Helium isotope geochemistry of oceanic volcanic rocks : implications for mantle heterogeneity and degassing

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1982The concentrations and isotopic compositions of helium have been measured in a number of mantle derived oceanic basalts. The goal of this research is to use the helium isotopic systematics to constrain the nature and origin of mantle heterogeneity in the oceanic mantle. Studies of helium partitioning in mid-ocean ridge basalt (MORB) glass, performed by crushing and melting in vacuo, show that a significant fraction of the helium resides within vesicles. Measured concentrations are therefore a function of original helium content, magmatic history, vesicle size and quantity, and grain size analyzed. The helium solubility inferred from the results is 3.7 x 10-4 cc STP/g-atm), which is significantly higher (by a factor of 5) than the enstatite value (Kirsten, 1968) most often used in the literature. Concentrations obtained from basaltic phenocrysts and glasses suggest that helium behaves as an incompatible element with respect to olivine, clinopyroxene, and plagioclase. Diffusion rates for helium in basaltic glass (in the temperature range 125-400˚C), determined using the method of stepwise heating, yielded an activation energy of 19.9 ± 1 Kcal/mole and 1nDo = -2.7 ± .6 (cgs units). Extrapolation of these results to ocean floor temperatures (0˚C) gives a diffusivity of 1.0 ± .6 x 10-17 cm2/sec, indicating that diffusion is an insignificant mechanism for helium loss from fresh basaltic glasses. The diffusion and partitioning studies suggest that these processes will not alter the helium isotopic ratios in basaltic melts. Therefore, the isotopic composition of the oceanic mantle can be inferred by extracting the helium from basaltic glasses and phenocrysts. A survey of the helium isotopic ratios in MORB glasses from all over the mid-ocean ridge system shows that there is considerable variation; the 3He/4He ratios range from 6.5 to 14.2 x atmospheric. The results from a number of oceanic island basalts show even more variability, with the 3He/4He ratios ranging from 5.0 x atmospheric (for alkali islands such as Gough and Tristan da Cunha) to 31.9 x atmospheric (for Loihi Seamount). The regional variability, and the correlations with 87Sr/86Sr can best be explained by the presence of three distinct reservoirs in the mantle which mix with one another. The three mantle source regions are believed to be 1) the depleted source for normal MORB (with 3He/4He -8.4 x atmospheric), presumed to be in the upper mantle; 2) an undepleted mantle reservoir with 3He/4He > 8.4 x atmospheric; and 3) a recycled oceanic crust reservoir with 3He/4He < 8.4 x atmospheric. A model for mantle structure that is consistent with the observations is proposed and discussed in light of the geophysical data. 3He concentrations for the different mantle reservoirs are inferred from the measurements, and suggest that the present-day 3He flux, and the 3He in MORB glasses, is ultimately derived from the lower mantle. Consideration of the 3He flux, available 3He/36Ar measurements, and the atmospheric 36Ar inventory, shows that present-day degassing rates are insufficient to generate the atmospheric argon. It is suggested that an episode of more rapid mant1e outgassing occurred in the past

Scale length of mantle heterogeneities : constraints from helium diffusion

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 269 (2008): 508-517, doi:10.1016/j.epsl.2008.03.010.A model of coupled He production/diffusion is used to constrain the question of whether Earth’s peridotitic mantle contains ubiquitous mesoscale veins or slabs of other lithologies. The high diffusion rates of helium preclude survival of He isotope heterogeneities on scales smaller than a few tens of meters, especially if they represent long term in-growth of 4He in the mantle. For 1.5 Gy residence times, and a diffusion coefficient of 10-10 m2/sec, 0.5 km slabs or 2 km cylinders will lose >90% of in-grown 4He. However, substantial 3He/4He variations may persist in slabs or be induced in adjacent mantle, depending on initial He, U and Th contents. We have modeled three cases of 3He/4He equilibration between mantle domains: an ocean crust (OC) slab in depleted upper mantle (DMM) or in enriched mantle (BSE), and a BSE slab in DMM. For a 1 km OC slab in DMM (8 Ra today), the slab today will have 3He/4He of only 3 Ra, and will have influenced the surrounding mantle with 4He for >7 km on either side. The average 3He/4He of this mixed zone will be <7 Ra, even when sampled by melts over a total width of 20-50 km. For the case of a 1 km BSE slab in DMM (8 Ra today), the slab will be 37 Ra today, and will have infected a mantle domain >16 km wide. Even with a 60 km melt sampling width, the average 3He/4He will be >15 Ra. Slabs may lose their He signature by diffusion, but their presence will be recorded in the surrounding mantle. We have evaluated 3 along-axis N-MORB ridge-crest data sets in this context (MAR 25.7-26.5°S; EPR 19-23°S; SWIR 16-24°E), with a view to defining scale-lengths of He isotope variability. The average 3He/4He variability for these 3 areas is very small, and independent of spreading rate: 0.13, 0.19 and 0.21 Ra (±1σ). Since these ridges range from ultraslow to very fast-spreading, the variability in size of along-axis magma chambers will lead inevitably to various scales of melt averaging. We conclude that these ridge areas are not sampling mantle that contains enriched veins or recycled oceanic crust slabs of significant size (> tens of meters). It appears difficult to sustain a view of the upper mantle as a ubiquitous mixture of veins and depleted matrix, with MORB always representing an averaging of this mixture.We are grateful for the consistent support of NSF that made this work possible (EAR - 0509891 to SRH; OCE - 0525864 to MDK)

