T32B-01 INVITED 10:20h
Hydrogen Mobility in Orthopyroxene in Comparison to Other Upper Mantle Minerals
The kinetics of hydrogen self-diffusion in orthopyroxene has been studied from 1 to 5000 bar for several chemically distinct synthetic samples. Samples were synthesized at 25 kbar and $1150\deg$C under H$_{2}$O and D$_{2}$O saturated conditions, respectively, and analysed by FTIR-spectroscopy and electron microprobe. For the kinetic studies, oriented wafers were produced and either exposed to H$_{2}$-atmosphere at $800\deg$C directly or equilibrated with H$_{2}$O or D$_{2}$O in a sealed Au-tube in an internally heated pressure vessel. After verification that H-D-exchange was fully reversible, diffusion coefficients were determined in both time dependent exchange experiments and from diffusion profiles. H-self-diffusion in pure enstatite turned out to be anisotropic, D[001] being approximately one order of magnitude faster than D[010]. Increasing pressure enhances H-mobility (-logD[010] = 12.9 (12.0) at 1 (2000) bar and $800\deg$C, respectively). Incorporation of Al and Cr in amounts comparable to natural mantle samples decreases H-mobility by one and two orders of magnitude, respectively. H-mobility in natural orthopyroxene from the Earth`s mantle is probably achieved by electron-hole diffusion coupled to a counter flux of protons, which is rate-limited by proton self-diffusion. Therefore, H-diffusivities determined in this study correspond to the same diffusion mechanism and are applicable to mantle processes. Compared to other nominally anhydrous mantle minerals, H-mobility in enstatite at $800\deg$C is slightly slower than in olivine and clinopyroxene [1-3], but may be faster at mantle temperatures if the higher activation energy is considered [4]. The data for Cr-free samples imply that H-diffusivities are fast enough to allow considerable H-exchange between a mantle xenolith consisting of mm-sized crystals and its host magma upon ascent. However, if it is assumed that Cr does not affect the activation energy for H-diffusion, Cr-rich orthopyroxenes may be protected from equilibration with the host magma. The anisotropic behavior of H-diffusion in orthopyroxene is very similar to that of clinopyroxene. Although pyroxenes show less pronounced anisotropic H-mobility than olivine, their generally higher water solubility may suggest that both may contribute to the electrical anisotropy of the asthenosphere inferred from magnetotelluric data [5]. [1] Mackwell SJ and Kohlstedt DL (1990) J Geoph Res 95, 5079-5088 [2] Carpenter Woods S, Mackwell S, Dyar D (2000) Amer Mineral 85, 480-487 [3] Hercule S and Ingrin J (1999) Amer Mineral 84, 1577-1587 [4] Stalder R and Skogby H (2003) Phys Chem Min 30, 12-19 [5] Bahr K and Duba A (2000) Earth Planet Sci Lett 178, 87-95
T32B-02 10:35h
Storage capacity of H$_{2}$O in the upper mantle and significance for melting at the 410 km discontinuity
Substantial data suggests that the transition zone has a large capacity to store H$_{2}$O in nominally anhydrous minerals, but the upper mantle is generally thought to have a much lower storage capacity for H$_{2}$O. Recent experimental and analytical advances suggest that the storage capacity of the upper mantle is also likely to be significant. First, improvements in analysis of H$_{2}$O in olivine [1,2] suggest that the solubility of H$_{2}$O in olivine is 3 times greater than previously believed [3], so the storage capacity of olivine reaches $\sim$0.5 wt.% at 410 km. Second, experimental measurements of coexisting mantle minerals at 1--2 GPa show that pyroxenes incorporate 10 times more H$_{2}$O than olivine [2,4]. If such strong pyroxene/olivine partitioning were to persist throughout the upper mantle, the storage capacity above 410 km could be greater than 1 wt.%, but more conservative assumptions still suggest that it is $>$0.4 wt.%. Thus, hydrous transition zone material advected above the 410 km discontinuity will not induce partial melting unless it has more than 0.4 wt.% H$_{2}$O and the H$_{2}$O retained in residual peridotite will be equivalent to the storage capacity of the upper mantle assemblage, i.e., a minimum of $\sim$0.4 wt.