Jed L. Mosenfelder

Rapid emplacement of the Oman ophiolite: Thermal and geochronologic constraints

In understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting, and the rate and duration of that thrusting. Hornblende 4oAr/39Ar ages demonstrate that the igneous oceanic crust in Oman crystallized and cooled to -825 K in 1-2 m.y. Hornblende ages from metamorphic rocks and from cross-cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below -825 K in 1-2 m.y. Motion along the sole thrust accounted for 200 km of displacement at a rate of 100-200 mm/yr. On the basis of age relationships and thermal considerations, we favor a two- stage model for the initial stages of Samall ophiolite emplacement: intraoceanic thrusting over < 2-m.y.-old lithosphere at 150 km/m.y. parallel to a spreading ridge for 1-2 m.y., followed by equally rapid and brief thrusting over cold and old lithosphere. Preservation of the Samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure, and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting.

Hydrogen incorporation in olivine from 2–12 GPa

Abstract We performed new experiments on incorporation of hydrogen in olivine at high pressures (2-12 GPa) and temperatures (1000-1300 °C). OH concentrations were calculated using the Bell et al. (2003) calibration applied to principal-axis infrared absorption spectra synthesized from polarized measurements on randomly oriented grains. Starting materials for the experiments included both fine-grained powders and larger single crystals. Hydrogen was incorporated during grain growth in the former case and by volume diffusion in the latter. The spectra of Fe-bearing olivines exhibit similar structure regardless of the starting material, and are dominated by bands in the wavenumber range from about 3500 to 3650 cm-1. We do not observe bands at 3525 and 3573 cm-1, which are predominant in many natural olivines as well as olivines annealed in experiments at lower pressures, and are attributed to humite-related defects. Furthermore, bands between 3300 and 3400 cm-1, attributed to high silica activity or high oxygen fugacity, are weak or non-existent. Our measurements indicate that OH solubility in Fe-bearing olivine is 2.5-4 times higher than that measured by Kohlstedt et al. (1996). Although this is largely due to the use of a new calibration in our study, correction of previous values is not straightforward. In the pure Mg-system, in contrast to Fe-bearing olivine, order-of-magnitude apparent differences in OH solubility can be obtained using different experimental procedures. This raises questions about attainment of equilibrium in experimental studies of hydrogen incorporation in nominally anhydrous minerals, particularly when crystals are grown from a hydrous melt.

Thermodynamic properties of Mg2SiO4 liquid at ultra-high pressures from shock measurements to 200 GPa on forsterite and wadsleyite

Polycrystalline samples of Mg_2SiO_4 forsterite and wadsleyite were synthesized and then dynamically loaded to pressures of 39–200 GPa. Differences in initial density and internal energy between these two phases lead to distinct Hugoniots, each characterized by multiple phase regimes. Transformation to the high-pressure phase assemblage MgO + MgSiO_3 perovksite is complete by 100 GPa for forsterite starting material but incomplete for wadsleyite. The datum for wadsleyite shocked to 136 GPa, however, is consistent with the assemblage MgO + MgSiO_3 post-perovksite. Marked increases in density along the Hugoniots of both phases between ∼130 and 150 GPa are inconsistent with any known solid-solid phase transformation in the Mg_2SiO_4 system but can be explained by melting. Density increases upon melting are consistent with a similar density increase observed in the MgSiO_3 system. This implies that melts with compositions over the entire Mg/Si range likely for the mantle would be negatively or neutrally buoyant at conditions close to the core-mantle boundary, supporting the partial melt hypothesis to explain the occurrence of ultra-low velocity zones at the base of the mantle. From the energetic difference between the high-pressure segments of the two Hugoniots, we estimate a Gruneisen parameter (γ) of 2.6 ± 0.35 for Mg_2SiO_4-liquid between 150 and 200 GPa. Comparison to low-pressure data and fitting of the absolute pressures along the melt Hugoniots both require that γ for the melt increases with increasing density. Similar behavior was recently predicted in MgSiO_3 liquid via molecular dynamics simulations. This result changes estimates of the temperature profile, and hence the dynamics, of a deep terrestrial magma ocean.