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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

Monogenetic near-island seamounts in the Galapagos Archipelago

Author Posting. © American Geophysical Union, 2020. 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 21(12), (2020): e2020GC008914, https://doi.org/10.1029/2020GC008914.Rarely have small seamounts on the flanks of hotspot derived ocean‐island volcanoes been the targets of sampling, due to sparse high‐resolution mapping near ocean islands. In the Galápagos Archipelago, for instance, sampling has primarily targeted the subaerial volcanic edifices, with only a few studies focusing on large‐volume submarine features. Sampling restricted to these large volcanic features may present a selection bias, potentially resulting in a skewed view of magmatic and source processes because mature magmatic systems support mixing and volcanic accretion that overprints early magmatic stages. We demonstrate how finer‐scale sampling of satellite seamounts surrounding the volcanic islands in the Galápagos can be used to lessen this bias and thus, better constrain the evolution of these volcanoes. Seamounts were targeted in the vicinity of Floreana and Fernandina Islands, and between Santiago and Santa Cruz. In all regions, individual seamounts are typically monogenetic, but each seamount field requires multigenerational magmatic episodes to account for their geochemical variability. This study demonstrates that in the southern and eastern regions the seamounts are characterized by greater geochemical variability than the islands they surround but all three regions have (Sr‐Nd‐He) isotopic signatures that resemble neighboring islands. Variations in seamount chemistry from alkalic to tholeiitic near Fernandina support the concept that islands along the center of the hotspot track undergo greater mean depths of melting, as predicted by plume theory. Patterns of geochemical and isotopic enrichment of seamounts within each region support fine‐scale mantle heterogeneities in the mantle plume sourcing the Galápagos hotspot.This work was carried out with funding from National Science Foundation Division of Ocean Sciences (OCE‐1634952 to V. D. Wanless, OCE‐1634685 to S. A. Soule). The authors have no competing interests to declare. We thank Sally Gibson and three anonymous reviewers for providing detailed and critical feedback on this manuscript.2021-05-0

No evidence of extraterrestrial noble metal and helium anomalies at Marinoan glacial termination

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 437 (2016): 76-88, doi:10.1016/j.epsl.2015.12.040.High concentrations of extraterrestrial iridium have been reported in terminal Sturtian and Marinoan glacial marine sediments and are used to argue for long (likely 3-12 Myr) durations of these Cryogenian glaciations. Reanalysis of the Marinoan sedimentary rocks used in the original study, supplemented by sedimentary rocks from additional terminal Marinoan sections, however, does not confirm the initial report. New platinum group element concentrations, and 187Os/188Os and 3He/4He signatures are consistent with crustal origin and minimal extraterrestrial contributions. The discrepancy is likely caused by different sample masses used in the two studies, with this study being based on much larger samples that better capture the stochastic distribution of extraterrestrial particles in marine sediments. Strong enrichment of redox-sensitive elements, particularly rhenium, up-section in the basal postglacial cap carbonates, may indicate a return to more fully oxygenated seawater in the aftermath of the Marinoan snowball earth. Sections dominated by hydrogenous osmium indicate increasing submarine hydrothermal sources and/or continental inputs that are increasingly dominated by young mantle-derived rocks after deglaciation. Sedimentation rate estimates for the basal cap carbonates yield surprisingly slow rates of a few centimeters per thousand years. This study highlights the importance of using sedimentary rock samples that represent sufficiently large area-time products to properly sample extraterrestrial particles representatively, and demonstrates the value of using multiple tracers of extraterrestrial matter.We are grateful for support from a 2008 WHOI Summer Student Fellowship for CAW. BPE acknowledges financial support from WHOI’s Ocean and Climate Change Institute (CH11320) and U.S. NSF SGER grant EAR-0821878. Fieldwork in NW Canada was licensed by the Aurora Research Institute and supported by a grant to PFH from the Astrobiology Institute of the US National Aeronautics and Space Administration (NASA). Fieldwork in NW Canada and Namibia was supported by grants EAR-9905495 and EAR-0417422 (to PFH) from the US NSF. We thank Jon Husson (Harvard University) and Ricardo Trindade (University of São Paulo, Brazil) for excellent support during fieldwork in Namibia in August of 2005

