Miocene basement exhumation in the Central Alps recorded by detrital garnet geochemistry in foreland basin deposits

The Neogene evolution of the European Alps was characterized by the exhumation of crystalline basement, the so-called external crystalline massifs. Their exhumation presumably controlled the evolution of relief, distribution of drainage networks, and generation of sediment in the Central Alps. However, due to the absence of suitable proxies, the timing of their surficial exposure and thus the initiation of sediment supply from these areas are poorly constrained. The northern Alpine foreland basin preserves the Oligocene to Miocene sedimentary record of tectonic and climatic adjustments in the hinterland. This contribution analyses the provenance of 25 to 14 Myr old alluvial fan deposits by means of detrital garnet chemistry. Unusually grossularand spessartine-rich garnet is found (1) to be a unique proxy for identifying detritus from the external crystalline massifs and (2) to occur abundantly in ca. 14 Myr old deposits of the foreland basin. In contrast to previous assumptions, we therefore propose that the external massifs were already exposed to the surface ca. 14 Myr ago

The role of trace elements in controlling H incorporation in San Carlos olivine

We conducted a series of hydroxylation experiments using mm-sized cuboids cut from six different crystals of San Carlos olivine with a range of trace-element concentrations. The cuboids were pre-annealed and then hydroxylated under identical conditions, ensuring that variation in the amounts of H incorporated depended only on the compositional variables. The pre-anneal was at 1400 °C, atmospheric pressure and an oxygen fugacity equivalent to Δlog FMQ + 1, with the subsequent hydroxylation at 800 °C and 1.5 GPa, for 3 days. Hydrogen was incorporated into all six crystals by the four main substitution mechanisms [Si], [Mg], [Ti] and [triv], with homogeneous H contents in the cores of the crystals, indicating H diffusion rates faster than 10− 11 m2/s. Total H as H2O in the homogeneous cores calculated by summing all the infrared absorbance bands ranges from 13 to 27 wt. ppm. The total H2O in the six pre-annealed crystals is poorly correlated with any measured compositional variable. However, when the H2O associated with individual infrared bands is compared, clear trends emerge. The intensity of absorption bands at 3572 and 3525 cm− 1 are strongly correlated with Ti concentrations, whose range in the six crystals exceeds an order of magnitude. Bands between 3400 and 3300 cm− 1, correlate negatively with Na+, but are positively correlated with the difference between molar Cr3+ and Na+. This highlights a previously unrecognised role for Na in suppressing H incorporation in natural olivines. The results confirm the important role that the trace constituents of olivine play in H incorporation. Two of these trace elements, Na and Ti, tend to be similarly enriched or depleted by partial melting or metasomatism of the mantle, but have opposite effects on H incorporation, with Ti enhancing it but Na suppressing it. Models estimating the effect of H in olivine on mantle rheology must, therefore, consider carefully the availability of these trace elements

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

Abstract The water solubility in olivine C H 2 O $$\left({C}_{{\mathrm{H}}_2\mathrm{O}}\right)$$ has been investigated at 1050 °C and 3 GPa as a function of water activity a H 2 O $$\left({a}_{{\mathrm{H}}_2\mathrm{O}}\right)$$ at subsolidus conditions in the piston-cylinder apparatus, with a H 2 O $${a}_{{\mathrm{H}}_2\mathrm{O}}$$ varied using H2O–NaCl fluids. Four sets of experiments were conducted to constrain the effect of a H 2 O $${a}_{{\mathrm{H}}_2\mathrm{O}}$$ on the four main substitution mechanisms. The experiments were designed to grow olivine in situ and thus achieve global equilibrium (G-type), as opposed to hydroxylating olivine with a pre-existing point-defect structure and impurity content (M-type). Olivine grains from the experiments were analysed with polarised and unpolarised FTIR spectroscopy, and where necessary, the spectra have been deconvoluted to quantify the contribution of each substitution mechanism. Olivine buffered with magnesiowüstite produced absorbance bands at high wavenumbers ranging from 3566 to 3612 cm−1. About 50% of the total absorbance was found parallel to the a-axis, 30% parallel to the b-axis and 20% parallel to the c-axis. The total absorbance and hence water concentration in olivine follows the relationship of C H 2 O ∝ a H 2 O 2 $${C}_{{\mathrm{H}}_2\mathrm{O}}\propto {a_{{\mathrm{H}}_2\mathrm{O}}}^2$$ , indicating that the investigated defect must involve four H atoms substituting for one Si atom (labelled as [Si]). Forsterite buffered with enstatite produced an absorbance band exclusively aligned parallel the c-axis at 3160 cm−1. The band position, polarisation and observed C H 2 O ∝ a H 2 O $${C}_{{\mathrm{H}}_2\mathrm{O}}\propto {a}_{{\mathrm{H}}_2\mathrm{O}}$$ are consistent with two H substituting for one Mg (labelled as [Mg]). Ti-doped, enstatite-buffered olivine displays absorption bands, and polarisation typical of Ti-clinohumite point defects where two H on the Si-site are charge-balanced by one Ti on a Mg-site (labelled as [Ti]). This is further supported by C H 2 O ∝ a H 2 O $${C}_{{\mathrm{H}}_2\mathrm{O}}\propto {a}_{{\mathrm{H}}_2\mathrm{O}}$$ and a 1:1 relationship of molar H2O and TiO2 in these experiments. Sc-doped, enstatite-buffered experiments display a main absorption band at 3355 cm−1 with C H 2 O ∝ a H 2 O 0.5 $${C}_{{\mathrm{H}}_2\mathrm{O}}\propto {a_{{\mathrm{H}}_2\mathrm{O}}}^{0.5}$$ and a positive correlation of Sc and H, indicating the coupled substitution of a trivalent cation plus a H for two Mg (labelled as [triv]). Our data demonstrate that extreme care has to be taken when inferences from experiments conducted at a H 2 O = 1 $${a}_{{\mathrm{H}}_2\mathrm{O}}=1$$ are applied to the mantle, where in most cases, a low a H 2 O $${a}_{{\mathrm{H}}_2\mathrm{O}}$$ persists. In particular, the higher exponent of the [Si] substitution mechanism means that the contribution of this hydrous defect to total water content will decrease more rapidly with decreasing a H 2 O $${a}_{{\mathrm{H}}_2\mathrm{O}}$$ than the contributions of the other substitution mechanisms. The experiments confirm previous results that the [Mg] mechanism holds an almost negligible amount of water under nearly all T-P-fO2-fH2O conditions that may be anticipated in nature. However, the small amounts of H2O we find in substituting by this mechanism are similar in the experiments on forsterite doped with either Sc or Ti to those in the undoped forsterite at equivalent a H 2 O $${a}_{{\mathrm{H}}_2\mathrm{O}}$$ (all buffered by enstatite), confirming the assumption that, thermodynamically, C H 2 O $${C}_{{\mathrm{H}}_2\mathrm{O}}$$ substituting by each mechanism does not depend on the water concentration that substitutes by other mechanisms

