Plagioclase Peridotites Research Articles

The geobarometric significance of plagioclase in mantle peridotites: A link between nature and experiments

Few previous studies on orogenic plagioclase peridotites, and recent low-P subsolidus experiments on lherzolites ( Borghini et al., 2010; J.Petrology 51, 229–254) have revealed that the composition of plagioclase is a marker of varying P conditions in mantle peridotites. Here we confirm this inference, by comparing microstructural–chemical features observed in natural plagioclase-facies recrystallized lherzolites from the External Liguride ophiolitic Unit (EL, Northern Apennine, Italy) with the results of previous ( Borghini et al., 2010) and new (this study) experiments on fertile lherzolite with the same bulk composition. Detailed microanalytical work and textural observations on the EL lherzolites revealed systematic chemical zoning in texturally associated plagioclase and pyroxene neoblasts. Anorthite-reverse zoning in plagioclase is coupled to Al, Na and Ca core-rim variations in granoblastic ortho- and clinopyroxenes, documenting two major stages of equilibration within the plagioclase-facies stability field. Consistent chemical variations in minerals are found in subsolidus experiments on fertile lherzolite in the pressure range 0.25–0.8 GPa. Results of experiments were fitted by multiple least-squares regression analysis to derive an expression for anorthite isopleths as a function of P and T in fertile lherzolites. Application of this equation to the EL lherzolites provided equilibrium pressure of 0.7 and 0.3 GPa for the two recrystallization stages, indicating exhumation from about 22 to 9 km depth. Results of combined experimental work and microstructural–chemical data on natural plagioclase lherzolites, highlight that the composition of plagioclase varies significantly in a rather narrow pressure range, and the shallow (< 1 GPa) P-T evolution of mantle peridotites can be therefore successfully traced by combining chemical zoning in plagioclase and geothermometric investigations on texturally associated pyroxenes. Experimentally-derived Na-Ca partitioning between plagioclase and clinopyroxene can be used to test the attainment of chemical equilibrium in neoblastic plagioclase-bearing mantle assemblages, thus enlarging the applicability of this approach in equilibrated plagioclase peridotites of different origins.

Ultramafic and mafic magmatism in southwestern Tuva

This paper presents the results of the first comprehensive geological, petrographic, mineralogical, geochemical, and geochronological studies of ultramafic and mafic magmatism in the poorly explored southwestern Tuva (evidence from the Birdag and Khayalyg massifs). These massifs intrude ortho- and paraschists, which are presumably Middle Proterozoic. Amphibole gabbros in the Khayalyg massifs contain numerous relict host-rock xenoliths. The massifs are dominated by meso- and leucocratic amphibole gabbros with minor ultramafic (serpentinous plagioclase harzburgites and plagioclase lherzolites) and transitional (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbronorites) rocks. The serpentinous plagioclase harzburgites and plagioclase lherzolites occur in the amphibole gabbros of the Birdag massif as small lenticular bodies. The latter are interpreted as xenoliths of ultramafic restites of earlier protrusion rather than mafic-melt differentiates. The wehrlites, olivine clinopyroxenites, hornblendites, and melanocratic olivine gabbronorites forming the outer zones of the xenogenic bodies of ultramafic restites are considered hybrid rocks. They resulted from the contact reactions of mafic melts and their fluids with the xenogenic bodies of ultramafic restites, which were feldspathized during this interaction. In the gabbros from both massifs, the chondrite-normalized content of MREE and HREE is lower and that of LREE is higher than those in N-MORB. The plagioclase peridotites, wehrlites, and olivine clinopyroxenites forming xenogenic bodies among the amphibole gabbros of the Birdag massif are richer in REE (especially LREE) than the ultramafic restites in ophiolite associations. This is because they were infiltrated by fluids enriched in these elements during their alteration under the influence of later mafic melts. The studies suggest that the Birdag and Khayalyg ultramafic–mafic massifs result from the spatial coexistence of (1) more ancient small protrusions of ultramafic restites, which occurred as allochthons among Middle Proterozoic(?) metamorphic rocks, (2) later gabbroic intrusions (from 494 ± 16 to 450–447.4 ± 5 Ma), and (3) hybrid transitional rocks (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbros) making up contact-reaction zones along their boundaries.

Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (‘melt layer’) is overlain by a ‘peridotite layer’ and a layer of vitreous carbon spheres (‘melt trap’). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170–1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T s ~ 1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr2O3 at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si ‘low-pressure’ signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros.

COMPOSITIONAL VARIATIONS IN PLATINUM-GROUP MINERALS AND GOLD IN LENSES OF MASSIVE PYRRHOTITE IN THE DZHUGDZHUR ANORTHOSITE, FAR-EASTERN RUSSIA

The accumulations of massive sulfides, mostly pyrrhotite, and the accompanying Pt–Pd and gold mineralization in the more eroded Lantar part of the Dzhugdzhur massif-type anorthosite, Aldan Shield, Russia, are characterized in detail for the first time. The metamorphic basement host-rocks of the anorthosite massif are crosscut by interlayers and injections of pyroxenite and plagioclase peridotite, which have a different extent of pyrrhotite dissemination. Upward, they give way to olivine-bearing gabbronorites interlayered with bodies of sulfide-bearing pyroxenite and gabbro. Above is a zone 350 m thick of pegmatitic gabbroic anorthosites which hosts the main bulk of the sulfide ores of Nyandoma and Batomga areas. These areas differ from one another by intensity of occurrence of the massive and disseminated pyrrhotite, chalcopyrite nests, and late pyrite-bearing assemblages. Platinum–palladium mineralization is represented by moncheite, sperrylite and kotulskite. Copper-, silver-, and Pt–Pd-bearing gold is found in the pyrite concentrate.

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

In this paper we present new data on the spatial variability of peridotite composition across a kilometer-scale mantle shear zone within the Lanzo massif (Western Alps, Italy). The shear zone separates the central from the northern part of the massif. Plagioclase peridotite shows gradually increasing deformation towards the shear zone, from porphyroclastic to mylonitic textures in the central body, while the northern body is composed of porphyroclastic rocks. The peridotite displays a large range of compositions, from fertile peridotite to refractory harzburgite and dunite. Deformed peridotites (proto-mylonite and mylonites) tend to be compositionally more homogeneous and fertile than weakly deformed peridotites. The composition of most plagioclase peridotites show rather high and constant (Ce/Yb) N ratios, and Yb N that cannot be explained by any simple melting model. Instead, refertilization modeling, consisting of melt increments from spinel peridotite sources, particularly with E-MORB melt, reasonably reproduces the plagioclase peridotite whole rock composition. Combined with constraints from Ce–Nb and Ce–Th systematics, we speculate that peridotites such as those from Lanzo record pervasive refertilization processes in the thermal boundary layer. In this scenario, mantle shear zones might act as important areas of melt focusing in the upper mantle that separates the thermal boundary layer from the conductively cooled mantle.

Structure and geochemistry of an Alaskan-type ultramafic–mafic complex in the Eastern Pontides, NE Turkey

The Karayasmak ultramafic–mafic association (KUMA) in NE Turkey represents an Alaskan-type, pre-Liassic intrusion in the Pulur metamorphic massif and is one of many such intrusive complexes that were emplaced into the continental basement of the Eastern Pontides belt during the pre-Liassic. Its main lithological units include plagioclase peridotite, melagabbronorite, banded gabbro and anorthosite (Phase-I), gabbro-pegmatite dikes (Phase-II), unmetamorphosed banded gabbro (Phase-III), cumulate olivine gabbronorite (Phase-IV), and non-cumulate gabbroic dikes and stocks (Phase-V). The contact relations between these units are gradational or intrusive, with the peridotites (Phase-I) enveloping the cumulate gabbronorites (Phase-IV) in the center of the elliptical body, displaying a concentric reverse-zoning pattern. Cr–Al spinel, plagioclase, and olivine are the first crystallizing phases in all cumulate rocks, whereas pyroxene, hornblende, phlogopite, magnetite, and ilmenite represent the intercumulus phases. The existence of hornblende and phlogopite, the high Wo and Cr 2O 3 contents of clinopyroxene, and the high Fo and NiO contents of olivine indicate that the KUMA units were the near-liquidus fractional crystallization products of hydrous basaltic magma. The Phase-I and Phase-III cumulates are slightly enriched in LILE and LREE, depleted in HFSE and HREE with respect to chondrite and primitive mantle, and show negative Nb, Zr, Hf and Ti anomalies. The Phase-IV cumulate gabbronorites are even more enriched in LILE and LREE and show less depletion in HFSE and HREE in comparison to the earlier phases. The KUMA units formed from high-Al hydrous basaltic magmas, which were derived from partial melting of a previously subduction-metasomatized subcontinental-lithospheric mantle. These magmas underwent crystal fractionation and mineral concentration processes in small magma chamber(s) at shallow crustal levels (< 5 kbar).

