Ophiolitic Peridotites Research Articles

Geochemical investigation of serpentinized oceanic lithospheric mantle in the Feather River Ophiolite, California: Implications for the recycling rate of water by subduction

The petrology and geochemistry of serpentinized harzburgites within the Feather River Ophiolite in northern California were investigated to constrain the origin of serpentinization. Trace-element systematics indicate that serpentinization was associated almost solely with relatively low temperature hydrothermal addition of seawater and not with the addition of metamorphic fluids associated with subduction or tectonic obduction. Major element systematics show almost negligible disturbance, suggesting low water/rock ratios rather than the very high water/rock ratios characteristic of serpentinization of exhumed mantle at the seafloor such as seen in abyssal peridotites. These observations were taken to indicate that serpentinization occurred by the addition of seawater to ultramafic protoliths while they were structurally within the oceanic lithosphere prior to tectonic accretion. Major and trace element systematics were used with simple melting models and assumptions about the thermal state of the mantle to put rough constraints on the paleo-depths at which the ultramafic protoliths were the last melted. Assuming that these peridotites were formed beneath a passive upwelling system, these paleo-melting depths were assumed to equate with paleo-serpentinization depths. For a model mantle potential temperature of 1350 °C, the paleo-serpentinization depth was estimated to be at least ∼ 40 km based on the minimum melting degree observed in the Feather River Ophiolite peridotites. This depth constraint on serpentinization was then combined with model estimates of the lateral extent of serpentinization assuming that serpentinization is associated with infiltration of water through faults and fractures. If these observations can be generalized, we estimate that recycling rate of serpentine-bound water in subduction zones is roughly one order of magnitude higher than that associated with mineralogically bound water in subducting oceanic crust and sediments. After accounting for serpentine recycling, it is shown that the recycling rate of water via subduction is roughly equal to the global volcanic dewatering rate, suggesting that the water budget of the ocean and Earth's interior may presently be close to steady state.

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.

Mantle redox in Cordilleran ophiolites as a record of oxygen fugacity during partial melting and the lifetime of mantle lithosphere

We examine controls on mantle oxygen fugacity ( fO 2) during the partial melting process that forms mantle lithosphere at spreading centers. We compare the paleo fO 2 at the time of melting inferred by V/Sc systematics of ophiolite peridotites, with the thermobarometric fO 2 recorded by olivine–orthopyroxene–spinel assemblages during simple cooling in relatively young oceanic lithosphere. Modelling of the V/Sc in the ophiolite peridotites from Alaska, Yukon and British Columbia is permissive of only a narrow range in log fO 2 during melting between NNO and NNO − 1 (where NNO is the nickel–nickel oxide buffer), depending on the choice of partition coefficients for Sc. This result is within uncertainty of the thermobarometric fO 2 recorded by most samples (within 1 log unit of NNO − 1). The same cannot be said for more complex peridotite residues from continental mantle, where V/Sc systematics show a narrow paleo fO 2 during formation but wide range of thermobarometric fO 2 after equilibration in the lithosphere. In continental mantle with a complex history, thermobarometric fO 2 is an ambiguous measure of that attendant during partial melting. Graphite-saturated melting in a system closed to oxygen controls melt Fe 3+/Fe 2+and CO 2 content, and creates a shift in fO 2 of about 2 log units [1] in a peridotite residue. In contrast, for the ophiolite mantle samples in this study, both paleo- and thermobarometric fO 2 are near values predicted by carbon–fluid equilibria, yet show no relationship with depletion, suggesting the melt-residue system in the mantle may be open to oxygen during the partial melting process.

Trace element distribution between orthopyroxene and clinopyroxene in peridotites from the Gakkel Ridge: a SIMS and NanoSIMS study

