Olivine-rich Troctolites Research Articles

Hydrothermal troctolite alteration at 300 and 400 °C: Insights from flexible Au-reaction cell batch experimental investigations

Abstract Troctolites are increasingly recognized as a common rock found in association with oceanic core complexes. They are similar to komatiite in composition, and hence troctolite alteration may provide insight into H2 production on Early Earth. We investigated the hydrothermal alteration of olivine-rich troctolites in two batch experiments (300 °C, and 400 °C – 40 MPa) by reacting forsteritic olivine and anorthite-rich plagioclase with salt solutions. The alteration process was evaluated based on concomitant fluid samples and solids retrieved upon the termination of the experiments. In both experiments, the initial rock powder was turned into a hard, compact mass through cementation by secondary phases. The heterogeneity of this mass was documented using µ-computed tomography and electron microscopy. Thermodynamic computations were conducted to determine the equilibrium phase assemblages and fluid compositions with increasing reaction turnover. Mineral zonation developed between the fast-reacting, fluid-dominated top portion of the solids and the more isolated portions at the bottom of the reaction cell. At 300 °C, the total reaction turnover after 1800 h was 77.5%. Serpentinization of olivine controlled the fluid composition after plagioclase had reacted away in the top layers. In contrast, a Ca- and Al-enriched assemblage of xonotlite and chlorite developed alongside unreacted plagioclase at the bottom. The porosity is very low in the top layers but high (around 15%) in the bottom part of the cemented mass. At 400 °C, the reaction turnover was only 51% as olivine was stable after plagioclase had reacted away. Clinopyroxene and andradite ± chlorite had formed in the top layers, whereas xonotlite, grossular, and chlorite had formed at the bottom. The permeability is more uniform and the mineral zonation less pronounced at 400 °C. These mineral zonations developed as a consequence of increased mobility of Ca, Al, Mg, and to a lesser extent of Fe in the experiment, which may be facilitated in the highly permeable granular materials when compared to a compact rock. Steady-state hydrogen concentrations were at least 20 mmol L−1 at 300 °C and <1 mmol L−1 at 400 °C. A lack of magnetite formation at the higher temperature is responsible for the low-H2 yields.

Fractionation of highly siderophile and chalcogen elements in the lower oceanic crust: Insights from the troctolites of the Alpine-Apennine Jurassic ophiolites

The present study aims at investigating the role of reactive porous flow in the fractionation of highly siderophile (HSE: Os, Ir, Ru, Rh, Pt, Pd, Au and Re) and chalcogen (S, Se, Te) elements during construction of the lower oceanic crust in (ultra-)slow spreading ridges. At this purpose, we analyzed the whole-rock HSE and chalcogen elements, and Re-Os isotopes in olivine-rich troctolites embedded within large-scale gabbroic sequences from the Alpine-Apennine Jurassic ophiolites. Leucotroctolites and chromitites associated with the olivine-rich troctolites were also investigated. The olivine-rich troctolites have initial γOs ranging from +0.2 to +5.9. Their primitive mantle-normalized patterns are characterized by a slight enrichment of Os over Ir, and an increase from Ir to Ru, Rh, Pt, Pd and Au, nearly flat Au-Te-Se-S, and weak Re depletion with respect to S. This HSE and chalcogen element, and Os isotopic signature was acquired in response to sulfide segregation, which was most likely triggered by reaction between an olivine-rich matrix and migrating MORB-type melts. The chromitite layers occurring within the olivine-rich troctolites exhibit high concentrations of platinum group elements (PGE: Os, Ir, Ru, Rh, Pt and Pd). Formation of the chromitites might produce melts depleted in PGE that reactively migrated within the olivine-rich matrix, and ultimately locally led to olivine-rich troctolites characterized by a slight increase of the PGE/chalcogen element fractionation. The leucotroctolites differ from associated olivine-rich troctolites in the lower concentrations of HSE and chalcogen elements, with subparallel primitive mantle-normalized patterns but slightly higher Se/Te values. The crystallization of the olivine-rich troctolites might release HSE and chalcogen elements-depleted melts that reacted with an olivine + plagioclase crystal mush to form the leucotroctolites. Alternatively, formation of the leucotroctolites involved a process of reacting melt flow characterized by a low melt/crystal matrix ratio, which led to a relatively low proportion of trapped melt and related sulfides. At (ultra-)slow spreading ridges, the crystallization of sulfides in high-Mg# lower crustal rocks may exert a significant control in shaping the HSE and chalcogen element signature of erupted basalts.

