Abstract. Although water controls the biology and geology of the surface, hydrogen is perhaps the most poorly constrained compositional variable in the bulk Earth. Its concentration in the upper mantle appears to be controlled by its solubility as hydroxyl in the nominally anhydrous silicate phases, olivine, pyroxene, garnet, wadsleyite, and ringwoodite. Here we describe a series of experiments showing that the solubility of H2O in olivine at 12 GPa increases with temperature to 8900 ppm by weight at 1250°C and decreases at higher temperature with the onset of melting. Sample characterization by infrared spectroscopy indicates that the primary hydration mechanism is the substitution of 2H+ for Mg2+. Similar results obtained from samples coexisting with clinohumite (low-silica) and with clinoenstatite (high-silica) indicate that silica activity has minimal effect on hydration under these conditions. Single-crystal X-ray diffraction measurements constrain the volume of hydration and indicate significant M-site vacancies. Hydrogen thus appears to become a geochemically compatible element as depths approach 400km.

The high-pressure stability of antigorite in the systems MSH and MASH was investigated using two structurally and chemically well-constrained natural samples. Careful sample selection and characterization ensured that the only significant difference between the samples was Al content, one sample being essentially Al free, and the other containing 3.06(2) wt% Al 2 O 3 . In the system MSH, the reaction antigorite = forsterite + clinoenstatite + water was bracketed, under water-saturated conditions, between 630 and 650 ∞C at 1.6 GPa, between 620 and 660 ∞C at 2.5 GPa, between 620 and 660 ∞C at 3.9 GPa, and between 4.5 and 5.0 GPa at 520 ∞C. In the system MASH, the reaction antigorite = forsterite + clinoenstatite + chlorite + water was bracketed, under water-saturated conditions, between 660 and 700 ∞C at 2.0 GPa, between 660 and 680 ∞C at 2.9 GPa, and between 5.0 and 5.5 GPa at 600 ∞C. At pressures above 5.8 GPa, intersection of this reaction with the reaction chlorite + clinoenstatite = pyrope + forsterite + water leads to an additional reaction whereby the Al component of the antigorite is transferred to pyrope upon antigorite breakdown. The addition of a few weight percent Al 2 O 3 into antigorite is shown to stabilize the antigorite structure to significantly higher temperatures and pressures, possibly by minimizing structural misfit among the component octahedral and tetrahedral sheets. This effect partially explains the considerable discrepancies noted between previous studies on the stability of antigorite at high pressure. In addition, antigorite breakdown in the system MASH transfers a significant volume of water to chlorite-bearing assemblages, thereby greatly increasing the range of temperatures over which water is tied up in hydrous phases relative to the system MSH.suggested by Zussman (1954) and Kunze (1956Kunze ( , 1958, antigorite can be considered to consist of alternating sinusoidal octahedral and tetrahedral layers. The tetrahedral layers reverse polarity every half wavelength. There are two types of reversal, named 8-reversal or 6-reversal after the occurrence of 8-and 6-membered tetrahedral rings. The 8-reversals effectively lead to the loss of 3 brucite units, and thus departure from the generalized serpentine formula. The range of chemical variation within antigorite is, however, small, and compositions depart only slightly from that of the generalized serpentine formula. Kunze (1961) expressed the composition of individual antigorites by the formula M 3m-3 T 2m O 5m (OH) 4m-6 , where m is the number of tetrahedra in a single chain defined by the wavelength a 0 , M = octahedral cations (Mg, Fe 2+ Ni, Mn 2+ , Al), and T = tetrahedral cations (Si, Al). Mellini et al. (1987) determined compositions of natural antigorite in the range M 2.79 T 2 O 5 (OH) 3.57 to M 2.87 T 2 O 5 (OH) 3.74 , corresponding to a variation in m from 14 to 23. TEM studies have shown that antigorite samples with one unique structural state are very rare; more commonly, a considerable range in m is noted. Antigorite with t...

