Jörg Hermann

Exhumation as fast as subduction

We produced a pressure-temperature-time path in order to determine the exhumation rate of the deepest subducted Alpine rocks. In situ dating of peak-metamorphic titanite in an eclogite facies calc-silicate rock indicates that subduction to pressures of ∼3.5 GPa was reached at 35.1 ± 0.9 Ma. Titanite formed during two decompression stages, at 1 ± 0.15 GPa and ∼0.4–0.5 GPa, and yielded ages of 32.9 ± 0.9 Ma and 31.8 ± 0.5 Ma, respectively. Combining the age data and making assumptions about the conversion of pressure to depth yield mean exhumation rates of 3.4 cm/yr and 1.6 cm/yr. These rates imply that exhumation acted at plate tectonic speeds similar to subduction, and was significantly faster than erosion. We suggest that fast exhumation is driven by a combination of tectonic processes involving buoyancy and normal faulting.

Fingerprinting the water site in mantle olivine

Nominally anhydrous minerals such as olivine contain trace amounts of water and may accommodate the entire water budget of the upper mantle. Here we report for the first time synthetic olivines, crystallized experimentally under upper mantle conditions, that reproduce the most common and intense infrared hydroxyl stretching bands (at 3572 and 3525 cm−1) observed in spinel peridotite mantle olivines. These bands arise from water accommodated at point defects associated with the trace element Ti, and we suggest that this is the most important defect site in the shallow upper mantle. Additional hydrated defects may occur at higher pressures. We also identify bands related to water associated with Fe3+; these are unlikely to reflect equilibrium with the mantle, and indicate water incorporation during exhumation or retrogression. Water must be present at the defect site appropriate for the mantle, at the conditions of interest, for partitioning, seismic wave speed, and deformation experiments on hydrous olivine to be relevant.

Composition of fluids during serpentinite breakdown in subduction zones: Evidence for limited boron mobility

We experimentally investigated the trace element compositions of fluids released during breakdown of subducted serpentinites. Serpentinites contain significant amounts of fluid-mobile elements such as B, Cs, As, and Ba, which during seafloor alteration are incorporated into mantle rocks. During the later high-pressure breakdown of the serpentinites, these trace elements are redistributed among the residual olivine, orthopyroxene, minor chlorite, and fluid. We find that B is far more compatible in these minerals than previously assumed; it has a fluid/residue partition coefficient (F/R D ) of 3–5. Most other fluid-mobile elements (Cs, As, Ba, Pb) are strongly enriched in the fluid and exhibit expected F/R D values of 30–250. Serpentinites are possibly the most important sink for B in subduction zones; the experimental results suggest that significant B is recycled into the deep mantle. Furthermore, high B concentrations in mantle olivines might be a fingerprint for previous metasomatism or serpentinization.

Three water sites in upper mantle olivine and the role of titanium in the water weakening mechanism

[1] Infrared spectroscopy on synthetic olivines has established that there are at least four different mechanisms by which hydrogen is incorporated into the crystal structure. Two mechanisms occur in the system MgO-SiO2-H2O associated with silicon and magnesium vacancies, respectively. A third mechanism is associated with trivalent cation substitution, commonly Fe3+ in natural olivine, while the fourth mechanism, which is the one most prevalent in natural olivines from the spinel-peridotite facies of the Earths upper mantle, is associated with Ti4+ [ Berry et al., 2005]. Here first principles calculations based on density functional theory are used to derive the structure and relative energies of the two defects in the pure MgO-SiO2-H2O system, and possible hydrogen-bearing and hydrogen-free point defects in Ti4+-doped forsterite. Calculated structures are used to compare the predicted orientation of the O-H bonds with the experimentally determined polarization. The energies are used to discuss how different regimes of chemical environment, temperature ( T), pressure ( P), and both water content and water fugacity (f(H2O)), impact on which of the different hydroxyl substitution mechanisms are thermodynamically stable. We find that given the presence of Ti impurities, the most stable mechanism involves the formation of silicon vacancies containing two protons charge balanced by a Ti4+ cation occupying an adjacent octahedral site. This mechanism leads to the water-mediated formation of silicon vacancies. As silicon is known to be the most slowly diffusing species in olivine, this provides a credible explanation of the observed water weakening effect in olivine.