Retrieving timescales of oceanic crustal evolution at Oceanic Core Complexes: Insights from diffusion modelling of geochemical profiles in olivine

The building of oceanic crust at Oceanic Core Complexes (OCC) has been described as a complex process involving multiple intrusions of magma over a protracted period of time. The migration of primitive magmas (i.e., Mid Ocean Ridge Basalts, MORBs) can lead to melt-rock interactions during reactive porous flow processes through the lithosphere. The timescales of these reactive processes and the subsequent cooling of the modified crystal matrix remain unconstrained. Diffusion modelling has been widely used to retrieve timescales of magmatic processes. In this study, we use diffusion models to constrain (i) the minimum timescales of melt-rock interactions and (ii) the cooling rates of the gabbroic sequence forming the oceanic crust at an OCC. We chose samples of the most primitive olivine-rich troctolites from the gabbroic sequence sampled in IODP Hole U1309D (Atlantis Massif OCC, Mid-Atlantic Ridge 30°N). Olivine-rich troctolites were interpreted as marking local partial assimilation of mantle intervals into the oceanic crust, and thus allowed to understand the dynamics of mantle assimilation and the formation of slow-spreading oceanic crust at the Atlantis Massif OCC. Olivines in olivine-rich troctolites represent relicts of pre-existing mantle olivine, while clinopyroxenes and plagioclases are crystallized during reactive percolation. Olivine chemical compositions show that olivine-rich troctolites inherit chemical heterogeneity from the mantle precursor. Flat geochemical profiles in olivine indicate complete chemical re-equilibration of olivine crystals with the locally modified percolating melt. Exception is made for most Ca profiles that show lower Ca contents at the olivine rim compared to the relative crystal core, as the result of subsolidus cooling. Three-dimensional (3D) diffusion models at magmatic conditions (T = 1210–1300 °C and P = 2 kbar) reveal that complete chemical re-equilibration of 4 mm-size mantle-derived olivine with percolating MORB-type melts can be attained within durations of less than 300 yr. The Ca-in-olivine geospeedometer reveal that cooling rates from ~1200 °C to ~1050 °C are constant downhole and on average 0.004 °C/yr; they are comparable with lower temperature cooling rates (850 °C–250 °C) estimated at the Atlantis Massif OCC. The minimum timescales from 3D models point to rather fast re-equilibration of olivine. The downhole chemical heterogeneity inherited from the precursor mantle, coupled with the timescales of diffusive re-equilibration suggest that the partial assimilation of the upwelling mantle and its incorporation into the oceanic crust occurred in the time-frame of a single melt input. Our cooling data are consistent with building of the oceanic crust at OCCs controlled by continuous uplift, in turn governed by long-lived detachment faults. The latter contribute to the rapid cooling of the assimilated mantle intervals and the magma bodies. Diffusion models of geochemical profiles in olivine from a single crustal section allow to reconstitute the early magmatic processes leading to mantle assimilation and early crystallization of gabbros, and the cooling history of the oceanic crust at OCC from magmatic conditions to hydrothermalism.

Reactive melt migration controls the trace element budget of the lower oceanic crust: insights from the troctolite-olivine gabbro association of the Pineto Ophiolite (Corsica, France)

The Pineto gabbroic sequence is a ~ 1.5 km-thick crustal section similar to those forming the oceanic core complexes at modern slow-spreading ridges. The sequence is subdivided into two sectors displaying different bulk compositions. The deeper sector consists of troctolites, with minor olivine gabbros and sparse olivine-rich troctolite intercalations. We carried out a thorough chemical characterization, including minerals and bulk rocks, of the troctolite/olivine gabbro association. Although the whole-rock compositions are mainly controlled by mineral proportions, the whole-rock variability of some minor (e.g., Cr, Ni) and trace (e.g., Y, REE) elements reveals formation by open-system crystallization processes. This interpretation is supported by the trace element mineral compositions. The incompatible trace element signature of clinopyroxene in the different rock types is nearly undistinguishable, whereas olivine and plagioclase show substantial variability of trace element compositions. In particular, olivine and plagioclase from the Ol-gabbros are typically depleted in incompatible elements compared to olivine and plagioclase from the troctolites. This chemical variability attests that fractional crystallization alone cannot have determined the compositions of these rocks, but requires an open system process where melts migrate and chemically interact with pre-existing crystal mushes leading to the redistribution of major and trace elements throughout the cumulate pile.

