Michel Grégoire

Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea : petrology and geochemistry of fluid-metasomatised mantle wedge xenoliths

Abstract Ultramafic, mafic and sedimentary xenoliths have been recovered from a recently erupted, shoshonitic submarine cinder cone (Tubaf and Edison volcanoes) from the Tabar–Lihir–Tanga–Feni island arc, located in the New Ireland basin of Papua New Guinea. These samples represent a proxy drill hole that can be re-assembled into an ‘ophiolite-type’ model of oceanic lithosphere. Petrographic and geochemical examination of the gabbroic and depleted mantle xenoliths indicates that the New Ireland fore-arc lithosphere is a fragment of ancient Pacific Plate generated at a mid-ocean ridge spreading centre and transported to the Pacific–Australian Plate margin. Convergent margin processes subjected the harzburgitic mantle wedge to hydrofracturing and hydration metasomatism at T=790–1030°C as a consequence of dewatering of a subducted slab. Advection of a high-density, H2O-rich fluid containing a substantial dissolved component (alkali aluminosilicate melt and aqueous carbon and sulphur species) through these mantle fractures caused a net transfer of soluble elements from the lower to upper mantle wedge and created a network of oxidised (ΔFMQ≈1.8–2.0) metasomatised peridotite enriched in orthopyroxene, clinopyroxene, phlogopite, amphibole, magnetite, and Fe–Ni sulphides. The vein mineral assemblage magnetite+sulphide indicates precipitation from a hydrous fluid with high SO2/H2S, consistent with the hydrous fluid being derived from dehydration of subducted, altered oceanic crust. Preferential partial melting of these metasomatically enriched mantle wedge regions could account for the highly oxidised, sulphur- and alkali-rich nature of the high-K calc-alkaline volcanoes of the Tabar–Lihir–Tanga–Feni island chain.

Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: Part 2. Trace element characteristics of slab-derived fluids

Abstract Spinel peridotite xenoliths recovered from the Tubaf and Edison volcanoes, south of Lihir Island in the Tabar–Lihir–Tanga–Feni island arc in Papua New Guinea, are predominantly fresh, refractory harzburgites. Many of the harzburgite xenoliths have cross-cutting vein networks and show evidence of modal metasomatism. These metasomatic veins contain a secondary mineral assemblage consisting of fibrous, radiating orthopyroxene and fine-grained Fe–Ni sulfide with minor olivine, clinopyroxene, phlogopite, amphibole and magnetite. Adjacent to the veins, primary clinopyroxene is cloudy while orthopyroxene exhibits replacement by secondary fibrous orthopyroxene, similar in habit to orthopyroxene occurring in the veins. The mineralogical and geochemical characteristics of the Tubaf mantle xenoliths are the product of two major processes: an early partial melting depletion event that was overprinted by oxidation and alkali enrichment related to percolation of slab-derived, hydrous melts. HREE and MREE concentrations in clinopyroxene from the least metasomatised harzburgites indicate that they are the residues from a 15% to 25% partial melting event, consistent with formation in a MOR setting. The secondary vein assemblages show strong enrichment in the LILE (primarily Sr, Ba, Rb, Th, U and Pb) and the REE (primarily La, Ce, Nd, Sm, Eu and Gd), while the HFSE (Nb, Ta, Zr, Hf, and Ti) are neither enriched nor depleted. The mineral precipitates in the vein assemblages have high LREE/HFSE and LILE/HFSE, and reflect the relative solubility of these elements in hydrous melts. These trace element characteristics are similar to those of the Tabar–Lihir–Tanga–Feni arc lavas, and display the commonly observed HFSE depletion of arc magmatism. These findings support the hypothesis that this so-called “arc signature” is primarily dependent on the relative solubility of elements in slab-derived, hydrous melts, and the enrichment of these soluble elements in metasomatised mantle regions that are prone to preferential partial melting.