%. This is far in excess of the H2O of the dominant (MORB-source) upper mantle (100--200 ppm), so it is unlikely that dehydrated $"$wet$"$ transition zone material can be a volumetrically significant source of upper mantle material. In contrast to pyroxene, garnet appears to have a low H$_{2}$O storage capacity. Increasing modal garnet at the expense of pyroxene between 300 and 400 km depth likely results in diminished H$_{2}$O storage capacity with increasing depth and so there is a local storage capacity minimum just above the 410 km discontinuity. If this minimum were not present, then hydrous material melted just above the 410 km discontinuity would continue to melt as it is advected throughout the upper mantle. But owing to the minimum, the residues of any such melting at 410 km would cease melting as they upwell, though they would begin to melt again at depths of 250-300 km, where they again exceed the local H2O storage capacity. [1] Bell et al. JGR, 2003. [2] Koga et al. G3, 2003. [3] Kohlstedt et al. CMP, 1996 [4] Aubaud et al. GRL, submitted
T32B-03 INVITED 10:50h
High-Pressure Raman Spectroscopic Studies of Hydrated Transition Zone Spinelloid and Spinel Phases
Wadsleyite (\beta-(Mg,Fe)$_{2}$SiO$_{4}$, spinelloid III) and ringwoodite (\gamma-(Mg,Fe) $_{2}$SiO$_{4}$,spinel) are thought to be the most abundant minerals in the Earth's mantle transition zone between 410 and 660 km depth. Though nominally anhydrous their crystal structures can accommodate significant amounts of OH: wadsleyite up to 3.3 wt% H$_{2}$O and ringwoodite up to 2.3 wt%. Wadsleyite II (spinelloid IV) is a hydrous, Fe-bearing silicate phase that might occur between the stability regions of wadsleyite and ringwoodite. We present Raman spectra from 50 to 4000 cm$^{-1}$ of high-quality single-crystals of hydrous Mg end-member and hydrous Fe-bearing wadsleyite, wadsleyite II, and ringwoodite up to 60 GPa. The experiments were performed with a diamond-anvil cell and rare-gas pressure-transmitting media. The Raman spectra of wadsleyite and wadsleyite II exhibit subtle differences. In the OH stretching region the spectrum of wadsleyite II appears more complex than the spectrum of wadsleyite consisting of at least 6 modes. The high-pressure behaviour of hydrous iron-bearing ringwoodite is unique: near 40 GPa additional Raman bands emerge in the region 550-580 cm$^{-1}$. Comparisons of Raman spectra of hydrated transition zone phases with varying iron and hydrogen content reveal that pure protonation has only a minor effect on the lattice dynamics whereas coupled iron and proton substitution can lead to additional levels of transformation complexity.
T32B-04 11:05h
Hydration of Olivine at 12 GPa
Although natural olivine of upper mantle origin generally contains less than 300 ppm H$_{2}$O by weight, olivine synthesized at temperature and pressure conditions near the 410 km discontinuity can contain up to 20 times this amount of H, enough to affect elastic properties, crystal structures, and unit cell volumes. Hydrous olivine has been synthesized with Fo95 composition at 12GPa and $1250\deg$ C in equilibrium with clinohumite in magnesia excess conditions and with clinoenstatite in silica excess conditions. Crystal structure refinements indicate that the hydration mechanisms are different in the two different conditions. The silica-excess sample appears to contain M2 cation vacancies, whereas the magnesia-excess sample shows tetrahedral site vacancies up to two percent, consistent with 5000 ppm by weight H$_{2}$O. Further, the partially vacant coordination polyhedra are significantly larger than their fully occupied counterparts. Cell parameter refinements indicate expansion of the unit cell by up to 0.3 percent with hydration in both samples, with tetrahedral vacancies causing an increase in both {\it b} and {\it c} axes and contraction of {\it a} relative to the crystal showing octahedral vacancies. If the excess volume were eliminated on compression to 12 GPa the resulting bulk modulus would be decreased by about 10 percent relative to anhydrous olivine. Static compression measurements are in progress. These results indicate that olivine can be a major host for H at depths approaching the Transition Zone in the upper mantle.