Melt-dunite interactions at 0.5 and 0.7 GPa: experimental constraints on the origin of olivine-rich troctolites

Olivine-rich troctolites and dunites are diffuse lithologies at crust-mantle transition of the oceanic lithosphere. Disequilibrium textures coupled to mineral compositions indicate that melt-rock reactions play an important role in their origin. The prevailing view is that olivine-rich troctolites are related to extensive melt impregnation of a precursor mantle dunite.In order to provide experimental constraints, we performed reactive dissolution and crystallization experiments by juxtaposing three variably evolved MORB-type glasses with a melt-bearing dunite at 1300 °C and then cooling to 1150 °C at constant pressure (0.5 and 0.7 GPa). Additionally, an isothermal experiment (0.7 GPa, 1250 °C) provides a snapshot of olivine-melt reaction after the high-temperature step.Runs result in glass-bearing gabbro overlain by olivine-rich troctolite (at 0.5 GPa) or dunite (at 0.7 GPa) showing disequilibrium textures comparable with natural occurrences typically related to melt-rock reaction, e.g. embayed and resorbed subhedral olivine with lobate contacts against plagioclase and clinopyroxene, often occurring as large poikiloblasts including rounded olivines. Modal abundance of interstitial phases and mineral chemistry are strongly controlled by the extent of reacting melt infiltrated into the dunite matrix, i.e. the melt/olivine ratio. We found that higher pressure further limits olivine dissolution and results in a lower high-T porosity, decreasing the final abundance of interstitial phases.Olivine composition is mostly buffered by the starting San Carlos olivine, resulting in high XMg (0.88–0.90). NiO content decreases at increasing melt/olivine ratio, matching the composition of olivine in natural samples. Remarkably, at very low melt/olivine ratio, NiO can exceed the starting San Carlos value as a result of the increase of Ni olivine-melt partition coefficient during cooling after olivine assimilation in the reacted melt. This might potentially produce high-Ni, at high XMg, magmatic olivine.Melt composition affects the chemistry of interstitial minerals that show large compositional variability (anorthite in plagioclase, TiO2 in clinopyroxene) as a result of local equilibrium driven by trapped melt. Remarkably, mineral co-variation trends (e.g. anorthite in plagioclase vs. olivine XMg) match those of some natural olivine-rich troctolites occurrences.Experimental results corroborate and constrain the lead role of melt-rock reactions in the origin of olivine-rich rocks at mantle crust transition in the oceanic lithosphere.

From mantle peridotites to hybrid troctolites: Textural and chemical evolution during melt-rock interaction history (Mt. Maggiore, Corsica, France)

Recent studies investigate the replacive formation of hybrid troctolites from mantle peridotites after multiple stages of melt-rock reaction. However, these studies are not conducted in a field-controlled geological setting displaying the clear evolution from the protolith to the end-product of the reactions. The Mt. Maggiore peridotitic body exposes a clear evolution from spinel lherzolite to plagioclase-bearing lithotypes (plagioclase peridotites, olivine-rich troctolites and troctolites) during two continuous episodes of melt-rock interaction. In the spinel facies, the reactive percolation of a LREE-depleted melt leads to the dissolution of mantle pyroxenes and the growth of olivine crystals, forming replacive spinel dunites. The progressive evolution from spinel lherzolite to harzburgite to replacive dunite is accompanied by a change of olivine Crystallographic Preferred Orientation (CPO), from axial-[100] in the lherzolite to axial-[010] olivine CPO in the dunites, indicative of deformation in presence of melt. The initial percolating melt composition is consistent with single melt increments after 6% partial melting of a depleted mantle source. Reactive melt percolation leads to a progressive enrichment in the melt M-HREE absolute concentrations, while preserving its LREE depletion, consistent with the enriched analyzed HREE composition of olivine in the spinel dunite.In the shallower plagioclase facies, the melts modified by reactive melt percolation impregnate the spinel-facies lithotypes, leading to the dissolution of olivine and crystallization of plagioclase and orthopyroxene in the peridotites. This impregnation stage is also observed in the spinel dunites, forming hybrid olivine-rich troctolites and troctolites. The dissolution-precipitation reactions forming hybrid troctolites cause a progressive textural evolution of the olivine matrix, with the disruption of deformed coarse grains into undeformed small rounded grains. This textural evolution is not accompanied by clear changes in the olivine CPO, indicating low instantaneous melt/rock ratios during the impregnation process. Olivine, plagioclase and clinopyroxene REE compositions analyzed in troctolite fit a process of impregnation with a progressive closure of the porosity (at decreasing melt mass), leading to the crystallization of trapped melt and REE enrichments during the last crystallization increments.