Armalcolite-bearing, Ti-rich metasomatic assemblages in harzburgitic xenoliths from the Kerguelen Islands: implications for the oceanic mantle budget of high-field strength elements

Abstract Some mantle clinopyroxene- and spinel- harzburgite xenoliths from the Kerguelen Islands contain an unusual metasomatic mineral association consisting of feldspar, olivine II, chromite II and Ti-oxides (rutile, ilmenite and armalcolite). These metasomatic minerals occur in veins along the grain boundaries of the olivine, orthopyroxene and clinopyroxene, but may cut across both these anhydrous silicates and pre-existing metasomatic amphibole and phlogopite. To our knowledge, this is the first time that such a mineral association, representing a type of mantle metasomatism that is distinct from those commonly attributed to potassium-rich hydrous fluids in cratonic mantle lithosphere or to carbonated Fe-Ti-rich silicate melts in non-cratonic mantle lithosphere, is reported from peridotite samples in the oceanic mantle. The vein-forming minerals are inferred to have crystallized at a minimum pressure of about 1.3 GPa and 1150–1200°C from a strongly alkaline magma, with low water activity buffered by the wall-rock harzburgites. The oxygen fugacity is well-constrained at NNO −2.5 to −3.0 log units. Exchange reactions of the parental melt with the peridotite wall-rock released silica (from orthopyroxene), chromium (from clinopyroxene and original Cr-Al spinel) and magnesium (from orthopyroxene and clinopyroxene); this element exchange was essential to form the armalcolite paragenesis. Such Ti-rich veins cause strong local enrichment in some incompatible trace elements, including LILE, that are largely concentrated in feldspar (e.g., Ba, Sr, LREE) and HFSE (Ti, Nb, Zr) in rutile and armalcolite, even though these comprise approximately only 0.05 wt% of the rock.

Feldspar–Ti-oxide metasomatism in off-cratonic continental and oceanic upper mantle

Abstract Metasomatism is responsible for enrichment of lithospheric mantle in incompatible elements. The most common metasomatic products in mantle xenoliths and peridotite massifs worldwide are amphibole and mica providing mineral hosts for alkalies, Ba, Nb. In contrast, xenoliths of spinel peridotite entrained in basaltic rocks in southern Siberia (Russia) and the Kerguelen islands (Indian ocean) have metasomatic aggregates of alkali feldspar and Ti-rich oxides (rutile, armalcolite, ilmenite) that commonly replace Al-spinel and orthopyroxene or make up cross-cutting veins. Importantly, the feldspar-rich aggregates also replace amphibole and mica formed by earlier metasomatic episodes. Armalcolite has not been previously reported in off-cratonic peridotite xenoliths. The unusual mineralogical composition of these metasomatic products defines a specific type of mantle metasomatism, distinct from those commonly attributed to H 2 O-rich fluids, carbonate melts or Fe–Ti-rich silicate melts. This type of metasomatism operates both in continental and oceanic mantle and may be related to alkali-rich fluids/melts with low water activity (probably due to high CO 2 /H 2 O ratios). Electron microprobe analyses have shown that the metasomatic rutile and armalcolite may have up to 4% of Nb 2 O 5 and ZrO 2 . Feldspar analysed by laser ablation ICP-MS typically has high contents of light REE, Rb, Ba, Sr. Overall, the feldspar-rich metasomatic aggregates may be strongly enriched in incompatible trace elements, with HFSE largely hosted by the Ti-rich oxides. Precipitation of this mineral assemblage can lead to unusual fractionations among incompatible elements both in the metasomatic assemblage and associated fluids.

Compaction in a mantle with a very small melt concentration: Implications for the generation of carbonatitic and carbonate-bearing high alkaline mafic melt impregnations

