Orogenic Peridotites Research Articles

Platinum-group element micronuggets and refertilization process in Lherz orogenic peridotite (northeastern Pyrenees, France)

Highly siderophile elements (Platinum-group elements, Au and Re) are currently assumed to reside inside base metal sulfides (BMS) in the convecting upper mantle. However, fertile lherzolites sampled by Pyrenean orogenic peridotite massifs are unexpectedly rich in 0.5–3 µm large micronuggets of platinum-group minerals (PGM). Among those, sulfides from the laurite-erlichmanite series (Ru, Os(Ir)S(As)2), Pt–Ir–Os alloys and Pt–Pd–Te–Bi phases (moncheite–merenskyite) are predominant. Not only the BMS phases but also the PGM micronuggets must be taken into account in calculation of the PGE budget of orogenic fertile lherzolites. Laurite is a good candidate for equilibrating the whole-rock budget of Os, Ir and Ru while accounting for supra-chondritic Ru/IrN. Textural relationships between PGMs and BMS highlight heterogeneous mixing between refractory PGMs (laurite/Pt–Ir–Os alloys) inherited from ancient refractory lithospheric mantle and late-magmatic metasomatic sulfides precipitated from tholeiitic melts. “Low-temperature” PGMs, especially Pt–Pd bismuthotellurides should be added to the list of mineral indicators of lithosphere refertilization process. Now disseminated within fertile lherzolites, “lithospheric“ PGMs likely account for local preservation of ancient Os model ages (up to 2 Ga) detected in BMS by in-situ isotopic analyses. These PGMs also question the reliability of orogenic lherzolites for estimating the PGE signature of the Primitive Silicate Earth.

Trace-element geochemistry of mantle olivine and application to mantle petrogenesis and geothermobarometry

Trace-element compositions of olivine from 75 mantle rocks of diverse origin, including xenoliths from kimberlites, basaltic lavas and orogenic peridotites, were determined by laser ablation ICP-MS to study systematic variations between mantle lithologies, partitioning mechanisms in olivine and their potential for geothermobarometry and unravelling mantle processes. Samples were selected to cover a wide range of forsterite contents (89.1–93.4), equilibration temperatures and pressures (750–1450 °C; 15–80 kbar). Trace elements in olivine can be divided into three groups. Group I elements (Ni, Mn, Co, Cu, Zn and Li) show small concentration ranges and olivine is the major host mineral. These are mostly divalent elements and have ionic radii close to that of Mg. Group II elements (Cr, Al, V, Sc, Ca and Na) show large concentration ranges, which are mainly controlled by the equilibration temperature of the host rock. The elements are strongly concentrated in co-existing mantle minerals (garnet, clinopyroxene and spinel) and show a narrow range of bulk rock concentrations. They fit less comfortably in the olivine lattice than Group I elements because of their charge or size. Differences between garnet and spinel-facies rocks are apparent for Al, Ca and Sc. Group III elements (Ti, Zr, Nb and Y) show large ranges of concentration in olivine as well as in co-existing minerals, and are strongly dependent on bulk rock contents. Concentration differences between olivine from garnet and spinel-facies rocks are apparent for all these elements. They are strongly incompatible in olivine and other rock-forming mantle minerals because of their charge or size. Various mantle lithologies can be discriminated using olivine composition. Spinel, garnet and garnet–spinel peridotites can be distinguished in olivine Sc–Zr and MnO–Al 2O 3 diagrams, whereas volcanic olivine is distinguished by high Ca and Al contents (picritic olivine) or high Nb contents (kimberlitic olivine). Since concentrations of Group III elements in olivine are diagnostic of whole-rock contents they can be used to trace the petrogenetic history of the rock. For instance, Ti contents and Cr# (Cr/(Cr + Al)) of olivine correlate with the amount of melt extracted from a mantle residue, although refertilisation may subsequently have increased Ti contents in high- T peridotites from the base of the lithosphere. The olivine dataset can be used to examine substitution reactions. Uptake of Al and Cr appears to be largely charge-balanced by Na in garnet-facies olivine, and olivine Cr# strongly correlates with that of co-existing minerals, in particular clinopyroxene and spinel. In spinel-facies olivine a large excess of trivalent cations is present in olivine, which can be fully attributed to excess Al. This suggests a Tschermak-style substitution, in which replacement of Mg by Al in the octahedral site is charge-balanced by replacement of Si by Al in the tetrahedral site. Partition coefficients of Group II elements are highly temperature sensitive with most of the element variability being shown by olivine. This allows the definition of simple geothermometers based solely on the concentrations of these elements in olivine. The most widely applicable of these is Al-in-olivine for garnet peridotites, following the expression T Al-ol ( ° C ) = 9423 + 51.4 P + 1860 Cr # ol ( 13.409 − ln [ Al ] ol ) − 273 with P in kbar, Al ol the Al concentration of olivine in ppm, and Cr# ol is Cr/(Cr + Al) in olivine. This thermometer predicts the temperature with a residual of 15 °C based on calibration with two-pyroxene and Al-in-opx geothermobarometers (Brey and Köhler, 1990). Although calibrated using lherzolites only, the thermometer performs well for clinopyroxene-free harzburgites and also spinel peridotites. An alternative thermometer is presented for the case where the presence of Cr 2+ is expected, e.g., for olivine inclusions in diamonds. The geochemical and thermobarometric information recorded by olivine can be a useful tool in studies of the petrogenesis of lithospheric mantle, olivine xenocrysts in mantle-derived magmas, the formation of diamonds and diamond exploration using detrital olivine.