Compressor Controls

Discussion Grou

Neon isotopic composition of the mantle constrained by single vesicle analyses

© 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 449 (2016): 145-154, doi:10.1016/j.epsl.2016.05.052.The origin of volatiles on Earth is still a matter of debate. Noble gases are an efficient geochemical tool to constrain Earth formation processes due to their inertness. Several studies have focused on the neon isotopic composition of the lower mantle because the 20Ne/22Ne ratio is thought to reflect that of Earth’s primordial components. Two models to explain the origin of light noble gases on Earth have been proposed: either solar wind implantation onto the Earth's solid precursors or dissolution into the mantle of a primordial atmosphere captured from solar nebula gas. In order to test these two models, we analyzed the noble gas compositions (helium, neon and argon) of two submarine oceanic island basalt glasses from Fernandina volcano (Galápagos archipelago), which have among the most primitive/unradiogenic terrestrial helium and neon isotopic compositions. Several sample pieces are studied both by step-crushing and by laser ablation analyses of single vesicles. Results of step-crushing are consistent with those of laser ablation analyses, but the latter results provide new insights into the origin of atmospheric contamination. The single-vesicle laser-ablation measurements overlap with the step crushing results, but have systematically higher 40Ar/36Ar, and 3He/36Ar, suggesting less atmospheric contamination using this method. The single vesicle data therefore suggest that atmospheric contamination is introduced by exposure to the modern atmosphere, after sample collection. 3He/4He values are about 23 times the atmospheric ratio (R/Ra) for the two Fernandina (Galápagos) samples, in agreement with previous studies. We obtain 20Ne/22Ne and 40Ar/36Ar isotopic ratios as high as 12.91 and 9400, respectively, for the mantle source of the Galápagos hotspot. The new data show that step-crushing and laser ablation analyses are complementary methods that should be used together to derive the noble gas ratios in uncontaminated samples. The results of neon compositions are consistent with previous hotspot studies and support the model of solar wind implantation associated with sputtering to explain helium and neon origins on Earth.M.D.K. acknowledges support from NSF OCE in collecting the samples and allowing his participation in this study (OCE-1259218 and OCE-1232985). M.M. acknowledges the financial support from the UnivEarthS Labex program of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02).2018-06-0

Driver Selection for Mechanical Drive Service

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Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project

Author Posting. © American Geophysical Union, 2004. 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 5 (2004): Q04G14, doi:10.1029/2002GC000439.This paper presents new magmatic helium isotopic compositions in a suite of lavas from phase II of the Hawaiian Scientific Drilling Project (HSDP2) core, which sampled Mauna Kea volcano to a maximum depth of 3098 m below sea level. Most of the measurements were performed by in vacuo crushing of olivine phenocrysts, but include submarine pillow glasses from the 2200 to 2500 meter depth interval, and orthopyroxene phenocrysts from an intrusive at 1880 m. The magmatic 3He/4He ratios range from 6 to 24.7 times atmospheric (Ra), which significantly extends the range of values for Mauna Kea volcano. The 3He/4He ratios are lowest (i.e., close to MORB values of ∼8 Ra) near the top of the Mauna Kea section and rise slowly, to 10–12 Ra, at 1000 m below sea level, consistent with results from the HSDP1 core. At depths greater than 1000 m in the core, primarily in the submarine lavas, there are brief periods when the 3He/4He ratios are higher than 14.5 Ra, always returning to a baseline value. Twelve such excursions were identified in the core; all but one are in the submarine section, and most (7) are in the deepest section, at depths of 1950 to 3070 m. The baseline 3He/4He value rises from 10–12 Ra near 1000 m depth to 12–14 Ra at 3000 m. The helium spikes are found only in lavas that are older than 380 Ka in age, based on an age model derived from Ar-Ar data (W. D. Sharp et al., manuscript in preparation, 2003). Excluding the excursions defined by single lava flows (3) and intrusive units (3), the average spike duration is approximately 15 (±9) Ka (n = 6). The high 3He/4He spikes are interpreted as pulses of magma from the center of the actively upwelling Hawaiian hot spot. The short duration of the high 3He/4He excursions suggests that Mauna Kea was never directly over high the 3He/4He component of the plume (during the HSDP2 eruptive period), presumed to be the plume center. Assuming that the Mauna Kea helium spikes result from melting of heterogeneities within the plume, their short duration implies that the length scales of heterogeneities in the solid upwelling mantle are between 60 m and 12 km (for upwelling rates of 2 to 40 cm/yr). The high 3He/4He are associated with high 208Pb/204Pb, and relatively low 143Nd/144Nd, Zr/Nb, and SiO2. The correlations with major elements, trace elements and isotopes demonstrate that helium is coupled to the other geochemical variations, and that the Mauna Kea isotopic variability is caused by heterogeneities within the upwelling plume.This work was supported by EAR/NSF through the Continental Dynamics and Instrumentation and Facilities programs