Xenoliths entrained in alkaline basalts and kimberlites give strong evidence that mantle carbonatitic and carbonated high alkaline mafic silicate melts, which are initially produced at very low degrees of partial melting (≪1%), percolate and accumulate to form impregnations with a melt concentration of up to 10%. At present no compaction model has explained such huge local amplification of melt concentration. Recently, Bercovici et al. [1] have shown that the commonly used equations of compaction are not sufficiently general to describe all melt percolation processes in the mantle. In particular, they show that, when the melt concentration in the mantle is very low, the pressure jump ΔP between the solid and liquid fractions of the mantle mush is very important and plays a driving role during compaction. 1-D compaction waves generated with two different systems of equations are computed. Three types of wave-trains are observed, i.e. (1) sinusoidal waves; (2) periodic waves with flat minima and very acute maxima (‘witch hat waves’); (3) periodic solitary waves with flat maxima and extremely narrow minima (‘bowler hat waves’). When the initial melt distribution in the mantle is quite homogeneous, the compaction waves have sinusoidal shapes and can locally amplify the melt concentration by a factor less than two. When there is a drastic obstruction at the top of the wetted domain, the pressure jump ΔP between solid and liquid controls the shape of the waves. If the computation assumes the equality of pressure between the two phases (ΔP=0), the compaction wave has a ‘bowler hat shape’, and locally amplifies the melt concentration by a factor less than 5. Alternatively, simulations taking into account the pressure jump between phases ΔP predict compaction waves with ‘witch hat shape’. These waves collect a large quantity of melt promoting the development of magmons with local melt concentration exceeding 100× the background melt concentration. It is inferred that in a mantle with very low concentrations of carbonatitic or high alkaline mafic silicate melt the magmons are about 1 km thick and reach, in less than 1 Ma, a melt concentration of about 10%. The magmons are likely generated below the lithosphere at some distance away from the center of hot spots. This can explain the development of mantle carbonatitic eruptions in the African rift and the carbonatite and high alkaline mafic silicate volcanic activity in oceanic islands.

Differences between Archean and Proterozoic lithospheres: Assessment of the possible major role of thermal conductivity

We study heat transfer through the conductive lithosphere and convective mantle on the basis of a 2-D convection model in order to better understand the differences between Archean and Proterozoic lithospheres. The original improvement in the modeling consists of a precise track of the cutoff temperature between conduction and convection. The conductive lithosphere is undeformable, and the convective mantle has a constant viscosity. The conductive lithosphere in Archean cratons is assumed to have a lower radiogenic heat production in the crust and/or the conductive mantle, and/or a higher cutoff temperature (attributed to a stiffening of the more depleted mantle) relative to Proterozoic terrains. We also investigate the effect of a fourth factor never considered before: an enhancement of the vertical thermal conductivity in the conductive Archean mantle due to a vertical lineation. Our model successfully predicts the observations on the Canadian and South African shields. A high vertical thermal conductivity in the Archean lithospheric mantle possibly associated to a radiogenic crust depletion explains many observations: i.e., (1) the development of a thick Archean cold root, (2) a dipping cold convective plume beneath Archean cratons, (3) a uniform mantle heat flux along Archean and Proterozoic terrains, and (4) strong high seismic velocity anomalies over Archean cratons. In addition, a lower radiogenic heat production of the lithospheric mantle or a larger cutoff temperature in Archean cratons is not favored by our models. Finally, we investigate how a cold root and a sub-Archean cold mantle plume react to the shear induced by large-scale mantle flow and to the collision with a hot mantle plume. Models show that shear removes the cold plume from below the Archean craton, but cold dipping instabilities are periodically generated inside the convective boundary layer below the Archean cratonic lid. However, the thick Archean lithospheric root may survive several orogeneses. In addition, the models show that it takes at least 200 Myr for a hot mantle plume to erode the thick lithospheric root. This suggests that an Archean root may only disappear if the plate remains above a hot plume for a very long time.

Petrological and geochemical constraints on the composition of the lithospheric mantle beneath the Syrian rift, northern part of the Arabian plate