Quantitative absorbance spectroscopy with unpolarized light: Part I. Physical and mathematical development

Abstract A new approach to the use of spectroscopic absorbance measurements for anisotropic crystals allows results to be extracted using unpolarized light incident on random crystal orientations. The theory of light propagation in anisotropic absorbing crystals is developed from Maxwell’s equations to devise an expression for the transmittance of linearly polarized light traveling in an arbitrary direction in weakly absorbing media. This theory predicts the distribution of transmittance and absorbance as a function of direction and polarization angle of incident light. It is shown how a previously deduced empirical expression, commonly used in infrared spectroscopy, is a good approximation to the full theory under a wide range of conditions. The new theory shows that principal polarized absorbances correspond to the eigenvalues of an absorbance ellipsoid. An expression is derived for the unpolarized absorbance as a function of the angles describing incident light direction, Aunpol(ϕ,ψ), and the principal polarized absorbances, Aa, Ab, Ac in an anisotropic crystal Aunpol(ϕ,ψ) = ½[Aa(cos2ϕcos2ψ + sin2ψ) + Ab(cos2ϕsin2ψ + cos2ψ) + Acsin2ϕ]. Integration of this expression over all incident angles leads to a simple relationship between total measured unpolarized absorbance and the three principal polarized absorbances. Using this theory, a procedure is proposed for estimating both total (Aa + Ab + Ac) and principal absorbances from spectroscopic measurements of absorbance using unpolarized light on a set of randomly oriented crystals.

The oscillatory intergrowth of feldspars in titanian andradite, Little Dromedary, NSW, Australia

Veins bearing titanian andradite were found in a metasomatically altered clinopyroxenite cumulate of an alkali magmatic suite (Little Dromedary, NSW, Australia). The titanian andradite is up to 2.5 cm in size, mainly subhedral and displays a large concentric, oscillatory intergrowth with layers consisting of plagioclase and alkali feldspar. Detailed field work and petrography showed that there is a gradual change in paragenesis from the cumulitic clinopyroxenite with interstitial plagioclase to an apatite- and scapolite-bearing plagioclase-titanian andradite-pyroxene rock, which hosts the titanian andradite veins. Clinopyroxene composition changes from a magmatic Ti-bearing subsilicic ferrian aluminian diopside to a metamorphic Ti-free, Al-poor diopside. Plagioclase changes composition from oligoclase (Ab 81 An 14 Or 5 ) in the pyroxenite to bytownite (Ab 14 An 86 ) in the garnet-pyroxene rock and to oligoclase (Ab 76 An 21 Or 3 ) in the vein. Orthoclase occurs exclusively in the vein and contains 12% albite component. The titanian andradite-bearing veins and the change in paragenesis, bulk rock and mineral composition provide evidence for the reaction of a late magmatic, oxidizing hydrothermal fluid with the magmatic pyroxenite. The temperature of alteration is constrained by the composition of the coexisting plagioclase and alkali feldspar in titanian andradites to ∼ 420 °C. Crystallization of the titanian andradite indicates that Ti was transported on a metre scale from the magmatic clinopyroxene to the vein by a fluid. Ti solubility may have been enhanced by ligands such as F - , Cl - , PO 4 3- , SO 4 2- and CO 3 2- , which are all present in the fluid as documented by their incorporation in metamorphic apatite, scapolite, and carbonate coexisting with titanian andradite. KCl and NaCl salt inclusions in feldspar within the titanian andradite were found with transmission electron microscopy (TEM) and indicate that the fluid was partly supersaturated in these components. The alternating intergrowth of titanian andradite and feldspars mirrors an oscillating evolution of the fluid. TEM images of the garnet-feldspar interface suggest that garnet has been partly dissolved prior to or during feldspar precipitation, supporting the hypothesis of significantly changing fluid composition during vein formation. The concentric intergrowth is interpreted as the result of both the changing chemistry of the fluid and the different growth behaviour of garnet and feldspar.