The Monte Maggiore peridotite (Corsica, France): a case study of mantle evolution in the Ligurian Tethys

Abstract The Monte Maggiore peridotite represents subcontinental mantle that underwent tectonic and magmatic evolution during the rifting stage of the Jurassic Ligurian Tethys oceanic basin. Pristine garnet peridotites were first equilibrated under spinel-facies conditions. During continental extension they were diffusely infiltrated by asthenospheric melts that consisted of single fractional melt increments (6% melting degree) showing depleted MORB (mid-ocean ridge basalt) signature. Diffuse melt migration of undersaturated melts at spinel-facies conditions formed reactive spinel peridotites, and melt impregnation at plagioclase-facies conditions formed impregnated plagioclase peridotites. Further focused melt migration occurred within high-porosity dunite channels. Subsequently, the single melt fractions underwent coalescence to form aggregate MORB melts that were intruded into shallow magma chambers. They underwent fractional crystallization and formation of variably evolved Mg-rich and Fe–Ti-rich magmas. Mg- and Fe–Ti-gabbroic dykes were formed by intrusion along fractures of these magmas. Melt-percolated peridotites and gabbroic rocks are isotopically homogeneous, suggesting that melts which percolated and intruded the mantle lithosphere derived from isotopically homogeneous asthenospheric mantle sources. The magmatic cycle, that is, asthenosphere partial melting, lithosphere diffuse melt percolation and dyke intrusion, occurred during Late Jurassic times (163–150 Ma) and represents the youngest events of lithosphere–asthenosphere interaction so far documented in ophiolitic peridotites from the Ligurian Tethys. The Ligurian Tethys basin never reached a mature oceanic stage, that is, the genetic link between exposed oceanic crustal rocks and refractory mantle peridotites.

Post-deformational impregnation of depleted MORB in Nain lherzolite (Central Iran)

Plagioclase lherzolites of Nain melange, Iran, show peculiar textures that indicate melt impregnation: (1) droplet or bleb-like grains of plagioclase distributed in the peridotite matrix, (2) plagioclase-bearing clinopyroxenite seams, and (3) trails of plagioclase crosscutting pyroxene porphyroclasts. The textural characteristics show post-deformational igneous formation of plagioclase, and possibly, associated clinopyroxene, from the impregnating melt. The melt has precipitated the clinopyroxenite seams and chemically modified all the peridotite minerals. Highly refractory compositions of the precipitated minerals suggest involvement of a highly depleted MORB-like melt. The melt was an increment of partial melt produced by 8% to 10% fractional melting from the MORB source. This is in contrast to the involvement of ordinary MORB in melt impregnation in abyssal plagioclase peridotites. Integration of increments of mantle partial melts to form MORB was possibly incomplete in the very incipient mid-ocean ridge as in the short-lived Nain back-arc basin.

Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere

The existence of different mantle domains exposed in ocean–continent transition zones provides a framework for understanding the generation of ultramafic seafloor along magma-poor rifted margins. In this study we present detailed petrological and geochemical data on peridotites from the Eastern Central Alps ophiolites in Switzerland and Italy to identify different mantle domains, to estimate the extent of refertilization, and to test whether refertilization is associated with a thermal signature that has important implications for geophysical interpretations of ocean–continent transitions. The compositions of clinopyroxene, orthopyroxene and spinel clearly reflect the different mantle domains. Relative to clinopyroxenes from spinel peridotites, clinopyroxenes from plagioclase peridotites have lower Na2O and Sr contents, but higher middle to heavy rare earth element ratios and Zr concentrations, and different Sc–V relationships. Spinels in plagioclase peridotites have higher TiO2 and lower Mg-numbers compared with those in spinel peridotites. Mineral–mineral trace element partitioning suggests that spinel peridotites equilibrated at substantially lower temperatures than plagioclase peridotites. The temperature difference between the spinel and plagioclase peridotites indicates an important thermal boundary between the two. The geochemical data show systematic spatial variations. A heterogeneous, ‘subcontinental domain’ with no syn-rift melt imprint is separated from a ‘refertilized domain’ that exhibits a complex history of regional-scale melt infiltration and melt–rock reaction, which has erased most of the ancient history. Simple calculations suggest that up to 12% of mid-ocean ridge basalt-type melt can be stored in plagioclase peridotite, relative to a depleted residue. Such a ‘lithospheric sponge’ provides an explanation for the fertile compositions of the peridotites and the rare occurrence of volcanic rocks in magma-poor rifted margins. We suggest that magma-poor vs magma-rich margins are largely determined by the efficiency of melt extraction and not so much by melt generation processes, given a similar initial composition of the upwelling mantle. It is proposed that refertilization increases textural diversity and chemical heterogeneity related to shallow crystallization in the mantle lithosphere.

