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We have modeled the genesis of primary mantle-derived magma to explore the controls exerted on its Ni-Cu ore potential by water content, pressure, and mantle potential temperature (Tp). During decompression melting, Ni concentration in primary magma decreases with an increasing degree of melting, which is in contradiction to long-held understanding obtained from previous isobaric melting models. Pressure exerts a first-order control on the ore potential of primary plume-derived melt, such that plumes rising beneath thick lithosphere with melting paths terminating at relatively high pressure generate Ni-rich melts. Additionally, as plumes with higher Tp produce more Ni-rich melt at a higher pressure, the magmatism related to hotter plume-centers may have the greatest ore potential. On the other hand, the strong dependence of Cu behavior upon the presence or absence of residual sulfide is partly countered in decompression melting. Significant influences of mantle-contained water on Ni and Cu partitioning are restricted to low-degree melting. While release of H2O in lithosphere delamination may trigger voluminous magmatism, the Ni concentration in the melt is far lower than in melt generated from plumes. Furthermore, if isobaric melting dominates when the subcontinental lithospheric mantle (SCLM) is heated by underlying hotter plumes, the plume-lithosphere interaction plays no active role in the Ni ore potential of primary magma because derived melt volumes are relatively small. In subduction zones, flux-melting of the mantle wedge tends to generate cool Ni-poor melts, however hot subduction zones may produce magmas with increased metal concentrations. Overall, the anticipated ranges of Ni contents in primary melts are strongly controlled by tectonic setting, with a range of 100-300 ppm in subduction zones, 230-450 ppm in mid-ocean ridges, and 500-1300 ppm in plume suites. There are only minor differences in the Cu concentrations of primitive magmas generated from diverse tectonic settings, despite the variations in Cu partitioning behaviors.

Speckling of sphalerite with micrometer-sized blebs of chalcopyrite is usually referred to as \"chalcopyrite disease.\" Fe-rich sphalerites are particularly prone to chalcopyrite disease. Considering the low degree of solid solution between sphalerite and chalcopyrite, exsolution is discarded as a process to explain the development of chalcopyrite disease. Diffusion-controlled replacement of Fe by Cu, and sphalerite-chalcopyrite co-precipitation are invoked as the most probable mechanisms. Although metamorphism is expected to dispel inhomogeneities through recrystallization, chalcopyrite disease interestingly appears unaffected and to be quite common in metamorphosed sulfide ores. We have conducted experiments on different bulk compositions in the system ZnS-PbS-FeS-Cu2S-As2S3 at 600 °C and annealed the run products containing melt at 350 °C to evaluate the role of sulfide partial melting, if any, in the development of chalcopyrite disease. The results indicate that chalcopyrite blebs developed only in those sphalerites that contained Fe and in which S atoms were in excess over Fe + Zn atoms. Also it was observed that the occurrence of Fe-bearing sphalerite and the sulfide partial melt (that invariably was S-deficient and Cu-enriched) in direct contact with each other was necessary for the chalcopyrite blebs to form. We propose nonstoichiometry-driven diffusion of Cu as the mechanism and sulfide partial melting as the principal causative factor behind the development of chalcopyrite disease in sphalerite. Chalcopyrite disease thus may be used as an easily identifiable potential indicator of sulfide partial melting in metamorphosed base metal sulfide deposits.

The alteration of magmatic monazite and its consequences for monazite geochronology are explored in granitoids from the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Biotite-granite samples were collected in two plutons emplaced slightly before the main tectono-metamorphic event: the first one, a massive granite (Shimokuhara) adjoins schistose rocks affected by greenschist facies metamorphism; and the second, a gneissose granite (Namera) adjoins migmatitic gneiss that experienced upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization along cracks and veinlets. It is variably replaced by allanite+apatite±xenotime±Th-U-rich phases. The outermost rims of primary domains yield a weighted average Pb/ U date of 102 ± 2 Ma while the Th-U phases show Th-U-Pb dates of 58 ± 5 and 15 to 14 ± 2–3 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by secondary domains enriched in Ca, Y, U, P, and containing numerous inclusions. The secondary domains preserve concordant Pb/ U dates spreading from 102 ± 3 to 91 ± 2 Ma while primary domain analyses are commonly discordant and range from 116 to 101 Ma. Monazite alteration textures in the two granites chiefly reflect differences in their post-magmatic histories. In the massive granite, monazite replacement occurred via a nearly stoichiometrically balanced reaction reflecting interaction with an aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration of the granitic assemblage, likely below 500 °C. In the gneissose granite, a small amount of anatectic melt, probably derived from the neighboring metasedimentary rocks, was responsible for pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. Regardless of whether monazite underwent replacement or recrystallization, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 and 106 ± 5 Ma). Conversely, the age of monazite alteration is not easily resolved. Monazite replacement in the massive granite might be constrained using the Th-U-rich alteration products; with due caution and despite probable radiogenic Pb loss, the oldest date of 58 ± 5 Ma could be ascribed to chloritization during final exhumation of the granite. The spread in apparently concordant Pb/ U dates for secondary domains in the gneissose granite is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long high-temperature (HT) history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.