Clinopyroxenes (cpx) in abyssal and ophiolitic peridotites are commonly analyzed for lithophile trace element abundances in order to estimate degrees of melting and porosity conditions during melt extraction, assuming that these data reflect near-solidus conditions. During cooling, however, cpxs always exsolve into parallel lamellae of low-Ca enstatite and high-Ca diopside. This may potentially lead to redistribution of the initial trace element budget. Since orthopyroxene (opx) cannot significantly host most incompatible trace elements, exsolution will lead to an enrichment in the cpx lamellae. In order to address a possibly exsolution-controlled partitioning between cpx and opx, we have obtained major and trace element mineral compositions on 14 plagioclase-free ocean floor mantle rocks. They cover the entire abyssal peridotite compositional spectrum from very fertile to highly depleted compositions. The mean volume proportion of opx lamellae in cpx porphyroclasts lies around 15% of the original cpx. For the light to middle rare earth elements, the enrichment in the measured cpx exsolution is exclusively controlled by these phase proportions. Relative to these highly incompatible trace elements, solely Ti and Yb partition significantly into opx. Lamellar interpyroxene partition coefficients, estimated from NanoSIMS analyses, are around three times as high as the ones for near-solidus bulk pyroxene. The equilibration temperatures for the exsolution lamella are slightly higher than 800°C. The bulk cpx can be reconstructed using the lamellar proportions and their relative partitioning. The implication of such a reconstruction is that the cpx rare earth element patterns shift almost in parallel to lower values. These shifts, however, do not affect mantle melting models proposed thus far for mid-ocean ridges.

Complete mantle section of a slow‐spreading ridge‐derived ophiolite: An example from the Isabela ophiolite in the Philippines

Abstract The Isabela ophiolite shows a complete ophiolite sequence exposed along the eastern coast of northern Luzon, the Philippines. It forms the Cretaceous basement complex for the northeastern Luzon block. This ophiolite is located at the northern end of a trail of ophiolites and ophiolitic bodies along the eastern margin of the Philippine Mobile Belt. This paper presents new findings regarding the nature and characteristics of the Isabela ophiolite. Peridotites from the Isabela ophiolite are relatively fresh and are composed of spinel lherzolites, clinopyroxene‐rich harzburgites, depleted harzburgites and dunites. The modal composition, especially the pyroxene content, defines a northward depletion trend from fertile lherzolite to clinopyroxene‐rich harzburgites and more refractory harzburgites. Variation in modal composition is accompanied by petrographic textural variations. The chromium number of spinel, an indicator of the degree of partial melting, concurs with petrographic observations. Furthermore, the Isabela ophiolite peridotites are similar in spinel and olivine major‐element geochemistry and clinopyroxene rare earth‐element composition to abyssal peridotites from modern mid‐oceanic ridges. Petrological and mineral compositions suggest that the Isabela ophiolite is a transitional ophiolite subtype, with the fertile lherzolites representing lower sections of the mantle column that are usually absent in most ophiolitic massifs. The occurrence of the fertile peridotite presents a rare opportunity to document the lower sections of the ophiolitic mantle. The variability in composition of the peridotites in one continuous mantle section may also represent a good analogy of the melting column in the present‐day mid‐oceanic ridges.

THE COMPOSITE MANTLE EVOLUTION OF THE ERRO-TOBBIO PERIDOTITE (VOLTRI MASSIF, LIGURIA ALPS, ITALY)

THE COMPOSITE MANTLE EVOLUTION OF THE ERRO-TOBBIO PERIDOTITE (VOLTRI MASSIF, LIGURIA ALPS, ITALY)

MELT DIFFUSE REFERTILIZATION AND FOCUSED MIGRATION IN LITHOSPHERIC MANTLE OF A FOSSILE RIFTED MARGIN: THE MT. NERO PERIDOTITE (EXTERNAL LIGURIDES, ITALY)

MELT DIFFUSE REFERTILIZATION AND FOCUSED MIGRATION IN LITHOSPHERIC MANTLE OF A FOSSILE RIFTED MARGIN: THE MT. NERO PERIDOTITE (EXTERNAL LIGURIDES, ITALY)