[1] The microstructure and crystallographic preferred orientation (CPO) of quartz were quantified in 17 samples of natural monomineralic tabular veins. The veins opened and were deformed, up to shear strain g > 15, in a small temperature window (about 25°C) above 500°C, as established by Ti-in-quartz thermometry. The veins filled a set of fractures within the Adamello tonalite (southern Alps, Italy) and localized homogeneous simple shear during postmagmatic cooling. The local (square millimeter scale) and bulk (square centimeter) CPO were investigated by computer-integrated polarization microscopy (CIP) and X-ray texture goniometry. Weakly deformed veins (WDV: g < 1) consist of millimeter-to centimeter-sized crystals with a strong CPO showing a c-axis girdle slightly inclined, mostly with the shear sense, to the foliation (XY) plane and a strong maximum close to the lineation (X). Moderately deformed veins (MDV: 2 < g < 3) consist of elongated nonrecrystallized ribbon grains and most have a CPO showing a strong Y maximum of c axes some with weak extension into a YZ girdle. Strongly deformed veins (SDV: g = 4 to 15) are pervasively to completely recrystallized to fine (34-40 mm grain size) aggregates with a strong CPO similar to that of MDV. The slip systems during plastic deformation were dominantly prism hai with subordinate rhomb and basal hai slip. Recrystallization occurred rather abruptly for 3 < g < 4. In contrast to dislocation creep experiments in quartz (and other minerals), a steady-state recrystallized fabric is achieved at early stages of deformation (g ≈ 4) as there is no evidence, with increasing strain, of strengthening of the CPO, of rotation of the fabric skeleton, or of change in grain size. WDV represent weakly deformed relicts of veins with an initial CPO believed to have developed during crystal growth but unsuitably oriented for prism hai slip during subsequent shear. MDV and SDV appear to derive from veins different from WDV, where the vein crystals grew with orientation favorable for prism hai slip. The relationship between the initial growth CPO and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism hci and prism hai slip, with the temperature of growth causing growth of crystals well oriented for slip. The initial CPO of veins, from which quartz mylonites are commonly derived, plays a critical role in the fabric evolution. The strong growth-and strain-induced CPOs of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal hai slip would have been dominant.

The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth's mantle, hydrosphere, atmosphere, and biosphere for ~4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6-4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6-2.2 GPa and 900-950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ~30-45-km-thick Eoarchean oceanic crust, modified Earth's mantle and Eoarchean environments and ecosystems.

granites where redox conditions have been shown to change from close to the magnetite-28 hematite buffer to close to the nickel-nickel oxide buffer respectively (Stephens et al., 1985). Variations in the Mn concentration of apatites appear to be largely independent of differences 36 in the Mn concentration of the melt. Apatite Mn concentrations may therefore provide an 37 independent oxybarometer that is amenable to experimental calibration, with major relevance 38 to studies on detrital mineral suites, particularly those containing a record of early Earth 39 redox conditions, and on the climatic impact of historic volcanic eruptions. 40

[1] The magnetic properties of olivine-hosted Fe-Ni particles have been studied to assess the potential of "dusty olivine" to retain a pre-accretionary remanence in chondritic meteorites. Both body-centered (bcc) and face-centered cubic (fcc) Fe-Ni phases were formed by reduction of a terrestrial olivine precursor. The presence of Ni complicates the magnetic properties during heating and cooling due to the fcc-bcc martensitic transition. First-order reversal curve (FORC) diagrams contain a central ridge with a broad coercivity distribution extending to 600 mT, attributed to non-interacting single-domain (SD) particles, and a "butterfly" structure extending to 250 mT, attributed to single-vortex (SV) states. SD and SV states were imaged directly using electron holography. The location of the SD/SV boundary is broadly consistent with theoretical predictions. A method to measure the volume of individual SD particles using electron holography is presented. Combining the volume information with constraints on coercivity, we calculate the thermal relaxation characteristics of the particles and demonstrate that the high-coercivity component of remanance would remain stable for 4.6 Ga, even at temperatures approaching the Curie temperature of pure Fe. The high coercivity of the particles, together with the chemical protection offered by the surrounding olivine, is likely to make them resistant to shock remagnetization, isothermal remagnetization and terrestrial weathering, making dusty olivine a credible recorder of pre-accretionary magnetic fields.

No abstract

No abstract