The Oxidation State of Metasomatized Mantle Wedge: Insights from C-O-H-bearing Garnet Peridotite

Oxygen fugacity (fO2) is an important parameter in determining the relative stabilities of phase assemblages. Whereas a number of studies have been devoted to determining the redox state of low-pressure assemblages in the mantle system, the fO2 of the supra-subduction mantle wedge is still poorly known. An essential input for fO2 estimates is the determination of the ferric–ferrous iron content of key mantle minerals such as garnet, which can be measured using the ‘flank method’ technique on an electron microprobe. We selected samples of orogenic peridotites from the ultrahigh-pressure Sulu belt (Eastern China) and from the Ulten Zone (Italian Alps) for detailed case studies; these correspond to slices of metasomatized mantle wedge sampled at different depths. They are characterized by the assemblage phlogopite + magnesite +amphibole in equilibrium with olivine, orthopyroxene and Fe3+-bearing garnet. The ‘flank method’ measurements indicate that these pyrope-rich garnets contain Fe3+/ΣFe up to 0·12–0·14. For peridotite mineral assemblages fO2 can be evaluated from equilibria involving the Fe3+-garnet component skiagite (Fe2+3Fe3+2Si3O12) on the basis of Fe3+–Al substitution on the octahedral site, which is sensitive to the garnet oxidation state. We modelled non-ideal mixing of Al and Fe3+ on the octahedral site and non-ideal mixing on the dodecahedral site, with a symmetric regular solution model for reciprocal solid solutions of Ca–Fe2+–Mg–Al–Fe3+-garnet. This allowed us to calculate garnet-peridotite fO2, given the presence of Fe3+ in garnet. Our results indicate that the Sulu and Ulten peridotites record high oxygen fugacities (FMQ to FMQ + 2) compared with garnet peridotite xenoliths from the sub-cratonic mantle equilibrated at similar pressures. The determination of the oxygen fugacity of these hydrate–carbonate-bearing garnet peridotites allowed us to estimate the speciation of C–O–H metasomatic fluids derived from the subducting slab, which are enriched in CO2. The fO2 evaluation of the metasomatized mantle-wedge peridotites, representing the oxygen chemical potential μO2, provides a first step in unravelling the relationship between μO2 and the metasomatic phase assemblage.

Non-chondritic HSE budget in Earth's upper mantle evidenced by abyssal peridotites from Gakkel ridge (Arctic Ocean)

Constraints on the highly siderophile element (HSE: including Os, Ir, Ru, Pt, Pd and Re) budget of the upper mantle upwelling beneath mid-ocean ridges have until now been based on samples that have experienced varying degrees of alteration, partial melting and secondary igneous processes. Here we present results from a set of abyssal peridotites that have been relatively unaffected by these phenomena. Eighteen abyssal peridotites from two localities along the ultra-slow spreading Gakkel Ridge (Arctic Ocean) were selected for this study. Samples from one locality (PS66-238) are extremely fresh, while those from HLY0102-D70 are typical abyssal serpentinites. Comparison of HSE data between fresh peridotites and weathering products supports the contention that HSE are stable during serpentinization, but that Pd and Re are mobile during subsequent weathering. HSE budgets of spinel lherzolites suggest that all platinum group elements (PGEs: including Os, Ir, Ru, Pt and Pd) are compatible during very low degree of partial melting (5–8%), whereas Re behaves as an incompatible element. Harzburgites from each locality were subjected to ~ 12% fractional partial melting, which is lower than that expected for total consumption of sulfides in mantle peridotites (ca. 16%). The harzburgites are depleted in PPGE (Pt, Pd) and Re relative to IPGE (Os, Ir, Ru), which reflects the extraction of sulfide melt along with the silicate melt. Low bulk-rock contents of both Na 2O and TiO 2 in the fresh PS66-238 lherzolites indicate that they were not transformed from harzburgites through melt refertilization, but represent pristine fertile mantle rocks. Their systematic suprachondritic Ru/Ir and Pd/Ir ratios, but chondritic Pt/Ir ratios, cannot be ascribed to partial melting or secondary igneous processes. This signature may reflect the primary HSE signature of the Earth's upper mantle. Non-chondritic HSE patterns discovered in PS66-238 lherzolites are consistent with the primitive upper mantle (PUM) inferred from the study of orogenic peridotites and mantle xenoliths.

Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone, Eastern Alps, Italy)

We document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO2-bearing fluids enriched in incompatible elements released by the crust. The P–T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO2-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO2-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’ assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P–T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge.

Miocene incorporation of peridotite into the Hercynian basement of the Maghrebides (Edough massif, NE Algeria): Implications for the geodynamic evolution of the Western Mediterranean

A laser ablation ICP-MS U–Pb age of 17.84 ± 0.12 Ma (late Burdigalian) was obtained from monazites separated from a leucocratic diatexite collected in close proximity to a small peridotite massif incorporated into the lower crustal sequence of the Edough Massif (north-eastern Algeria), a southern segment of the peri-Mediterranean Alpine Belt. Monazites extracted from a neighbouring deformed leucogranite intruding early Paleozoic phyllites yield a consistent age of 17.4 ± 1.3 Ma. Zircons occurring in the same leucogranite, with magmatic characteristics, have an age of 308 ± 7 Ma interpreted as dating magmatic crystallisation of the leucogranite and reflecting partial melting during the Hercynian orogeny. Low Th/U domains (Th/U < 0.10) from the same grains substantiate recrystallisation during a younger metamorphic event whose upper age limit is 286 ± 11 Ma. These results emphasize the polycyclic evolution of basement rocks preserved in the crystalline units of the western Mediterranean and indicate that part of their metamorphic features were inherited from older, Hercynian, events. Taken together with published Ar–Ar dates, the late Burdigalian age of monazites indicates a rapid cooling rate of c. 370 °C/Ma and is regarded as closely approximating the emplacement of the peridotites into the Hercynian basement. The monazite ages are significantly younger than those recorded for orogenic peridotites from the Betic-Rif orocline and for the timing of lithospheric extension forming the Alboran sea. It is also younger than rifting and back-arc extension opening the Liguro–Provençal basin. The late Burdigalian age is interpreted as dating the incipient rifting event that opened the Algerian basin, which is consequently not a continuation of the Liguro–Provençal basin. At the scale of the western Mediterranean, these observations concur with current models supporting slab roll-back and an eastwards migration of extension in the western Mediterranean, but suggest that the Algerian basin opened as a result of torsion and stretching of the Thethyan slab due to its steepening under the Alboran microplate.

Chemical composition and ultrahigh-P metamorphism of garnet peridotites from the Sulu UHP terrane, China: Investigation of major, trace elements and Hf isotopes of minerals

