’ is used to provide an objective definition of the plume geometry. Using this plume geometry, the plume composition can be analysed in terms of the proportion of material that has been entrained from different depths. We show that the plume composition can be well described using a simple empirical relationship, which depends only on a single parameter, the sampling coefficient, Sc. High-Sc plumes are composed of material which originated from very deep in the fluid domain, while low-Sc plumes contain material entrained from a range of depths. The analysis is also used to show that the geometry of the plume can be described using a similarity solution, in agreement with previous studies. Finally, numerical simulations are used to vary both the Rayleigh number and viscosity contrast independently. The simulations allow us to predict the value of the sampling coefficient for mantle plumes; we find that as a plume reaches the lithosphere, 90% of its composition has been derived from the lowermost 260−750 km in the mantle, and negligible amounts are derived from the shallow half of the lower mantle. This result implies that isotope geochemistry cannot provide direct information about this un-sampled region, and that the various known geochemical reservoirs must lie in the deepest few hundred kilometres of the mantle., This work was funded by the National Science Foundation (grant EAR-055199), the MAPS Dean’s Office at UCL and the CIDER workshop (EAR-1135452)., 2016-12-29

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 579–599, doi:10.1002/2014GC005576., The three-dimensional velocity and temperature fields surrounding an isolated thermal plume in a fluid with temperature-dependent viscosity are measured using Particle-Image Velocimetry and thermochromatic liquid crystals, respectively. The experimental conditions are relevant to a plume rising through the mantle. It is shown that while the velocity and the isotherm surrounding the plume can be used to visualize the plume, they do not reveal the finer details of its structure. However, by computing the Finite-Time Lyapunov Exponent fields from the velocity measurements, the material lines of the flow can be found, which clearly identify the shape of the plume head and characterize the behavior of the flow along the plume stem. It is shown that the vast majority of the material in the plume head has undergone significant stretching and originates from a wide region very low in the fluid domain, which is proposed as a contributing factor to the small-scale isotopic variability observed in ocean-island basalt regions. Lastly, the Finite-Time Lyapunov Exponent fields are used to calculate the steady state rise velocity of the thermal plume, which is found to scale linearly with the Rayleigh number, in contrast to some previous work. The possible cause and the significance of these conflicting results are discussed, and it is suggested that the scaling relationship may be affected by the temperature-dependence of the fluid viscosity in the current work., This work was funded by the National Science Foundation (grant EAR-055199) and the MAPS Dean's Office at UCL., 2015-09-04

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 2301–2318, doi:10.1002/2013GC005221., The Samoan islands of Ta'u and Ofu/Olosega (Ofol hereafter) are single shield volcanoes that have erupted alkali basalt for the past 70 and 440 kyr, respectively. They are 20 km apart, and are the easternmost subaerial expressions of the Samoan plume. The isotopic data for these islands are published; we report here the first major and trace element data for Ofol. The two islands are similar isotopically and in trace elements. Their high 3He/4He marks them as being a FOZO mantle end-member. By comparing data from both volcanoes, we test the efficacy of melting models in constraining the mantle compositions and their P-T of melting. We show that the mantle sources are similar, with spidergrams that peak at Ta (4× BSE), and Lu ∼0.5× BSE. Melts and mantle sources both have Lu/Hf ratios that are too low to support measured 176Hf/177Hf ratios, pointing to a young enrichment event in the mantle source. Degrees of melting are 6.5% for Ta'u and 5.2% for Ofol. P-T of melting show a wide, overlapping range, but define a precise array; average values are 1475°C—77 km for Ta'u and 1550°C—110 km for Ofol. The deepest P-T estimate is 4.2 GPa and 1550°C. The P-T array is either a melting adiabat, or a mixing line of melts equilibrated at various depths. Kinetic modeling shows melt re-equilibration will be likely for ascent velocities (m/yr) less than 40/(conduit radius in cm)2. P-T estimates for melting may typically be minimum values., We thank the National Science Foundation for their many years of support of our studies in Samoa (OCE- 0351437, EAR-0509891, EAR-0652707, EAR-0318137, OCE-1153894)., 2014-12-06