Melt transport and mantle assimilation at Atlantis Massif (IODP Site U1309): Constraints from geochemical modeling

Olivine-rich troctolites (>70% olivine) reveal that extensive melt impregnation of pre-existing olivine-rich lithologies participate to the building of slow spread oceanic crust. To constrain their origin and their impact on the structure and geochemistry of oceanic crust, we realized a multi-scale petro-structural, geochemical, and numerical modeling study of olivine-rich troctolites drilled at IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N). Discrete intervals of olivine-rich troctolites display sharp to diffuse contacts with neighboring troctolites or gabbros. Their texture is characterized by plastically deformed (high temperature imprint), corroded coarse-grained to undeformed fine-grained olivine embayed in poikilitic clinopyroxene and plagioclase. Olivine crystallographic preferred orientations show weak [001] clusters. Olivine has variable major and minor element compositions, but similar fractionated REE (DyN/YbN = 0.04–0.11). We distinguished three types of olivine-rich troctolites based on microstructure, texture and mineral composition. Olivine-rich troctolites 1 and 2 display sharp contacts with adjacent lithologies. Type 1 has modal olivine <75%, occurring mainly as single rounded grains with primitive compositions (Mg# = 85–86), and associated with high Mg# clinopyroxene. Type 2 has higher olivine modes (>75%), dominantly forming aggregates, showing more evolved compositions (Mg# = 83–84) and associated with slightly lower Mg# clinopyroxene. These variations of olivine modes and compositions are in contrast to common trends of magmatic crystallization that predicts decreasing modal olivine with melt differentiation towards evolved compositions. Type 3 has diffuse contacts with gabbroic veins and modal olivine overlapping those of types 1 and 2. Chemical traverses along principal crystallographic axes of olivine are flat, suggesting local equilibrium between olivine and neighboring phases. Mineral modes and compositions, together with textures and microstructures, suggest that olivine-rich troctolites formed after melt-rock interactions in a reactive porous flow process. Their compositions are best modeled by percolation of primitive MORBs into Hole U1309D impregnated and compositionally heterogeneous harzburgites, triggering orthopyroxene dissolution, followed by olivine assimilation and concomitant crystallization of clinopyroxene and plagioclase. Modeling shows that Ni variations in olivine at constant Mg# are mantle inherited. Compositions of olivine-rich troctolite 1 are fitted assuming higher olivine assimilation (Ma = 0.06–0.13) in contrast to olivine-rich troctolites 2 and 3 (Ma = 0.01–0.02). Olivine-rich troctolite 3 was ‘buffered’ by crystallizing reacted melts, progressively more evolved as temperature decreased during a late stage process. We interpret olivine-rich troctolites from the Atlantis Massif as marking local assimilation of harzburgitic mantle into the gabbroic sequence during a period of enhanced magmatism at depth. Our study shows that the distribution and variable compositions of olivine-rich troctolites result from the incipient stages of this process when local spatial variations in mantle rock permeability, probably related to pyroxene distribution, controlled in turn melt transport and mantle-melt interactions.

Petrogenesis of ultramafic rocks and olivine-rich troctolites from the East Taiwan Ophiolite in the Lichi mélange

