Olivine-rich Rocks Research Articles

The structure of the asteroid 4 Vesta as revealed by models of planet-scale collisions

Asteroid 4 Vesta seems to be a major intact protoplanet, with a surface composition similar to that of the HED (howardite-eucrite-diogenite) meteorites. The southern hemisphere is dominated by a giant impact scar, but previous impact models have failed to reproduce the observed topography. The recent discovery that Vesta's southern hemisphere is dominated by two overlapping basins provides an opportunity to model Vesta's topography more accurately. Here we report three-dimensional simulations of Vesta's global evolution under two overlapping planet-scale collisions. We closely reproduce its observed shape, and provide maps of impact excavation and ejecta deposition. Spiral patterns observed in the younger basin Rheasilvia, about one billion years old, are attributed to Coriolis forces during crater collapse. Surface materials exposed in the north come from a depth of about 20 kilometres, according to our models, whereas materials exposed inside the southern double-excavation come from depths of about 60-100 kilometres. If Vesta began as a layered, completely differentiated protoplanet, then our model predicts large areas of pure diogenites and olivine-rich rocks. These are not seen, possibly implying that the outer 100 kilometres or so of Vesta is composed mainly of a basaltic crust (eucrites) with ultramafic intrusions (diogenites).

Melt-Rock Reaction in the Mantle: Mantle Troctolites from the Parece Vela Ancient Back-Arc Spreading Center

The Godzilla Megamullion is a giant oceanic core complex located in an extinct slow-spreading segment of the Parece Vela Basin (Philippine Sea). It exposes lower crust and mantle, offering an opportunity to unravel the architecture, composition, and magmatic processes operating in the shallow mantle of a back-arc basin. Here we present data on primitive troctolites and associated olivinegabbros close to the breakaway zone of the megamullion. The troctolites are subdivided into olivine (olivine460 vol. %) and plagioclase (plagioclase460 vol. %) sub-types. Both have textures and mineral compositions suggesting formation through melt^rock reaction.We suggest that the Ol-troctolites formed by dissolution and disaggregation of dunite with crystallization of new plagioclase and diopside by a mid-ocean ridge basalt (MORB)-like melt, producing a high-Mg melt.The Pl-troctolites probably formed by local dissolution of an anorthositic cumulate and crystallization of olivine, diopside and new plagioclase owing to mixing the high-Mg melt residual to the formation of the Ol-troctolites with melt crystallizing an anorthositic cumulate. We show that the Oland Pl-troctolites and associated Ol-gabbros were strongly influenced by melt^rock reaction with the enclosing peridotite, reflecting solidification by reaction and conductive cooling of ascending melts in the mantle. The effects of the melt^rock reaction account for a sharp decrease in plagioclase anorthite content with respect to diopside Mg# and olivine forsterite content. Contrasting olivine nickel contents can constrain the environment of formation of olivine-rich rocks in the lower crust and mantle, the extent and nature of the processes by which they form, and the melt compositions involved in the reaction process. Comparison of olivine nickel contents in troctolite and gabbro occurrences in oceanic and ophiolite settings reveals a set of relationships consistent with this. Olivine gabbros from the 1508 m Hole 735B Atlantis Bank gabbro section have low nickel contents, whereas those from the 1400 m Hole U1309D Atlantis Massif gabbro section have high nickel. This is consistent with less-reacted infiltrating melt at the latter, and suggests that U1309D represents a deeper section of the lower crust near the crust^mantle transition, whereas the former has been shown to have crystallized near the dike^gabbro transition. Godzilla troctolites and gabbros bear clear similarities to East Pacific Rise (EPR) troctolites and gabbroic segregations drilled in dunites crosscutting residual mantle peridotite at Hess Deep Site 895.The latter represent multi-stage crystallization of stagnant melts in conduits beneath the EPR.Thus, it appears that the processes that formed the Godzilla troctolites occur beneath ocean ridges across the spreading rate spectrum.