http://ruby.colorado.edu/~smyth/Research/Talks/Talks.html
T32B-05 11:20h
True Equilibrium Experiments on Dense Hydrous Magnesium Silicates (DHMS) in the MgO-SiO$_{2}$-H$_{2}$O System, and the Stability of Phase A, E, and Clinohumite in Mantle Compositions
Although none of the so-called Dense Hydrous Magnesium Silicates (DHSM) have been found as minerals in nature, they are of crucial importance for understanding both hydration and dehydration processes in the Earth's mantle. Since Ringwood and Major (1967), numerous experimental studies have focused on the determination of phase relations involving DHMS-phases and their stability fields within MgO-SiO$_{2}$-H$_{2}$O. However, most of these results are based on synthesis and unreversed experiments. In addition, fluid-saturated high pressure experiments suffer from high solubilities of the solid components in the fluid, which result in chemical zonations through Soret-diffusion. As a result, phases in excess of those allowed by the phase rule (in a homogeneous system) are commonly present in run products, and strong zonations in the run products may cause erroneous interpretations of the phase relations and equilibria. We overcome the problem of zonation and 'excess' phases by inverting the capsule at experimental conditions within the gravity field ('rocking' multi-anvil). Rocking and tilting of a chemically stratified fluid induces Rayleigh-Taylor instabilities, causing chemical re-homogenisation of the fluid. If rocking of the multi-anvil is frequent enough (1 turn/min) the chemical gradient in the fluid is reset before dissolution/reprecipitation processes cause zonations, we thus obtain a homogeneous run product containing exactly 2 solid phases (+fluid), which represent chemical equilibrium throughout the entire capsule. Reversed equilibrium experiment have been carried out with the 'rocking' multi-anvil at conditions near $900\deg$C, 11 GPa, around the invariant point where the stability fields of phase A+enstatite, phase E+forsterite, and forsterite+enstatite (+fluid, in model mantle compositions) were thought to encounter. Mineral compositions of the experimental charges were determined by electron microprobe, and phase identities confirmed by Raman spectroscopy. The results show that the above invariant point is metastable and that there is a narrow stability field of clinohumite (for mantle bulk compositions). The stability relations between phase A, phase E, enstatite, forsterite and clinohumite have been revised.
T32B-06 11:35h
Water controls fields of metastable olivine in subducting slabs
Presence of metastable olivine in subducting slabs has important implications for buoyancy force against subduction, stress state and strength of the slabs, and origin of deep earthquakes. We have examined pressure, temperature, and water content dependences of the $\alpha$-$\beta$ transformation rate in Mg$_{2}$SiO$_{4}$ using in-situ X-ray diffraction method at SPring-8 to discuss fields of metastable olivine in subducting slabs. Graphite and Ag-Pd alloy were used as the sample capsule in nominally dry and wet runs, respectively. Time-resolved X-ray diffraction patterns were successfully obtained during the transformation. Water contents in recovered samples were measured using a micro Fourier-transform infrared spectroscopy. We have determined growth rates in the $\alpha$-$\beta$ transformation at 13.4-15.8 GPa, 730-1100$\deg$C, and 750-4900 wt. ppm H$_{2}$O. In case that the water content is about 750 wt. ppm H$_{2}$O, the activation energy and activation volume for growth were estimated to be 348 kJ/mol and 1.7 cm$^{3}$/mol (Kubo et al., 2004). This growth kinetics is almost consistent with the previous study of Mosenfelder et al. (2001), in which the olivine can survive to a depth of around 600 km in the slab for the coldest thermal model. We observed that the growth in the $\alpha$-$\beta$ transformation of olivine is significantly enhanced by water. The growth rate is roughly proportional to water content to the 3 power. The enhancement associated with an increase of an order of magnitude in water content is identical to that with a 200$\deg$C increase in temperature. Thus, water can control fields of metastable olivine in slabs. When the water content of cold slabs is less than about 750 wt. ppm H$_{2}$O, the depth of the olivine transformation would be greater than 600 km, whereas fields of metastable olivine become much smaller if cold slabs contain much water. References: Kubo et al. (2004) Am. Mineral., 89, 285-293; Mosenfelder et al. (2001) Phys. Earth Planet. Inter., 127, 165-180.