We present major, trace element, and Pb‐Sr‐Nd‐Hf isotope data for Quaternary basalt and basaltic andesite lavas from cross‐chain volcanoes in the northern Izu (N‐Izu) arc. Lavas from Izu‐Oshima, Toshima, Udonejima, and Niijima islands show consistent chemical changes with depth to the Wadati‐Benioff zone, from 120 km beneath Izu‐Oshima to 180 km beneath Niijima. Lavas from Izu‐Oshima at the volcanic front (VF) have elevated concentrations of large ion lithophile elements (LILEs), whereas rear‐arc (RA) lavas are rich in light rare earth elements (LREEs) and high field strength elements (HFSEs). VF lavas also have more radiogenic Pb, Nd, Sr, and Hf isotopic compositions. We have used the Arc Basalt Simulator version 3 (ABS3) to examine the mass balance of slab dehydration and melting and slab fluid/melt‐fluxed mantle melting and to quantitatively evaluate magma genesis beneath N‐Izu. The results suggest that the slab‐derived fluids/melts are derived from ∼20% sediment and ∼80% altered oceanic crust, the slab fluid is generated by slab dehydration for the VF magmas at 3.3–3.5 GPa/660°C–700°C, and slab melt for RA magmas is supplied at 3.4–4.4 GPa/830°C–890°C. The degree of fluxed melting of the mantle wedge varies between 17% and 28% (VF) and 6% and 22% (RA), with a slab flux fraction of 2%–4.5% (VF fluid) to 1%–1.5% (RA melt), and at melting depths 1–2.5 GPa (VF) and 2.4–2.8 GPa (RA). These conditions are consistent with a model whereby shallow, relatively low temperature slab fluids contribute to VF basalt genesis, whereas deeper and hotter slab melts control formation of RA basalts. The low‐temperature slab dehydration is the cause of elevated Ba/Th in VF basalt due mainly to breakdown of lawsonite, whereas deeper breakdown of phengite by slab melting is the cause of elevated K and Rb in RA basalts. Melting in the garnet stability field, and at lower degrees of partial melting, is required for the elevated LILEs, LREEs, and HFSEs observed in the RA basalts. Less radiogenic Sr, Nd, Hf, and Pb in RA basalts are all attributable to lesser slab flux additions. The low H2O predicted for RA basalt magmas (<1.5 wt %) relative to that in VF basalt magmas (5–8 wt %) is also due to melt addition rather than fluid. All these conclusions are broadly consistent with existing models; however, in this study they are quantitatively confirmed by the geochemical mass balance deduced from petrological ABS3 model. Overall, the P‐T‐X(H2O) structure of the slab and the mantle wedge exert the primary controls on arc basalt genesis.

