Résultat de la recherche
The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere
Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (Fifteen–Twenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44–2.8 μg g−1) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 μg g−1), depleted in Li (most values below 1 μg g−1) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4–65.0 μg g−1), H2O and Cl contents and to lower Li contents (0.07–3.37 μg g−1) of peridotites, implying that—contrary to alteration of oceanic crust—B is fractionated from Li and that the B and Li inventory should depend essentially on rock–water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17–307 kg in a column of 1 m × 1 m × thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25–904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.
Li, B and Be Contents of Harzburgites from the Dramala Complex (Pindos Ophiolite, Greece): Evidence for a MOR-type Mantle in a Supra-subduction Zone Environment
The Pindos ophiolite represents oceanic lithosphere obducted during the Jurassic. The Dramala mantle section mainly consists of highly depleted spinel harzburgite and minor plagioclase-bearing harzburgite. Textural observations and major element compositions of minerals indicate that the harzburgites experienced impregnation by a mafic, depleted melt and subsequent high-temperature (high-T) hydration and cooling (>750°C) forming pargasite and edenitic hornblende. During further cooling (from 350–400°C to < 100°C), talc + tremolite ± serpentine ± olivine, serpentine + magnetite, and finally plagioclase alteration phases formed. To test the hypothesis of a supra-subduction zone origin for the Dramala mantle, we measured Li, B and Be contents of minerals by secondary ion mass spectrometry. Whole-rock contents were measured using inductively coupled plasma–mass spectrometry and prompt gamma neutron activation analysis. We observe low Li and B contents of primary minerals (olivine, orthopyroxene, clinopyroxene) consistent with values for unmetasomatized mantle minerals; only Li contents of clinopyroxene (up to 3•7 µg/g) are slightly elevated. The bulk Li contents (0•5–1•1 µg/g) are in the upper range of values for unmetasomatized mantle, whereas B contents (<0•04–1•1 µg/g) are variable and slightly elevated compared with the unmetasomatized mantle as a result of serpentinization. Beryllium abundances in all minerals are very low (<0•005 µg/g), except for pargasite, where a maximum Be content of 0•012 µg/g was measured. The selective addition of Li to clinopyroxene can be related to the interaction with a depleted melt, and/or to partitioning of Li into clinopyroxene upon cooling. During high-T hydration and cooling, the fluid calculated to be in equilibrium with the pargasite or edenitic hornblende (based on Li, Be and B) could have been reaction-modified seawater. Low-T hydration may have led to a very minor increase in bulk B content of most samples and to the formation of serpentine with highly variable B contents (0•1–28 µg/g). Low-T hydration decreased the Li content of orthopyroxene, and Li was probably leached from some samples. The lack of correlation between degree of serpentinization and bulk B contents as well as the presence of high- and low-B serpentine can be explained by low fluid–rock ratios, decreasing T during serpentinization and lack of equilibrium as a result of fast obduction–exhumation. The low light-element contents of primary minerals and whole-rock samples clearly argue against a supra-subduction zone (SSZ) origin of the Dramala mantle section, and against the previous hypothesis of hydrous melting of the Pindos mantle above a subduction zone. We therefore conclude that the Dramala harzburgites represent a mid-ocean ridge (MOR)-type mantle, and not an SSZ-type mantle, juxtaposed with MOR-type and SSZ-type oceanic crust, either in a back-arc or in an intra-oceanic subduction zone setting.
Emplacement of ultramafic rocks into the continental crust monitored by light and other trace elements: An example from the Geisspfad body (Swiss-Italian Alps)
In order to evaluate the influence of continental crustal rocks on trace element budgets of serpentinized peridotites incorporated into the continental crust, we have analyzed the chemical composition of whole rock samples and minerals of the Geisspfad ultramafic complex (Swiss-Italian Alps). This complex represents a relict oceanic succession composed of serpentinites, ophicarbonates and metabasic rocks, emplaced into crustal gneisses during Alpine collision. Following peak metamorphic amphibolite facies conditions, fluid flow modified some of the trace element contents of ophicarbonates and deformed serpentinites close to the contact with country rocks. The fluid originated from the surrounding continental crustal rocks as documented by the increase of Pb in the serpentinites, and by the strongly negative ∂D values (− 112‰) of some ultramafic rocks close to the contact with surrounding gneisses. Little or no modification of the fluid mobile elements Li, B or U was observed in the serpentinite. In-situ analysis of light elements of serpentinite minerals indicate redistribution of light elements coupled to changes of mineral modes towards the outer 100–150 m of the massif. In the centre of the massif, Li is preferentially concentrated in olivine, while Be and B are hosted by tremolite. In contrast, at the outer rim of the massif, Li and Be are preferentially incorporated into diopside, and B into antigorite. This redistribution of light elements among the different minerals is visible in the serpentinite, at a maximum distance of ~ 100–150 m from the ophicarbonate–metabasite contact. Our results show that interaction of ultramafic rocks and crust-derived fluids can be easily detected by studies of Pb and ∂D in whole rocks. We argue that small ultramafic bodies potentially record an emplacement-related trace element signature, and that crustal light element values in ultramafic rocks are not necessarily derived from a subducting slab.