T32B-07 11:50h
Hydrogen in Olivines From the Colorado Plateau: Implications for Water in the Mantle and the Alpe Arami Controversy
Petrological, geochemical and geophysical data indicate that portions of the upper mantle beneath the Colorado Plateau are highly hydrated, which has important implications for mantle viscosity and geodynamics (e.g. [1]). One line of evidence for this comes from the study of H in nominally anhydrous minerals. It has been shown that mantle garnets from the Plateau have higher H contents than those from other continental areas. Recently, we began a reinvestigation of H in olivines from the Buell Park (Arizona) and Green Knobs (New Mexico) diatremes, using polarized IR spectroscopy. Two varieties of olivine have been described [2] at these localities: green-colored crystals containing trace amounts of H, and brown olivines that contain substantially more H in the form of lamellae of Ti-clinohumite (Ti-cl). Our new observations show that there is a continuum between these two varieties. H concentrations range from 50 to 530 wt ppm H$_{2}$O. The olivine with the lowest H content exhibits strong absorption bands in the 3400 to 3650 cm$^{-1}$ wavenumber region that closely resemble (in location and pleochroism) those found in olivines hydrated in our laboratory and by Kohlstedt and co-workers. The olivines with higher H contents contain these same bands as well as additional peaks at 3571, 3524, 3402, 3319, and 3230 cm$^{-1}$ that are attributed to the Ti-cl lamellae. The 3402 cm$^{-1}$ band is the strongest absorber both in these olivines and in spectra of Ti-cl single crystals. In addition to the nm-scale Ti-cl lamellae, these olivines contain oriented, micrometer-scale inclusions of chromite and cr-diopside. These precipitates are indicative of cooling and high silica activity [3]. This contrasts with assertions [4,5] that the high-wavenumber peaks in olivine are related to low silica activity, whereas low-wavenumber peaks indicate high silica activity. The specific attribution of the 3230 cm$^{-1}$ peak to high silica activity [5] may also be in error. This peak is present in olivines in this study as well as olivines from several other localities (Zabargad Island, Egypt; Vesuvius, Italy; Monastery Farm, South Africa). In all of these cases it is correlated with a peak at 3402 cm$^{-1}$ that suggests the presence of Ti-cl inclusions or lamellae [6]. The dominance of low wavenumber peaks at nominally low silica activity in some experimental studies [4,5] may be related to other factors such as oxygen fugacity, Fe-content, or the presence of unidentified hydrous inclusions. Some olivines from the Alpe Arami garnet peridotite contain oriented ilmenite + chromite inclusions, and the inferred high TiO$_{2}$ content of the precursor olivines has been attributed to ultra-high pressure metamorphism at 10 GPa or more (e.g. [7]). Our observations support an alternative view [8] that the ilmenite exsolution is related to breakdown of Ti-cl lamellae in the olivine, not necessarily at such high pressures. Further work is underway using analytical TEM to investigate the structures and chemistry of these lamellae, and further elucidate the mechanisms by which mantle olivine may incorporate both Ti and H. References: [1] Dixon (2004) EPSL 222: 451-467 [2] Kitamura (1987) Nature 328: 143-145 [3] Markl (2001) Am Min 86: 36-46 [4] Matveev (2001) J Petr 42: 721-729 [5] Lemaire (2004) CMP 147: 48-57 [6] Miller (1987) PCM 14:461-472 [7] Bozhilov (2003) Am Min 88: 596-603 [8] Risold (2001) CMP 140: 619-628
T32B-08 12:05h
Deformation Microstructures of Garnet Peridotites From Norwegian Caledonides and Evidences for High Water Content in the Deep Upper Mantle
Water has marked effects on both physical and chemical properties of minerals and rocks and hence the inference of distribution of water is an important subject for geological studies. Rocks carried from deep upper mantle may provide a clue as to the distribution of water in the deep mantle. However, the high diffusivity of water in minerals provides a challenge because water can be lost or added easily during their transport to the surface. Deformation microstructures have different response times for time-varying physical and chemical environment including water content. Here we attempt to read the history of deformation conditions of garnet peridotites from Norwegian Caledonides, which contain majoritic garnet, to reveal water content in the deep upper mantle. One of the peridotites from the Otroy island shows a olivine lattice-preferred orientation (LPO) with a maximum for the [001] subparallel to the stretching lineation and the (100) plane subparallel to the foliation. This pattern is similar to that found in olivine experimentally deformed at high water content ($>$1000 ppm H/Si) and relatively low stress (Jung and Karato, 2001). Spatial relationship between precipitates and subgrain boundaries in garnets indicates that the high-strain deformation occurred at high pressures ($>$5GPa). Dislocation microstructures in the olivine show well-defined glide loops on the (001) plane, suggesting the dominant slip system of [100](001). This is distinct from the observed olivine fabric, and this fabric was found at modest water content (200-1000 ppm H/Si) and low stress by our recent deformation experiments. Lattice-preferred orientation records conditions for long-term large strain deformation, whereas dislocation microstructures tend to reflect the conditions of the latest stage, short-term deformation (or annealing). Therefore, we concluded that large strain deformation with high water content ($>$1000 ppm H/Si) at deeper portion ($>$150 km) is followed by low strain deformation at lower water content in the shallower portions. These observations indicate that the deep upper mantle in this region had considerably higher water content than the upper mantle near mid-ocean ridges.