Hydrogen concentration analyses using SIMS and FTIR: Comparison and calibration for nominally anhydrous minerals (original) (raw)
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Chemical Geology, 2009
To extend our knowledge of partitioning of hydrogen between nominally anhydrous mantle minerals and basaltic melts to higher pressures, we conducted 8 experiments at 3–5 GPa and 1350–1440 °C on a modified mid-ocean ridge basalt (MORB) + 2 wt.% H2O. H contents in olivine, orthopyroxene, clinopyroxene, and garnet were measured by counting 16O1H ions with low-blank SIMS, and total H2O concentrations in coexisting glasses were measured by FTIR, SIMS, and electron microprobe. The glasses have water contents ranging from 4.1 to 8.9 wt.%. Eleven measured pyroxenes have H2O concentrations ranging from 430 to 1430 ppm, two garnets have concentrations of 73 and 260 ppm, and one olivine has a concentration of 29 ppm. Mineral/melt hydrogen partition coefficients are as follows: DHcpx/melt ranges from 0.014–0.021, DHopx/melt ranges from 0.009–0.019, DHgt/melt ranges from 0.001–0.0033, and DHol/melt has a value of 0.0006. Mineral/mineral hydrogen partitioning data are as follows: DHcpx/opxranges from 1.2–2, DHcpx/gt has values of 5.5 and 14, and a single DHcpx/ol has a value of 27. DHpyx/melt correlates with the concentration of Al2O3 in pyroxene and with the concentration of Al on tetrahedral sites. Combining the mineral/melt hydrogen partitioning data from this study, and estimates of mineral modes and compositions along the solidus, we estimate that DHperid/melt increases from 0.006 at 1 GPa up to 0.01 near the garnet lherzolite/spinel lherzolite transition, owing to the maximum in Al2O3 concentration in pyroxenes. With increasing pressure DHperid/melt for garnet lherzolite diminishes to 0.005 at 5 GPa, as pyroxenes become less aluminous and less abundant. As a result, incipient hydrous melts beneath oceanic ridges with bulk mantle H2O contents of 50–200 ppm will have solidus partial melts with H2O contents between 0.5 and 3.8 wt.%, while oceanic island mantle with 300–1000 ppm will have solidus partial melts with H2O content between 3 and 20 wt.%.
Molecular hydrogen in minerals as a clue to interpret ∂D variations in the mantle
Nature Communications
Trace amounts of water dissolved in minerals affect density, viscosity and melting behaviour of the Earth’s mantle and play an important role in global tectonics, magmatism and volatile cycle. Water concentrations and the ratios of hydrogen isotopes in the mantle give insight into these processes, as well as into the origin of terrestrial water. Here we show the presence of molecular H2 in minerals (omphacites) from eclogites from the Kaapvaal and Siberian cratons. These omphacites contain both high amounts of H2 (70 to 460 wt. ppm) and OH. Furthermore, their ∂D values increase with dehydration, suggesting a positive H isotope fractionation factor between minerals and H2–bearing fluid, contrary to what is expected in case of isotopic exchange between minerals and H2O-fluids. The possibility of incorporation of large quantities of H as H2 in nominally anhydrous minerals implies that the storage capacity of H in the mantle may have been underestimated, and sheds new light on H isotope...
Molecular hydrogen in mantle minerals
Geochemical Perspectives Letters, 2016
Current models assume that hydrogen was delivered to Earth already in oxidised form as water or OH groups in minerals; similarly, it is generally believed that hydrogen is stored in the present mantle mostly as OH. Here we show by experiments at 2-7 GPa and 1100-1300 °C that, under reducing conditions, molecular hydrogen (H 2) has an appreciable solubility in various upper mantle minerals. This observation suggests that during the accretion of the Earth, nebular H 2 could have been delivered to the growing solid planet by direct dissolution in a magma ocean and subsequent incorporation in silicates. Moreover, the presence of dissolved molecular H 2 in the minerals of the lower mantle could explain why magmas sourced in this region are rich in hydrogen, despite the fact that lower mantle minerals contain almost no OH groups.
