A Highly Depleted and Subduction-Modified Mantle Beneath the Slow-Spreading Mohns Ridge

The Mohns Ridge is a very slow-spreading ridge that, together with the Knipovich Ridge, marks the boundary between the North American and Eurasian plates in the Norwegian-Greenland Sea. In this study, we report the major and trace element composition of spatially associated basalts and peridotites from a gabbro-peridotite complex ∼20 km west of the Mohns Ridge rift flank. Formation of the ∼4–5 Myr crustal section involved accretion of normal mid-ocean ridge basalts with Na-content suggesting derivation from a depleted mantle source. This is consistent with the degree of partial melting estimated for clinopyroxene poor harzburgites using the Cr-number of spinel (14%–18%) and rare earth element modeling of orthopyroxene (16%–24%) and reconstructed whole-rock composition (14%–20%). If all the melting took place beneath the paleo-Mohns Ridge, a crustal thickness of ∼7–8 km is expected, which is nearly double the observed thickness. Orthopyroxene trace elements are not consistent with typical fractional melting expected for mid-ocean ridges but rather resemble that seen in supra-subduction zone peridotites. The geochemistry of both the basalts and the peridotites suggests that a water-rich slab flux in the past has influenced the mantle source. In turn, this caused hydrous melting which increased the depletion of the pyroxene components, leading to a highly depleted mantle that is now underlying much of the Arctic Mid-Ocean Ridges and represents the source for the spreading related magmatism.publishedVersio

Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?

Granulite is preserved over large areas of partially eclogitized and hydrated rocks on Holsnøy, Bergen Arcs, Norway. The interfaces between granulite and eclogite are sharp on a hand-specimen scale and contain microstructural and compositional evidence for the mechanism of eclogitization. The interface studied here is undeformed with a continuous foliation from granulite through an eclogite ‘finger’ that protrudes into the granulite. Diopside in the granulite evolves continuously to omphacite in eclogite by increasing jadeite composition at a well-defined sequence of microstructures that involve pyroxene-amphibole intergrowths and symplectites. Plagioclase in the granulite develops a high density of zoisite and kyanite inclusions that increase in abundance prior to plagioclase breakdown in eclogite. The transition between granulite and eclogite is interpreted as indicating a pressure gradient. The observation that granulite is preserved adjacent to eclogite although it shows sufficient evidence of hydration such that metastability may not be a factor, suggests that eclogitization involves the generation of increased pressure due to reaction and rock weakening. The pyroxene and feldspar microstructures in the transition zone between granulite and eclogite are very similar to the transition zones between granulite and amphibolite elsewhere in the Bergen Arcs. Localized variation in pressure could be an explanation for concurrent eclogitization and amphibolitization of granulite at the same crustal level during orogenesis

Functionalizing Collagen Membranes with MSC-Conditioned Media Promotes Guided Bone Regeneration in Rat Calvarial Defects.

Functionalizing biomaterials with conditioned media (CM) from mesenchymal stromal cells (MSC) is a promising strategy for enhancing the outcomes of guided bone regeneration (GBR). This study aimed to evaluate the bone regenerative potential of collagen membranes (MEM) functionalized with CM from human bone marrow MSC (MEM-CM) in critical size rat calvarial defects. MEM-CM prepared via soaking (CM-SOAK) or soaking followed by lyophilization (CM-LYO) were applied to critical size rat calvarial defects. Control treatments included native MEM, MEM with rat MSC (CEL) and no treatment. New bone formation was analyzed via micro-CT (2 and 4 weeks) and histology (4 weeks). Greater radiographic new bone formation occurred at 2 weeks in the CM-LYO group vs. all other groups. After 4 weeks, only the CM-LYO group was superior to the untreated control group, whereas the CM-SOAK, CEL and native MEM groups were similar. Histologically, the regenerated tissues showed a combination of regular new bone and hybrid new bone, which formed within the membrane compartment and was characterized by the incorporation of mineralized MEM fibers. Areas of new bone formation and MEM mineralization were greatest in the CM-LYO group. Proteomic analysis of lyophilized CM revealed the enrichment of several proteins and biological processes related to bone formation. In summary, lyophilized MEM-CM enhanced new bone formation in rat calvarial defects, thus representing a novel 'off-the-shelf' strategy for GBR