Lunar silicic rocks were first identified by granitic fragments found in samples brought to Earth by the Apollo missions, followed by the discovery of silicic domes on the lunar surface through remote sensing. Although these silicic lithologies are thought to make up a small portion of the lunar crust, their presence indicates that lunar crustal evolution is more complex than originally thought. Models currently used to describe the formation of silicic lithologies on the Moon include in situ differentiation of a magma, magma differentiation with silicate liquid immiscibility, and partial melting of the crust. This study focuses on testing a crustal melting model through partial melting experiments on compositions representing lithologies spatially associated with the silicic domes. The experiments were guided by the results of modeling melting temperatures and residual melt compositions of possible protoliths for lunar silicic rocks using the thermodynamic modeling software, rhyolite-MELTS. Rhyolite-MELTS simulations predict liquidus temperatures of 950-1040 °C for lunar granites under anhydrous conditions, which guided the temperature range for the experiments. Monzogabbro, alkali gabbronorite, and KREEP basalt were identified as potential protoliths due to their ages, locations on the Moon (i.e., located near observed silicic domes), chemically evolved compositions, and the results from rhyolite-MELTS modeling. Partial melting experiments, using mixtures of reagent grade oxide powders representing bulk rock compositions of these rock types, were carried out at atmospheric pressure over the temperature range of 900-1100 °C. Because all lunar granite samples and remotely sensed domes have an elevated abundance of Th, some of the mixtures were doped with Th to observe its partitioning behavior. Run products show that at temperatures of 1050 and 1100 °C, melts of the three protoliths are not silicic in nature (i.e., they have <63 wt% SiO2). By 1000 °C, melts of both monzogabbro and alkali gabbronorite approach the composition of granite, but are also characterized by immiscible Si-rich and Fe-rich liquids. Furthermore, Th strongly partitions into the Fe-rich, and not the Si-rich glass in all experimental runs. Our work provides important constraints on the mechanism of silicic melt formation on the Moon. The observed high-Th content of lunar granite is difficult to explain by silicate liquid immiscibility, because through this process, Th is not fractionated into the Si-rich phase. Results of our experiments and modeling suggests that silicic lunar rocks could be produced from monzogabbro and alkali gabbronorite protoliths by partial melting at T < 1000 °C. Additionally, we speculate that at higher pressures (P ≥ 0.005 GPa), the observed immiscibility in the partial melting experiments would be suppressed.

The pressure–temperature ( P – T ) conditions for producing adakite/tonalite–trondhjemite–granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10–15 kbar and 800–1,050 °C to investigate the major and trace elements of melts and residual minerals and further constrain the P – T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase + amphibole ± garnet) at 10–15 kbar and 800 °C, garnet granulite (plagioclase + amphibole + garnet + clinopyroxene + orthopyroxene) at 12.5 kbar and 900 °C, two-pyroxene granulite (plagioclase + clinopyroxene + orthopyroxene ± amphibole) at 10 kbar and 900 °C and 10–12.5 kbar and 1,000 °C, garnet pyroxenite (garnet + clinopyroxene ± amphibole) at 13.5–15 kbar and 900–1,000 °C, and pyroxenite (clinopyroxene + orthopyroxene) at 15 kbar and 1,050 °C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with <20 wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements (HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are >20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10–40 wt% melting of lower crust composition at 10–12.5 kbar and 800–900 °C and 15 kbar and 800 °C closely resemble adakite/TTG rocks. TiO 2 contents of the 1,000–1,050 °C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10–12.5 kbar and 1,000 °C and 15 kbar and 1,050 °C have elevated HREE, whereas partial melts at 13.5–15 kbar and 900–1,000 °C in equilibrium with >20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P – T conditions for producing adakite/TTG from mafic lower crust are 800–950 °C and 10–12.5 kbar (corresponding to a depth of 30–40 km), whereas a depth of >45–50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.

The seismic low-velocity zone (LVZ) of the upper mantle is generally associated with a low-viscosity asthenosphere that has a key role in decoupling tectonic plates from the mantle 1 . However, the origin of the LVZ remains unclear. Some studies attribute its low seismic velocities to a small amount of partial melt of minerals in the mantle 2 , 3 , whereas others attribute them to solid-state mechanisms near the solidus 4 – 6 or the effect of its volatile contents 6 . Observations of shear attenuation provide additional constraints on the origin of the LVZ 7 . On the basis of the interpretation of global three-dimensional shear attenuation and velocity models, here we report partial melt occurring within the LVZ. We observe that partial melting down to 150–200 kilometres beneath mid-ocean ridges, major hotspots and back-arc regions feeds the asthenosphere. A small part of this melt (less than 0.30 per cent) remains trapped within the oceanic LVZ. Melt is mostly absent under continental regions. The amount of melt increases with plate velocity, increasing substantially for plate velocities of between 3 centimetres per year and 5 centimetres per year. This finding is consistent with previous observations of mantle crystal alignment underneath tectonic plates 8 . Our observations suggest that by reducing viscosity 9 melt facilitates plate motion and large-scale crystal alignment in the asthenosphere. Analysis of global three-dimensional shear attenuation and velocity models implies that partial melting in the seismic low-velocity zone enables motion of oceanic plates by reducing the viscosity of the asthenosphere.