The Stability of Plagioclase in the Upper Mantle: Subsolidus Experiments on Fertile and Depleted Lherzolite

Plagioclase peridotites are important markers of processes that characterize the petrological and tectonic evolution of the lithospheric mantle in extensional tectonic settings. Studies on equilibrated plagioclase peridotites have documented continuous chemical changes in mantle minerals in response to plagioclase crystallization, potentially tracing the re-equilibration of mantle peridotites up to very low pressure.This experimental study provides new constraints on the stability of plagioclase in mantle peridotites as a function of bulk composition, and the compositional and modal changes in minerals occurring within the plagioclase stability field as a function of P^T^bulk composition. Subsolidus experiments have been performed at pressures ranging from 0·25 to 1·0 GPa, and temperatures ranging from 900 to 12008C on fertile and depleted anhydrous lherzolites modelled in the system TiO2^Cr2O3^Na2O^CaO^FeO^ MgO^Al2O3^SiO2 (Ti,Cr-NCFMAS). In the fertile lherzolite (Na2O/CaO 1⁄4 0·08; XCr 1⁄4 0·07) a plagioclase-bearing assemblage is stable up to 0·7 GPa, 10008C and 0·8 GPa, 11008C, whereas in the depleted lherzolite (Na2O/CaO 1⁄4 0·09; XCr 1⁄4 0·10) the upper limit of plagioclase stability is shifted to lower pressure.The boundary between plagioclase lherzolite and spinel lherzolite has a positive slope in P^T space. In a complex chemical system, the plagioclase-out boundary is multivariant and sensitive to the XCr value [XCr 1⁄4 Cr/(Cr þAl)] of spinel.This latter is controlled by the reaction MgCr2O4 þ CaAl2Si2O8 1⁄4 MgCrAlSiO6 þ CaCrAlSiO6, which is a function of the Cr^Al partitioning between spinel and pyroxenes, and varies with the XCr value and chromite/ anorthite normative ratio of the bulk composition. Within the plagioclase stability field, the Al content of pyroxenes decreases, coupled with an increase in the anorthite content in plagioclase, and Ti and XCr in spinel with decreasing pressure; these chemical variations are combined with systematic changes in modal mineralogy governed by a continuous reaction involving both plagioclase and spinel. As a consequence, the composition of plagioclase varies significantly over a rather narrow pressure range and is similar at the same P^T conditions in the investigated bulk-rocks.This suggests the potential application of plagioclase composition as a geobarometer for plagioclase peridotites.

Seismogenic Shear Zones in the Lithospheric Mantle: Ultramafic Pseudotachylytes in the Lanzo Peridotite (Western Alps, NW Italy)