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q07025, doi:10.1029/2010GC003438., The volcanic origin of the Samoan archipelago can be explained by one of three models, specifically, by a hot spot forming over a mantle plume, by lithospheric extension resulting from complex subduction tectonics in the region, or by a combination of these two processes, either acting sequentially or synchronously. In this paper, we present results of 36 high-resolution 40Ar/39Ar incremental heating age analyses for the initial (submarine) phase of Samoan volcanoes, ranging from 13.2 Ma for the westernmost Samoan seamounts to 0.27 Ma in the eastern Samoan volcanic province. Taken as a whole, our new age data point to a hot spot origin for the shield-building volcanism in the Samoan lineament, whereby seamounts younger than 5 Ma are consistent with a model of constant 7.1 cm/yr plate motion, analogous to GPS measurements for the Pacific Plate in this region. This makes our new 40Ar/39Ar ages of the submarine basalts all older compared to recent absolute plate motion (APM) models by Wessel et al. (2008), which are based on the inversion of twelve independent seamount trails in the Pacific relative to a fixed reference frame of hot spots and which predict faster plate motions of around 9.3 cm/yr in the vicinity of Samoa. The Samoan ages are also older than APM models by Steinberger et al. (2004) taking into account the motion of hot spots in the Pacific alone or globally. The age systematics become more complicated toward the younger end of the Samoan seamount trail, where its morphology bifurcates into two en echelon subtracks, termed the VAI and MALU trends, as they emanate from two eruptive centers at Vailulu'u and Malumalu seamount, respectively. Spaced ∼50 km apart, the VAI and MALU trends have distinct geochemical characters and independent but overlapping linear 40Ar/39Ar age progressions since 1.5 Ma. These phenomena are not unique to Samoa, as they have been observed at the Hawaiian hot spot, and can be attributed to a geochemical zoning in its underlying man, Financial support is provided by NSF‐OCE 0002875 and NSF‐OCE 0351437.

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q08001, doi:10.1029/2011GC003617., Isotope systematics of basalts provide information on the distribution of mantle components and the length scale of mantle heterogeneity. To obtain this information, high data and sampling density are crucial. We present hafnium and neodymium isotope data on more than 400 oceanic volcanics. Over length scales of several hundred to over one thousand kilometers hafnium and neodymium isotopes of mid-ocean ridge basalts are correlated and form an array of parallel trends on a global scale. On a larger scale these domains differ in the amount of highly depleted mantle material with radiogenic hafnium and neodymium isotope ratios. Compared to the Atlantic and Indian Ocean basins the asthenosphere of the Pacific basin seems to have a more uniform and a less radiogenic Hf isotopic composition for a given Nd isotopic composition. The parallel arrays of mid-ocean ridge basalts provide strong constraints on the makeup of the MORB mantle and are evidence for the presence of a highly depleted and highly radiogenic neodymium and hafnium component. This component, because of its highly depleted character, is unrecognized in the strontium-neodymium-lead isotope systems alone. Alternatively, the parallel arrays can have an ancient origin of systematic variations in the degree of depletion. Each array then represents the variations in this fossil melting regime. Individual ocean island basalt suites display different slopes in hafnium-neodymium isotope space, which are also best explained by varying amounts of highly residual mantle rather than isotopic differences in enriched mantle components as previously invoked. The ocean island basalt arrays diverge at the depleted end and project to radiogenic compositions that are similar to those of the asthenosphere through which they travel. This is strong evidence that the plume material interacts with its surrounding mantle as it ascends. The isotopic compositions of the ocean island and ridge basalts suggest that their systematics are i, This work was supported by NSF grants EAR 0635864 and OCE0648484 to V.S. and OCE0351437 to S.H.