OCEANIC PERIDOTITES ON MACQUARIE ISLAND: A UNIQUE RECORD OF MANTLE MELTING AND MELT PERCOLATION

Macquarie Island (Southern Ocean) is a very rare occurrence of young (9 Ma) ocean crust exposed above sea level. As such, it is a key source of information about magmatic and tectonic seafloor spreading processes. Residual mantle peridotites are also exposed on the island. This unique location thus allows us to study both the residue and the product of mantle melting underneath a spreading centre, and allows us to test the co-genetic relation between oceanic mantle and crust. We will present a detailed petrographic analysis of the Macquarie Island harzburgites, and trace element geochemical as well as Nd and Sr isotopic data on fractions of clinopyroxene crystals. The Macquarie Island peridotites are among the most depleted peridotites from young oceanic basins worldwide in terms of modal composition and relatively compatible element concentrations, comparable to harzburgites from Hess Deep. At the same time, they show enrichment in light rare earth elements. The spoon-shaped trace elements patterns show a remarkable similarity to those in ophiolitic mantle peridotites, which are often interpreted as resulting from melting in supra-subduction zone settings. Macquarie Island thus acts as a key link between ophiolites and young ocean floor. Importantly, the spreading environment of Macquarie Island is well characterized and a supra-subduction zone setting can be ruled out unequivocally. On the basis of trace element modeling, we propose that the Macquarie Island harzburgites represent samples of upper mantle material that have undergone high degrees of accumulated melt extraction (21-22%). Modelling further shows that light rare earth enrichment was probably not the result of hydrous, supra-subduction style melting. In particular, observed mobile/immobile element ratios are inconsistent with such a model. Instead, a chromatographic melt percolation model produces a good match with the observed trace element patterns. The Macquarie Island harzburgites are thus examples of mantle rocks that have recorded cryptic metasomatism and re-equilibration associated with porous (unchanneled) flow of MORB-like melts. We also show that Nd isotope ratios are controlled by the isotope ratios in the percolating melts. Therefore, the similarity in isotope ratios between crustal and mantle rocks on the island cannot be used to infer that the harzburgites are the source residue of the oceanic crust. The high degrees of melting recorded by the harzburgites are inconsistent with the thin crust (c. 3 km), slow spreading rate (c. 2 cm/yr full rate) and the enriched character of the gabbros, dykes and lavas on the island, and argue against a simple co-genetic link between mantle and crust. The oceanic crust was probably produced from local highly enriched domains (‘plums’) within the mantle, with no significant contribution from the enclosing depleted mantle. A major implication of the study is that Macquarie Island- like, spoon-shaped trace element patterns in peridotites cannot be used as a ‘supra-subduction zone signature’ in ophiolites.

STRUCTURAL AND COMPOSITIONAL FEATURES OF THE MT. MUSINÈ PERIDOTITES (LANZO MASSIF, WESTERN ALPS, ITALY)

Present knowledge indicates that the ophiolitic peridotites of the Lanzo South Massif were exhumed from spinel-facies conditions to shallow oceanic levels during the rifting stage of the Jurassic Ligurian Tethys. During exhumation, most of these peridotites were diffusely percolated and impregnated by asthenospheric melts. Field, structural and petrological-geochemical investigations, in the frame of a multidisciplinary investigation, have been dedicated to the southern flank of Monte Musine (Lanzo South, Western Alps), where the extreme structural and lithological variability and the mutual relationships between different rock types (i.e. petrogenetic processes) allow to reconstruct some steps of the composite structural and petrological evolution of this peridotite massif. The oldest structural elements recognizable in the field are represented by pyroxenite bands and by spinel-facies foliation, which are almost parallel in any outcrop. This feature essentially supports the presence of a long-time constant strain field at spinel facies conditions. Consistently, the percolating melt increments, which produced the plagioclase-rich impregnation of the spinel peridotites, utilized most probably the spinel-facies foliation plane to migrate by porous flow. A strong deformation event developed along shear zones, following the plagioclase impregnation, forming plagioclase peridotite tectonites which were subsequently used by silica-undersaturated asthenospheric melts, i.e. aggregated MORB-type melts, to migrate upwards along these structural discontinuities. These pyroxene-undersaturated melts interacted with the plagioclase tectonite mineral assemblage, dissolving plagioclase and clinopyroxene, and forming concordant harzburgite bands or, later, dissolving plagioclase and both pyroxenes, forming concordant dunite bands. Subsequent melt-related events, i.e. the formation of decametre- to hectometre-wide replacive dunite bodies, and intrusion events, i.e. the intrusion of MORB gabbroic dikes, were driven by a different strain field strongly discordant with the previous foliation and with the trend of the shear zones and the concordant replacive bands. Thus, the section of mantle lithosphere of Monte Musine record a composite scenario of melt-related and tectonic-metamorphic processes occurred during its exhumation to shallow oceanic levels of the Jurassic Ligurian Tethys.