We examine ultramafic and olivine-rich troctolite blocks of the East Taiwan Ophiolite (ETO) in the Lichi Melange. Although ultramafic rocks are extensively serpentinized, the primary minerals, such as olivine, orthopyroxene, clinopyroxene, spinel and plagioclase can be identified. The ultramafic rocks are classified into harzburgite (± clinopyroxene), dunite, and olivine websterite. Major and trace element compositions of the primary minerals in harzburgites, such as the Cr# [= Cr/(Cr + Al) atomic ratio] of chromian spinel (0.3–0.58) and incompatible elements-depleted trace element patterns of clinopyroxenes, indicate their residue origin after partial melting with less flux components. These compositions are similar to those from mid-ocean ridge peridotites as well as back-arc peridotites from the Philippine Sea Plate. The olivine websterite contains discrete as well as occasional locally concentrated plagioclase grains. Petrological characteristics coupled with similarity in trace element patterns of clinopyroxenes in the harzburgite and olivine websterite samples indicate that the olivine websterite is likely formed by clinopyroxene addition to a lherzolitic/harzburgitic peridotite from a pyroxene-saturated mafic melt. Dunite with medium Cr# spinels indicates cumulus or replacement by melt-peridotite reaction origins. Mineral composition of olivine-rich troctolite cannot be explained by simple crystallization from basaltic magmas, but shows a chemical trend expected for products after melt-peridotite interactions. Mineral compositions of the dunite and olivine-rich troctolite are also within chemical ranges of mid-ocean ridge samples, and are slightly different from back-arc samples from the Philippine Sea Plate. We conclude that peridotites in the ETO are not derived from the northern extension of the Luzon volcanic arc mantle. Further geochronological study is, however, required to constrain the origin of the ETO ophiolite, because peridotites are probably indistinguishable in petrology and mineralogy between the Philippine Sea and the South China Sea/Eurasian Plates.

New perspectives on the origin of olivine-rich troctolites and associated harrisites from the Ligurian ophiolites (Italy)

The mid-ocean ridge (MOR)-type gabbroic sequences from the Internal Ligurian ophiolites include olivine-rich troctolite lenses up to tens of metres thick. For one of these lenses, a portion of metre-scale thickness characterized by skeletal to dendritic olivines (harrisites) was observed near the contact with host gabbros. Spinels from the olivine-rich troctolites locally include Ti-pargasite to kaersutite frequently associated with phlogopite to Na-phlogopite. Spinel-hosted amphibole displays rare earth element (REE) patterns characterized by a negative Eu anomaly, thereby recording a magmatic process associated with plagioclase segregation. The amphiboles show variable depletion of light REE (LREE) relative to middle REE (MREE), and heavy REE (HREE) that are weakly enriched to depleted with respect to MREE. Crystallization of the inclusion-bearing spinels is attributed to cooling of hybrid melts that originated by interaction between primitive melts and gabbro-related melts relatively rich in SiO 2 and incompatible elements. Clinopyroxene and amphibole from the harrisites show extensive variability for trace element compositions, albeit characterized by subparallel incompatible element patterns. This chemical variability was most probably acquired in response to rapid crystal growth related to undercooling of the harrisite parental melt. We propose that the melt undercooling was related to interaction of a primitive melt batch with a gabbroic crystal mush. Supplementary material : Details of the analytical technique are available at http://doi.org/10.6084/m9.figshare.c.3284312

Mantle–crust interactions in the oceanic lithosphere: Constraints from minor and trace elements in olivine

Minor and trace element compositions of olivines are used as probes into the melt-rock reaction processes occurring at the mantle–crust transition in the oceanic lithosphere. We studied mantle and lower crustal sections from the Alpine Jurassic ophiolites, where lithospheric remnants of a fossil slow-spreading ocean are exposed. Olivines from plagioclase-harzburgites and replacive dunites (Fo=91–89mol%) and from olivine-rich troctolites and troctolites (Fo=88–84mol%) were considered. Positive correlations among the concentrations of Mn, Ni, Co, Sc and V characterize the olivines from the dunites. These chemical variations are reconciled with formation by melts produced by a mixed source consisting of a depleted peridotite and a pyroxene-rich, garnet-bearing component melted under different pressure conditions. We thereby infer that the melts extracted through these dunites channels were not fully aggregated after their formation into the asthenospheric mantle.Olivines from the olivine-rich troctolites and the troctolites are distinct by those in the dunites by lower Ni and higher concentrations of Mn and incompatible trace elements (Ti, Zr, Y and HREE). Fractional crystallization cannot reproduce the chemical variations of the olivines from the olivine-rich troctolites and the troctolites. In these rock-types, the olivines commonly show heterogeneous Ti, Zr, Y and HREE compositions, which produce variable Ti/Y and Zr/Y values. We correlate these olivine characteristics with events of reactive melt migration occurred during the formation of the primitive lower oceanic crust. We propose that the migrating melts formed at the mantle–crust transition via interaction with mantle peridotites during periods of low melt supply.