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q04003, doi:10.1029/2007GC001651., We report 238U-230Th-226Ra-210Pb-210Po, 232Th-228Ra and 235U-231Pa measurements for a suite of 14 geologically and geochemically well-characterized basaltic samples from the Samoan volcanoes Vailulu'u, Malumalu, and Savai'i. Maximum eruption ages based on the presence of parent-daughter disequilibria indicate that Vailulu'u is magmatically productive with young lavas (<8 Ka) resurfacing both its summit crater and lower flanks. 210Pb and 210Po measurements indicate that several flows have erupted within its summit crater in the past 100 years, with the newest observed flow being erupted in November of 2004. For lavas which have eruption ages that are demonstrably young, relative to the half-lives of 230Th, 231Pa, and 226Ra, we interpret their 238U -230Th, 235U-231Pa and 230Th - 226Ra disequilibria in terms of the magmatic processes occurring beneath the Samoan Islands. (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of (238U/232Th) and (230Th/232Th) is attributed to long-term source variation. The Samoan basalts are all alkaline basalts and show significant 230Th and 231Pa excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their (230Th/238U), (231Pa/235U) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites (particularly Hawaii) which show that (230Th/238U) and (231Pa/235U) of many OIBS can be explained by simple time-independent models. Interpretation of the 226Ra data requires time-dependent melting models. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Samoan basalts. Several young samples from the Vailulu'u summit crater also exhibit significant 210Pb deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times ar, Support for this research was provided by NSF grants EAR-9909473 (KWWS), EAR-0509891 (SRH), EAR- 0609670 (MKR) and EAR-0504362 (MKR).

Author Posting. © Oceanography Society, 2010. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 23, 1 (2010): 164-165., Vailulu’u seamount is an active underwater volcano that marks the end of the Samoan hotspot trail.

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q04027, doi:10.1029/2007GC001876., We report evidence for a global Ti, Ta, and Nb (TITAN) enriched reservoir sampled by ocean island basalts (OIBs) with high 3He/4He ratios, an isotopic signature associated with the deep mantle. Excesses of Ti (and to a lesser degree Nb and Ta) correlate remarkably well with 3He/4He in a data set of global OIBs, demonstrating that a major element signature is associated with the high 3He/4He mantle. Additionally, we find that OIBs with high 3He/4He ratios have moderately radiogenic 187Os/188Os (>0.135). The TITAN enrichment and radiogenic 187Os/188Os in high 3He/4He OIBs indicate that they are melts of a mantle domain that hosts a nonprimitive (nonchondritic) component. The observation of TITAN enrichment in the high 3He/4He mantle may be important in balancing the Earth's budget for the TITAN elements. Understanding the origin of the TITAN enrichment is important for constraining the evolution of the enigmatic high 3He/4He mantle domain., Funds for helium measurements were provided by NSF-OCE to M.D.K. Funds for major and trace element analyses were provided by NSF-EAR 0509891 to S.R.H.

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 10 (2009): Q03006, doi:10.1029/2008GC002324., We report 87Sr/86Sr and 143Nd/144Nd data on clinopyroxenes recovered from 10 ocean island lavas from three different hot spots (Samoa, Society, and Cook-Austral island chains). The clinopyroxenes recovered from eight of the 10 lavas analyzed in this study exhibit 87Sr/86Sr disequilibrium with respect to the host lava. The 87Sr/86Sr ratios in clinopyroxene separates are 95–3146 ppm (0.0095–0.31%) different from their respective host whole rocks. Clinopyroxenes in three lavas have 143Nd/144Nd ratios that are 70–160 ppm (0.007–0.016%) different from the host lavas. The 87Sr/86Sr and 143Nd/144Nd disequilibrium in one lava (the oldest lava considered in this study, Mangaia sample MGA-B-47) can be attributed to posteruptive radiogenic ingrowth, but the isotope disequilibrium in the other, younger lavas cannot be explained by this mechanism. In five of the lava samples, two populations of clinopyroxene were isolated (black and green, separated by color). In four out of five of these samples, the 87Sr/86Sr ratios of the two clinopyroxene populations are isotopically different from each other. In addition to 87Sr/86Sr disequilibrium, the two clinopyroxene populations in one of the lavas (Tahaa sample TAA-B-26) have 143Nd/144Nd ratios that are ∼100 ppm different from each other. Given the resilience of clinopyroxene to seawater alteration and the likelihood that the Sr and Nd isotope composition of fresh clinopyroxene separates provides a faithful record of primary magmatic compositions, the clinopyroxene-clinopyroxene isotope disequilibrium in these four lavas provides strong evidence that a mechanism other than seawater alteration has generated the observed isotopic disequilibrium. This study confirms the isotopic diversity in ocean island lavas previously observed in olivine-hosted melt inclusions. For example, the Sr isotopic variability previously observed in olivine-hosted melt inclusions is mirrored by the isotopic diversity in clinopyroxenes isolated from many of the, NSF grant EAR-0509891 to SRH is gratefully acknowledged.