Geochemistry of a continental site of serpentinization, the Tablelands Ophiolite, Gros Morne National Park: A Mars analogue

The presence of aqueously altered, olivine-rich rocks along with carbonate on Mars suggest that serpentinization may have occurred in the past and may be occurring presently in the subsurface, and possibly contributing methane (CH4) to the martian atmosphere. Serpentinization, the hydration of olivine in ultramafic rocks, yields ultra-basic fluids (pH⩾10) with unique chemistry (i.e. Ca2+–OH− waters) and hydrogen gas, which can support abiogenic production of hydrocarbons (i.e. Fischer–Tropsch Type synthesis) and subsurface chemosynthetic metabolisms. Mars analogue sites of present-day serpentinization can be used to determine what geochemical measurements are required for determining the source methane at sites of serpentinization on Earth and possibly on Mars. The Tablelands Ophiolite is a continental site of present-day serpentinization and a Mars analogue due to the presence of altered olivine-rich ultramafic rocks with both carbonate and serpentine signatures. This study describes the geochemical indicators of present-day serpentinization as evidenced by meteoric ultra-basic reducing groundwater discharging from ultramafic rocks, and travertine and calcium carbonate sediment, which form at the discharge points of the springs. Dissolved hydrogen concentrations (0.06–1.20mg/L) and methane (0.04–0.30mg/L) with δ13CCH4 values (-28.5‰ to -15.6‰) were measured in the spring fluids. Molecular and isotopic analyses of CH4, ethane, propane, butane, pentane and hexane suggest a non-microbial source of methane, and attribute the origin of methane and higher hydrocarbon gases to either thermogenic or abiogenic pathways.

Reply to Schuiling: Last things last

Schuiling's letter (1) concerning my commentary on geoengineering (2) makes two major claims: (i) decarbonization of the global industrial metabolism is not driven by climate protection but by the exhaustion dynamics of fossil fuels, and (ii) there is an attractive option for artificial CO2 removal from the atmosphere, namely, milling of olivine-rich rocks.

Mafic and ultramafic rocks, and platinum mineralisation potential, in the Longwood Range, Southland, New Zealand

Intrusive rocks in the Longwood Range represent a component of the Permian Brook Street Terrane. They include diffusely layered, cumulate-textured olivine gabbro, troctolite, and gabbro, and gradations into non-cumulate gabbro and gabbronorite. Volumetrically small ultramafic layers occur (plagioclase wehrlite), and thin veins of felsic rocks ranging from quartz diorite to trondhjemite. Primary olivine, plagioclase, clinopyroxene, and subordinate orthopyroxene and hornblende are commonly altered or metamorphosed to amphiboles, minor spinel, magnetite, chlorite, biotite and clinozoisite, and serpentine in olivine-rich rocks. Accessory primary Ti-bearing magnetite and ilmenite occur, and trace Cr-magnetite is characteristic of olivine-rich rocks. Trace pyrrhotite, chalcopyrite, pentlandite, and pyrite could reflect equilibrated late magmatic, and alteration-derived phases. Key petrochemical characteristics of the rock suite are high Mg, Al, Ca, and Sr contents, and low alkali, LILE, and sulfur contents. Platinum and Pd are locally enriched in drill-hole intercepts, but zones appear unrelated to rock type, magnetic properties, or to S, Cu, Ni, Cr, or Au values. Local platinum group element (PGE) enrichment in altered rocks implies metamorphic and/or hydrothermal redistribution. Pervasive PGE enrichment in Longwood rocks is an indicator of potential ‘fertility’, but evidence is currently lacking for the precipitation of primary stratiform PGE accumulations from a sulfide liquid saturated magma.

Reproducing hydrogeochemical conditions triggering the formation of carbonate and phyllosilicate alteration mineral assemblages on Mars (Nili Fossae region)