Large-Scale Stable Isotope Alteration Around the Hydrothermal Carbonate-Replacement Cinco de Mayo Zn-Ag Deposit, Mexico

Carbonate-hosted hydrothermal deposits typically show narrow visible mineralogical and textural alteration halos, which inhibit exploration targeting. In contrast, hydrothermal modification of the country rock’s stable isotope composition usually extends far beyond the limited visible alteration. Hence, stable isotope studies should be an effective tool to aid exploration for carbonate-hosted deposits. Here we present new insight into the development of a large stable isotope alteration halo based on 910 O and C isotope analyses of carbonate veins and hydrothermally altered limestone hosting the Cinco de Mayo Pb-Zn-Ag (Au, Cu) carbonate replacement deposit (CRD), in Chihuahua, Mexico. Our results demonstrate that stable isotope alteration is consistent with reactive, magmatic fluid flow into unaltered limestone and represents a powerful tool for the characterization of these hydrothermal ore systems. Synmineralization veins are texturally and isotopically distinct from those formed during pre- and postmineralization diagenesis and fluid flow and show distinct gradients along the direction of mineralizing fluid flow: this appears to be a promising exploration vectoring tool. Downhole variations in wall-rock isotope values reveal aquifers and aquicludes and outline the principal hydrothermal flow paths. Furthermore, wall-rock δ18OVSMOW systematically decreases toward mineralization from ~23‰ to <17‰ over a distance of ~10 km, providing another vectoring tool. The extent of the stable isotope alteration halo likely reflects the overall fluid volume and areal extent of a fossil hydrothermal system, which may be expected to scale with the mineral endowment. This suggests that constraining the size, shape, and degree of isotopic alteration has direct application to mineral exploration by outlining the system and indicating the potential size of a deposit

In situ sequestration of atmospheric CO 2 at low temperature and surface cracking of serpentinized peridotite in mine shafts

The investigation of carbonate formation at low temperature during weathering of ultramafic rocks has become increasingly important as it represents an analog study for a cost-efficient carbon disposal strategy. Here we present new insight into carbonate formation under subarctic surface conditions obtained from extensively carbonated chromite mine shafts in the Feragen ultramafic body, E Norway. Carbonation proceeded by formation of mm- to cm-thick carbonate surface coatings on the serpentinized peridotite host rock. The surface coatings consist in most cases of pure lansfordite (MgCO 3·3H 2O). Heavy O- and C-isotopes are strongly enriched in the carbonate coatings relative to drip water samples from the mines and water samples from subaerial ponds. The major element composition of the water samples reveal that infiltrating rainwater reacts rapidly with the peridotite mainly due to brucite dissolution. Within the mines, discharging alkaline water forms thin films on walls and ceilings that evaporate upon circulation of cold, dry air. In mines with only one opening the carbonation is limited to the entrance area. In mines with a more efficient air circulation due to multiple entrances carbonation is found throughout the mine. Evaporation of the mine water results in lansfordite saturation and precipitation and is accompanied by a Rayleigh-type distillation effect causing enrichment of the heavy C- and O-isotopes in the liquid from which the carbonates precipitate. Formation of the carbonate coatings involved fragmentation of the underlying serpentinite substrate. Fragmentation proceeded by subcritical cracking, i.e. by a combination of substrate weakening due to brucite dissolution and stress build-up due to lansfordite precipitation in fractures close to the surface. Reaction-induced fracturing represents a positive feedback mechanism as it creates additional reactive surface area during the carbonation. Mining operations in the area ceased in the mid 1920s indicating that the described carbonation occurs on a time-scale relevant for the disposal of anthropogenic CO 2. © 2012 Elsevier B.V