Subcontinental rift initiation and ocean-continent transitional setting of the Dinarides and Vardar zone: Evidence from the Krivaja–Konjuh Massif, Bosnia and Herzegovina

The Dinaride and Vardar zone ophiolite belts extend from the south-eastern margins of the Alps to the Albanian and Greek ophiolites. Detailed sampling of the Krivaja–Konjuh massif, one of the largest massifs in the Dinaride belt, reveals fertile compositions and an extensive record of deformation at spinel peridotite facies conditions. High Na2O clinopyroxene and spinel–orthopyroxene symplectites after garnet indicate a relatively high pressure, subcontinental origin of the southern and western part of Krivaja, similar to orogenic massifs such as Lherz, Ronda and the Eastern Central Alpine peridotites. Clinopyroxene and spinel compositions from Konjuh show similarities with fertile abyssal peridotite. In the central parts of the massif the spinel lherzolites contain locally abundant patches of plagioclase, indicating impregnation by melt. The migrating melt was orthopyroxene undersaturated, locally converting the peridotites to massive olivine-rich troctolites. Massive gabbros and more evolved gabbro veins cross-cutting peridotites indicate continued melt production at depth. Overall we infer that the massif represents the onset of rifting and early stages of formation of a new ocean basin. In the south of Krivaja very localized chromitite occurrences indicate that much more depleted melts with supra-subduction affinity traversed the massif that have no genetic relationship with the peridotites. This indicates that volcanics with supra-subduction affinity at the margins of the Krivaja–Konjuh massif record separate processes during closure of the ocean basin. Comparison with published compositional data from other Balkan massifs shows that the range of compositions within the Krivaja–Konjuh massif is similar to the compositional range of the western massifs of the Dinarides. The compositions of the Balkan massifs show a west to east gradient, ranging from subcontinental on the western side of the Dinarides to depleted mid-ocean ridge/arc compositions in the Vardar zone in the east. This is consistent with the hypothesis that both ophiolite belts originate in a single ocean, rather than from two separate basins. A distinct decrease in fertility occurs in the south of the Dinarides towards the Albanian ophiolites with supra-subduction affinity.

Moho transition zone in the Cretaceous Andaman ophiolite, India: A passage from the mantle to the crust

Abstract We examined the composition and lithological variability from a road section in south-Andaman which represents a pathway to the crustal section of the Cretaceous Andaman ophiolite. Like other well-studied ophiolites worldwide this transition zone is marked by association of olivine-rich troctolite, wehrlite, pyroxenite and gabbroic rocks. The mineral chemical variations document the evolution of this zone by melt–mantle interaction and fractional crystallization. Petrographic evidence suggests that water was introduced during the evolution of this transition zone. The olivine-rich troctolites record impregnation of MORB melt into a residual olivine-rich lithology (replacive dunite) that formed by an earlier episode of melt–peridotite interaction at a slow spreading ridge. The clinopyroxenites indicate formation from an extreme clinopyroxene saturated melt that might be genetically linked with the formation of olivine-rich protolith of the troctolitic rocks prior to melt impregnation. The wehrlite crystallized from the melt residual after the formation of clinopyroxenite. The composition of the impregnating melt that transformed the replacive dunite to olivine-rich troctolite is identical to the gabbroic rocks. We conclude that the association of these rock types from south-Andaman provides us with a snapshot of the switch over of geodynamic setting of the Andaman ophiolite (MOR to arc) as preserved presently between north-Andaman in the north and Rutland Island in the south.

Geochemistry of spinel-hosted amphibole inclusions in abyssal peridotite: insight into secondary melt formation in melt–peridotite reaction