Analysis of hydrogen in olivine by SIMS: Evaluation of standards and protocol

Abstract We measured hydrogen concentrations in 12 olivines using secondary ion mass spectrometry (SIMS and NanoSIMS), cross-calibrated against Fourier transform infrared (FTIR) spectroscopy and nuclear reaction analysis (NRA). Five of these samples are routinely used for calibration in other laboratories. We assess the suitability of these olivines as standards based on over 300 SIMS analyses, comprising 22 separate calibrations. Seven olivines with 0-125 ppm H2O give highly reproducible results; in contrast to previous studies, the data are fit to well-constrained calibration lines with high correlation coefficients (r2 = 0.98-1). However, four kimberlitic megacrysts with 140-245 ppm H2O sometimes yield 16O1H/30Si ratios that have low internal precision and can vary by up to a factor of two even in sequential analyses. A possible cause of this behavior is the presence of sub-microscopic inclusions of hydrous minerals, such as serpentine. In most cases, however, we link the anomalous results to the presence of sub-micrometer to micrometer-scale pores (as small as 100 nm), which we imaged using SEM and NanoSIMS. These pores are interpreted to be fluid inclusions containing liquid H2O, other volatiles (including fluorine), and/or hydrous phase precipitates. Ionization of the contents of the pores contributes variably to the measured 16O1H, resulting in analyses with erratic depth profiles and corresponding high uncertainties (up to 16%, 2σmean). After filtering of these analyses using a simple criterion based on the error predicted by Poisson counting statistics, all the data fit well together. Our results imply that the Bell et al. (2003) calibration can be applied accurately to all olivines with IR bands from -3400-3700 cm-1, without the need for band-specific IR absorption coefficients.

Quantitative polarized infrared analysis of trace OH in populations of randomly oriented mineral grains

Abstract Use of infrared spectroscopy as an accurate, quantitative method to measure concentrations of hydrous species in minerals requires consideration of the interactions of anisotropic crystals with infrared light. Ensuring that contributions are identified from species at all orientations in the crystal requires combining three measurements, taken with the electric field polarized along three mutually perpendicular directions. This is typically accomplished by determining the orientation of a crystal in advance, and then sectioning it perpendicular to its principal axes. In many instances, however, natural or experimental samples are not suitable for such handling. Here we demonstrate a method that instead uses at least three randomly sectioned grains, considered to be multiple samples of a homogeneous population. We explain the theory whereby: (1) the orientations of the polarization vectors of measurements taken on these grains are determined by comparison to oriented standards of the same mineral, and (2) the principal-axis spectra of the sample are synthesized from the randomly oriented spectra. By comparison to complementary electron backscatter diffraction (EBSD) data, we demonstrate that determination of orientations using the silicate overtone bands in Fourier-Transform infrared (FTIR) spectra is accurate and precise, with typical angular errors of 6°. We show that this precision is sufficient for the synthetic principal-axis spectra to be essentially indistinguishable from X-ray oriented standard spectra. We demonstrate the application of this technique to determining the OH concentrations in a population of hydrated olivine grains recovered from a high-pressure, high-temperature multi-anvil experiment.

Factors in the preservation of coesite: The importance of fluid infiltration

Abstract The survival of coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite. We used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in coesite are below the detection limit of ~100 ppm H2O. The silica phases surrounding coesite, however, show varying amounts of H2O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (<100 ppm H2O, red-violet CL); .mosaic. quartz, which is actually chalcedony (up to 0.4 wt% H2O, yellow/brown CL); and a rare, highly hydrated silica phase interpreted to be opal (~7 wt% H2O, dark blue CL). Very similar signatures are observed in a grospydite xenolith from the Roberts Victor kimberlite. The quartz replacing coesite in other UHP samples studied contains on the order of 500 ppm H2O or less, and most measurements are under the detection limit of our technique. We infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, after cracking of the host phase, which allows external fluids to infiltrate. Delay of fluid infiltration to low temperatures, where kinetics are slow even in the presence of water, is the most critical factor in the preservation of coesite.