Partitioning and budget of Li, Be and B in high-pressure metamorphic rocks
Partitioning and budget of Li, Be and B in high-pressure metamorphic rocks from the island of Syros (Greece) were studied, using secondary ion mass spectrometry, inductively coupled plasma optical emission spectrometry and prompt gamma neutron activation analysis. Partitioning between coexisting mineral phases was found to be rather constant and independent of element concentrations. For several mineral pairs, apparent partition coefficients vary in a narrow range, while concentrations vary by more than an order of magnitude. Hence, it was possible to establish sets of inter-mineral partition coefficients for Li, Be and B among 15 different high-pressure minerals. This data set provides important information on the behaviour of the light elements in different lithologies within subducting slabs from the onset of metamorphism to the eclogite stage. It is essential for modelling trace-element and isotope fractionation during subduction and dehydration of oceanic crust.
Boron, lithium and strontium isotopes as tracers of seawater–serpentinite interaction at Mid-Atlantic ridge, ODP Leg 209
Spinel harzburgites from ODP Leg 209 (Sites 1272A, 1274A) drilled at the Mid-Atlantic ridge between 14°N and 16°N are highly serpentinized (50–100%), but still preserve relics of primary phases (olivine ≥ orthopyroxene >> clinopyroxene). We determined whole-rock B and Li isotope compositions in order to constrain the effect of serpentinization on δ11B and δ7Li. Our data indicate that during serpentinization Li is leached from the rock, while B is added. The samples from ODP Leg 209 show the heaviest δ11B (+ 29.6 to + 40.52‰) and lightest δ7Li (− 28.46 to + 7.17‰) found so far in oceanic mantle. High 87Sr/86Sr ratios (0.708536 to 0.709130) indicate moderate water/rock ratios (3 to 273, on the average 39), in line with the high degree of serpentinization observed.
Applying the known fractionation factors for 11B/10B and 7Li/6Li between seawater and silicates, serpentinized peridotite in equilibrium with seawater at conditions corresponding to those of the studied drill holes (pH: 8.2; temperature: 200 °C) should have δ11B of + 21.52‰ and δ7Li of + 9.7‰. As the data from ODP Leg 209 are clearly not in line with this, we modelled a process of seawater–rock interaction where δ11B and δ7Li of seawater evolve during penetration into the oceanic plate. Assuming chemical equilibrium between fluid and a rock with δ11B and δ7Li of ODP Leg 209 samples, we obtain δ11B and δ7Li values of + 50 to + 60‰, − 2 to + 12‰, respectively, for the coexisting fluid. In the oceanic domain, no hydrothermal fluids with such high δ11B have yet been found, but are predicted by theoretical calculations. Combining the calculated water/rock ratios with the δ7Li and δ11B evolution in the fluid, shows that modification of δ7Li during serpentinization requires higher water/rock ratios than modification of δ11B.
Extremely heavy δ11B in serpentinized oceanic mantle can potentially be transported into subduction zones, as the B budget of the oceanic plate is dominated by serpentinites. Extremely light δ7Li is unlikely to survive as the Li budget is dominated by the oceanic crust, even at small fractions.
Detrital, metamorphic and metasomatic tourmaline in high-pressure metasediments from Syros (Greece): intra-grain boron isotope patterns determined by secondary-ion mass spectrometry
The boron isotopic composition of zoned tourmaline in two metasediments from the island of Syros, determined by secondary-ion mass spectrometry (SIMS), reflects the sedimentary and metamorphic record of the rocks. Tourmaline from a silicate-bearing marble contains small (≤20 μm) detrital cores with highly variable δ11B values (−10.7 to +3.6‰), pointing to a heterogeneous protolith derived from multiple sources. The sedimentary B isotopic record survived the entire metamorphic cycle with peak temperatures of ~500°C. Prograde to peak metamorphic rims are homogeneous and similar among all analysed grains (δ11B ≈ +0.9‰). The varying δ11B values of detrital cores in the siliceous marble demonstrate that in situ B isotope analysis of tourmaline by SIMS is a potentially powerful tool for provenance studies not only in sediments but also in metasediments. A meta-tuffitic blueschist bears abundant tourmaline with dravitic cores of detrital or authigenic origin (δ11B ≈ −3.3‰), and prograde to peak metamorphic overgrowth zones (−1.6‰). Fe-rich rims, formed during influx of B-bearing fluids under retrograde conditions, show strongly increasing δ11B values (up to +7.7‰) towards the margins of the grains. The δ11B values of metamorphic tourmaline from Syros, formed in mixed terrigenous–marine sediments, reflect the B signal blended from these two different sources, and was probably not altered by dehydration during subduction.