Spinel-hosted hydrous silicate mineral inclusions are often observed in dunite and troctolite as well as chromitite. Their origin has been expected as products associated with melt–peridotite reaction, based on the host rock origin. However, the systematics in mineralogical and geochemical features are not yet investigated totally. In this study, we report geochemical variations of the spinel-hosted pargasite inclusions in reacted harzburgite and olivine-rich troctolite collected from Atlantis Massif, an oceanic core complex, in the Mid-Atlantic Ridge. The studied samples are a good example to examine geochemical variations in the inclusions because the origin and geological background of the host rocks have been well constrained, such as the reaction between MORB melt and depleted residual harzburgite beneath the mid-ocean ridge spreading center. The trace-element compositions of the pargasite inclusions are characterized by not only high abundance of incompatible elements but also the LREE and HFSE enrichments. Distinctive trace-element partitioning between the pargasite inclusion and the host-rock clinopyroxene supports that the secondary melt instantaneously formed by the reaction is trapped in spinel and produces inclusion minerals. While the pargasite geochemical features can be interpreted by modal change reaction of residual harzburgite, such as combination of orthopyroxene decomposition and olivine precipitation, degree of the LREE enrichment as well as variation of HREE abundance is controlled by melt/rock ratio in the reaction. The spinel-hosted hydrous inclusion could be embedded evidence indicating melt–peridotite reaction even if reaction signatures in the host rock were hidden by other consequent reactions.

ORIGIN OF OLIVINE-RICH TROCTOLITES FROM THE OCEANIC LITHOSPHERE: A COMPARISON BETWEEN THE ALPINE JURASSIC OPHIOLITES AND MODERN SLOW SPREADING RIDGES

Olivine-rich troctolite bodies occur within lower crust and mantle sections of the Jurassic oceanic lithosphere exposed along the Alpine-Apennine belt. These rocks bear structural and compositional resemblances to the olivine-rich troctolites from slow spreading ridges. The olivine-rich troctolites from the Alpine-Apennine belt contain olivines (Fo = 89-87 mol%) with rounded to embayed morphology and clinopyroxene oikocrysts with high Mg# (90-88). The clinopyroxene oikocrysts have higher Cr2O3 (1.6-1.3 wt%) and lower Ti/Yb than clinopyroxenes in equilibrium typical MORB-type melts. These chemical characteristics were most likely acquired by reaction between an olivine-rich matrix and migrating melts crystallizing clinopyroxene and plagioclase. The pla- gioclases from the olivine-rich troctolites of the Alpine-Apennine belt are commonly poorer in anorthite component (71-61 mol%) than the plagioclases from slow spreading ridge olivine-rich troctolites. The migrating melts involved in the formation of the olivine-rich troctolites from the Alpine-Apennine belt were most likely slightly enriched in Na2O with respect to the basalts normally produced at slow spreading ridges. We attribute this Na2O enrichment to a low de- gree of melting of asthenospheric sources. The olivine-rich troctolites from fossil and modern oceanic lithosphere probably formed at the mantle-crust transi- tion. The occurrence of olivine-rich troctolite bodies within gabbroic sequences is reconciled with a process of dissection and entrapment of the mantle-crust transition during the growth of the lower crust.

Scientific Drilling

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 340T returned to the 1.4-km-deep Hole U1309D at Atlantis Massif to carry out borehole logging including vertical seismic profiling (VSP). Seismic, resistivity, and temperature logs were obtained throughout the geologic section in the footwall of this oceanic core complex. Reliable downhole temperature measurements throughout and the first seismic coverage of the 800–1400 meters below seafloor (mbsf) portion of the section were obtained. Distinct changes in velocity, resistivity, and magnetic susceptibility characterize the boundaries of altered, olivine-rich troctolite intervals within the otherwise dominantly gabbroic se-quence. Some narrow fault zones also are associated with downhole resistivity or velocity excursions. Small deviations in temperature were measured in borehole fluid adjacent to known faults at 750 mbsf and 1100 mbsf. This suggests that flow of seawater remains active along these zones of faulting and rock alteration. Vertical seismic profile station coverage at zero offset now extends the full length of the hole, including the uppermost 150 mbsf, where detachment processes are expected to have left their strongest imprint. Analysis of wallrock properties, together with alteration and structural characteristics of the cores from Site U1309, highlights the likely interplay between lithology, structure, lithospheric hydration, and core complex evolution. doi: 10.2204/iodp.sd.15.04.2013