[1] The cooccurrence of Mg-Fe-Ca carbonate and stratigraphically lower, phyllosilicate-bearing rock units, identified in the Nili Fossae region on Mars, may reveal a CO2-driven aqueous alteration process on ancient Mars. We reproduce hydrogeochemical conditions for concurrent carbonate and phyllosilicate formation in rock columns by using hydrogeochemical one-dimensional transport modeling, which is based on chemical equilibrium thermodynamics. Our models assume the isochemical, low-temperature equilibration of ultramafic rocks with pure water at varying atmospheric CO2 partial pressure, water-to-rock ratio, temperature, and rock composition. Equilibration among (1) mineral assemblages of olivine-rich rocks (18–28 wt% MgO), (2) pure water (water-to-rock ratio: 500–50 mol mol−1), (3) CO2 transferred from the atmosphere (CO2 partial pressure lower than ∼2 bars and higher than ∼0.01 bar) at ∼278 K, and (4) 100,000 years of diffusive mass transport create hydrogeochemical conditions enabling the concurrent formation of Mg-rich Mg-Fe-Ca solid-solution carbonate and stratigraphically lower phyllosilicate-rich alteration assemblages in rock columns. The modeled composition of alteration mineral assemblages, their depth distribution, and carbonate composition are similar to the observations from the Nili Fossae region. Higher equilibration temperature (373 K) results in the formation of Ca-dominated Ca-Mg carbonate instead of Mg-rich Mg-Fe-Ca carbonate and in an inverse stratification with phyllosilicate-rich units overlying carbonate-rich units. Modeling results suggest that rocks in the Nili Fossae region, hosting the carbonate and phyllosilicate mineral assemblage, have been exposed to a groundwater alteration process driven by atmospheric CO2 partial pressures ranging between ∼2 bars and ∼0.01 bar at low temperature (∼278 K).

An experimental study of the effects of surface tension in homogenizing perturbations in melt fraction

Static annealing experiments were conducted on fine-grained samples of a partially molten, olivine-rich rock to explore the role of interfacial tension driven flow in redistributing melt within the sample. A sample of fine-grained olivine + 20% chromite was prepared with an initially homogeneously distributed melt fraction of 0.04. When this sample was deformed in torsion, the melt segregated into distinct melt-rich bands uniformly spaced throughout the sample, as demonstrated in prior studies. Portions of this sample were then statically annealed for different lengths of time to observe the homogenization of the melt distribution. The evolution of the melt distribution in experimental samples was compared to numerical models based on formulations for interfacial tension driven flow that do or do not incorporate the effects of dissolution/precipitation coupled with diffusive mass transfer of components of the solid phase through the liquid phase (dissolution/diffusion/precipitation). The results indicate that, at the grain size and perturbation length scales in these samples, dissolution/diffusion/precipitation in response to the chemical potential gradient arising from the curvature of solid–liquid phase boundaries at triple junctions plays a significant role in accommodating interfacial tension driven flow. These experiments provide a valuable test for theory and allow us to place constraints on the homogenization rate of perturbations in melt fraction in rocks with a relatively simple composition. The results contribute to increasing our understanding of the nature of melt-rich, high-permeability pathways that may facilitate melt extraction from the upper mantle.

PGE geochemistry of the Eagle Ni–Cu–(PGE) deposit, Upper Michigan: constraints on ore genesis in a dynamic magma conduit