We report 87Sr/86Sr and 143Nd/144Nd data on clinopyroxenes recovered from 10 ocean island lavas from three different hot spots (Samoa, Society, and Cook-Austral island chains). The clinopyroxenes recovered from eight of the 10 lavas analyzed in this study exhibit 87Sr/86Sr disequilibrium with respect to the host lava. The 87Sr/86Sr ratios in clinopyroxene separates are 95–3146 ppm (0.0095–0.31%) different from their respective host whole rocks. Clinopyroxenes in three lavas have 143Nd/144Nd ratios that are 70–160 ppm (0.007–0.016%) different from the host lavas. The 87Sr/86Sr and 143Nd/144Nd disequilibrium in one lava (the oldest lava considered in this study, Mangaia sample MGA-B-47) can be attributed to posteruptive radiogenic ingrowth, but the isotope disequilibrium in the other, younger lavas cannot be explained by this mechanism. In five of the lava samples, two populations of clinopyroxene were isolated (black and green, separated by color). In four out of five of these samples, the 87Sr/86Sr ratios of the two clinopyroxene populations are isotopically different from each other. In addition to 87Sr/86Sr disequilibrium, the two clinopyroxene populations in one of the lavas (Tahaa sample TAA-B-26) have 143Nd/144Nd ratios that are ∼100 ppm different from each other. Given the resilience of clinopyroxene to seawater alteration and the likelihood that the Sr and Nd isotope composition of fresh clinopyroxene separates provides a faithful record of primary magmatic compositions, the clinopyroxene-clinopyroxene isotope disequilibrium in these four lavas provides strong evidence that a mechanism other than seawater alteration has generated the observed isotopic disequilibrium. This study confirms the isotopic diversity in ocean island lavas previously observed in olivine-hosted melt inclusions. For example, the Sr isotopic variability previously observed in olivine-hosted melt inclusions is mirrored by the isotopic diversity in clinopyroxenes isolated from many of the

Author Posting. © Nature Publishing Group, 2007. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 448 (2007): 684-687, doi:10.1038/nature06048., Substantial quantities of terrigenous sediments are known to enter the mantle at subduction zones, but little is known about their fate in the mantle. Subducted sediment may be entrained in buoyantly upwelling plumes and returned to the earth’s surface at hotspots, but the proportion of recycled sediment in the mantle is small and clear examples of recycled sediment in hotspot lavas are rare. We report here remarkably enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures (up to 0.720830 and 0.512285, respectively) in Samoan lavas from three dredge locations on the underwater flanks of Savai’i island, Western Samoa. The submarine Savai’i lavas represent the most extreme 87Sr/86Sr isotope compositions reported for ocean island basalts (OIBs) to date. The data are consistent with the presence of a recycled sediment component (with a composition similar to upper continental crust, or UCC) in the Samoan mantle. Trace element data show similar affinities with UCC—including exceptionally low Ce/Pb and Nb/U ratios—that complement the enriched 87Sr/86Sr and 143Nd/144Nd isotope signatures. The geochemical evidence from the new Samoan lavas radically redefines the composition of the EM2 (enriched mantle 2) mantle endmember, and points to the presence of an ancient recycled UCC component in the Samoan plume.