Earth and Planetary Science Letters, 1991
An ion probe analytical procedure has been established to measure in-situ D/H ratios in hydroxylated minerals on the scale of a few tens of #m with a precision of _+10%0. This technique has been applied to determine the D/H ratio of amphiboles occurring as rare disseminated minerals in peridotites. These amphiboles include pargasite from the Lherz peridotite, Pyrdndes (France), and kaersutite and pargasite in ultramafic xenoliths and megacrysts from the Massif Central (France), Nunivak Island (Alaska) and Salt Lake Crater (Hawaii). The 6D value measured in Lherz (-92 and -65) and Nunivak Island (-93 to -69) are in the accepted range for upper mantle minerals. The higher 6D (-59 to -28) from the Massif Central xenoliths may involve contamination of the subcontinental lithosphere by subduction of seawater-altered oceanic crust. The very low 6D (down to -125) observed for Hawaiian xenoliths could reflect the presence of a previously unidentified deep hydrogen component in the mantle. For the Massif Central and Salt Lake Crater, 8DsMow variations up to 70%~ were measured both on the scale of single crystals (< 200 /~m) and among crystals of the same sample. Chemical variations are minor and not correlated with 8D. Combining the diffusion data for hydrogen in amphiboles with these results implies that the time interval between the onset of exchange and volcanism was shorter than a few hundred years.
Geochimica et Cosmochimica Acta, 2009
The present study illustrates the interest of using the elastic recoil detection analysis (ERDA) method to characterize any geological sample matrix with respect to hydrogen. ERDA is combined with Rutherford back scattering (RBS) and particle induced X-ray emission (PIXE), allowing the simultaneous characterization of the matrix with respect to major and trace elements (Z > 15). Analyses are performed by mapping of a 4 Â 16 lm 2 incident beam of 4 He + on large areas (50 Â 200 lm 2 ). The method is almost not destructive and requires no calibration with respect to well known hydrous samples. Hydrous and nominally anhydrous phases in contact with each other in the same sample may both be characterized. The depth of the analyses is limited to several lm beneath the surface, allowing tiny samples to be investigated, provided their sizes are larger than the incident beam. Our setup has been improved in order to allow H determination on a micrometric scale with a 5-15% relative uncertainty and a detection limit of 94 wt ppm H 2 O. We present multi-elemental mappings on a large panel of samples: (1) natural and analogue synthetic glasses from Stromboli volcano (0.44-4.59 wt% H 2 O), natural rhyolitic glasses (1466-1616 wt ppm H 2 O); (2) magmatic rhyolitic melt inclusions from Guadeloupe Island (4.37-5.47 wt% H 2 O) and their quartz host crystal (2020 ± 230 wt ppm H 2 O); (3) nominally anhydrous natural (82-260 wt ppm H 2 O) and experimentally hydrated (240-790 wt ppm H 2 O) olivines; natural clinopyroxenes (159-716 wt ppm H 2 O); natural orthopyroxenes (201-452 wt ppm H 2 O); a natural garnet (90 wt ppm H 2 O)
Rapid Communications in Mass Spectrometry, 2017
We have used ahigh-precision, high efficiency method for themeasurementof the 2 H/ 1 H ratios of hydrous silicates (amphiboles) and nominally anhydrous minerals (NAM) such as clinopyroxene, garnet and diamond, which are usually extremely resistant to pyrolysis. This opens up new fields of investigation to better understand the conditionsof formation fordeep-Earth minerals. METHOD: The technique described hereinvolvesIsotopic Ratio Mass Spectrometry (IRMS) online in continuous flow mode with an Elemental Analyzer (EA) using-purge and trap‖ technology rather than conventional packed column-GC gas separation. The system is equipped with a special high temperature furnace reaching 1500°C, with a longer hot zone and improved temperature stability. Emphasis is put on the efficiency of the system to reliably pyrolyserefractory minerals difficult to analyse with other conventional systems. RESULTS:While conventional systems usually fail to generate hydrogen suitable for isotopic analyses, with the technique presented here we were able to measure 2 H/ 1 H ratios from 4diamond samples ( 2 H=-60,-77,-84 and-79‰ V-SMOW; average SD =4.5‰; n=2),3 garnet samples ( 2 Hfrom-70 to-63‰), and 9 clinopyroxenes ( 2 H from-92 to-58‰) associated with7 amphiboles ( 2 Hfrom-76 to-27‰) from single mantle rock. CONCLUSIONS:The possibility of using such a system to reliably measure 2 H/ 1 H ratios from refractory minerals, which are usually extremely difficult to analyse, offers a new tool of investigation for giving us unrivaled clues to study the Earth's deep interiors.