The Eagle Ni–Cu–(PGE) deposit is hosted in mafic–ultramafic intrusive rocks associated with the Marquette–Baraga dike swarm in northern Michigan. Sulfide mineralization formed in association with picritic magmatism in a dynamic magma conduit during the early stages in the development of the ~1.1 Ga Midcontinent Rift System. Four main types of sulfide mineralization have been recognized within the Eagle deposit: (1) disseminated sulfides in olivine-rich rocks; (2) rocks with semi-massive sulfides located both above and below the massive sulfide zone; (3) massive sulfides; and (4) sulfide veins in sedimentary country rocks. The disseminated, massive and lower semi-massive sulfide zones are typically composed of pyrrhotite, pentlandite and chalcopyrite, whereas the upper semi-massive sulfide ore zone also contains pyrrhotite, pentlandite, and chalcopyrite, but has higher cubanite content. Three distinct types of sulfide mineralization are present in the massive sulfide zone: IPGE-rich, PPGE-rich, and PGE-unfractioned. The lower and upper semi-massive sulfide zones have different PGE compositions. Samples from the lower semi-massive sulfide zone are characterized by unfractionated PGE patterns, whereas those from the upper semi-massive sulfide zone show moderate depletion in IPGE and moderate enrichment in PPGE. The mantle-normalized PGE patterns of unfractionated massive and lower semi-massive sulfides are subparallel to those of the disseminated sulfides. The results of numerical modeling using PGE concentrations recalculated to 100% sulfide (i.e., PGE tenors) and partition coefficients of PGE between sulfide liquid and magma indicate that the disseminated and unfractionated massive sulfide mineralization formed by the accumulation of primary sulfide liquids with similar R factors between 200 and 300. In contrast, the modeled R factor for the lower semi-massive sulfide zone is <100. The fractionated sulfide zones such as those of the IPGE-rich and PPGE-rich massive sulfides and the upper semi-massive sulfide zone can be explained by fractional crystallization of monosulfide solid solution from sulfide liquids. The results of numerical modeling indicate that the sulfide minerals in the upper semi-massive sulfide zone are the products of crystallization of fractionated sulfide liquids derived from a primary sulfide liquid with an R factor similar to that for the disseminated sulfide mineralization. Interestingly, the modeled parental sulfide liquid for the IPGE-rich and PPGE-rich massive sulfide zones has a higher R factor (~400) than that for the unfractionated massive sulfide mineralization. Except one sample which has unusually high IPGE and PPGE contents, all other samples from the Cu-rich sulfide veins in the footwall of the intrusion are highly depleted in IPGE and enriched in PPGE. These signatures are generally consistent with highly fractionated sulfide liquids expelled from crystallizing sulfide liquid. Collectively, our data suggest that at least four primary sulfide liquids with different R factors (<100, 200–300, ~400) were involved in the formation of the Eagle magmatic sulfide deposit. We envision that the immiscible sulfide liquids were transported from depth by multiple pulses of magma passing through the Eagle conduit system. The sulfide liquids were deposited in the widened part of the conduit system due to the decreasing velocity of magma flow. The presence of abundant olivine in some of the sulfide ore zones indicates that the ascending magma also carried olivine crystals. Sulfide saturation was attained in the parental magma due in large part to the assimilation of country rock sulfur at depth.

Dependence of dislocation creep of dunite on oxygen fugacity: Implications for viscosity variations in Earth's mantle

[1] Significant variations in flow behavior are known to exist both vertically and laterally in Earth's upper mantle. The sources of such variation may be thermal, compositional, or reflect differences in the chemical activity of components, such as oxygen, silica, and water. We report on the effects of oxygen fugacity on dislocation creep of dunite, with a view to understanding potential strength heterogeneity in the mantle. Although room pressure experiments on single crystals of olivine have shown a clear dependence of creep rate on oxygen fugacity, no prior deformation study of polycrystalline olivine-rich rocks has demonstrated such a dependency under high-pressure conditions. In this study we performed a series of dry creep experiments on a natural dunite under carefully controlled thermochemical conditions, including oxygen fugacity. The samples, cored from coarse-grained Aheim dunite with a grain size of ∼0.9 mm, were deformed under triaxial compression at oxygen fugacities fixed by either the iron/wustite or the nickel/nickel oxide solid state buffers, temperatures between 1150° and 1277°C, and differential stresses up to 300 MPa. The results of a global fit to all experimental data indicate a power law dependence of creep rate on oxygen fugacity, with an oxygen fugacity exponent of m = 0.20 ± 0.01, n = 3.6 ± 0.1, A = 102.6±0.3 s−1 MPa−3.6 Pa−0.2, and an activation energy for creep of 449 ± 7 kJ/mol. This activation energy is significantly less than the commonly used value of 535 kJ/mol because the earlier experiments made no corrections for the effects of oxygen fugacity. When applied to planetary interiors, an increase in oxygen fugacity by a factor of ∼103.5, from the iron/wustite to the fayalite-magnetite-quartz buffers, will result in a factor of ∼5 decrease in viscosity.

