Dynamic Metasomatism Experiments Investigating the Interaction between Migrating Potassic Melt and Garnet Peridotite

Dynamic metasomatism experiments were performed by reacting a lamproite melt with garnet peridotite by drawing melt through the peridotite into a vitreous carbon melt trap, ensuring the flow of melt through the peridotite and facilitating analysis of the melt. Pressure (2–3 GPa) and temperature (1050–1125 °C) conditions were chosen where the lamproite was molten but the peridotite was not. Phlogopite was formed and garnet and orthopyroxene reacted out, resulting in phlogopite wehrlite (2 GPa) and phlogopite harzburgite (3 GPa). Phlogopites in the peridotite have higher Mg/(Mg + Fe) and Cr2O3 and lower TiO2 than in the lamproite due to buffering by peridotite minerals, with Cr2O3 from the elimination of garnet. Compositional trends in phlogopites in the peridotite are similar to those in natural garnet peridotite xenoliths in kimberlites. Changes in melt composition resulting from the reaction show decreased TiO2 and increased Cr2O3 and Mg/(Mg + Fe). The loss of phlogopite components during migration through the peridotite results in low K2O/Na2O and K/Al in melts, indicating that chemical characteristics of lamproites are lost through reaction with peridotite so that emerging melts would be less extreme in composition. This indicates that lamproites are unlikely to be derived from a source rich in peridotite, and more likely originate in a source dominated by phlogopite-rich hydrous pyroxenites. Phlogopites from an experiment in which lamproite and peridotite were intimately mixed before the experiment did not produce the same phlogopite compositions, showing that care must be taken in the design of reaction experiments.

Subduction initiation-induced rapid emplacement of garnet-bearing peridotites at a nascent forearc: Petrological and Os-Li isotopic evidence from the Purang ophiolite, Tibet

Garnet-bearing peridotites commonly occur in the deeper parts of mature or thickened oceanic lithosphere, and are rarely exhumed and emplaced onto the seafloor. The Purang ophiolitic peridotites in south Tibet contain rare symplectite pseudomorphs after garnet, offering a unique window into the still poorly understood evolution of the deep oceanic lithosphere. Here, integrated petrologic and Os-Li isotopic data are used to constrain the evolution and dynamics of emplacement for these garnet peridotite protoliths. The Purang peridotites show wide variations of chemical compositions (spinel Cr#: 0.2−0.8) and Os model ages (up to 2.0 Ga), thus representing a piece of heterogeneous oceanic mantle lithosphere. Dunite channels show two distinctive groups of Cr# of spinels and Os-isotope compositions, with the low- to medium-Cr# (0.2−0.6) and high-Cr# (0.7−0.8) dunites reflecting the reaction of host lherzolites/harzburgites with percolating mid-ocean ridge basalt−like and boninitic melts, respectively. This confirms recent subduction initiation-related melt percolation in the Purang peridotites. Coexisting olivines and pyroxenes in the peridotites show systematic Li elemental and isotopic disequilibrium, suggesting fast cooling of the peridotites to Li closure temperature shortly after the melt percolations, likely during exhumation of the peridotites onto the seafloor. This supports a close link between subduction initiation and tectonic emplacement of the Purang peridotites. Combined with other geological evidence, we suggest the Purang peridotites may originate from the deep part of old, thick oceanic lithosphere of the Neo-Tethys. This thick oceanic lithosphere was progressively weakened and thinned likely during widespread plume-lithosphere interaction, triggering the transformation of garnet peridotite protoliths to spinel peridotites. Subsequently, initiation of a new subduction zone along the lithospheric weakness caused rapid ascent and emplacement of the Purang peridotites at a nascent forearc.

Ni-in-garnet geothermometry in mantle rocks: a high pressure experimental recalibration between 1100 and 1325 °C

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule $$\left( {{\text{lnD}}_{{{\text{grt}}/{\text{olv}}}}^{{{\text{Ni}}}} ; \frac{{{\text{Ni}}_{{{\text{grt}}}} }}{{{\text{Ni}}_{{{\text{olv}}}} }}} \right)$$ lnD grt / olv Ni ; Ni grt Ni olv , the Ca mole fraction in garnet ($${\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}};$$ X grt Ca ; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ($${\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}};$$ X grt Cr ; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:$$T \left(^\circ{\rm C} \right)=\frac{-8254.568}{\left(\left( {\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}} \times 3.023 \right)+\left({\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}} \times 2.307 \right)+\left({\mathrm{lnD}}_{\frac{\mathrm{grt}}{\mathrm{olv}}}^{\mathrm{Ni}} - 2.639 \right)\right)}-273\pm 55$$ T ∘ C = - 8254.568 X grt Ca × 3.023 + X grt Cr × 2.307 + lnD grt olv Ni - 2.639 - 273 ± 55 where $${\mathrm{D}}_{\mathrm{grt}/\mathrm{olv}}^{\mathrm{Ni}}= \frac{{\mathrm{Ni}}_{\mathrm{grt}}}{{\mathrm{Ni}}_{\mathrm{olv}}},$$ D grt / olv Ni = Ni grt Ni olv , Ni is in ppm, $${\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}}$$ X grt Ca = Ca/(Ca + Fe + Mg) in garnet, and $${\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}}$$ X grt Cr = Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.