GEOCHEMICAL CONSTRAINTS ON THE MULTISTAGE MELT MIGRATION THROUGH OPHIOLITIC PERIDOTITE OF LANZO SOUTH BODY (WESTERN ALPS)

GEOCHEMICAL CONSTRAINTS ON THE MULTISTAGE MELT MIGRATION THROUGH OPHIOLITIC PERIDOTITE OF LANZO SOUTH BODY (WESTERN ALPS)

Chronology, Petrology and Isotope Geochemistry of the Erro-Tobbio Peridotites (Ligurian Alps, Italy): Records of Late Palaeozoic Lithospheric Extension

Mantle peridotites from the Erro–Tobbio (ET) ophiolitic unit (Voltri Massif, Ligurian Alps) record a tectono-metamorphic decompressional evolution, indicated by re-equilibration from spinel- to plagioclase- to amphibole-facies conditions, and progressive deformation from granular to tectonite to mylonite fabrics. The peridotites are considered to represent subcontinental lithospheric mantle that was tectonically denuded during rifting and opening of the Jurassic Ligurian Tethys ocean, similar to the Northern Apennine (External Ligurides) ophiolitic peridotites. We performed chemical and isotopic investigations on selected granular and tectonite spinel peridotites and plagioclase tectonites and mylonites, with the aim of defining the nature of the mantle protoliths, and to date the onset of exhumation of the ET peridotites. Spinel- and plagioclase-bearing tectonites and mylonites exhibit heterogeneous bulk-rock major and trace element composition, despite rather homogeneous mineral chemistry, thus indicating that the ET mantle protoliths record a composite history of partial melting and melt migration by reactive porous flow. The lack of correlation between the observed geochemical heterogeneity and the structural type (granular, tectonite, mylonite) indicates that the inferred reactive porous flow event preceded the exhumationrelated lithospheric history of the Erro–Tobbio mantle. The tectonometamorphic evolution caused systematic chemical changes in minerals: (1) Al decrease in orthopyroxene; (2) Al decrease, and Cr and Ti increase in spinels; (3) Al and Sr decrease, Cr, Ti, Zr, Sc, V and middle to heavy rare earth element increase and development of a negative Eu anomaly in clinopyroxene. The studied samples have Nd isotope compositions consistent with a mid-ocean ridge basalt mantle reservoir. Sm/Nd isotope data on plagioclase and clinopyroxene separates (and corresponding whole rocks) from two plagioclase peridotites, representative of the plagioclase-bearing mylonitic extensional shear zone, have yielded ages of 273 � 16Ma and 313 � 16Ma, for the plagioclase-facies recrystallization stage, significantly older than the expected Jurassic age. This indicates that the Erro–Tobbio peridotites represent subcontinental lithospheric mantle that was tectonically exhumed from spinel-facies depths to shallower lithospheric levels during Late Carboniferous– Permian times. Our results are consistent with the previously documented evidence for an extensional regime in the Europe–Adria lithosphere during Late Palaeozoic time, and they represent the first record that extensional mechanisms were also active at lithospheric mantle levels.

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.

Contrasting bulk and mineral chemistry in depleted mantle peridotites: evidence for reactive porous flow