At Mt. Moncuni (Lanzo Massif, Western Alps) plagioclase peridotites and early mid-ocean ridge basalt (MORB) gabbroic dykes are deformed by shear zones containing cataclastic bands and both fault-vein and injection-vein pseudotachylytes, which are crosscut by late MORB porphyritic dykes. Fault-vein pseudotachylytes have thicknesses of the order of 1 mm; injection-vein pseudotachylytes have a typical thickness of 1–10 cm and contain spinifex textures. Structural, petrological and geochemical data show that the pseudotachylytes formed by near-complete melting of the host peridotite, at ambient temperature–pressure conditions (T = 600 ± 100°C, P < 0·5 GPa) close to the brittle–ductile transition of ultramafic rocks, during exhumation of the lithospheric mantle in the early stages of formation of the Ligurian Tethys oceanic basin. Estimates of the average volume fraction of unmelted clasts and of the ambient and liquidus temperature, together with thermophysical parameters, allow the determination of the melting energy per unit volume. Coseismic displacement is not observable at Mt. Moncuni, and consequently the dynamic shear resistance cannot be inferred. We show that commonly proposed relations between fault-vein thickness and displacement are of limited value, given the difficulty in identifying ‘single-event’ pseudotachylytes and the mobility of the melt. However, we also show that dynamic shear resistance can be predicted to decrease sharply if the melt coats the whole fault plane, partly as a consequence of the nonlinear viscosity of silicate melts at high strain rates. The Mt. Moncuni pseudotachylytes are the result of upper mantle seismicity at shallow depth (z < 20 km) over a time period of at most 5 Myr. Estimation of the total seismic energy release and moment (caused by an unspecified number of small to moderate earthquakes) requires an assessment of the total pseudotachylyte volume. This is highly uncertain, with a probable qualitative error margin of ±1 order of magnitude. The inferred values of cumulative seismic energy release and moment are of the order of 1015 ± 1 J and 1019 ± 1 N m, respectively, resulting in a seismic energy release rate of approximately 108 ± 1 J/a. This value is compatible with present-day seismic rates at extensional plate margins.

Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite

Abstract A widely used assumption of mantle geochemistry and the theory of partial melting at oceanic settings is the existence of isotopic equilibrium between mantle source and melt. Yet, recent diffusion studies and isotopic investigations of ophiolites, abyssal peridotites and associated MORBs have cast doubts on this assumption, by providing evidence for isotopic disequilibrium between residual peridotites and MORBs. Here we present Sr and Sm–Nd isotope data on mantle peridotites and gabbroic intrusions from the Mt. Maggiore (Alpine Corsica, France) Tethyan ophiolite, which document Nd isotopic homogeneity, implying isotopic equilibrium, on a 1-kilometer scale. The peridotites record multi-stage melt–rock interaction and melt intrusion occurring at different lithospheric depths. Samples studied are residual cpx-poor spinel lherzolites, reactive spinel harzburgites, impregnated plagioclase peridotites and related gabbronoritic veinlets, later gabbroic dykes. Strontium isotopes in peridotites and gabbros are highly variable, due to interaction with sea-water derived fluids, and cannot be used to test melt-residue isotopic equilibrium. In contrast, Nd isotopes are unaffected by sea-water alteration. Peridotites display present-day high 147 Sm/ 144 Nd (0.49–0.59) and 143 Nd/ 144 Nd (0.513367–0.513551) ratios, with no appreciable differences between residual and reactive spinel peridotites, and between spinel and plagioclase peridotites. Gabbroic dykes have present-day Nd isotopic compositions typical of MORB ( 143 Nd/ 144 Nd = 0.513122–0.513138). Internal (plag–whole rock–cpx) Sm–Nd isochrons for olivine gabbro dykes and a gabbronoritic veinlet yield Jurassic ages (162 ± 10 and 159 ± 15 Ma in ol-gabbros, 155 ± 6 Ma in gabbronorite), and initial e Nd = 8.9–9.7 indicative of a MORB-type source. Sm–Nd isotopic compositions of peridotites conform to the linear array defined by the gabbroic rocks, and yield initial (160 Ma) e Nd values of 7.6–8.9, again consistent with a MORB source. This Sm–Nd isotopic homogeneity may indicate a close approach to solid-melt equilibration on a 1-kilometer scale in the mantle, or it may represent an inherited isotopic homogeneity of the mantle source. In either case, melting and melt–rock interaction were the most likely processes to produce nearly complete equilibration between peridotite and melt. In contrast with recently accumulated evidence of apparent melt-source isotopic disequilibrium, generally on scales of several kilometers, here we show a setting where isotopic homogeneity between source rock and melt was attained. Whether this result can be applied elsewhere will depend on additional detailed studies in other regions.

