La Grille Research Articles

Geochemical evidence for a lithospheric origin of the Comoros Archipelago (Indian Ocean) as revealed by ultramafic mantle xenoliths from La Grille volcano

Petrology and fluid inclusions (FI) geochemistry are increasingly used in tandem to constrain the compositional features and evolution of the lithospheric mantle. In this study, we combine petrography and mineral chemistry with analyses of noble gases (He, Ne and Ar) and CO2 in olivine, orthopyroxene- and clinopyroxene-hosted FI, as well as radiogenic isotope (Sr-Nd-Pb) systematics of ultramafic xenoliths collected at La Grille volcano in Grande Comore Island, aiming at better characterizing one of the most enigmatic and controversial portions of the western Indian Ocean lithospheric mantle. Xenoliths have been divided in three groups on the basis of their textural features: Group 1 (Opx-bearing), Group 2 (Opx-free) and Group 3 (Cumulate). Overall, petrographic observations and mineral phase compositions indicate that the sampled lithospheric portion experienced variable degrees of melting (from 5% to 35%), recorded by Group 1 most refractory harzburgites and lherzolites, as well as modal metasomatic processes as evidenced by the crystallization of cpx at the expense of opx in Group 1 fertile lherzolites and wehrlite and by Group 2 xenoliths. Crystallization of slightly oversaturated basic silicate melts seems also to have occurred, as shown by Group 3 xenoliths. A positive trend between temperature and ƒO2 is evident, with Group 2 and 3 xenoliths testifying for hotter and more oxidised conditions than Group 1. The variability of the 4He/40Ar* ratio (0.02–0.39) in Group 1, significantly below typical values of a fertile mantle (4He/40Ar* = 1–5), can be explained by the variable degrees of partial melting coupled to metasomatic enrichment that may account for modifying 4He/40Ar*, as also indicated by the mineral composition. He-Ar-CO2 relationships support the presence of a metasomatic CO2-rich process post-dating the melt extraction and the cumulate formation. The air-corrected 3He/4He isotopic ratios (6.30 to 7.36 Ra) are intermediate between the MORB mantle signature (8 ± 1Ra) and the SCLM (6.1 ± 0.9 Ra). The Ne and Ar isotopic signatures (20Ne/22Ne, 21/Ne/22Ne and 40Ar/36Ar) are consistent with mixing between an air-derived component and a MORB-like mantle, supporting the hypothesis for a lithospheric origin of the Comoros magmas, and arguing against any deep mantle plume-related contribution. This is also corroborated by combining Ne with He isotopes, showing that La Grille ultramafic xenoliths are far from the typical plume-type compositions. Sr-Nd-Pb isotope systematics in opx and cpx from La Grille additionally support a MORB-type signature for the lithospheric mantle beneath the area.

Melilite-bearing lavas in Mayotte (France): An insight into the mantle source below the Comores

Ocean island basalts (OIB) from the Comores archipelago (Indian Ocean) display mineralogical and geochemical features different from the other Indian OIB. We present here new geochronological data (40Ar/39Ar), major and trace element compositions and isotopic (Sr, Nd, Pb) ratios of silica-undersaturated alkaline rocks from Mayotte, the oldest island of the Comores. Two trends are defined using major element composition: (1) a highly silica-undersaturated trend which includes basanites, (melilite-bearing) nephelinites, intermediate lavas and phonolites from the southern part of the island and (2) a moderately silica-undersaturated trend which is mainly represented by alkali basalts, basanites, intermediate lavas and phonolites from the northern part of the island. Both trends could be explained, to some extent, by variable degrees of partial melting. Normative larnite-bearing olivine melilitites and nephelinites exhibit, in addition to their high silica-undersaturation, elevated concentrations in CaO (>12wt.%) and P2O5 (up to 1.35wt.%). These exceptional rocks would result from low degree deep partial melting of a CO2-metasomatized source in the presence of carbonate (probably dolomite) and apatite. Igneous rocks from southern and northwestern shield volcanoes are characterized by a radiogenic Pb composition, revealing the existence of a HIMU (high μ=238U/204Pb) component in their source. Its influence decreases from the main building stage (>10.6-~3.0Ma) to the post-shield stage for the benefit of a depleted MORB-mantle (DMM) component, especially in the north central rocks. This feature would reflect increasing melting degrees of the depleted dominant source, bearing small-scale HIMU heterogeneities progressively consumed with time. The HIMU signature might have been introduced in the Comorian lithospheric mantle by thermal erosion or delamination of a continental lithosphere during the Gondwana break-up. The other islands of the Comores archipelago (Moheli, Anjouan and La Grille type lavas from Grande Comore) display also a DMM-HIMU mixing trend. Only a few lavas from Grande Comore (Karthala type) and one sample from Mayotte show the clear EM1 contribution (87Sr/86Sr>0.7035) of the Comorian plume.

Osmium isotopes in Grande Comore lavas: A new extreme among a spectrum of EM-type mantle endmembers