A series of recent papers have indicated that reconstructed bulk compositions of abyssal peridotites define chemical correlations, namely increasing FeO tot and decreasing SiO 2 with increasing MgO, which cannot be produced by simple extraction of partial melts. However, no general consensus exists on the reliability of these trends, because they could be artifacts of the adopted calculations, and on the origin of abyssal peridotites. Specifically, it has been inferred that abyssal peridotite compositions are consistent with combined histories of partial melting and subsequent melt migration which caused either olivine addition, or dissolution and precipitation reaction, by equilibrium porous flow [Niu et al., Earth Planet. Sci. Lett. 152 (1997) 251–265; Asimow, Earth Planet. Sci. Lett. 169 (1999) 303–319]. We report combined bulk-rock and mineral chemical data for ophiolitic peridotites from the Erro-Tobbio (ET) Unit (Voltri Massif, Ligurian Alps), which represent lithosphere remnants of the Jurassic Ligurian Tethys embryonic ocean. These peridotites include sp-lherzolites and sp-harzburgites, and display overall depleted geochemical signature. However, comparison between bulk rock and corresponding mineral compositions reveals that these rocks cannot be residues of simple (equilibrium or fractional) melt extraction. Mineral compositions are similar in all the samples. By contrast, the bulk rock compositions define striking correlations, i.e. increasing FeO tot, Ni, Co, and decreasing Al 2O 3, SiO 2, CaO, Sc, Cr, Yb N, with increasing MgO: the MgO–FeO tot and MgO–SiO 2 correlations are similar to those recognized in abyssal peridotites. Thus, the ET peridotites provide evidence that the above trends are indeed consistent with similar variations in on-land peridotites; also, these trends cannot simply result from progressive melt depletion, because constituent minerals in the different ET samples have rather uniform composition. Calculated bulk modes indicate that the observed chemical variations are coupled to systematic modal changes, namely decrease in cpx and opx, and increase in olivine, at increasing bulk MgO. The ET peridotites also display decrease of the cpx/opx ratio at increasing bulk MgO, and this argues against a process of simple olivine addition. By contrast, some peculiar bulk-mineral compositional features – e.g. (i) nearly constant olivine Mg* [=Mg/(Mg+Fe tot)] values, at increasing modal olivine, (ii) opposite bulk Cr and Ni correlations, (iii) bulk Cr and Yb decrease, and parallel Ni increase, strikingly correlated with progressive modal cpx decrease and concomitant modal olivine increase – are consistent with the expected chemical and modal effects, during a process of interaction between depleted peridotites and melts migrating by equilibrium porous flow (involving pyroxene dissolution and olivine precipitation reactions). Bulk-rock and mineral chemistry data in the ET peridotites thus indicate that the major element correlations inferred by Niu et al. for the abyssal peridotites are reliable, and most likely result from a combined history of partial melting and melt interaction by reactive porous flow.

Seismic anisotropy of the upper mantle 1. Factors that affect mineral texture and effective elastic properties

Flow‐induced mineral alignment in the Earth's mantle affects the nature of seismic wave propagation. Since measurements of seismic travel time and shear wave splitting are a key means by which the structure of the upper mantle can be imaged, it is important to understand the factors that contribute to variability in elastic properties. Seismic anisotropy associated with lattice‐preferred orientations of minerals in peridotite is the focus of this study. One way to better understand anisotropy in a convecting upper mantle is to simulate texture evolution based on certain assumptions. Simulations of the development of olivine and orthopyroxene alignment along streamlines of a mantle flow field illustrate how continuously varying strain conditions affect the resulting orientation distribution. There are various uncertainties in rock texture simulations, and the aim here is to investigate how much model assumptions may influence the results. A model of upper mantle flow in the vicinity of an oceanic spreading center is used to illustrate these points. First we assess how sensitive results are on assumptions of different polycrystal plasticity models, specifically lower bounds and viscoplastic self‐consistent approaches. We also investigate how recrystallization during deformation might affect the texture that develops along a streamline. The effects of grain growth and nucleation produce, as expected, textures that are different from the deformation‐only case. However, the basic P wave structure of the predicted anisotropy is similar between models for streamlines in a slow‐spreading, passive flow model as is commonly used for simulating flow near this type of plate boundary. Shear wave splitting patterns are more complex and differ somewhat more between the models in the off‐axis region. We also compare predictions for texture development of olivine‐only aggregates to that of mixed composition with 70% olivine and 30% orthopyroxene. Although we consider only one aspect of such polyphase deformation, i.e., plastic deformation on specified crystal slip systems, our results are consistent with field observations of the orientation distributions in ophiolitic peridotites. Finally, we determine how simplifying assumptions about the symmetry of the elastic anisotropy can bias interpretations of seismic travel time and shear wave splitting. Whereas a hexagonal approximation can lead to underestimates of the degree of anisotropy, an orthorhombic approximation is found to closely match the results predicted for a general elastic tensor corresponding to an orientation distribution whose symmetry is as low as monoclinic or triclinic.

Ch4-rich fluid inclusions in the Yushigou mantle peridotite and their implications, North Qilian Mountains, China

Studies were carried out on the early phase of fluid inclusions which occur in residual olivines in harzburgite from the Yushigou ophiolitic mantle peridotite, the North Qilian Mountains. Components of these inclusions, analyzed by micro laser Raman spectroscopy, are dominantly CH, (70%–95%) with minor H2, N2, H2S, CO2,C2H4, C2H6, and C3H6 but there are no H2O,CO and SO2. The highly CH4-rich fluid was probably derived from an ancient deep mantle. This discovery plays an important role in all-round understanding of the fluid property of the upper mantle, especially the ancient oceanic upper mantle.