Li, Be, and B abundances in minerals of peridotite xenoliths from Marsabit (Kenya): Disequilibrium processes and implications for subduction zone signatures
The light elements Li, Be, and B have been analyzed in situ in minerals from three groups of peridotite xenoliths hosted in Quaternary basanites from the Marsabit volcanic field (northern Kenya). Group I and II are fertile lherzolites that experienced deformation, decompression, and cooling in the context of Mesozoic rifting (Group I), followed by heating, static recrystallization, and associated cryptic metasomatism (Group II) as a result of Tertiary-Quaternary rifting and magmatism. Group III xenoliths are spinel harzburgites and dunites that experienced strong cryptic and modal metasomatism. The Li-Be-B systematics in minerals of Group I and II are similar to unmetasomatized subcontinental lithospheric mantle. In contrast, Group III samples are characterized by significant enrichment in all light elements and disequilibrium partitioning between different phases. Light element concentrations levels are similar to that expected for mantle rocks metasomatized by melts and fluids released from subducting slabs, while light element/rare earth element ratios (especially Li/Yb) approach those of typical Island Arc basalts. However, detailed investigation of textures and chemical zoning shows that at least Li concentrations in primary minerals were modified (i.e., decoupled from Yb) during late-stage melting and/or fluid percolation related to Tertiary-Quaternary alkaline magmatism in Marsabit (formation of melt pockets consisting of silicate glass, clinopyroxene, olivine, and chromite), ultimately followed by xenolith entrapment and transport to the surface. Mass balance calculations show that the melt pockets formed at the expense of earlier metasomatic phases. During this process the melt pockets mostly preserved the B, Be, and rare earth element budget of the precursor phase assemblage, whereas Li was added. Elevated B/Be and low Ce/B of metasomatic phases prior to late melting could result from metasomatism by a slab fluid. However, similar characteristics are expected for evolved Si- and CO2-rich fluids derived from basanite melt-peridotite interaction, not related to any subduction zone process. The results of this study imply that the inference of a “slab signature” exclusively based on trace element data of metasomatized peridotite is ambiguous.
Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite
Garnet-bearing and garnet-free pyroxenite xenoliths from Quaternary basanites of Marsabit, northern Kenya, were analysed for microstructures and mineral compositions (major and trace elements) to constrain the thermal and compositional evolution of the lithospheric mantle in this region. Garnet-bearing rocks are amphibole-bearing websterite with ~5–10 vol% orthopyroxene. Clinopyroxene is LREE-depleted and garnet has high HREE contents, in agreement with an origin as cumulates from basaltic mantle melts. Primary orthopyroxene inclusions in garnet suggest that the parental melts were orthopyroxene-saturated. Rock fabrics vary from weakly to strongly deformed. Thermobarometry indicates extensive decompression and cooling (~970–1,100°C at ~2.3–2.6 GPa to ~700–800°C at ~0.5–1.0 GPa) during deformation, best interpreted as pyroxenite intrusion into thick Paleozoic continental lithosphere subsequently followed by continental rifting (i.e., formation of the Mesozoic Anza Graben). During continental rifting, garnet websterites were decompressed (garnet-to-spinel transition) and experienced the same P–T evolution as their host peridotites. Strongly deformed samples show compositional overlaps with cpx-rich, initially garnet-bearing lherzolite, best explained by partial re-equilibration of peridotite and pyroxenite during deformation and mechanical mingling. In contrast, garnet-free pyroxenites include undeformed, cumulate-like samples, indicating that they are younger than the garnet websterites. Major and trace element compositions of clinopyroxene and calculated equilibrium melts suggest crystallisation from alkaline basaltic melt similar to the host basanite, which suggests formation in the context of alkaline magmatism during the development of the Kenya rift.