Anorthitic plagioclase and pargasitic amphibole in mantle peridotites from the Yungbwa ophiolite (southwestern Tibetan Plateau) formed by hydrous melt metasomatism

Plagioclase crystals with an anorthite content up to An 99 have been discovered in mantle peridotites from the Yungbwa ophiolite, which crops out along the Indus–Yarlung Zangbo Suture, southwestern Tibetan Plateau. Amphiboles of pargasitic composition also occur in these plagioclase peridotites. Microtextures support the view that both plagioclase and amphibole were formed metasomatically by hydrous melts entering the Yungbwa peridotites, during which pyroxene was dissolved and olivine was precipitated. The extreme Ca-rich nature of the plagioclase indicates that the metasomatic melts had high Ca/Na ratios, whereas low Ti contents in accompanying spinel and amphibole indicate severe depletion of Ti in the hydrous melts. This suggests that the metasomatic melts were derived from a refractory mantle source. Clinopyroxene trace element data indicate that the metasomatic melts were also enriched in light rare earth elements relative to both middle and heavy rare earth elements. Therefore, the melts show geochemical characteristics similar to arc magmas. This implies that the mantle peridotites in the Yungbwa ophiolite experienced a two-stage evolution; they were formed at a mid-ocean ridge and subsequently entered a subduction zone setting. Their history thus records the opening and closing of the Neo-Tethys Ocean.

Melt Migration and Intrusion during Exhumation of the Alboran Lithosphere: the Tallante Mantle Xenolith Record (Betic Cordillera, SE Spain)

Microstructural and in situ mineral chemistry studies on mantle peridotite xenoliths from the Late Neogene alkaline volcanic center of Cabezo Tallante (SE Spain) reveal an exceptional record of a multi-stage history of deformation, recrystallization, melt–rock interaction and melt intrusion tracking the progressive exhumation of this lithospheric mantle sector. Xenoliths include porphyroclastic to equigranular spinel peridotites, impregnated plagioclase peridotites, and composite xenoliths made up of peridotites intruded first by gabbronorite veins and later by amphibole-bearing pyroxenites. The earliest stage involved subsolidus re-equilibration from garnet- to spinel-facies conditions, represented by rounded opx + spinel ± cpx clusters indicative of precursor garnet. The spinel-facies equilibration was followed by development of a porphyroclastic fabric, accentuated in many xenoliths by spinel trails, in response to shear deformation that may be related to the early stages of Neogene extension. Porphyroclastic spinel peridotites subsequently underwent multiple episodes of reactive porous melt percolation documented by crystallization of undeformed olivine replacing pyroxene porphyroclasts, and of undeformed poikilitic orthopyroxene at the expense of both pyroxene porphyroclasts and newly crystallized olivines. The porphyroclastic and melt–rock reaction textures are progressively obliterated by an equigranular structure developed as the result of static, possibly melt-assisted, annealing recrystallization. Clinopyroxenes in equigranular peridotites (i.e. the most equilibrated with the percolating melts) display slight light rare earth element (REE) depletion and almost flat middle to heavy REE spectra (LaN/YbN = 0·37–0·62; SmN/YbN = 0·89–1·23). Computed equilibrium liquids have an enriched tholeiitic affinity, consistent with the sub-alkaline magmatism of the Alboran Domain. Overall, the tectonic and magmatic stages recorded in spinel peridotites from Tallante are remarkably consistent with the evolution documented in the Ronda peridotites of the western Betics. Reactive porous flow and annealing recrystallization were followed by an impregnation event, documented by crystallization of interstitial (plag ± opx ± ol) aggregates in porphyroclastic and equigranular xenoliths; this indicates further exhumation to shallower depths. Diffuse melt percolation was followed by intrusion of melts with distinct chemical affinity. The first event is documented by thin gabbronoritic–noritic veins, showing opx reaction rims against the host peridotite. Comparable gabbronorites were previously ascribed to slab-derived melts. The norite veins are crosscut by centimeter-thick dikelets of amphibole pyroxenite. Geobarometric estimates and the observed crystallization order (ol–cpx–amph–plag) point to 0·7–0·9 GPa for pyroxenite intrusion. Computed melts in equilibrium with clinopyroxene show alkaline affinity, similar to the host Tallante alkali basalts. Textural and geochemical features in the xenoliths thus indicate that the progressive uplift of the Tallante lithospheric mantle was accompanied by interaction with melts of different sources, reflecting the magmatic evolution of the Alboran Domain in response to lithosphere extension and thinning leading to the formation of the Betic–Rif arc.