The Zhimafang peridotite body recovered by a pre-pilot borehole (PP1) of the Chinese Continental Scientific Drilling Project occurs in gneiss from the southern Sulu ultrahigh-P (UHP) terrane, east-central China. Detailed petrographic observation of the freshest lherzolites reveals the existence of pre-peak mantle relics, rare enstatite inclusions (Opx1) in garnet, and coarse-grained diopside (Cpx1) in the peridotites. Matrix phases Ol + Grt + Opx + Cpx ± Phl represent the assemblage of the peak-stage. At least > 1.5 GPa pressure increase is recognized from the mantle setting to the peak-stage of subduction-zone UHP metamorphism at ~ 800 °C and 6.8 GPa for the peridotites. All peridotitic minerals have high Mg# (Ol, 91–93, Cpx, 93–94 and Opx, 92–94). Garnets are depleted in LREE, and have increased abundances from Pr through the MREE to a plateau of heavy REE (HREE) at about 10 times chondrite. In contrast, clinopyroxenes display humped REE patterns with high (La/Yb) N values of 27–68. Phlogopite is a major carrier of Ba (5205 ppm) and Th (5.5 ppm). Partition coefficients ( D Cpx/Grt) for REE between clinopyroxene and garnet decrease from LREE (~ 100) to HREE (< 1 but > 0.1), suggesting that clinopyroxene and garnet equilibrated at low- T (< 1100 °C). Zircons from three phlogopite and/or magnesite-bearing peridotites show very low 176Lu/ 177Hf values (between 10 − 3 and 10 − 6 ), and a wide range in 176Hf/ 177Hf ratios (0.28221–0.28250). The zircons with low 176Hf/ 177Hf and relatively high 176Lu/ 177Hf document that metasomatism affected the peridotite protolith. The model ages ( T DM) of zircons mostly cluster around 1.3–1.4 Ga, which probably reflects the metasomatic source. These chemical data combined with previous published bulk-rock major and trace element data indicate that the Zhimafang peridotites probably represent depleted and metasomatized subcontinental lithospheric mantle (SCLM) beneath the Sino-Korean craton. Investigation of the Sulu UHP metamorphic garnet peridotites provides additional insights into the Earth's geochemical and tectonic evolution, and enlarges our understanding of the exhumation of orogenic peridotites.

Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclase^spinel websterites. The ‘’ part of the Ronda peridotite contains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitionstowardstheirhost peridotites. Previousstudieshave suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modellingstudy ofa layered outcrop ofgroup C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization melt^ rock reactions. Mg-number [¼ Mg/(Mg þ Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (� 0� 65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (40� 1and41, respectively) to balance Mg^Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convexupward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized). Based on (Ce/Nd)N values, two transient, enriched domains between the light REE (LREE)depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxenite^peridotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions.The latterare probably volatile-rich, small-volume melt fractions residual after the refertilization reactionsthatgeneratedgroup C pyroxenites, which migrated throughout the massifcincluding the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the ‘layered granular’subdomain and ‘plagioclase-tectonite’domain) are on average morefertilethan the residual,‘coarsegranular’subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.

Petrology of local concentration of chromian spinel in dunite from the slow-spreading Southwest Indian Ridge

This is the first detailed report on local concentration of chromian spinel in a dunite from an ultraslow spreading ridge, the Southwest Indian Ridge (SWIR). The sample was collected from an outcrop with detailed observations using submersible SHINKAI 6500 of the Japanese Marine Science Technology Center. The dunite occurs as a tabular-shaped layer in a lherzolite host outcrop. Spinel is found as a string of small micropods 2–3 centimeters in size. These spinel micropods make a layer in the middle part of a spinel-poor dunite ( 0.4) compared with other peridotite samples in the studied area (lherzolites to harzburgite with low-Cr# spinel, typically Cr# ≤ 0.3). The occurrence and chemical compositions of clinopyroxene in the enstatite-poor harzburgite suggest that some clinopyroxenes crystallized from infiltrated interstitial melts. The host peridotites are interpreted as a residue of relatively low degrees of partial melting consistent with a location along the SWIR far from a mantle hot spot. This was then followed by crystallization of clinopyroxene from interstitial melt in the dunite. Irrespective of their small size, the lithological relationships between the spinel micropods and the host peridotites are the same as those for podiform chromitite in ophiolites and orogenic peridotites. The spinel Cr# in the micropods (0.3) is compatible with the lower range of those in basalts from SWIR far from hot spot. The spinel micropods were mainly formed by interaction between relatively depleted peridotite and a locally significant volume of basaltic melt traversing the upper mantle. This study coupled with the previous works on chromitites suggest that podiform chromites occur in every geodynamic setting, though economic concentrations of chromite (Cr-rich spinel) are unlikely to occur in the mantle at ultraslow spreading ridges.

The Lherz spinel lherzolite: Refertilized rather than pristine mantle

Differentiation of the Earth's mantle and formation of continental and oceanic crust occur principally through partial melting and extraction of basaltic melt. Among the mantle rocks occurring at the Earth's surface, as tectonically-emplaced massifs, abyssal peridotites or xenoliths, the harzburgites (< 5% clinopyroxene) are widely considered as refractory mantle residues left after extraction of a basaltic component. In contrast, the most fertile lherzolites (15% clinopyroxene) are generally regarded as pristine mantle, only weakly affected by partial melting. In this paper we present new convergent structural and geochemical data from the Lherz massif, the type-locality of lherzolite, indicating that the lherzolites from Lherz do not represent pristine mantle. Detailed structural mapping clearly shows that the lherzolites are secondary rocks formed at the expense of the harzburgites. Variations of major, minor and trace elements through the harzburgite–lherzolite contacts indicate that the lherzolites were formed through a refertilization process involving interaction of refractory, lithospheric mantle with upwelling asthenospheric partial melts. Combined with previously published indications of refertilization in orogenic peridotites, our new observations in Lherz suggest that most lherzolite massifs represent secondary (refertilized) rather than pristine mantle. Together with geochemical data on mantle xenoliths, this indicates that melt transport and melt-rock reaction play a key role on the rejuvenation and erosion of the lithospheric mantle.