Low silica activity for hydrogen generation during serpentinization: An example of natural serpentinites in the Mineoka ophiolite complex, central Japan

The textural evolution in the serpentinite of the Mineoka ophiolite complex has been investigated to constrain the natural environment for hydrogen production in the serpentinite-hosted hydrothermal vent systems. Textural relations of the serpentinites from the Mineoka ophiolite indicate at least two stages in the process of serpentinization, with the replacement of olivine by a mesh texture of serpentine and brucite, followed by the development of magnetite-bearing or -free serpentine veins. The generation of hydrogen during serpentinization, which accompanies the formation of magnetite, involves a silica-depletion reaction, as evidenced by the low abundance of serpentine in the magnetite-bearing veins and the absence of magnetite in pseudomorphs of orthopyroxene. Direct evidence for the production of hydrogen and strongly reducing conditions is provided by CH 4 and H 2-bearing inclusions in relic olivine crystals; the production of methane and hydrogen may have provided a suitable environment for microbial activity in hydrothermal vent systems along the seafloor. Our results indicate that low silica activity plays a key role in the generation of hydrogen during serpentinization, and that low silica activity environments are possible in olivine-rich rocks such as dunite, or during local disequilibrium in other silica-poor rocks in the mantle lithosphere.

Variations in Os isotope ratios of pyrrhotite as a result of water–rock and magma–rock interaction: Constraints from Virginia Formation–Duluth Complex contact zones

Os isotope ratios in pyrrhotite-bearing pelitic rocks of the ∼1.85 Ga Virginia Formation are variable, with perturbations linked to the emplacement of the ∼1.1 Ga Duluth Complex. Pyrrhotite in footwall rocks of the contact aureole show evidence for a mixing event at 1.1 Ga involving a low 187Os/ 188Os fluid. However, because rocks with perturbed pyrrhotite Os isotope ratios occur 1½ km or more from the Duluth Complex, the fluid is unlikely to have been of magmatic origin. Fluid inclusions in layer-parallel quartz veins provide evidence of the involvement of a boiling fluid at temperatures between ∼300 and 400 °C. Analyses of fluid inclusions via LA-ICP-MS show that the fluids contain up to 1.7 wt% Na, 1.1 wt% K, 4330 ppm Fe, 2275 ppm Zn, and 415 ppm Mg. The veins also contain pyrite or pyrrhotite, plus minor amounts of chalcopyrite, bornite, pentlandite, and sphalerite. The Re–Os isotopic ratios of pyrite from the veins indicate that they crystallized from low 187Os/ 188Os fluids (<0.2). δ 18O values of vein quartz range from 7.7‰ to 9.5‰, consistent with an origin involving fluid with a relatively low δ 18O value between 2‰ and 5‰. Meteoric water with such a low δ 18O value could have interacted with the igneous rocks of the Complex and would have acquired Os with a low 187Os/ 188Os ratio. Strongly serpentinized olivine-rich rocks of the Complex are commonly characterized by such low δ 18O values and we propose that the fluid involved in serpentinization was also responsible for the perturbation of the Os isotopic system recorded by pyrrhotite in the Virginia Formation. Two important observations are that only pyrrhotite-bearing assemblages in the contact aureole show isotopic perturbation and that intervals showing Os exchange are spatially restricted, and not uniformly distributed. Os exchange and mixing has occurred only where temperatures were sufficient to convert pyrite to pyrrhotite, and where time-integrated water–rock ratios in the aureole were high enough to provide a supply of Os. Troctolitic and gabbroic rocks of the Partridge River Intrusion, Duluth Complex, are characterized by Os isotope ratios that are indicative of variable degrees of crustal contamination ( γ Os values of ∼0–543). Xenoliths of carbonaceous and sulfidic pelitic rocks of the Virginia Formation found in the igneous rocks provide evidence that Os was released by organic matter and pyrite in the sedimentary rocks and assimilated by mantle-derived magma. However, residual pyrrhotite produced as a result of pyrite breakdown in the xenoliths is characterized by 187Os/ 188Os ratios that are much lower than anticipated and similar to those of pyrrhotite in the contact aureole. The Os exchange and addition shown by pyrrhotite in the xenoliths highlight an unusual cycle of Re–Os liberation during devolatilization, kerogen maturation, and pyrite to pyrrhotite conversion (processes that contribute to magma contamination), followed by Os uptake by pyrrhotite during back reaction involving magma and/or fluid characterized by a relatively low 187Os/ 188Os ratio. The extreme Os uptake recorded by pyrrhotite in the xenoliths, as well as the lesser degree of uptake recorded by pyrrhotite in the contact aureole, is in line with the high Os diffusivity in pyrrhotite experimentally determined by Brenan et al. (2000). Our data confirm that Os isotope ratios in pyrrhotite-bearing rocks may be readily perturbed. For this reason caution should be exercised in the interpretation of Os isotope ratios in rocks where pyrrhotite may be the primary host of Os.