Secular evolution of subcontinental lithospheric mantle

Chemical and petrographic observations on mantle-derived xenoliths and xenocrysts define a fundamental distinction between Archean cratonic mantle and Phanerozoic circumcratonic mantle. Archean xenoliths are more depleted on average, and those from South Africa and Siberia, the best-studied areas, have higher Si/Mg than Phanerozoic xenoliths of similar Mg#; subcalcic harzburgites are wellrepresented in Archean xenolith and xenocryst suites, but rare in younger ones. Analysis of >13,000 garnet xenocrysts from volcanic rocks worldwide shows a correlation of garnet composition with the tectonothermal age of the crust penetrated by the volcanic rocks. Typically, garnet suites from volcanics penetrating Archean crust have higher Cr#, lower Y, Y/Ga and HREE/Sc, and higher Zr/Y than those from volcanic rocks in Proterozoic and Phanerozoic terranes. These differences, and the rarity of subcalcic garnets in mantle beneath Proterozoic and Phanerozoic crust, are consistent with a decrease in mean modal cpx/gnt and (cpx+gnt) from Archean to Proterozoic to Phanerozoic mantle. Temperatures calculated from the Ni content of peridotitic garnets (TNi) can be referred to a local paleogeotherm to map the distribution of garnet types with depth. These maps show that Archean mantle sections are typically strongly stratified, with concentrations of harzburgitic rocks and depleted lherzolites in the middle to lower parts of the sections, and more fertile lherzolites at depth. Proterozoic mantle sections are less depleted and typically less strongly stratified, while the garnet-facies parts of Phanerozoic sections are still less depleted and essentially homogeneous. Inversion of TNi and composition data on garnets allows mapping of the vertical distribution of the XFo of coexisting olivine in mantle sections. Archean sections typically show decreasing XFo with depth; Proterozoic sections are less regular, and many show the lowest XFo in the upper parts of the sections. These differences imply changes in the processes that have produced subcontinental lithospheric mantle (SCLM) at different times in Earth history. In xenolith suites, the Cr content of peridotitic garnet correlates well with the Al content of the host peridotite, and Al is closely related to the contents of other major and minor elements. These correlations allow calculation of the mean composition of a mantle section, given the Cr content of garnet xenocrysts from that section. The calculated mean composition of SCLM beneath Archean, Proterozoic and Phanerozoic terrains shows a secular evolution in all measures of depletion, such as Al, Ca, Mg#, and Fe/Al; Proterozoic SCLM is intermediate in composition between Archean and Phanerozoic SCLM. Cenozoic SCLM, exemplified by garnet peridotites from young extensional areas (China, Siberia, Australia) is only mildly depleted (²10% melt extraction) relative to Primitive Mantle. Similar compositions are found in some exposed spinel lherzolite massifs in extensional settings, such as Zabargad Island. Shallow spinel-facies SCLM beneath some Phanerozoic terrains, especially in Europe (both xenolith suites and exposed massifs), is more depleted and may represent reworked Proterozoic SCLM; this is consistent with some Re- Os depletion ages. Archean xenolith suites show positive correlations between Al, Fe and Cr that are not present in younger mantle samples; these correlations indicate that no Cr-Al phase was present on the liquidus during the melting event that produced Archean mantle. We suggest that most Archean SCLM was derived by high-degree melting at depths ³150 km, and that subduction processes have not been significant in its genesis. Phanerozoic circumcratonic xenolith suites are typically much less depleted than ophiolitic and abyssal peridotites; this suggests that subducted oceanic mantle is not a major component of Phanerozoic SCLM. Instead, the fertile composition of circumcratonic xenolith suites, and comparison with the Zabargad situation, suggests that most existing Phanerozoic SCLM probably has been generated in extensional post-orogenic environments. Density modelling shows that Phanerozoic SCLM, once it has cooled to typical conductive geotherms, will be gravitationally unstable with respect to the asthenosphere. This instability provides a mechanism for removing such lithosphere and replacing it with upwelling fertile material on a short time scale. In contrast, typical sections of Archean and Proterozoic SCLM are gravitationally stable, allowing their long-term preservation. The broad correlation of SCLM composition with crustal age implies quasi-contemporaneous formation of crustal volumes and their underlying SCLM, and crust-mantle coupling over periods of aeons beneath Archean and Proterozoic terranes; it also requires an evolution in fundamental processes involved in the formation of continents and their roots. This evolution may be related to the secular cooling of Earth, and a consequent decrease in the melting temperature and depth of melting in mantle plumes.