Metamorphism and deformation along the emplacement thrust of the Samail ophiolite, Oman

Abstract The base of the Samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement. The classic exposure of this metamorphic sole at Wadi Tayin consists of a few meters of garnet-clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite, overlying 150 m of lower amphibolite facies mafic and sedimentary schist. The sole is bounded by faults but is internally coherent. Peak metamorphic temperatures decrease exponentially, from ∼ 825°C at the top of the sole to ∼ 500°C at its base, as documented by changes in grain size, hornblende composition, the disappearance of chlorite and epidote, garnet-clinopyroxene element partitioning, and garnet-hornblende partitioning. The inverted peak temperature gradient decreases from > 4000°C/km in the upper 50 m to ∼ 100°C/km in the lower 150 m. Contrary to earlier reports, greenschist-facies tectonites are not present at Wadi Tayin, although the entire sole underwent greenschist-facies retrogression, driven by the influx of fluids from the overlying peridotite or underlying carbonate. Calculations based on an amphibolite constitutive relation reveal that, although the section represents a lithologic continuum, the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column.

Analysis of hydrogen and fluorine in pyroxenes

Abstract Studies of coexisting, nominally anhydrous minerals in mantle samples show that clinopyroxene is an especially important host for hydrogen. Recent experimental studies have also shown that clinopyroxene may contain significant amounts of fluorine, which has implications for the F budget of the mantle. More accurate quantification of H and F is therefore a desirable goal. We measured H in 13 natural clinopyroxenes using Fourier transform infrared (FTIR) spectroscopy. 16O1H/30Si and 19F/30Si were also measured in the samples using secondary ion mass spectrometry (SIMS). H data were compared between the two techniques and F was calculated with reference to F-bearing silicate glass standards. Four of the clinopyroxenes are used as standards for SIMS calibration in multiple laboratories, and three have been measured previously using hydrogen manometry and/or elastic recoil detection analysis. Compared to clinopyroxenes in previous surveys comparing FTIR and SIMS, the 13 samples cover a broader range in chemistry and band positions in the O-H vibrational spectrum. They also all lack detectable amphibole lamellae, which are otherwise commonly present in this mineral group. In contrast to orthopyroxene, the SIMS and FTIR data for clinopyroxene show significantly better correlations (r2 = 0.96-0.98) when the frequency-dependent IR calibration of Libowitzky and Rossman (1997) is applied, as opposed to the Bell et al. (1995) calibration (r2 = 0.92-93). We derive a frequency-dependent molar absorption coefficient with parameters different from those of Libowitzky and Rossman’s calibration, which was established using data on stoichiometric hydrous phases and gives poor agreement with the manometrically determined value for PMR-53. Comparison of data for PMR-53 to our SIMS calibrations for orthopyroxene and olivine suggests that the matrix effect among these phases is less than 20% relative. Fluorine concentrations vary depending on geological context, with the highest concentrations (up to 214 ppm) found in diopsides from crustal metamorphic environments. Mantle samples follow similar geographic trends as olivines and orthopyroxenes, with higher F in xenocrysts from Kilbourne Hole (46 ppm) and South African kimberlites (up to 29 ppm) compared to the Colorado Plateau (8 ppm). On the basis of chemical correlations, we propose two different incorporation mechanisms for F: (1) coupled subsititution with Al3+ and/or Fe3+ in tetrahedral sites; and (2) coupled substitution with monovalent cations (Na and K) in the M2 site. The second substitution is more relevant to mantle augites than crustal diopsides. Our measured F concentrations are much lower than those in some clinopyroxenes synthesized in recent high P-T studies. Nevertheless, our data support suggestions that the F budget of the mantle can be entirely accommodated by incorporation in nominally anhydrous/fluorine-free minerals.