Geodynamic significance of ultramafic xenoliths from Eastern Serbia: Relics of sub-arc oceanic mantle?

A suite of highly depleted peridotite xenoliths in East Serbian Palaeogene basanites represents the lithospheric mantle beneath the Balkan Peninsula. The xenoliths are harzburgites, clinopyroxene-poor lherzolites and rare dunites. They contain mostly 91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al2O3 contents in orthopyroxene (mostly 1–2 wt.%). They have experienced some mantle metasomatism which has slightly obscured their original composition. Nevertheless, the general characteristics of the xenoliths imply a composition which is significantly more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peridotites. Geological and compositional evidence suggests that the xenoliths do not represent Archean mantle. The existence of Proterozoic mantle cannot be entirely excluded, although it is in disagreement with geological evidence. On the other hand, the studied xenoliths are compositionally very similar to peridotites of modern oceanic sub-arc settings. The existence of such a depleted lithospheric mantle segment is also inferred from the presence of rare orthopyroxene-rich xenoliths in the same suite. These are interpreted to have originated as lithospheric precipitates of high-Mg, SiO2-saturated magmas that require a highly depleted mantle source. Such source is typically required by boninitic-like magmas of intraoceanic suprasubduction settings. A proposed geodynamic model to explain these observations involves accretion or underplating of the lower parts of the Tethyan oceanic lithosphere during the Upper Jurassic closure of the eastern branch of the Vardar ocean.

Origin of eclogite and garnet pyroxenite from the Moldanubian Zone of the Bohemian Massif, Czech Republic and its implication to other mafic layers embedded in orogenic peridotites

The granulite terrane of the Czech part of the Gfohl unit includes numerous small bodies of mantle derived peridotite, some of which contain layers or lenses of eclogite and garnet pyroxenite. These eclogitic rocks have generally been considered to be high-pressure crystal cumulates formed in the upper mantle. We present new analyses of whole-rock major and trace element contents for three kynanite-quartz eclogite samples taken from the Nove Dvory garnet peridotite body. Integrating these data with previously published analyses from the literature on eclogitic rocks from this terrane, we demonstrate that a magnesian group of eclogites, including these three new samples, were originally formed as cumulus gabbros, which were later transformed to eclogites in the mantle. A gabbroic origin for some mafic layers (Type II) has been advocated for other orogenic peridotites, such as Beni Bousera (Morocco), Ronda (Spain), and Horoman (Japan). By comparing these sets of data with those from the Bohemian Massif, we propose a simple method of identifying groups of metagabbros by utilizing MgO-normalization in oxide ratio plots for whole-rock major element analyses.

IN MEMORY OF LAURO MORTEN

There are people you can not forget. Lauro Morten was one of them. At a first glance he looked like a rough man, forged in the tradition of mountain people. Indeed, he loved mountains and he spent most of his energy to understand their evolution. He was always enthusiastic in planning new field excursions in every part of the world. We remember his company during long trips on minibus and the lively discussions in the field. We appreciated his honesty and directness in scientific discussion. His scientific career lasted over 40 years and contributed to the improvement of the Italian petrologic community. His first work, dated 1971, dealt with the ultramafic xenoliths collected in the Tertiary volcanics from the Lessini Mts. area. The study of mantle rocks, both as xenoliths enclosed in basaltic volcanics and exposed as orogenic ophiolitic peridotite massifs, was his main research interest that he carried out until his last days. In 1986 he accepted a full professor position at the University of Calabria in Cosenza, where he organized and chaired the Fourth International Eclogite Conference (1993). In those years he addressed other research topics, including the granulite rocks of the Catena Costiera and the ophiolitic units occurring in northern Calabria. In Calabria he was appointed as Head of the Department of Earth Sciences and he was also in charge of the courses for the diploma degree in Geological Sciences. He was deeply involved in the establishment and cultural growth of a local petrological school, which benefited from his brilliant and generous teaching. In his last years Lauro Morten served as full professor of Petrology and Petrography at the University of Bologna. He went back to the basement of his beloved Nonsberg range. His most recent research was devoted to mantle metasomatism and crustmantle interactions. In particular, he always encouraged his post-graduated students to be scientifically critical, honest and thoughtful. Even after he was diagnosed his illness, he continued to work in cooperation with scientists from all over the world. His family, colleagues, students and friends miss his cleverness and company. Thanks a lot, Lauro, for communicating your love of the beauty of rocks and the Nonsberg range, where you now rest in peace.

Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway

The buoyancy and strength of sub-continental lithospheric mantle is thought to protect the oldest continental crust (cratons) from destruction by plate tectonic processes. The exact origin of the lithosphere below cratons is controversial, but seems clearly to be a residue remaining after the extraction of large amounts of melt. Models to explain highly melt-depleted but garnet-bearing rock compositions require multi-stage processes with garnet and clinopyroxene possibly of secondary origin. Here we report on orogenic peridotites (fragments of cratonic mantle incorporated into the crust during continent-continent plate collision) from Otrøy, western Norway. We show that the peridotites underwent extensive melting during upwelling from depths of 350 kilometres or more, forming a garnet-bearing cratonic root in a single melting event. These peridotites appear to be the residue after Archaean aluminium depleted komatiite magmatism.

Transformation of Archaean Lithospheric Mantle by Refertilization: Evidence from Exposed Peridotites in the Western Gneiss Region, Norway

Orogenic peridotites occur enclosed in Proterozoic gneisses at several localities in the Western Gneiss Region (WGR) of western Norway; garnet peridotites typically occur as discrete zones within larger bodies of garnet-free, chromite-bearing dunite and are commonly closely associated with pyroxenites and eclogites. The dunites of the large Almklovdalen peridotite body have extremely depleted compositions (Mg-number 92–93� 6); the garnet peridotites have lower Mg-number (90� 6–91� 7) and higher whole-rock Ca and Al contents. Post-depletion metasomatism of both rock types is indicated by variable enrichment in the light rare earth elements, Th, Ba and Sr. The dunites can be modelled as residues after very high degrees (>60%) of melt extraction at high pressure (5–7GPa), inconsistent with the preservation of lower degrees of melting in the garnet peridotites. The garnet peridotites are, therefore, interpreted as zones of melt percolation, which resulted in refertilization of the dunites by a silicate melt rich in Fe, Ca, Al and Na, but not Ti. Previous Re–Os dating gives Archaean model ages for the dunites, but mixed Archaean and Proterozoic ages for the garnet peridotites, suggesting that refertilization occurred in Proterozoic time. At least some Proterozoic lithosphere may represent reworked and transformed Archaean lithospheric mantle.

Ferric iron in orogenic lherzolite massifs and controls of oxygen fugacity in the upper mantle

The bulk Fe 2O 3 contents and inter-mineral distributions of Fe 3+ were investigated in a suite of samples from several orogenic lherzolite massifs, including Beni Bousera (Morocco), Ronda (Spain) and Lherz (France). Ferric iron contents were determined for each phase by Mössbauer spectroscopy and these results were combined with microprobe data and modal abundances to determine the bulk Fe 2O 3 contents. The notion that Fe 3+ is moderately incompatible during partial melting in the mantle is supported by the observed decrease in bulk Fe 2O 3 content with increasing MgO, as well as by the generally lower Fe 3+ content of clinopyroxene in samples with low modal abundances of this phase. The partitioning of Fe 3+ between orthopyroxene and clinopyroxene is consistent with literature data for spinel peridotite xenoliths. The partitioning of Fe 3+ between clinopyroxene and spinel is composition dependent, changing with the Cr/Al of spinel. Thus partitioning is expected to be different in the spinel peridotite and plagioclase peridotite facies of the upper mantle. The ƒO 2 of orogenic massifs varies over several log units relative to the FMQ buffer. The values recorded by spinel-based and clinopyroxene-based oxygen barometry are generally comparable, indicating redox equilibrium between these two phases even at the relatively low temperature conditions existing in parts of the lithospheric mantle. Calculated bulk Fe 2O 3 contents range from 0.03 to 0.27 wt.%. The combination of modal abundance and major element composition means that orthopyroxene is a major contributor to the bulk Fe 2O 3 budget of peridotites, although clinopyroxene and spinel are much richer in Fe 3+ on a per formula unit basis. Residual Cr-rich spinel is the dominant source of Fe 3+ in plagioclase peridotites. In terms of the geochemical behaviour of Fe 3+, it can be concluded that orogenic peridotites exhibit essentially the same behaviour as spinel peridotite xenoliths. In terms of the controlling factors of ƒO 2 in the upper mantle, our data set records a certain degree of decoupling of ƒO 2 from whole rock Fe 2O 3 content, even if a correlation between these two parameters is generally apparent. This decoupling is because, whereas both whole rock Fe 2O 3 content and ƒO 2 are influenced by partial melting and melt extraction, additional processes such as metasomatism and phase changes can effectively reset ƒO 2 without always causing a concomitant change in bulk Fe 2O 3 content. Modelling the oxidation of spinel reveals that the ƒO 2 can be readily reset under initially reducing conditions, but the incorporation of progressively more and more Fe 3+ in spinel is required to further raise ƒO 2. A quasi-limit of ΔlogƒO 2 = FMQ + 1 is expected. These results are consistent with the general redox behaviour observed for spinel peridotites. Our data imply that Fe 3+–Fe 2+ equilibria have an important, if not dominant influence on ƒO 2 in the spinel peridotite facies of the upper mantle.