Abstract A suite of mantle xenoliths from the Neogene–Quaternary volcanic province of Jabel El Arab (Syria) is dominated by spinel±amphibole harzburgites, with rare lherzolites and wehrlites that were equilibrated at temperatures of 900–1100 °C. The major elements of pristine minerals and trace element compositions of clinopyroxene and amphibole indicate that the lithospheric mantle experienced various degrees of melt extraction (olivine Mg-number=89.4–91.8, spinel Cr-number=10.4–46.4), followed by a multistage metasomatic history. The primary clinopyroxene has variable and high Mg-number (89.1–93.6) and highly variable major element concentrations (Al2O3 2.7–7.6 wt%, Na2O 0.5–2.5 wt% and Cr2O3 0.4–2.5 wt%). Three groups of harzburgites were identified on the basis of petrographical, mineralogical and geochemical data. Group I harzburgites have compositions indicating a residual origin after polybaric partial melting with F <20%, which started in the garnet stability field and continued in the spinel stability field. Group II harzburgites are interpreted as a result of a percolation mechanism involving the infiltration of large volumes of undifferentiated basaltic melts through the residual lithosphere. Finally, the mineral major element compositions and the selectively enriched trace element contents of clinopyroxenes in group III harzburgites (high (La/Sm)N and Th, U, Sr and low high field strength element contents) are attributed to a percolation mechanism involving small volume melt fractions. Such small melt fractions correspond to CO2-bearing alkaline silicate magmas that have evolved to CO2-rich melts during repeated percolation-reaction within the Syrian lithospheric mantle. Shortly before eruption, some xenoliths were infiltrated by small silicate melt fractions, which produced discrete reaction zones composed of cpx±ol±sp±glass surrounding reacting primary spinels. The glass in the melt pockets has a trachytic to trachy-andesitic composition and its composition suggests that glass is derived from melting of pre-existing amphibole in the lithospheric mantle, triggered by infiltration of a Na-rich metasomatic agent.

Petrological evolution of the European lithospheric mantle

Several different databases and models have been developed over many years of petrological study carried out by several European and non-European groups on mantle xenoliths, peridotite massifs, ophiolites and mafic magmas spanning in age from Archaean to Recent times. This volume aims to bring together these different approaches and to integrate the geochemical perceptions of the European upper mantle. The papers include regional petrological studies of the European lithospheric mantle, from Spain to the Pannonian Basin, from Corsica and Serbia as far north as Svalbard. Six contributions are based on studies of mantle xenoliths, while the remaining three deal with ophiolitic and peridotitic complexes. A further article provides an update on the textural classification of mantle rocks using a computer-aided approach and there is an introductory overview.

Composite xenoliths from Spitsbergen: evidence of the circulation of MORB-related melts within the upper mantle

Abstract The Sverrefjell Quaternary volcano in Spitsbergen contains composite xenoliths showing lherzolite rocks cross-cut by websterite veins. These two rock types are characterized by similar major element compositions of olivines, orthopyroxenes, clinopyroxenes and spinels, as well as similar trace element composition for clinopyroxene. The clinopyroxenes of both rock types mostly display upwards convex or spoon-shaped REE (rare earth elements) patterns with a systematic enrichment in La over Ce (CeN/YbN 0.72–1.32; SmN/YbN 0.86–1.93 and LaN/CeN 1.27–1.93), except for one sample (SV-69) in which clinopyroxenes show a pattern characterized by low LREE compare to HREE (CeN/YbN 0.33–0.35). Metasomatic processes appear to be the most reasonable origin to form the lherzolite–websterite associations. We therefore propose that the Spitsbergen mantle has undergone at least two events: (1) a sub-alkaline (tholeiitic) metasomatism followed by (2) an alkaline metasomatic event.

Origin of the adakite–high-Nb basalt association and its implications for postsubduction magmatism in Baja California, Mexico: Discussion

Constraining the origin of the adakite–high-Nb basalt (HNB) association in Baja California, Mexico, is critical to a better understanding of global arc magmatism. Currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted Farallon-Cocos plate, thus providing support for the slab-melting origin of adakites elsewhere. Moreover, a tectono-magmatic model involving the production of both adakite and HNB from slab melts offers a comprehensive explanation for the origin of the atypical, arc-related, postsubduction magmatism in Baja California. This paper proposes alternative models for the origin of HNB and postsubduction magmatism in Baja California, wherein the unusual geologic setting of western Mexico and westward movement of North America permitted the influx of Pacific asthenosphere beneath the adjacent Gulf of California after the cessation of subduction. Unlike the previous tectono-magmatic model, the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in Baja California as tholeiites and HNB, respectively. Fractional crystallization of some of the HNB magmas plus assimilation of tholeiitic materials produced Nb-enriched basalts (NEB). The influx of Pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower Baja California crust, producing adakite rocks, and the preexist-ing metasomatized mantle wedge, producing bajaites and calc-alkaline magmas.