Stress orientation-dependent reactions during metamorphism

© 2019 Geological Society of America. Grain-scale pressure variations have been recognized as an important driver for the formation of distinct mineral assemblages during high- and ultrahigh-pressure metamorphism. However, the effects of differential stress acting during hydration of granulite remain underexplored. Here, we present textural evidence for the orientation dependence of two distinct amphibolite-facies plagioclase grain boundary replacement assemblages that formed in response to differential stress during the early stage of lower-crustal hydration. The two assemblages, A1 (zoisite, kyanite, and quartz) and A2 (plagioclase and K-feldspar), are indicative of contemporaneous formation at local equilibrium conditions at ~700 °C and 1.1 GPa and 0.9 GPa, respectively. Mineral replacement was accompanied by minor redistribution of chemical components by the alteration fluid in response to the heterogeneous stress field and local equilibria. Thus, our observations provide new insight into the driving forces for pressure solution and indicate that differential stress and fluid-induced mass transfer may define the evolution of metamorphic assemblages

Trace-element mobilization during Ca-metasomatism along a major fluid conduit: Eclogitization of blueschist as a consequence of fluid-rock interaction

In the subduction complex of the Tianshan mountains, western China, massive blueschist is cross-cut by an eclogite-facies major fluid conduit surrounded by a reaction zone which is mainly composed of omphacite and garnet. Petrological as well as geochemical evidence suggest that formation of the vein and the eclogitic selvage around the vein was caused by fluid infiltration under peak metamorphic conditions of 21 ± 1.5 kbar and 510 ± 30 °C. The combination of whole-rock with mineral trace-element data as well as mass-balance calculations indicate that substantial differences exist between the unaltered host rock and the part of the system which was altered by fluid-rock interaction. These differences include: (1) depletion of mainly large-ion lithophile elements (LILE) and Li of up to 60% relative to their concentrations in the unaltered host rock; (2) an extreme enrichment of CaO (~115%), Sr and Pb (&gt;300%) in the altered parts of the vein-wall-rock system; (3) redistribution of heavy rare earth elements (HREE) from partly replaced rutile and recrystallized titanite in the blueschist-eclogite transition zone into newly grown garnet rims in the eclogitic selvage around the vein; (4) transformation of high Nb/Ta rutile into low Nb/Ta titanite which is associated with preferred mobilization of Nb over Ta; and (5) decoupling of Zr and Hf from Nb and Ta; the latter are depleted by ~30% relative to the unaltered blueschist host rock whereas the former are depleted by only ~10%. The prerequisite for the transformation of Ca-poor blueschist (6-7 wt.% CaO) into Ca-rich eclogite (up to 13 wt.% CaO) was the infiltration of a Ca-rich fluid. The release of trace elements can be attributed to partitioning of these elements into the passing fluid phase during dissolution-reprecipitation processes in the course of eclogitization. The reactivity of the precursor mineral assemblage and the chemical gradients between the reacting and passing fluid of the conduit are mainly responsible for trace-element mobilization in the studied samples. The suite of trace elements released upon fluid-induced eclogitization of the reactive wall-rock resembles that in island arc magmas showing strong enrichment of LIL elements, strong depletions in HFS elements and intermediate concentrations of REE. © 2009 Elsevier Ltd. All rights reserved

Kinetic study of chalcopyrite dissolution with iron(III) chloride in methanesulfonic acid

© 2018 The suitability of methanesulfonic acid as a copper lixiviant with ferric chloride as an oxidant was studied by analysing the leaching kinetics and by characterising solid residues from leach tests on a chalcopyrite-rich ore sample. The effects of temperature, initial acidity, ferric-ion concentration and particle size were determined. The leach kinetics were dependent on the temperature and particle size, whereas the acid and ferric concentrations had a minimal impact on the leaching rate within the ranges studied. Although a sulfur layer formed on the solid residue, the reaction mechanism could be modeled with the shrinking-core model with surface chemical-reaction control, which implies that lixiviant flow through the sulfur layer did not control the reaction rate. The apparent activation energy was 101 kJ mol-1as calculated by the Arrhenius and 'time-to-a-given-fraction' methods. The activation parameters of the reaction were an enthalpy (?H++) of 99.4 kJ mol-1, and an entropy (?S++) of -197 J mol-1K-1as calculated by using transition state theory and the Eyring equation