Paired melt lenses at the East Pacific Rise and the pattern of melt flow through the gabbroic layer at a fast-spreading ridge

From geophysics, two thin and narrow sill-like melt lenses occur directly beneath the ridge axis on the East Pacific Rise (EPR) near 9°N. One is at the base of sheeted dikes, and the other at the base of the crust in the mantle transition with largely crystalline material in between. Along-axis continuity indicates that the upper one is a steady-state feature along much of the EPR.What causes melt to pool at two levels in the crust, with little melt in between? Normative densities of gabbro sampled from Hess Deep, eastern Pacific, at solidus temperatures are greater than melt densities calculated from compositions of basalt glass from the EPR. Therefore, abyssal-tholeiite liquids of all compositions are buoyant in a hot gabbroic crystalline matrix with no more than a few percent melt. There is therefore no level in the gabbroic layer at which pure basaltic melt attains neutral buoyancy, and by this means produces sill-like rather than dike-like intrusives. Instead, the upper melt lens appears to collect at a combined compositional and permeability barrier, defined by the coarse-grained base of the dike sequence sampled on the northern rift margin at Hess Deep. About 200m of massive dike rocks with ophitic to subophitic coarse-grained texture occur just above gabbronorite and oxide–gabbro cumulates which are the crystallized relics of highly differentiated ferrobasaltic and ferroandesitic liquids in the melt lens. At temperatures of crystallization of cumulates at the level of the melt lens (1100–1000°C), the basal dike rocks of less differentiated basaltic composition just above this were 90–98% crystalline, and they annealed to the point where dike contacts cannot be seen on outcrop surfaces. These slightly molten rocks need to be fractured by rift dilatancy or overpressure from beneath for magmas to ascend beyond the melt lens. Neutral buoyancy occurs in fine-grained dikes above where densities of escaping magma finally match those of the fractured and porous formation and this is the level that can be laterally intruded.The base of the crust, on the other hand, presents an abrupt reduction in the density contrast between ascending melt and surrounding rock, and a higher concentration of melt pockets and channels. Neutral buoyancy can be attained where primitive gabbros are first encountered if the ascending magmas are picritic, laden with olivine phenocrysts. High olivine concentrations in the narrow bodies of moving magma can be attained by a combination of sinking, relative plagioclase flotation, and flow differentiation. Sill-like intrusions that crystallize to dunite and plagioclase–peridotite cumulates will result. Such rocks are found but their structural orientation is unknown at the base of the intra-rift ridge at Hess Deep; sill-like analogs are found in some ophiolites. Lateral leakage of deep melt lenses into transform faults might account for both primitive picritic tholeiites and strongly plagioclase-phyric basalts found, for example, at Siqueiros Fracture Zone at 8°30′N along the EPR.

PGM in chromite ore of the Sopcheozero deposit, Kola Peninsula

The Sopcheozero chromite deposit is hosted in dunite of the Monchegorsk layered intrusion as a sheetlike body of disseminated ore with a chromite grade varying from 20 to 60%. The total PGM content in the ore attains 0.5–0.8 g/t. The composition of host rocks varies from plagioclase peridotite to dunite, but PGM were found only in chromite-bearing dunite. PGM inclusions were detected in the interstices of chromite and olivine grains and within grains themselves. The data obtained confirm the known tendency toward variation in PGM composition with increasing sulfur and light PGE contents in the residual magmatic melt. The first particles of refractory Ir, Os, and Ru intermetallides appeared at the final stage of olivine crystallization, whereas laurite (Ru,Os,Ir)S2 and pentlandite (Fe,Ni)9S8 were formed at the final stage of chromite crystallization, when the sulfur concentration in the residual melt became sufficient.