Re–Os and S systematics of spinel peridotite xenoliths from east central China: Evidence for contrasting effects of melt percolation

Abstract Os isotopic compositions and Re and Os concentrations were determined for Cenozoic basalt-borne ultramafic xenoliths from the Subei Basin of east central China. Re–Os analyses were coupled with whole rock major and trace element and S abundance determinations, and with characterization of rock textures, modal phase proportions and sulfide petrography. The two main sampling areas, Lianshan and Panshishan, separated by only 6 km, have similar textures and major and moderately incompatible lithophile trace element compositions. The Os isotopic ratios of these two areas plot on the same trends relating 187Os/188Os to indices of melt extraction such as whole rock Yb content. These Os isotopic systematics suggest that both areas were affected by an early Proterozoic (∼1.8 Ga) melt extraction event. Thus the two areas apparently shared the same long term lithospheric history. Nevertheless, the sulfide abundances and whole rock S, Os and Re concentrations are strikingly lower in Lianshan than in Panshishan, and the two localities have different incompatible lithophile trace element signatures. These differences resulted from contrasting melt percolation styles between the two areas. Lianshan was affected by extensive percolation of sulfur undersaturated melts that removed Re, Os and S, while Panshishan experienced interaction with low degree or highly evolved melts that added Re, Cu and S, but had no affect on Os abundances. The lack of correlation between 187Os/188Os and 187Re/188Os, compared with the good correlation between 187Os/188Os and Yb, indicates that the perturbation of the Re and Os concentrations was fairly recent, and perhaps related to Mesozoic or Cenozoic lithospheric thinning in eastern China. The Lianshan Os concentrations are typical of those of off-cratonic mantle xenoliths, while the Panshishan Os concentrations are closer to those of orogenic peridotites. This suggests that the low Os concentrations, and by extension, the low concentrations of all of the highly siderophile elements (HSE) typically observed in ultramafic mantle xenoliths, may result from recent melt percolation processes, probably directly or indirectly related to the magmatism that brought the xenoliths to the surface. Thus ultramafic xenoliths may not provide reliable estimates of the HSE contents of the upper mantle, and variations in HSE abundances between xenolith localities should not be used to define global scale processes. Os and other HSE abundances may prove to be sensitive indicators of melt percolation, and may provide information about the degree of sulfur saturation of the melts. Despite the loss of Os during recent percolation, the Os isotopic ratios of most samples are nearly unmodified by this process, confirming the utility of this system for dating ancient melt extraction events.

UHP GARNET ORTHOPYROXENITES (DABIE SHAN, CHINA) AS MONITORS OF PERCOLATION OF HYDROUS SI-RICH MELTS IN DEEP SUBDUCTION ENVIRONMENTS

UHP GARNET ORTHOPYROXENITES (DABIE SHAN, CHINA) AS MONITORS OF PERCOLATION OF HYDROUS SI-RICH MELTS IN DEEP SUBDUCTION ENVIRONMENTS

Titanium solubility in olivine in the system TiO2–MgO–SiO2: no evidence for an ultra-deep origin of Ti-bearing olivine

