Erik H. Hauri

Mantle Plumes and Entrainment: Isotopic Evidence

Many oceanic island basalts show sublinear subparallel arrays in Sr-Nd-Pb isotopic space. The depleted upper mantle is rarely a mixing end-member of these arrays, as would be expected if mantle plumes originated at a 670-kilometer boundary layer and entrained upper mantle during ascent. Instead, the arrays are fan-shaped and appear to converge on a volume in isotopic space characterized by low 87Sr/86Sr and high 143Nd/144Nd, 206Pb/204Pb, and 3He/4He ratios. This new isotopic component may be the lower mantle, entrained into plumes originating from the core-mantle boundary layer.

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle

The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earths mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-ocean-ridge basalt. The incompatible behaviour of carbon dioxide, together with the CO2/Nb and CO2/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.

Experimental and natural partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene and basaltic melts

Abstract Partition coefficients for Th, U, Pb, rare-earth elements (REE), high field strength elements (HFSE), alkaline-earth elements, Sc, Cr, V and K were measured by ion microprobe techniques in two experiments on a natural high-alumina basalt composition from Medicine Lake, California. All elements were measured at natural abundance levels except Th, U and Pb, which were each present in the starting mix at 1-wt% levels. The results show that garnet retains U preferentially over Th ( D gt melt U = 0.0059 , D gt melt Th = 0.0014 ), while clinopyroxene shows the opposite sense of partitioning ( D cpx melt U = 0.0127 , D cpx melt Th = 0.014 ). The experimental Th, U and Pb partition coefficients for garnet-melt and cpx-melt are consistent with garnet-cpx pairs from garnet-bearing ultramafic rocks which exhibit U-Pb isochrons, thus demonstrating equilibrium ( D gt cpx U = 0.30 , D gt cpx Th = 0.072 , D gt cpx Pb = 0.016 ). The partition coefficients for Th and U between clinopyroxene and basaltic melt vary systematically as a function of the tetrahedral Al content of clinopyroxene. Garnet/melt values for Th, U and Pb agree with previous determinations, indicating that mid-ocean ridge basalt (MORB) generation begins in the stability field of garnet lherzolite. However, high 226 Ra 230 Th ratios in MORB require very small porosities near the region where the melts lose chemical equilibrium with the mantle. Partitioning data for HFSE and REE suggest that this region of melt segregation is not in the spinel lherzolite field. This requires either rapid transport of MORB magmas from ϵ 70 km, or some degree of disequilibrium during melt generation and/or transport.

ReOs isotope systematics of HIMU and EMII oceanic island basalts from the south Pacific Ocean

The ReOs and complementary Sr, Nd and Pb systematics of 24 oceanic island basalts from the islands of Savaii, Tahaa, Rarotonga, Rurutu, Tubuai and Mangaia are investigated. Re concentrations range from 100 to 1621 ppt (parts per trillion), while Os concentrations vary from 26 to 750 ppt. The Re and Os concentration variations suggest that fractionation and accumulation of olivine, or a lowRe/Os phase in conjunction with olivine, is important in determining the Os concentration and theRe/Os ratio of the erupted basalt. 187Os186Os in EMII basalts from Samoa and Tahaa varies from 1.0261 to 1.1275. These ratios are mostly within estimates for depleted upper mantle, and do not constrain the involvement of recycled continental crust in the origin of the EMII signature.187Os186Os ratios in HIMU basalts from Rurutu, Tubuai and Mangaia range from 1.1159 to 1.2473, and provide strong evidence for the role of subducted oceanic crust in the HIMU source. The PbPb systematics constrain the range of possible ages and238U204Pb andTh/U ratios of the subducted crust; this crust is estimated to pass through the subduction zone withRb/Sr,Sm/Nd,Lu/Hf andTh/U ratios similar to fresh MORB. The homogeneity of the Os isotopic compositions in the Tubuai and Mangaia basalts indicates that interaction of these basalts with low187Os186Os mantle had an insignificant effect on the Os isotopic composition of the erupted magmas. This requires a network of channels, veins or cracks capable of delivering melt from the source region (plume) to the surface fast enough to avoid interaction with the depleted upper mantle and the oceanic lithosphere. The possible identification of the HIMU signature (high206Pb204Pb, low87Sr86Sr) with recycled oceanic crust suggests the possible presence of segments of recycled crust, with independent histories, in other oceanic mantle sources, including that of some mid-ocean ridge basalts.