The effect of mantle composition on density in the extending lithosphere

The density distribution of the lithosphere is non-linear and discontinuous due to complex mineralogy and, most importantly, phase transitions. We evaluate the influence of changes in mantle composition on lithospheric density and its evolution during horizontal stretching, using thermodynamic calculations of the density as a function of pressure, temperature and composition. We also develop a simple parameterization based on end-member mineral reactions and geometric relationships between the geotherm and the phase boundary for comparison. The garnet–spinel peridotite transition leads to a moderate decrease in density of the mantle part of the lithospheric column at the initial stages of stretching. When the crust is sufficiently thinned and temperature is relatively high, plagioclase peridotite becomes stable in the upper part of the mantle. The density reduction due to the plagioclase-in reaction is controlled by bulk Al 2O 3 in the mantle and by the depth of the plagioclase-in reaction, which is mainly governed by the Na 2O/Al 2O 3 ratio. Since Na 2O and Al 2O 3 increase with the fertility of the mantle the phase transition effect is most pronounced for relatively fertile mantle (and strong extension) and can lead to 2.3% density reduction. This is equivalent to heating the entire lithosphere by 700 °C if only the effect of thermal expansion on density is taken into account. The formation of plagioclase peridotite can explain syn-rift uplift in sedimentary basins that experienced large mantle stretching without invoking an unrealistically strong increase in temperature. It might also be responsible for the break-up unconformity observed at continental margins.

Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere–lithosphere history of partial melting and subsequent multi-stage melt–rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting (F = 5–10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling (T = 970–1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt–rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt–rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt–rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt–rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt–rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the lithospheric depths at which interaction occurred.

The Jurassic Ligurian Tethys, a fossil ultraslow‐spreading ocean: the mantle perspective

Abstract Alpine–Apennine ophiolites derive from the Jurassic Ligurian Tethys oceanic basin formed by lithosphere extension and failure in the pre-Triassic Europe–Adria system. The basin was floored by mantle peridotites and was characterized by along-axis alternation of avolcanic and volcanic segments. Lithosphere extension and thinning caused asthenosphere adiabatic upwelling and decompressional melting. Mid-ocean ridge basalt (MORB)-type melts diffusely percolated through and reacted with the overlying lithospheric peridotites, which were strongly modified, both depleted (harzburgites and dunites) and enriched (plagioclase peridotites), by melt–peridotite interaction and melt refertilization. The stratigraphic–structural features (mantle at the sea floor and alternation of avolcanic and volcanic segments) coupled with petrological features (presence of alkaline melts and strongly heterogeneous, melt-modified peridotites) allow us to interpret the Ligurian Tethys as a Jurassic analogue of modern ultraslow‐spreading oceans . The Liguria Mode for the inception of an oceanic basin consists of: (1) the rifting (continental) stage , dominated by extension of continental lithosphere and tectonic exhumation of lithospheric mantle; (2) the drifting (transition) stage , characterized by melt-related processes (i.e. inception of asthenosphere partial melting and MORB melt percolation through the overlying mantle lithosphere); (3) the spreading (oceanic) stage , characterized by failure of the continental crust, sea‐floor exposure of mantle peridotites and discontinuous MORB extrusion.

The pre-oceanic evolution of the Erro-Tobbio peridotite (Voltri Massif, Ligurian Alps, Italy)

Abstract This paper presents the results of field, structural, petrologic and geochemical investigations on the Erro-Tobbio (E-T) ophiolitic peridotite (Voltri Massif, Ligurian Alps, Italy). This massif represents a mantle section equilibrated at spinel-facies conditions in the subcontinental lithosphere of the Europe-Adria system prior to the Early Jurassic that has been exhumed and emplaced at the sea-floor during rifting and opening of an ocean basin. The E-T massif comprises km-scale volumes of peridotites with structural and compositional characteristics pointing to melt–peridotite interaction. Their formation is thought to result from the interaction of pristine lithospheric peridotites with MORB-type melts ascending by porous flow, leading to the development of reactive spinel harzburgites, impregnated plagioclase peridotites and replacive spinel dunites. The melt-related events were followed by MORB melt intrusion. Field relationships between sheared lithospheric peridotites, including coarse tectonites as well as fine-grained mylonites developed during lithosphere extension, and melt-modified peridotites suggest that melt-related processes occurred during exhumation of the E-T mantle. These melt-related processes likely included both diffuse percolation and focused intrusion and are considered to be a consequence of MORB-forming partial melting of the asthenosphere induced by near-adiabatic decompressional upwelling related to lithosphere extension and thinning. Field, structural and petrological data allow us to conclude that the entire pre-oceanic evolution of deformation, metamorphism and magmatism recorded by the E-T mantle started during the Early-Middle Jurassic and was related to lithospheric extension leading to the Late Jurassic opening of the Ligurian Tethys ocean.