The origin of U-shaped rare earth patterns in ophiolite peridotites: assessing the role of secondary alteration and melt/rock reaction

Ten samples of serpentinized harzburgite and lherzolite from the Trinity ophiolite complex (California) can be classified into two groups using their REE patterns and initial neodymium and strontium isotopic ratios. Samples with the most refractory major element compositions (Al 2O 3 = 0.6–1.8%; CaO < 1.2%) have U-shaped REE patterns and show low to very low ε Nd(T) values (+2 to −7), but high to very high 87Sr/ 86Sr(T) ratios (up to 0.715), whereas rocks with more fertile major element compositions (Al 2O 3 = 2–3.2%; CaO = 2–2.6%) give REE patterns showing increasing depletion from Lu to La and have high ε Nd(T) values (+6 to +8), but low 87Sr/ 86Sr(T) ratios (0.703–0.704). The low neodymium and high strontium isotopic ratios of those peridotites having U-shaped REE patterns demonstrate that the LREE enrichments observed in these rocks are the result of contamination by continental crust and not the consequence of peculiar fractionation during melting or chromatographic effects during mantle ascent. A clinopyroxene concentrate prepared from a bulk sample of lherzolite with ε Nd(T) = −7 and 87Sr/ 86Sr(T) = 0.708 gives MORB-like isotopic signatures: ε Nd(T) = +14 and 87Sr/ 86Sr(T) = 0.7035. Furthermore, two highly serpentinized samples with U-shaped REE patterns show strongly negative δD values (−95 and −110‰) much typical of peridotites altered by continental metamorphic fluids or/and meteoritic waters. From these results, we conclude that the crustal contamination corresponds to a low-temperature alteration event which took place during or after obduction of the Trinity ophiolite complex onto the continent and is not the consequence of any high-temperature invasion of the ultramafic rocks by melts/fluids derived from subducted continental materials.

Chemistry and origin of trapped melts in ophioiitic peridotites

Melt impregnation and peridotite-melt interaction are ubiquitous phenomena in the oceanic-type lithospheric mantle. Nevertheless, the nature of the infiltrating melts is still poorly understood. We performed detailed textural and chemical investigations (by means of electron and ion microprobe) on: (1) impregnated plagioclase-bearing ophiolitic peridotites from the Internal Ligurides (Northern Apennine, Italy) and Mt. Maggiore (Corsica), and (2) olivine cumulates (consisting of 85 vol% olivine plus interstitial plagioclase and rare poikilitic clinopyroxene) from magmatic pods intruded within the Mt. Maggiore peridotites. Field evidence indicates that such cumulates crystallized from the melts, which impregnated the surrounding peridotites. Melt impregnation in the peridotites is verified by the occurrence of peculiar microstructures: (1) plagioclase blebs and/or veins confined along grain boundaries or crosscutting mantle minerals; (2) partial dissolution of mantle clinopyroxene and replacement by orthopyroxene and plagioclase aggregates, which indicate disequilibrium between melts and host peridotites. Interaction with melts also produces chemical modifications in mantle clinopyroxenes, i.e., Ti, M(middle)- to H(heavy)-REEs (and, to a lesser extent, Zr, Y, and Sc) enrichment coupled to depletion in Al. Minerals crystallized from the melts have depleted Geochemical signature: plagioclases are highly Anorthitic (An 88An 94 in the peridotites; An 81An 85 in the cumulates), and show extremely low Sr (<26 ppm) and LREE ( < 1 × Cl; Ce N SM N = 0.13−0.50 ) concentrations. Interstitial clinopyroxenes in the cumulates are characterized by high Mg values (90.6–91.7): their REE spectra show significant LREE depletion ( Ce N Yb N = 0.027−0.039 ), high M- to H-REE concentrations (15–30 × C1) and pronounced negative Eu N anomalies. Geochemical modeling indicates that the impregnating liquids probably consisted of unmixed depleted melt increments produced by 6–7% fractional melting. The results of this study therefore suggest that the impregnating melts originated at deeper mantle levels and presumably represented the last melt increments of a fractional melting process. There is thus a higher probability that they will remain incorporated in the upper mantle. They subsequently ascended, partly crystallized as cumulate pods, and interacted with the studied peridotites dissolving mantle clinopyroxene and precipitating orthopyroxene. Such a process has been increasingly invoked in studies of melt/rock reaction zones from ophiolitic peridotites.