IODP Expedition 340T: Borehole Logging at Atlantis Massif Oceanic Core Complex

Integrated Ocean Drilling Program (IODP) Expedition 340T returned to the 1.4-km-deep Hole U1309D at Atlantis Massif to carry out borehole logging including vertical seismic profiling (VSP). Seismic, resistivity, and temperature logs were obtained throughout the geologic section in the footwall of this oceanic core complex. Reliable downhole temperature measurements throughout and the first seismic coverage of the 800–1400 meters below seafloor (mbsf) portion of the section were obtained. Distinct changes in velocity, resistivity, and magnetic susceptibility characterize the boundaries of altered, olivine-rich troctolite intervals within the otherwise dominantly gabbroic se-quence. Some narrow fault zones also are associated with downhole resistivity or velocity excursions. Small deviations in temperature were measured in borehole fluid adjacent to known faults at 750 mbsf and 1100 mbsf. This suggests that flow of seawater remains active along these zones of faulting and rock alteration. Vertical seismic profile station coverage at zero offset now extends the full length of the hole, including the uppermost 150 mbsf, where detachment processes are expected to have left their strongest imprint. Analysis of wallrock properties, together with alteration and structural characteristics of the cores from Site U1309, highlights the likely interplay between lithology, structure, lithospheric hydration, and core complex evolution. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.15.04.2013" target="_blank"&gt;10.2204/iodp.sd.15.04.2013&lt;/a&gt;

Building of the deepest crust at a fossil slow-spreading centre (Pineto gabbroic sequence, Alpine Jurassic ophiolites)

This work presents new field and petrological data on a poorly known lower crustal section from the Alpine Jurassic ophiolites, the Pineto gabbroic sequence from Corsica (France). The Pineto gabbroic sequence is estimated to be *1.5 km thick and mainly consists of clinopyroxene-rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gab- bros in its deeper sector. The sequence also encloses olivine-rich troctolite and mantle peridotite bodies at dif- ferent stratigraphic heights. The composition and the lith- ological variability of the Pineto gabbroic sequence recall those of the lower crustal sections at slow- and ultra-slow- spreading ridges. The gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros, which approximately constitute 2/3 of the section. In particular, the lower sector of the Pineto gab- broic sequence shows the existence of large-scale frag- ments of the deepest oceanic crust displaying a highly primitive bulk composition. The mineral chemical varia- tions document that the origin and the evolution of the Pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation. The clinopyroxenes from the olivine gabbros and the olivine-rich troctolites also record the infiltration of olivine-dissolving, Cr2O3-rich melts that presumably formed within the mantle, into replacive dunite bodies. Cooling rates of the troctolites and the olivine gabbros were evaluated using the Ca in olivine geospee- dometer. We obtained high and nearly constant values of -2.2 to -1.7 C/year log units, which were correlated with the building of the Pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold litho- spheric mantle.

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Olivine-rich Troctolites from Ligurian Ophiolites (Italy): Evidence for Impregnation of Replacive Mantle Conduits by MORB-type Melts

The gabbroic plutons of the Internal Ligurian ophiolites (Northern Apennines, Italy) include olivine-rich troctolite bodies up to 80 m thick. The rounded to embayed morphology of olivine (Fo ¼ 87^ 88 mol %) and spinel from the olivine-rich troctolites, together with the high Mg-number (88^90) and Cr contents (2400^ 3000 ppm) of associated poikilitic clinopyroxene, indicates their formation through reaction between an olivine^spinel matrix and an infiltrating melt.This interpretation is corroborated by the occurrence of plagioclase-rich veins that exhibit diffuse margins against the host olivine-rich troctolites and centimetre-scale spinel-bearing dunitic clots, which are interpreted as the crystallization products of the infiltrating melts and relics of pre-existing olivine-rich material, respectively. The incompatible element signature of clinopyroxene from the olivine-rich troctolites shows that the infiltrating melts were chemically similar to mid-ocean ridge basalt (MORB). Spinel from the olivine-rich troctolites contains inclusions of Ti-pargasite/ kaersutite, phlogopite/aspidolite and orthopyroxene; inclusions of ilmenite and loveringite are also locally present. These spinels most probably formed from hybrid melts derived from interaction between melts rich in incompatible elements and new injections of primitive melt. We propose that this interaction occurred in the mantle, in melt conduits of replacive origin near the base of the conductive thermal boundary layer, and led to formation of a matrix made up of olivine and minor inclusion-bearing spinel. This matrix was subsequently impregnated by MORB-type melts saturated in plagioclase þ clinopyroxene to produce the olivine-rich troctolites. The amount of melt added to the olivine^spinel matrix to produce the olivine-rich troctolites is estimated to be � 25 wt %.

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.