Osmium-isotope variations in Hawaiian lavas: evidence for recycled oceanic lithosphere in the Hawaiian plume

Isotopic heterogeneity in Hawaiian shield lavas reflects the presence of two distinct recycled components in the Hawaiian plume, both from the same packet of recycled oceanic lithosphere. Radiogenic Os-isotopes and anomalously heavy oxygen-isotopes in Koolau lavas reflect melt generation from recycled oceanic crust plus pelagic sediment. In contrast, Kea lavas have unradiogenic Os-isotopes but anomalously light oxygen-isotopes. Oxygen–osmium–lead isotope correlations preclude generation of the Kea isotopic signature from asthenospheric upper mantle or the in situ lithospheric mantle or crust. Instead, melting of recycled, hydrothermally altered ultramafic lower crust or lithospheric mantle in the Hawaiian plume can produce Kea-type lavas. The preservation of both upper- and lower-crustal oxygen isotope signatures in plume-derived Hawaiian lavas indicates that chemical heterogeneities with length scales of only a few kilometers can be preserved in the convecting mantle for long periods of time, probably on the order of 1 Ga or more.

Volatile content of lunar volcanic glasses and the presence of water in the Moon’s interior

The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO2, H2O, F, S and Cl) contents of the most primitive basalts in the Moon—the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon’s formation and its thermal and chemical evolution.

Fluid dynamic and geochemical aspects of entrainment in mantle plumes

A similarity solution has been developed for vertical, steady-state mantle plume conduits by considering the boundary layer flow emanating from a point source of heat in an axisymmetric geometry. This model includes the effects of temperature and shear stress on viscosity, and incorporates depth-dependent viscosity and thermal expansivity. Plumes with variable viscosity have upward velocities of 0.30–100 m/yr and radii of 30–250 km, depending on temperature, rheology and buoyancy flux. These results demonstrate the small lateral scale of plume features relative to the resolution of most large-scale convection models and seismological studies. All of the plumes studied showed significant entrainment of ambient mantie surrounding the plume conduit, driven by the radial conduction of heat from the plume. This heat raises the buoyancy and lowers the viscosity of the ambient mantel, thereby entraining it into the conduit flow. For buoyancy fluxes of 0.1–10 Mg/s, similar to the range estimated for plumes in the Earths mantle, we calculate a range of entrainment of >90% to <5% ambient mantle, correlating negatively with buoyancy flux. Examination of the streamlines of mantle material which is entrained into thermal plumes indicates that most of the entrained fraction originates from approximately the lower half of the layer traversed by the plume, and shows minor entrainment of upper level material. This is especially true for non-Newtonian and depth dependent rheologies, and for depth-dependent thermal expansivity. Upwelling of depleted upper mantle, viscously coupled to the plume flow, is proposed as a mechanism for generating post-shield stage alkalic basalts erupted on oceanic island chains and their associated flexural arches. The existing Sr-Nd-Pb isotope data for oceanic basalts indicate the presence of a component which is common to hotspot basalts worldwide, and which is distinct from the upper mantle source of mid-ocean ridge basalts. This component (termed “FOZO” by Hart et al. (1992)) has moderately depleted Sr and Nd signatures, radiogenic Pb isotopes, and elevated 3He/4He ratios. The high He isotope ratios of FOZO suggest an origin for this component in the lower mantle, and would appear to provide independent evidence to support the fluid dynamic observations for significant entrainment of lower mantle in plumes and exclusion of upper mantle. If the composition of FOZO is representative of the isotopic composition of the lower mantle, then it would appear that this reservoir has been differentiated relative to estimates for the bulk silicate earth (BSE). This may be due either to melting and differentiation at higher levels in the mantle, or to fractionation of high pressure phases from a terrestrial magma ocean.