Chromitite and ultramafic rock compositional zoning through a paleotransform fault, Poum, New Caledonia

The major mylonitic shear zone of the northern part of the ophiolitic peridotite sheet of New Caledonia has been described as a paleotransform fault (Secher, 1981). Located along this structure, the Poum ultramafic massif shows a wide spectrum of peridotite compositions ranging from plagioclase lherzolite to dunite across a 2-km-thick transition zone. The mylonitized plagioclase lherzolite contains abundant pyroxenite and mafic veins. The Poum ultramafic massif includes more than 30 chromitite orebodies. Their composition vanes according to the nature of the host peridotite: Al rich in the plagioclase lherzolite zone (31-43% Al 2 O 3 and 25-36% Cr 2 O 3 ), Cr rich (39-60% Cr 2 O 3 ) in the spinel lherzolite and clinopyroxene harzburgite zones, and still richer in the harzburgite-dunite zone (49-64% Cr 2 O 3 ). In peridotites, the accessory Cr spinel also displays a Cr enrichment from lherzolite to dunite. Segregations of Cr-rich chromite (50-60% Cr 2 O 3 ) are present in orthopyroxenite veins displaying boninitic affinities. The Poum chromitites display low platinum-group element (PGE) contents and Pd/Ir ratios as do the chromitite segregations of the orthopyroxenite veins, suggesting a common parent magma or/and common fractionation processes. The compositional zoning of the Poum massif can be explained by the contrasting behavior of ultramafic rocks and upwelling mafic melts, away from the axis of the transform fault, using the current ideas about the genesis of boninitic melts and the formation of dunite and harzburgite by magma-rock interactions during melt percolation (Nicolas and Prinzhofer, 1983; Kelemen, 1990; Kelemen et al., 1992; Takazawa et al, 1992). In the lithospheric lherzolite zone that was cooled and mylonitized along the older wall of the transform fault, melts were channeled along fractures leaving Al-rich chromitite pods. By contrast, pervasive percolation along the lithosphere-asthenosphere boundary allowed important melt-rock interactions resulting in the progressive formation of harzburgite and dunite and generating Mg-Cr-rich melts from which precipitated Cr-rich chromitites.

Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes: A chromatographic approach

Mantle rock studies provide evidence of interaction with upwelling magmas. In orogenic lherzolites, one of the most conspicuous effects of these interactions is the development of harzburgite and dunite bands. Recent studies have suggested that these bands were formed at the expense of the host lherzolites by melt-rock reactions associated with magma percolation. In order to better understand the geochemical effects associated with percolation-reaction processes, we propose a numerical model of melt infiltration that takes into account modal variations in time and space resulting from melt-rock reactions. Melt volume variations are considered by means of porosity variations, and a local equilibrium approach is used for trace element exchange between melt and minerals. The transport of trace elements by the interstitial melt is described by a mass balance equation while the modal variations are constrained by the mineralogical trends observed in refractory peridotites massifs. The model is applied to REE, Cr and Ni in percolated peridotites affected by an olivine-forming reaction, with the aim of reproducing the evolution of these elements in refractory peridotites from the Ronda massif. Our modelling can explain the negative correlation between the LREE/HREE ratio and the HREE content and between Cr and Ni in the Ronda refractory peridotites. Our results validate the hypothesis that, in the Ronda, the bands of refractory peridotites represent porous-flow channels formed by olivine-forming melt-rock reaction, at increasing melt volume. Because similar geochemical features are observed in ophiolitic peridotites and in mantle xenoliths, it is likely that melt-rock reactions associated with magmatic infiltration are widespread and represent important mantle processes.