We report evidence for a global Ti, Ta, and Nb (TITAN) enriched reservoir sampled by ocean island basalts (OIBs) with high 3He/4He ratios, an isotopic signature associated with the deep mantle. Excesses of Ti (and to a lesser degree Nb and Ta) correlate remarkably well with 3He/4He in a data set of global OIBs, demonstrating that a major element signature is associated with the high 3He/4He mantle. Additionally, we find that OIBs with high 3He/4He ratios have moderately radiogenic 187Os/188Os (>0.135). The TITAN enrichment and radiogenic 187Os/188Os in high 3He/4He OIBs indicate that they are melts of a mantle domain that hosts a nonprimitive (nonchondritic) component. The observation of TITAN enrichment in the high 3He/4He mantle may be important in balancing the Earth's budget for the TITAN elements. Understanding the origin of the TITAN enrichment is important for constraining the evolution of the enigmatic high 3He/4He mantle domain.

We report 238U-230Th-226Ra-210Pb-210Po, 232Th-228Ra and 235U-231Pa measurements for a suite of 14 geologically and geochemically well-characterized basaltic samples from the Samoan volcanoes Vailulu'u, Malumalu, and Savai'i. Maximum eruption ages based on the presence of parent-daughter disequilibria indicate that Vailulu'u is magmatically productive with young lavas (<8 Ka) resurfacing both its summit crater and lower flanks. 210Pb and 210Po measurements indicate that several flows have erupted within its summit crater in the past 100 years, with the newest observed flow being erupted in November of 2004. For lavas which have eruption ages that are demonstrably young, relative to the half-lives of 230Th, 231Pa, and 226Ra, we interpret their 238U -230Th, 235U-231Pa and 230Th - 226Ra disequilibria in terms of the magmatic processes occurring beneath the Samoan Islands. (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of (238U/232Th) and (230Th/232Th) is attributed to long-term source variation. The Samoan basalts are all alkaline basalts and show significant 230Th and 231Pa excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their (230Th/238U), (231Pa/235U) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites (particularly Hawaii) which show that (230Th/238U) and (231Pa/235U) of many OIBS can be explained by simple time-independent models. Interpretation of the 226Ra data requires time-dependent melting models. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Samoan basalts. Several young samples from the Vailulu'u summit crater also exhibit significant 210Pb deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times ar

Author Posting. © The Author, 2006. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Journal of Petrology 27 (2006): 1673-1704, doi:10.1093/petrology/egl024., This paper presents field, geochemical and isotopic (Sr, Nd, Pb) results on basalts from the Antipodes, Campbell and Chatham Islands, New Zealand. New 40Ar/39Ar age determinations along with previous K-Ar dates reveal three major episodes of volcanic activity on Chatham Island (85-82, 41-35, ~5 Ma). Chatham and Antipodes samples comprise basanite, alkali and transitional basalts that have HIMU-like isotopic (206Pb/204Pb >20.3-20.8, 87Sr/86Sr <0.7033, 143Nd/144Nd >0.5128) and trace element affinities (Ce/Pb 28-36, Nb/U 34-66, Ba/Nb 4-7). The geochemistry of transitional to Q-normative samples from Campbell Island is explained by interaction with continental crust. The volcanism is part of a long-lived (~100 Myr), low-volume, diffuse alkaline magmatic province that includes deposits on the North and South Islands as well as portions of West Antarctica and SE Australia. All of the continental areas were juxtaposed on the eastern margin of Gondwanaland at >83 Ma. A ubiquitous feature of mafic alkaline rocks from this region is their depletion in K and Pb relative to other highly incompatible elements when normalized to primitive mantle values. The inversion of trace element data indicates enriched mantle sources that contain variable proportions of hydrous minerals. We propose that the mantle sources represent continental lithosphere that host amphibole/phlogopite-rich veins formed by plume and/or subduction related metasomatism between 500 and 100 Ma. The strong HIMU signature (206Pb/204Pb >20.5) is considered to be an in-grown feature generated by partial-dehydration and loss of hydrophile elements (Pb, Rb, K) relative to more magmaphile elements (Th, U, Sr) during short-term storage at the base of the lithosphere., This study was supported by National Science Foundation Grants OPP-9419686 and OPP-0003702 awarded to KSP.