Hydrogen diffusion mechanisms in quartz: insights from H–Li, 2H–H and 2H–H–Li exchange experiments

The diffusivity and diffusion mechanisms of hydrogen together with with deuterium and lithium, parallel to the c axis of quartz, were investigated experimentally at 800 degrees C, 0.1 GPa with the activity of H2O or (H2O)-H-2 approximate to 1 [H-2 is used throughout this work to describe deuterium rather than D, to avoid confusion with the diffusion coefficient, D]. The pH was set using mixtures of H2O (or (H2O)-H-2) and HCl. Three types of experiment were conducted: (1) H-in/Li-out; (2) H-2-in/H-out; and (3) H-2-in/H + Li out, using three different natural quartz crystals as starting materials. Profiles of H, H-2 and Li were measured using Fourier-transform infrared (FTIR) spectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). H, H-2 and Li are charge-compensated by Al3+ replacing Si4+, or by excess O2-. The total atomic concentration of monovalent cations appears to remain constant over the duration of the experiments. The resulting diffusion profiles are different for the three experimental designs and three starting materials, and some show complex shapes inconsistent with simple diffusion. A multi-site diffusion-reaction model is developed, with the theory based on previous models that have been derived mainly on the basis of conductivity measurements. In these models, the monovalent cations move away from their charge-balancing ion then diffuse rapidly to another site. The mobility of the monovalent cations is described by both a diffusion coefficient and an equilibrium constant that enables dissociation of the immobile charge-balanced defects. This model can describe complex step-shaped profiles formed in H-in/Li-out experiments, profiles with local maxima ('humped' profiles) in H-2-in/H + Li out experiments, and error function-shaped profiles in H-2-in/H-out and previously published Li-in/H-out experiments. Our data provide support for models previously proposed for quartz. Studies of the lengths and forms of diffusion profiles from such experiments provide a useful complement to assertions from conductivity experiments

Frozen melt–rock reaction in a peridotite xenolith from sub-arc mantle recorded by diffusion of trace elements and water in olivine