SIMS analysis of volatiles in silicate glasses, 2: isotopes and abundances in Hawaiian melt inclusions

Ion microprobe measurements of the concentrations of H2O, CO2, F, S and Cl and the isotopic composition of hydrogen are reported for populations of olivine-hosted melt inclusions from five lava samples from the Hawaiian volcanoes Loihi, Kilauea, Mauna Loa and Koolau. After reheating of the melt inclusions and correction for the effects of post-entrapment modification, the melt inclusions have MgO contents ranging from 8.9% to 15.1% and averaging 11%, significantly higher in MgO than most submarine glasses. The melt inclusions show large ranges in H2O (0.03–0.84 wt.%), CO2 (5–862 ppm), F (308–1000 ppm), S (156–3330 ppm) and Cl (8 ppm to 1.11 wt.%), accompanied by large ranges in δD (−165‰ to +40‰). Laboratory reheating experiments on Loihi inclusions show that diffusive loss of hydrogen can occur from olivine-hosted melt inclusions on hour- to day-long time scales via proton diffusion through olivine, with consequent positive shifts in the D/H ratios of the residual hydrogen in the melt inclusion. Most melt inclusions from subaerial Kilauea and Mauna Loa samples have signatures of low H2O (0.05–0.2 wt.%) and high δD (up to +40%) compared to published analyses of submarine glasses, suggesting diffusive H loss during slow cooling of inclusions shortly after eruption. Koolau melt inclusions have the lowest δD values yet measured in oceanic basalts (−61‰ to −165‰). Shallow-level degassing produces a H2O–δD relationship in most Koolau melt inclusions which can be explained by open-system (Rayleigh) degassing with a vapor-melt D/H fractionation factor of 1.024, similar to previous estimates. Shallow degassing is also indicated in some inclusions by parallel depletions in H2O and S, but degassed melt inclusions from all volcanoes display a wide range in CO2 concentrations, indicating kilometer-scale vertical convection of melts within Hawaiian magma reservoirs. The measured δ13C of three CO2-bearing melt inclusions from Koolau volcano are depleted (−12‰ to −29‰) and correlated with δD, possibly consistent with open-system degassing of CO2-rich magmas and subsequent mixing with less-degassed magmas. Assimilation of seawater-derived components is indicated in a small number of melt inclusions which exhibit high Cl and Cl/K ratios, with an extreme example from Loihi (1.11 wt.% Cl, 0.48 wt.% H2O, δD=−118‰). A subset of melt inclusions have escaped the confounding effects of H diffusion, shallow degassing and crustal contamination, and provide evidence for heterogeneity of D/H ratios in the Hawaiian mantle, which appear to correlate with published radiogenic (Sr, Nd, Pb, Os) and oxygen isotope data. If the apparent Hawaiian δD variability can be confirmed to be a source signature, then the Hawaiian D/H data indicate that heterogeneities within the Hawaiian plume are required to be large in scale (tens of kilometers) and/or young in age (<1 Ga) in order to preserve hydrogen isotopic variability against the extremely rapid diffusivity of hydrogen in the mantle.

SIMS analysis of volatiles in silicate glasses 1. Calibration, matrix effects and comparisons with FTIR

This paper describes microanalysis techniques using secondary ion mass spectrometry (SIMS) to measure the abundances and isotopic compositions of hydrogen, carbon, fluorine, sulfur and chlorine in volcanic glasses. SIMS measurement of total H_2O and total CO_2 abundances compare very well with measurements on the same glasses using vibrational spectroscopy techniques (FTIR). A typical 10-min SIMS measurement for volatile abundances is made on a singly polished specimen, sputtering a crater 15–30 μm in diameter and 2–3 μm deep, utilizing 1–5×10^(−9) g of sample material. Detection limits are routinely <30 ppm H_2O, <3 ppm CO_2, and <1 ppm F, S and Cl. Measurements of δD, δ^(13)C and δ^(34)S in volcanic glasses are currently reproducible and accurate to 2–5‰, depending on the concentration of the element. Because of their spatial selectivity, the SIMS methods allow resolution of magmatic volatile signatures from those carried by secondary phases, which can sometimes plague traditional vacuum extraction methods that require large amounts of sample (tens to hundreds of milligrams). Ease of sample preparation, rapid analysis and high sensitivity allow SIMS to be applied to volatile analysis of small samples such as melt inclusions, in which large numbers of individual analyses are often required in order to obtain a representative sample population. Combined abundance and isotopic composition data for volatile elements provide coupled constraints on processes relevant to magma genesis and evolution, including degassing, magma contamination, mixing, and source variability.