Relocation of aftershocks of the 2001 Bhuj earthquake: A new insight into seismotectonics of the Kachchh seismic zone, Gujarat, India

Abstract In view of an anomalous crust–mantle structure beneath the 2001 Bhuj earthquake region, double-difference relocations of 1402 aftershocks of the 2001 Bhuj earthquake were determined, using an improved 1D velocity model constructed from 3D velocity tomograms based on data from 10 to 58 three-component seismograph stations. This clearly delineated four major tectonic features: (i) south-dipping north Wagad fault (NWF), (ii and iii) south-dipping south Wagad faults 1 and 2 (SWF 1 , SWF 2 ), and (iv) a northeast dipping transverse fault (ITF), which is a new find. The relocated aftershocks correlate satisfactorily with the geologically mapped and inferred faults in the epicentral region. The relocated focal depths delineate a marked variation to the tune of 12 km in the brittle–ductile transition depths beneath the central aftershock zone that could be attributed to a lateral variation in crustal composition (more or less mafic) or in the level of fracturing across the fault zone. A fault intersection between the NWF and ITF has been clearly mapped in the 10–20 km depth range beneath the central aftershock zone. It is inferred that large intraplate stresses associated with the fault intersection, deepening of the brittle–ductile transition to a depth of 34 km due to the presence of mafic/ultramafic material in the crust–mantle transition zone, and the presence of aqueous fluids (released during the metamorphic process of eclogitisation of lower crustal olivine-rich rocks) and volatile CO 2 at the hypocentral depths, might have resulted in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.

Evidence for low viscosity garnet-rich layers in the upper mantle

Abstract The rheological properties of upper mantle rocks play an important role in controlling the dynamics of the lithosphere and mantle convection. Experimental studies and microstructures in naturally deformed mantle rocks usually imply that olivine controls the upper mantle rheology. Here we show for the first time evidence from the geometry of folded compositional layers in mantle rocks from Western Norway that garnet-rich rocks can have lower solid-state viscosities than olivine-rich rocks. Modeling of melt-free and dry rheology of garnet and olivine confirms that the reversed viscosity contrast between garnet-rich and olivine-rich layers for this folding event can be achieved over a relatively wide range of temperatures at low stress conditions when the fine-grained garnet deforms by diffusion creep while the coarse-grained olivine deforms by dislocation creep and/or diffusion creep. In general, modeling of the fold viscosity contrast shows that in the stable subcontinental lithospheric mantle or convecting mantle such a reversed viscosity contrast can be formed due to diffusion creep processes in fine-grained garnets in a dry mantle environment or at conditions where the garnet-pyroxene layer is partially molten, i.e. close to solidus–liquidus conditions in the upper mantle. Alternatively in cold plate tectonic settings, e.g. in subduction zones, some water-weakening is a feasible mechanism to create the reversed viscosity contrast between garnet and olivine.

Stress-driven Melt Segregation in Partially Molten Olivine-rich Rocks Deformed in Torsion

Torsion experiments on partially molten aggregates of olivine þ chromite þ 4 vol. % mid-ocean ridge basalt provide new insights into the interactions between deformation and melt segregation. When samples are sheared, melt segregates into distinct melt-rich bands oriented � 208 antithetic to the macroscopic shear plane. In one series of experiments, samples were deformed at similar shear strain rates (or stresses) to a range of finite shear strains to explore the evolution of melt-rich bands. In another series of experiments, samples were deformed to similar finite shear strains at a range of strain rates to explore the effect of strain rate (or stress) on band spacing and microstructure. We relate variations in strain rate to the compaction length and show that band spacing increases with increasing compaction length. These experiments provide new information on the evolution of melt distribution, the partitioning and localization of strain, and the scaling of experimental results to the Earth’s mantle.

Geochemical and petrographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge (IODP Hole U1309D, 30°N)

IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (~ > 70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poikilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82–88 in olivine-rich troctolites, to 78–83 in gabbros) and is in Mg–Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Yb cpx ~ 5–11 × Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges.