The finding of ilmenite rods in olivine from orogenic peridotites has sparked a discussion about the processes of incorporation and exsolution of titanium in olivine. We have experimentally investigated the solubility of Ti in olivine as a function of composition, temperature and pressure in the synthetic TiO2–MgO–SiO2 system. Experiments at atmospheric pressure in the temperature range 1,200–1,500°C showed that the highest concentration of TiO2 is obtained when olivine coexists with spinel (Mg2TiO4). The amount of TiO2 in olivine in the assemblages olivine + spinel + periclase and olivine + spinel + ilmenite at 1,500°C was 1.25 wt.%. Changes in the coexisting phases and decreasing temperature result in a significant reduction of the Ti solubility. Olivine coexisting with pseudobrookite (MgTi2O5) and a Ti–Si-rich melt at 1,500°C displays a fourfold lower TiO2 content than when buffered with spinel. A similar decrease in solubility is obtained by a decrease in temperature to 1,200°C. There is a negative correlation between Ti and Si and no correlation between Ti and Mg in Ti-bearing olivine. Together with the established phase relations this suggests that there is a direct substitution of Ti for Si at these temperatures, such that the substituting component has the stoichiometry Mg2TiO4. The unit cell volume of olivine increases systematically with increasing TiO2 content demonstrating that the measured TiO2 contents in olivine are not caused by micro-inclusions but by incorporation of Ti in the olivine structure. Least squares fitting of 20 olivine unit cell volumes against the Ti content yield the relation: V (Å3)=290.12(1) + 23.67(85) NTi. The partial molar volume of end-member Mg2TiO4 olivine (NTi=1) is thus 47.24±0.13 cm3. The change of the Ti solubilty in olivine coexistent with rutile and orthopyroxene with pressure was investigated by piston cylinder experiments at 1,400°C from 15 to 55 kbar. There is no increase in TiO2 contents with pressure and in all the experiments olivine contains ~0.2 wt.% TiO2. Moreover, a thermodynamic analysis indicates that Ti contents of olivine coexisting with rutile and orthopyroxene should decrease rather than increase with increasing pressure. These data indicate that the ilmenite exsolution observed in some natural olivine does not signify an ultra-deep origin of peridotite massifs.

A multi-technique study of platinum group element systematic in some Ligurian ophiolitic peridotites, Italy

Fe–Ni–Cu sulfide mineralogy has been investigated along with bulk-rock and in-situ PGE analyses by ICP MS and LA-ICP-MS in eight lherzolites from the Internal (IL) and External Liguride (EL) ophiolites (Italy). The two EL lherzolites are fertile (2–4% partial melting) and slightly serpentinized while the six IL cpx-poor lherzolites have experienced 5–10% of partial melting, impregnation by instantaneous melt fractions [Geochim. Cosmochim. Acta 61 (1997) 4557] and have been highly serpentinized. The EL lherzolites show broadly chondritic PGE relative abundances with a slight to pronounced enhancement of the light PGE (Ru, Rh and Pd) relative to the heavy PGE (Os, Ir and Pt) (Ru N/Ir N=1.13; Rh N/Ir N=1.08–1.10; Pd N/Ir N=1.24–1.62; N=CI-chondrite normalized). Their magmatic sulfide modal abundances and S contents, similar to the orogenic peridotites values, are consistent with their very low degree of partial melting. The occurrence of Cu–Rh–Pd-rich pentlandite, however, demonstrates that, even for low degree of partial melting, a Cu–Ni-rich sulfide liquid can segregate, leaving the residual monosulfide solid solution (Mss) (now transformed into Cu-poor pentlandite) depleted in Rh and Pd (Rh N/Ir N and Pd N/Ir N<1). The IL cpx-poor lherzolites display a broadly flat PGE patterns from Os to Pt with a slight enhancement of Ru and Rh (Ru N/Ir N=1.05–1.38; Rh N/Ir N=1.01–1.31). Pd N/Ir N ratios range from chondritic to superchondritic (1.02–2.99) and cannot be interpreted in terms of partial melting models. Rh–Pd–Cu–Ni-rich pentlandite grains are associated with large corroded cpx crystals ascribed to exotic melt percolation by Rampone et al. [Geochim. Cosmochim. Acta 61 (1997) 4557]. It is concluded that precipitation of Cu–Ni–Rh–Pd-rich sulfides has significantly enhanced the Pd concentrations as well as the magmatic sulfide modal abundances. Such processes, previously documented in abyssal peridotites from slow-spreading mid-oceanic ridges, characterize residues from low to moderate melting degrees (5–10%) of the oceanic mantle as a whole, either from mature ocean or from short-lived oceanic basins.