The oceanic mantle as an important repository for the light elements Li, Be and B
It is important to quantify the Li, Be and B content of oceanic peridotites, in order to evaluate their contributions to the light element input in subduction zones (compared to oceanic crust). In previous studies, the input related to oceanic mantle was usually neglected, because no strong data are available for the light element contents (minerals, whole rock samples). The objective of this thesis is to provide a dataset of Li, Be and B contents of minerals and whole rock samples from fresh and serpentinized oceanic mantle, and to determine processes which can potentially modify the light element signature of the oceanic mantle. The Li, Be and B content of the oceanic mantle can be modified during processes acting close to mid-ocean ridges, like mafic melt percolation/impregnation and serpentinization. The Li, Be and B content can also be changed during emplacement of oceanic mantle into the continental crust. In order to study these processes, oceanic mantle from various tectonic settings was studied: (i) Pindos ophiolite (Greece) for melt-related processes, (ii) Pindos and Vourinos ophiolites (Greece), Mid-Atlantic ridge (MAR) ODP Leg 209 for serpentinization, (iii) Geisspfad ultramafic body (Alps) for the effect of the emplacement into the continental crust. The study of the Dramala harzburgites (Pindos), recording high degree of partial melting prior to melt percolation, shows that there is a Li enrichment of the depleted harzburgite during the crystallization of clinopyroxene cumulate (Li in Cpx ≤ 3.7 µg/g), related to percolation of N-MORB melt. Subsequent impregnation by ultra-depleted melt did not change the Li, Be and B content of the harzburgites. Light element contents of the fresh Dramala harzburgite after melt-related processes are low (Li: 0.9-1.0 µg/g, Be: <0.003 µg/g, B: <0.03 µg/g). These low contents are certainly due to the high degree partial melting, while melt impregnation and/or percolation does not strongly modify the light element content of whole rock samples. During serpentinization, there is a B enrichment in whole rock samples (no Li or Be enrichment), while Li, Be and B contents of the primary mantle phases stay constant. The major B carrier phase is serpentine (≤ 28 µg/g). The quantity of B incorporated into serpentinized harzburgite probably depends on the nature of serpentinization (temperature, pH, water/rock ratio). B contents in serpentine/serpentinites from Dramala serpentinized harzburgites are low compared to serpentinites from the MAR. Samples from Dramala show low whole rock B contents in highly serpentinized harzburgites (up to 1.1 µg/g) and heterogeneous B content in serpentine (0.1-28 µg/g). It probably reflects serpentinization occurring at high temperature and low water/rock ratio. In contrast, serpentinization in the MAR samples led to high B content in serpentine (≤ 200 µg/g) and serpentinites (10-65 µg/g), probably related to low temperatures and high water/rock ratio. The Geisspfad serpentinites showed that Li, Be and B contents of oceanic serpentinites are modified during emplacement into the continental crust by fluids related to retrograde metamorphism (evident from Li, Be and B contents in minerals/whole rock samples). These fluids can penetrate ultramafic bodies or travel along the contact between the ultramafics and the surrounding crustal rocks. It shows that only large ultramafic bodies can potentially maintain their prograde light element systematics in the core. In conclusion, light element content of the fresh oceanic mantle is low, except for Li (can be enriched during N-MORB melt impregnation). The oceanic mantle is variably enriched in B during serpentinization, depending on temperature, pH and water/rock ratio. Due to its big volume compared to the oceanic crust, the oceanic mantle could strongly contribute to the Li and B input into subduction zones.
Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya)
Mantle-derived xenoliths from the Marsabit shield volcano (eastern flank of the Kenya rift) include porphyroclastic spinel peridotites characterized by variable styles of metasomatism. The petrography of the xenoliths indicates a transition from primary clinopyroxene-bearing cryptically metasomatized harzburgite (light rare earth element, U, and Th enrichment in clinopyroxene) to modally metasomatized clinopyroxene-free harzburgite and dunite. The metasomatic phases include amphibole (low-Ti Mg-katophorite), Na-rich phlogopite, apatite, graphite and metasomatic low-Al orthopyroxene. Transitional samples show that metasomatism led to replacement of clinopyroxene by amphibole. In all modally metasomatized xenoliths melt pockets (silicate glass containing silicate and oxide micro-phenocrysts, carbonates and empty vugs) occur in close textural relationship with the earlier metasomatic phases. The petrography, major and trace element data, together with constraints from thermobarometry and fO2 calculations, indicate that the cryptic and modal metasomatism are the result of a single event of interaction between peridotite and an orthopyroxene-saturated volatile-rich silicate melt. The unusual style of metasomatism (composition of amphibole, presence of graphite, formation of orthopyroxene) reflects low P –T conditions (~850–1000°C at < 1•5 GPa) in the wall-rocks during impregnation and locally low oxygen fugacities. The latter allowed the precipitation of graphite from CO2. The inferred melt was possibly derived from alkaline basic melts by melt–rock reaction during the development of the Tertiary–Quaternary Kenya rift. Glass-bearing melt pockets formed at the expense of the early phases, mainly through incongruent melting of amphibole and orthopyroxene, triggered by infiltration of a CO2-rich fluid and heating related to the magmatic activity that ultimately sampled and transported the xenoliths to the surface.