Subarc magmatic and hydration processes inferred from a hornblende peridotite xenolith in spessartite from Kyoto, Japan

Hornblende peridotite xenolith in the spessartite dike in the Tamba belt, northern Kyoto city, is mainly composed of olivine with high Fo and NiO contents (Fo87−94 and 0.22-0.51 wt%). The olivine encloses chromian spinels with a high Cr/(Cr + Al) atomic ratio, 0.66-0.80, partly replaced and digested by hornblendes with a low Mg/(Mg + total Fe) atomic ratio, 0.68-0.81. Microscopic and geochemical features of the hornblende peridotite xenolith indicate a reaction between a dunitic cumulate from a high-Mg and -Cr melt and an evolved hydrous high-Mg melt to form hornblendes. Olivines are lower in Fo when in contact with hornblendes. Rare earth element (REE) patterns of the hornblendes, common to the xenolith and host spessartite, indicate that all the involved melts are similar but different in the degree of differentiation. The strong hydration of olivine in the xenolith suggests a wide distribution of hornblende-rich rocks, such as hornblendites, formed by the hydration of olivine-rich rocks, such as dunites, around the Moho beneath the arc. This may give rise to a layer with a Vp intermediate between typical lower crustal and mantle values.

Orbital Identification of Carbonate-Bearing Rocks on Mars

Geochemical models for Mars predict carbonate formation during aqueous alteration. Carbonate-bearing rocks had not previously been detected on Mars' surface, but Mars Reconnaissance Orbiter mapping reveals a regional rock layer with near-infrared spectral characteristics that are consistent with the presence of magnesium carbonate in the Nili Fossae region. The carbonate is closely associated with both phyllosilicate-bearing and olivine-rich rock units and probably formed during the Noachian or early Hesperian era from the alteration of olivine by either hydrothermal fluids or near-surface water. The presence of carbonate as well as accompanying clays suggests that waters were neutral to alkaline at the time of its formation and that acidic weathering, proposed to be characteristic of Hesperian Mars, did not destroy these carbonates and thus did not dominate all aqueous environments.

Mineralogical, petrological, and geochemical studies of the Limahe mafic–ultramatic intrusion and associated Ni–Cu sulfide ores, SW China

The Limahe Ni–Cu sulfide deposit is hosted by a small mafic–ultramafic intrusion (800 × 200 × 300 m) that is temporally associated with the voluminous Permian flood basalts in SW China. The objective of this study is to better understand the origin of the deposit in the context of regional magmatism which is important for the ongoing mineral exploration in the region. The Limahe intrusion is a multiphase intrusion with an ultramafic unit at the base and a mafic unit at the top. The two rock units have intrusive contacts and exhibit similar mantle-normalized trace element patterns and Sr–Nd isotopic compositions but significantly different cumulus mineralogy and major element compositions. The similarities suggest that they are related to a common parental liquid, whereas the differences point to magma differentiation by olivine crystallization at depth. Sulfide mineralization is restricted to the ultramafic unit. The abundances of sulfides in the ultramafic unit generally increase towards the basal contacts with sedimentary footwall. The δ34S values of sulfide minerals from the Limahe deposit are elevated, ranging from +2.4 to +5.4‰. These values suggest the involvement of external S with elevated δ34S values. The mantle-normalized platinum-group element (PGE) patterns of bulk sulfide ores are similar to those of picrites associated with flood basalts in the region. The abundances of PGE in the sulfide ores, however, are significantly lower than that of sulfide liquid expected to segregate from undepleted picrite magma. Cr-spinel and olivine are present in the Limahe ultramafic rocks as well as in the picrites. Mantle-normalized trace element patterns of the Limahe intrusion generally resemble those of the picrites. However, negative Nb–Ta anomalies, common features of contamination with the lower or middle crust, are present in the intrusion but absent in the picrites. Sr–Nd isotopes suggest that the Limahe intrusion experienced higher degrees of contamination with the upper crust than did the picrites. The results of this study permit us to suggest that the parental magma of the Limahe intrusion was derived from picritic magma by olivine fractionation and contamination in a staging chamber at mid-crustal levels. Depletion of PGE in the sulfide ores in the Limahe intrusion is likely due to previous sulfide segregation of the parental magmas in the staging chamber. Sulfide mineralization in the Limahe intrusion is related to second-stage sulfide segregation after the fractionated magmas acquired external S from pyrite-bearing country rocks during magma ascent to the Limahe chamber. The abrupt change in mineralogical and chemical compositions between the ultramafic unit and the overlying unit suggests that at least two separate pulses of magma were involved in the development of the Limahe intrusion. We propose that the Limahe intrusion was once a wider part of a dynamic conduit that fed magma to the overlying subvolcanic dykes/sills or lavas. The ultramafic unit formed by the first, relatively more primitive magma, and the mafic unit formed by the second, relatively more fractionated magma. Immiscible sulfide droplets that segregated from the first magma settled down with olivine crystals to form the sulfide-bearing, olivine-rich rocks in the base of the intrusion. The overlying residual liquids were then pushed out of the chamber by the second magma. Critical factors for the formation of an economic Ni–Cu sulfide deposit in such a small intrusion include the dynamic petrologic processes involved and the availability of external sulfur. The Limahe deposit reminds us that small, multiphase, mafic–ultramafic intrusions in the region should not be overlooked for the potential of economic Ni–Cu sulfide deposits.