Inferring the ambient state of the lithospheric mantle from xenoliths may be misleading if the magmatic activity responsible for the exhumation has modified the xenolith. Changes due to melt–xenolith interactions during exhumation are usually identifiable because of the short timescales involved, but changes due to earlier pulses of magma passing through the lithosphere may be more insidious because the longer timescales allow extensive overprinting of the ambient petrography. Here we describe an intermediate stage of melt–rock reaction recorded in a porphyroclastic peridotite xenolith from the upper mantle wedge adjacent to the West Bismarck Island Arc. The texture and chemistry, frozen during rapid exhumation, reflects the progressive change from harzburgite to clinopyroxene-bearing dunite caused by the infiltration of hydrous basaltic melt into the xenolith. The unusually low equilibration temperature of the xenolith (∼650 °C) prior to this event is reflected in very low concentrations of incompatible elements in the olivine, enhancing the diffusion profiles of these elements into the olivine from the interaction with the melt. The Ca concentration profiles correspond to a timescale of approximately three months, which we interpret as the time between the first infiltration of melt into the lithospheric mantle at this locality and the subsequent exhumation of the xenolith by a second pulse of magma. The diffusion profiles of other trace elements (Li, Na, Sc, Ti, V, Cr, Mn, Ni, Cu, Y) and major elements (Fe–Mg) confirm that all these elements diffuse through olivine at rates that differ from each other by less than one order of magnitude. Infrared spectroscopy reveals OH¯ (“water”) contents and incorporation mechanisms typical of other arc mantle peridotites in the olivine cores. Water contents increase towards the crystal rims, similarly to the profiles of other measured trace elements. The increase in water concentration is accompanied by subtle changes in the infrared bands, caused by an increasing proportion of the OH¯ being bonded to Cr³⁺ towards the rim. The OH¯ coupled with Cr³⁺ follows the zoning pattern of Cr, except at the crystal rim, where it decreases due to late-stage degassing during eruption. This relationship shows that water incorporation in olivine was a metastable process controlled by the availability of suitable sites. In general, the amount of water incorporated into mantle olivine may be expected to depend not only on the intensive variables of H₂O fugacity, temperature and pressure, but also on the concentrations of enabling trace components like Cr³⁺ and Fe³⁺ in the olivines. The water in the olivine core is associated with Cr³⁺ and Fe³⁺ in approximately equal proportions.This research was supported by Natural Environment Research Council Algorithm Doctoral Studentship NERC/10/000497448 to Pete Tollan

Point defect populations of forsterite revealed by two-stage metastable hydroxylation experiments

Hydroxylation is a method that allows “decoration” of the pre-existing point defect structure of nominally anhydrous minerals, such as olivine. We tested this method on synthetic forsterite (Fo: Mg2SiO4 ) crystals. To control starting point defect structures, Fo crystals were pre-annealed at different temperatures ( 1100−1500∘C ), silica activity conditions (forsterite–enstatite Fo–En and forsterite–periclase Fo–Per) and oxygen fugacity (0.21 and 10−6 bars). Then low-temperature hydroxylation (900 °C, 1.5 GPa) of the crystals successfully allowed the decoration with protons of pre-existing point defect structures, as subsequently revealed by infrared spectroscopy. Protons are arranged in three different point defect stoichiometries in Fo, related to Mg and Si vacancies ([Mg] and [Si], respectively) as well as to a trivalent cation-associated substitution mechanism ([triv]). Over the timescale and equilibrium conditions studied, hydroxylation does not reset the point defect structure inherited from pre-anneal. The data further show that the concentrations of [Mg]-, [Si]- and [triv]-hydrated defects are function of pre-anneal silica activity and temperature. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of the crystals revealed diffusion of Al and Fe into the crystals during the pre-annealing, a phenomenon clearly promoted at high aSiO2 . The data confirm a very fast mechanism of Al diffusion in Fo during pre-annealing, and suggest a strong coupling between H+ and Al3+ during hydroxylation. Overall, they show the strong importance of aSiO2 and temperature in the incorporation of trace cations in forsterite, and the subsequent effects of incorporation of trace cations on Mg- and Si-related point defects in Fo. The dry point defect population of Fo is determined by interactions between the trace trivalent cations and dry Si and Mg vacancies. Without trace elements, T only has a limited effect on Mg- and Si-related point defect populations. Finally, approaching or potentially slightly exceeding the Fo–En solidus leads to strong changes in the trace element concentration and point defect population in Fo, which may be related to either partial melting or pre-melting effects.Funding by the ARC Laureate Fellowship FL130100066 to Hugh O’Neill is acknowledged

A shock recovery experiment and its implications for Mercury's surface: The effect of high pressure on porous olivine powder as a regolith analog

We conducted classic dynamic high - pressure experiments on porous San Carlos (SC) olivine powder to examine if and how different shock stages modify corresponding reflectance mid – infrared (MIR) spectra. Microscopic investigation of the thin sections produced of our shocked samples indicates local peak pressures of >60 GPa along with all lower grade shock stages. Spectral analyses of optically identified shock areas were documented and compared in terms of Christiansen Feature (CF) and the position of olivine – diagnostic Reststrahlenbands (RBs). We found that one RB (fundamental vibrations of the orthosilicate - ion) of olivine occurring at 980 cm−1 (corresponding to ≈ 10.2 μm) shows the least energetic shift in the investigated MIR spectra and could therefore serve as a proxy for the presence of olivine in remote sensing application. Furthermore, a peak located at ≈ 1060 cm−1 (≈ 9.4 μm) shows a significant intensity change probably related to the degree of shock exposure or grain orientation effects, as we observe a decline in intensity of this band from our averaged reference olivine spectra of our IRIS database (diffuse reflectance measurement) down to spectra of grains showing mosaicism and recrystallized areas. We also report the presence of a weak band in some of the olivine spectra located at ≈ 1100 cm−1 (9.1 μm) that has an influence on the position of the CF when spectral data of olivine are averaged