Deep subduction and exhumation of continental crust in the Alps

<p>The Adula Nappe in the Central Alps and the Pohorje Nappe in the Eastern Alps are among the highest-pressure metamorphic complexes in the Alps. In both cases, Variscan continental crust containing post-Variscan intrusions was subducted, during the Cenomanian-Turonian in the case of Pohorje and during the Eocene in the case of Adula.</p><p>The Pohorje Nappe is exceptional in that ultrahigh pressures of 3.0 to 4.0 GPa are recorded by different rocks contrasting in rheology: competent lenses of kyanite eclogite and garnet peridotite as well as the surrounding incompetent matrix of diamond-bearing paragneiss. If pressure had been strongly non-lithostatic, rheologically different rock types would be expected to record different pressures. This is not the case, which rather suggests near-lithostatic pressure and, consequently, subduction to >100 km depth. Lu-Hf ages for UHP metamorphism in eclogite and garnet peridotite are similar (c. 96–92 Ma). Paragneiss yielded Permian to Triassic zircon cores and Cretaceous (c. 92 Ma) rims grown during UHP metamorphism. Hence, the rocks were subducted and exhumed together as a coherent, although strongly deformed unit.</p><p>The Adula Nappe originated from the southern passive continental margin of Europe. It was buried in and exhumed from a south-dipping subduction zone after Europe-Adria continent collision. Previous interpretations as a tectonic mélange were based on the mixture of gneiss with eclogite and garnet peridotite lenses. However, the eclogites also record an older (Variscan) metamorphism and thus do not represent Mesozoic oceanic crust but pre-Alpine continental basement, just like the gneisses. The Alpine subduction culminated around 37 Ma. Alpine metamorphic pressures show a strong gradient from c. 1.2 GPa at the front of the nappe in the North to >3 GPa in garnet peridotite and eclogite in the southernmost part (e.g. Alpe Arami), over a north-south distance of only c. 40 km. In contrast to Pohorje, indications of UHP metamorphism have not yet been found in the gneissic matrix surrounding eclogite and peridotite. During exhumation, the nappe was intensely sheared and folded but stayed coherent and did not mix with the surrounding units.  The exhumation of the Adula from deep in the subduction zone is recorded by mylonitic shearing in the gneissic matrix. Structures, strain, and textures indicate strongly three-dimensional, non-plane-strain flow. Differential loading, not buoyancy, is proposed to have caused the exhumation.</p><p>The main results from these two case studies are: (1) Subduction of continental crust to mantle depth is real and not a misinterpretation of non-lithostatic pressure; (2) not all subducted units are mélanges but some stay coherent during subduction and exhumation.</p>

HP melting of eclogites and metasomatism of garnet peridotites in the Monte Duria area (Central Alps, N Italy): a proxy for the mafic crust-to-mantle mass transfer at subduction zones

<p>In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) Grt-peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both mafic and ultramafic rocks share a common HP peak at 2.8 GPa and 750 °C and post-peak static equilibration at 1.2 GPa and 850 °C (Tumiati et al., 2018).</p><p>Grt-peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO<sub>2</sub>, K<sub>2</sub>O, CO<sub>2</sub> and H<sub>2</sub>O. Peridotites also display LREE fractionation (La/Nd = 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. At Borgo, retrogressed Grt-peridotites show low strain domains characterised by garnet compositional layering, cut by a subsequent low-pressure chlorite foliation, in direct contact with migmatised eclogites. Kfs+Pl+Qz+Cpx interstitial pocket aggregates and Cpx+Kfs thin films around symplectites after omphacite parallel to the Zo+Omp+Grt foliation in the eclogites suggest that they underwent partial melting at HP.</p><p>The contact between garnet peridotites and associated eclogites is marked by a tremolitite layer. Tremolitites also occur as variably stretched layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, indicating that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl+Tc+Chl+Tr pseudomorphs after garnet, both crystallized in a static regime postdating the boudins formation, suggesting that they derive from a Grt-bearing precursor. Tremolitites have Mg#>0.90 and Al<sub>2</sub>O<sub>3</sub>=2.75 wt.% pointing to ultramafic compositions but also show enrichments in SiO<sub>2</sub>, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we calculated a log aH<sub>2</sub>O-X pseudosection at fixed P=3GPa and T=750°C to model the chemical interaction between the garnet peridotite and the eclogite-derived melt. Our results show that the interaction produces a Opx+Cpx+Grt assemblage + Amp+Phl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous Grt-websterite with amphibole and phlogopite. Both peridotites and tremolitites also show a selective enrichment in LILE recorded by amphiboles in the spinel stability field, indicating that a fluid-assisted metasomatic event occurred at LP conditions, leading to the formation of a Chl-foliation post-dating the garnet layering in peridotites, and the retrogression of Grt-websterites in tremolitites.</p><p>The Monte Duria area is a unique case study where we can observe eclogite-derived melt interacting with peridotite at HP and relatively HT, and could thus represents a proxy for the crust-to mantle mass transfer at great depths in subduction zones.</p><p> </p><p>Tumiati, S., Zanchetta, S., Pellegrino, L., Ferrario, C., Casartelli, S., Malaspina, N., 2018. Granulite-facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): Clues from srilankite- and sapphirine-bearing symplectites. J. Petrol. 59.</p>