Low-temperature, high-stress deformation of olivine under water-saturated conditions

Low-temperature high-stress rheology of olivine was investigated under water-saturated conditions by large strain shear deformation experiments using the Griggs-type apparatus. Samples at T = 1273–1373 K and P = 2 GPa under water-saturated conditions deformed under a significantly lower applied stress than expected from the power-law relation and the strain rate increases more rapidly with stress. These observations suggest that an exponential flow law operates under the present experimental conditions, in which the stress dependence of activation enthalpy is characterized by the Peierls stress. The Peierls stress is determined between 1.6 and 2.9 GPa depending on the choice of the form of stress dependence of the activation enthalpy. These values are significantly smaller than the Peierls stress under dry conditions (9.1 GPa). We suggest that water (hydrogen) incorporated in the olivine lattices decreases the Peierls stress, and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water. The reduction of the Peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine. The transition between the power-law and the exponential creep occurs at relatively low stress ∼100 MPa at a geologically relevant strain rate of 10 −15 s −1 when temperature is lower than 1273 K and under water-rich conditions. Hence, the exponential creep might be dominated in olivine-rich rocks at low-temperature and high-stress under water-rich conditions, and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle.

Rheological Consequences of Redox State

Within the upper mantle of Earth, there is a gradient from a relatively more oxidizing near-surface region, with oxygen fugacities near those of the fayalite-magnetite-quartz buffer (FMQ), to more reducing conditions at depth near the iron-wustite buffer (IW). Oxygen fugacity appears to vary laterally, as well as vertically, by as much as a factor of 10 4 . As flow within the interior of the Earth and other terrestrial planets occurs due to the (mostly) solid-state deformation of rocks, an understanding of the effect of oxygen fugacity on creep is critical in modeling planetary interior dynamical behavior. This is especially important for the asthenosphere, that anomalously weak region in the uppermost mantle that accommodates isostasy and largely decouples mantle convection from plate motions. Experimental studies of the rheological behavior of iron-bearing minerals have demonstrated that oxygen fugacity can play an important role in deformation. We have shown that olivine rich rocks deformed near FMQ deform in the dislocation creep regime about a factor of 6 faster when buffered near FMQ than at IW. Experiments on olivine single crystals and aggregates indicate that this difference in behavior results from an increase in the concentration of silicon vacancies under more oxidizing conditions, as dislocation creep is rate-limited by the climb of dislocations, which is controlled by diffusion of silicon defects. Although fewer data are available for the effects of oxygen fugacity on pyroxene deformation, clinopyroxene appears to be stronger under more oxidizing conditions, while the data on orthopyroxene deformation show no dependence on oxygen fugacity. These results indicate that vertical and lateral variations in oxygen fugacity may result in, at most, an order of magnitude difference in viscosity, while other factors, such as water fugacity and lithology may be more significant.