Hydrogen incorporation in natural mantle olivines
Geophysical Monograph Series, 2006
Constraints on water storage capacity and actual content in the mantle must be derived not only from experimental studies, but also from investigation of natural samples. Olivine is one of the best-studied, OH-bearing "nominally anhydrous" minerals, yet there remain multiple hypotheses for the incorporation mechanism of hydrogen in this phase. Moreover, there is still debate as to whether the mechanism is the same in natural samples vs. experimental studies, where concentrations can reach very high values (up to ~o.6 wt% H 2 0) at high pressures and temperatures. We present new observations and review IR and TEM data from the literature that bear on this question. Hydrogen incorporation in natural olivine clearly occurs by multiple mechanisms, but in contrast to some previous assertions we find that there are strong similarities between the IR signatures of experimentally annealed olivines and most natural samples. At low pressures (lower than ~2 GPa) in both experiments and natural olivines, hydrogen incorporation might be dominated by a humite-type defect, but the nature of the defect may vary even within a single sample; possibilities include point defects, planar defects and optically detectable inclusions. IR bands between 3300 and 3400 cm-1 , ascribed previously to the influence of silica activity, are apparently related instead to increased oxygen fugacity. At higher pressures in experiments, the IR band structure changes and hydrogen is probably associated with disordered point defects. Similar IR spectra are seen in olivines from xenoliths derived from deeper parts of the mantle (below South Africa and the Colorado Plateau) as well as in olivines from the ultra-high pressure metamorphic province of the Western Gneiss Region in Norway.
American Mineralogist, 2019
Small amounts of H 2 O, on the order of tens to hundreds of parts per million, can significantly influence the physical properties of mantle rocks. Determining the H 2 O contents of nominally anhydrous minerals (NAMs) is one relatively common technique that has been applied to estimate mantle H 2 O contents. However, for many mantle NAMs, the relation between H 2 O activity and H 2 O content is not well known. Furthermore, certain mantle minerals may be prone to H 2 O loss during emplacement on Earth's surface. The goal of this study is to apply mineral equilibria to estimate values of a H2O in rocks that originated below the Moho. The chemical compositions of olivine + orthopyroxene + clinopyroxene + amphibole + spinel ± garnet were used to estimate values of temperature (T), pressure (P), a H2O , hydrogen fugacity (f H2), and oxygen fugacity (f O2) in 11 amphibole-bearing mantle xenoliths from the southwestern U.S.A. Application of amphibole dehydration equilibria yields values of a H2O ranging from 0.05 to 0.26 for these 11 samples and the compositions of coexisting spinel + olivine + orthopyroxene yield ∆logf O2 (FMQ) of-1 to +0.6. For nine of the samples, values of f H2 were estimated using amphibole dehydrogenation equi-libria, and these values of f H2 ranged from 6 to 91 bars. Values of f H2 and f O2 were combined, using the relation 2H 2 O = 2H 2 + O 2 , to estimate a second value of a H2O that ranged from 0.01 to 0.57 for these nine samples. Values of a H2O , estimated using these two methods on the same sample, generally agree to within 0.05. This agreement indicates that the amphibole in these samples has experienced little or no retrograde H-loss and that amphibole equilibria yields robust estimates of a H2O that, in these xenoliths, are generally <0.3, and are often 0.1 or less.
Postmelting hydrogen enrichment in the oceanic lithosphere
Science Advances, 2021
The large range of H 2 O contents recorded in minerals from exhumed mantle rocks has been challenging to interpret, as it often records a combination of melting, metasomatism, and diffusional processes in spatially isolated samples. Here, we determine the temporal variations of H 2 O contents in pyroxenes from a 24-Ma time series of abyssal peridotites exposed along the Vema fracture zone (Atlantic Ocean). The H 2 O contents of pyroxenes correlate with both crustal ages and pyroxene chemistry and increase toward younger and more refractory peridotites. These variations are inconsistent with residual values after melting and opposite to trends often observed in mantle xenoliths. Postmelting hydrogen enrichment occurred by ionic diffusion during cryptic metasomatism of peridotite residues by low-degree, volatile-rich melts and was particularly effective in the most depleted peridotites. The presence of hydrous melts under ridges leads to widespread hydrogen incorporation in the oceanic lithosphere, likely lowering mantle viscosity compared to dry models.