Constraining the source, genesis, and evolution of Archaean felsic crust is key to understanding the growth and stabilization of cratons. The Akia Terrane, part of the North Atlantic Craton, West Greenland, is comprised of Meso-to-Neoarchaean orthogneiss, with associated supracrustal rocks. We report zircon U–Pb and Lu–Hf isotope data, and whole-rock geochemistry, from samples of gneiss and supracrustals from the northern Akia Terrane, including from the Finnefjeld Orthogneiss Complex, which has recently been interpreted as an impact structure. Isotope data record two major episodes of continental crust production at ca. 3.2 and 3.0 Ga. Minor ca. 2.7 and 2.5 Ga magmatic events have more evolved εHf, interpreted as reworking of existing crust perhaps linked to terrane assembly. Felsic rocks from the Finnefjeld Orthogneiss Complex were derived from the same source at the same time as the surrounding tonalites, but from shallower melting, requiring any bolide-driven melting event to have occurred almost simultaneously alongside the production of the surrounding crust. A simpler alternative has the Finnefjeld Complex and surrounding tonalite representing the coeval genesis of evolved crust over a substantial lithospheric depth. Hafnium isotope data from the two major Mesoarchaean crust-forming episodes record a contribution from older mafic Eoarchaean crust. Invoking the involvement of an Eoarchaean root in the growth of younger Mesoarchaean crust puts important constraints on geodynamic models of the formation of the discrete terranes that ultimately assembled to form Earth’s cratons.

The Earth’s uppermost asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or silicate melts. We report the electrical conductivity of both H 2 O-bearing (0–6 wt% H 2 O) and CO 2 -bearing (0.5 wt% CO 2 ) basaltic melts at 2 GPa and 1,473–1,923 K measured using impedance spectroscopy in a piston-cylinder apparatus. CO 2 hardly affects conductivity at such a concentration level. The effect of water on the conductivity of basaltic melt is markedly larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation log σ = 2.172 − (860.82 − 204.46 w 0.5 )/( T  − 1146.8), where σ is the electrical conductivity in S/m, T is the temperature in K, and w is the H 2 O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.006 between minerals and melt, 2 vol% of melt will account for the observed electrical conductivity in the seismic low-velocity zone. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube-like structures, even less than 1 vol% of hydrous melt, may be sufficient to produce the observed conductivity. We also show that ~1 vol% of hydrous melts are likely to be stable in the low-velocity zone, if the uncertainties in mantle water contents, in water partition coefficients, and in the effect of water on the melting point of peridotite are properly considered.

Experiments on MORB + 4 wt% H2O at 0.8–2.8 GPa and 700–950 °C (Liu in High pressure phase equilibria involving the amphibolite–eclogite transformation. PhD dissertation, Stanford University, Stanford, California, 1997; Liu et al. in Earth Planet Sci Lett 143:161–171, 1996) were reexamined for their major and trace element melt compositions and melting relations. Degree of melting diminishes at greater pressures, with corresponding evolution of melt from andesitic at the lowest pressures and hottest temperatures to high-silica rhyolitic at the greatest pressure and coolest temperature. Quartz contributes greatly to the production of near-solidus melts of basaltic eclogite, with the result that melt productivity falls markedly following quartz exhaustion. This limits the extent of melting attainable in the basaltic eclogite portions of sub-arc subducting plates to no more than ~ 2 × the modal wt% quartz in the mafic eclogite protolith. Synthesized residual mineral assemblages lack an epidote-series mineral at temperatures > 750 °C, and as a result, melts from the rutile eclogite and rutile-amphibole eclogite facies have elevated concentrations of light rare earth elements, U, Th, have elevated Ba, K, and Sr, high Sr/Y, and are strongly depleted in Nb, Y, and the heavy rare earth elements. Models of eclogite partial melt reacting with peridotite of the mantle wedge reproduce major and trace element characteristics of parental arc magmas so long as the proportions of infiltrating melt to peridotite are relatively high, consistent with channelized ascent. Melt mass is estimated to increase roughly three- to ten-fold, consistent with H2O concentrations of 3–7 wt% in the magmas produced by reaction. Partial melts of subducting basaltic eclogite are predicted to have positive Sr concentration anomalies, relative to Ce and Nd, that persist through melt-peridotite reactions. Primitive arc magmas commonly have positive Sr anomalies, whereas such anomalies are smaller in estimates of the bulk continental crust. Overall, Sr anomalies diminish passing from primitive to more evolved arc volcanic rocks, consistent with extensive mineral-melt differentiation (crystallization, partial remelting) involving plagioclase. On the order of 50 wt% differentiation would be necessary to eliminate Sr positive anomalies, based on geochemical variations in the Cascade and western Aleutian magmatic arcs. Loss to the mantle of cumulates and restites with high Sr anomalies, in abundances broadly equal to the mass of the preserved crust, would be required to form the continents via processes similar to present-day subduction magmatism.