Analysis of hydrogen and fluorine in pyroxenes: I. Orthopyroxene

Abstract Pyroxenes have the capacity to incorporate both hydrogen and fluorine in their structures, and accurate measurement of these volatile elements can be used to constrain geophysical and petrologic processes in planetary bodies. For example, pyroxenes may be used to constrain the volatile contents of melts from which they crystallized. However, the experimental determination of H and F in pyroxenes is difficult, particularly at the relatively low levels present in natural samples. Here we evaluate methods for determining both H and F in orthopyroxene. We measured trace concentrations of H (~40-400 ppm H2O) and F (<1-17 ppm) in a suite of nine orthopyroxenes from varying geological environments, using secondary ion mass spectrometry (SIMS). The SIMS data for H (measured as 16O1H, referenced to 30Si and 18O) are cross calibrated against Fourier transform infrared (FTIR) spectra, in turn calibrated against either manometry (Bell et al. 1995) or the frequency-dependent molar absorption coefficient derived by Libowitzky and Rossman (1997). Despite the fact that our samples exhibit a wide range of IR band structures, with varying percentages of absorbance split among low (2600-3350 cm-1) and high (3350-3700 cm-1) wavenumber bands, the SIMS data are fit with the same precision and virtually the same regression slope regardless of which IR calibration is used. We also confirm previous suggestions that the matrix effect for SIMS analyses between orthopyroxene and olivine is small (≤20%). Anomalously high yields of 16O1H in some analyses can be attributed to the presence of amphibole lamellae, and these analyses must be filtered out with different criteria than for olivine due to differences in the geometrical relationship of host to inclusion. For F, our derived values are highly dependent on analytical uncertainties related to the use of silicate glasses as standards. Regardless of the accuracy of our calibration, we see systematic differences in F concentrations in orthopyroxenes and olivines depending on their geological context. Samples derived from crustal environments and from Colorado Plateau minette diatremes have very low F (≤3 ppm), while higher contents can be found in megacrysts from South African kimberlites (up to 17 ppm in orthopyroxene and 47 ppm in olivine) and in xenocrysts from the Rio Grande Rift (Kilbourne Hole, 7-9 ppm in orthopyroxene).

Strength of (Mg,Fe)2SiO4 wadsleyite determined by relaxation of transformation stress

Abstract The growth of a reaction rim around a crystal undergoing a polymorphic phase transformation generates elastic strain energy as a consequence of the reaction volume change. The strain energy inhibits the transformation by counteracting the chemical free energy driving force for growth. Stress relaxation enables growth to continue, at a rate controlled primarily by the mechanical properties of the rim. We studied this process in high-pressure experiments in order to constrain the rheology of wadsleyite, one of the high-pressure polymorphs of olivine. Single crystal spheres of San Carlos olivine, surrounded by a quasi-hydrostatic pressure medium of NaCl or Au, were partially transformed to wadsleyite at 1100°C and 16–17 GPa to determine the rate of growth as a function of time. Microstructural observations indicate that the relict olivine remains largely undeformed, while the wadsleyite rims deform by dislocation creep. Application of an elastoplastic model to fit the observed decrease in growth rates with time constrains the yield strength of wadsleyite at these conditions to between 4 and 6 GPa. A decrease in growth rates was also observed in another study in which varying amounts of water were added to the samples [Kubo, T., Ohtani, E., Kato, T., Shinmei, T., Fujino, K., 1998a. Experimental investigation of the α–β transformation of San Carlos olivine single crystal. Phys. Chem. Miner. 26, 1–6; Kubo, T., Ohtani, E., Shinmei, T., Fujino, K., 1998b. Effects of water on the α–β transformation kinetics in San Carlos olivine. Science 281, 85–87]. Modeling these results gives values for the yield strength of wadsleyite between 2 and 5 GPa under various conditions. A strength difference of ∼1 GPa is indicated by experiments at 1030°C conducted under nominally dry (200 ppm H2O) and water-added (500 ppm H2O) conditions, supporting the hypothesis that hydrolytic weakening is an important process in wadsleyite. The calculated yield strengths for wadsleyite indicate that it is three to six times stronger than olivine at the same conditions, based on an extrapolation of the low-temperature plasticity flow-law for olivine. These results imply a strong increase in the strength of subducting slabs in the mantle transition zone, provided that the deformation mechanism is the same as in the experiments. Furthermore, inhibition of growth by transformation stress may be an important factor in extending the depth range over which metastable olivine transforms to its high-pressure polymorphs in subducting slabs.