Osmium isotopes for volcanic rocks from Grande Comore, the youngest island of the Comores Archipelago, located between East Africa and Madagascar, show systematic differences between the two volcanoes of the island. 187Os/ 188Os ratios (0.15–0.16) in shield-stage alkali basalts from Karthala are among the highest yet measured in ocean island basalts with > 40 ppt Os. They are uniform over the whole range of Os abundances of 20–100 ppt and form nearly linear correlations with 87Sr/ 86Sr, 143Nd/ 144Nd and 3He/ 4He ratios. In contrast, 187Os/ 188Os ratios in alkaline lavas from La Grille volcano are more variable (0.129–0.159) and negatively correlate with Os abundances, but not with Sr–Nd–He isotope ratios. Os isotope variability of La Grille samples is consistent with shallow contamination of mantle xenolith-bearing lavas with low 187Os/ 188Os lithospheric mantle and contamination of low Os magmas with high 187Os/ 188Os crust. Karthala magmas are free of such shallow contamination despite the lower Os concentration in some of its lavas. 187Os/ 188Os ratios (excluding contaminated samples) indicate that Grande Comore lavas represent mixtures between melts from the Comores plume and the oceanic lithospheric mantle, with La Grille mostly lithosphere-derived and Karthala plume-derived. The inferred isotopic composition of the metasomatized hydrous lithosphere is typical of the upper mantle. The high 187Os/ 188Os ratio (0.159) of the Comores plume endmember requires dominance of pyroxenite (estimated to be ~ 60%) over peridotite in the melt source. The Comores plume forms yet another extreme among ocean island basalts in Sr–Nd–Pb–He–Os isotope space. In the global context, individual islands with enriched mantle affinities trend towards their own unique enriched mantle endmember. The global data thus do not support the existence of narrowly defined EMI and EMII components [Zindler and Hart (1986), Chemical Geodynamics. Ann. Rev. Earth Planet. Sci. 14, 493–571] but rather indicate that enriched components in the mantle exhibit a spectrum of mantle endmembers.

Grand Comore Island: A well-constrained “low 3He/ 4He” mantle plume

We report He isotope ( 3He/ 4He) variations in samples from alkali basaltic and basanitic lava flows from Grande Comore Island complemented by existing [1,2] [C. Class, S.L. Goldstein, Plume–lithosphere interactions in the ocean basins: constraints from the source mineralogy. Earth Planet. Sci. Lett., 150 (1997) 245–260, C. Class, S.L. Goldstein, R. Altherr, P. Bachèchlery, The process of plume–lithosphere interaction in the ocean basins—the case of Grande Comore. J. Petrol., 39 (5) (1998) 881–903] and new Sr–Nd–Pb isotope ratios and major and trace element abundances. He isotope data in samples from Tristan da Cunha and Gough islands and the Huri Hills in Kenya are reported also. Grande Comore 3He/ 4He ratios vary between 5.05 and 7.08 R A ( 4He/ 3He ≈ 141,000–101,000). Chemical and Sr–Nd–Pb isotopic variations of Grande Comore lavas were previously shown to reflect melts derived from the deep mantle plume and the shallow lithospheric mantle [1–3] [C. Class, S.L. Goldstein, Plume–lithosphere interactions in the ocean basins: constraints from the source mineralogy. Earth Planet. Sci. Lett., 150 (1997) 245–260, C. Class, S.L. Goldstein, R. Altherr, P. Bachèchlery, The process of plume–lithosphere interaction in the ocean basins-the case of Grande Comore. J. Petrol., 39 (5) (1998) 881–903, C. Claude-Ivanaj, B. Bourdon, C.J. Allègre, Ra–Th–Sr isotope systematics in Grande Comore Island: a case study of plume–lithosphere interaction. Earth Planet. Sci. Lett. 164 (1998) 99–117]. The lithosphere-dominated end-member (La Grille volcano) shows uniform 3He/ 4He ratios within error of 6.75–7.08 R A ( 4He/ 3He ≈ 106,000–101,000) over a range of [He] = 36–428 × 10 − 9 ccSTP/g. The plume end-member (of the Karthala volcano suite), as constrained by Sr, Nd, Pb isotope ratios, shows uniformly lower 3He/ 4He ratios with 5.05–5.41 R A ( 4He/ 3He ≈ 141,000–132,000) over a range of [He] = 11–136 × 10 − 9 ccSTP/g. All samples show good correlations between Sr–Nd–He isotope ratios, indicating that the Grande Comore 3He/ 4He ratios are not significantly influenced by crustal contamination and reflect recent mixing between plume- and lithosphere-derived melts. The lithosphere beneath Grande Comore has retained its MORB-like helium isotopic composition, which suggests that the previously identified amphibole-forming metasomatism of the lithospheric mantle [1] [C. Class, S.L. Goldstein, Plume–lithosphere interactions in the ocean basins: constraints from the source mineralogy. Earth Planet. Sci. Lett., 150 (1997) 245–260] occurred prior to the arrival of the Comoro plume. The well-constrained 3He/ 4He = 5.2 ± 0.2 R A ( 4He/ 3He ≈ 137,000 ± 5000) of the Comoro plume confirms the existence of “low 3He/ 4He” mantle plumes. A global compilation of OIB shows that OIB from “low 3He/ 4He” plumes form one “side” of the roughly triangular distribution of the global OIB data set in 143Nd/ 144Nd versus 206Pb/ 204Pb and 208Pb/ 204Pb, encompassing the lowest Nd to the highest Pb isotope ratios. It is also shown that “low 3He/ 4He” plumes are more enriched in Th and U relative to other plumes.

Carbonatite Metasomatism of the Oceanic Upper Mantle: Evidence from Clinopyroxenes and Glasses in Ultramafic Xenoliths of Grande Comore, Indian Ocean

Petrological and geochemical data are presented for a suite of ultramafic xenoliths (lherzolites and wehrlites) from La Grille volcano, Grande Comore island. These xenoliths have peculiar textural features characterized by reaction rims or glassy patches, where clinopyroxene grows at the expense of orthopyroxene. The glasses vary from very strongly silica-oversaturated to strongly undersaturated varieties. They are generally homogeneous within a single xenolith, are extremely alkali rich, with Na2O contents (up to 14 wt %) higher than any previously encountered, and are highly evolved (mg-number 34–37). The major element (especially Na) and trace element abundances, together with some remarkable Ti and Zr negative anomalies in chondrite-normalized diagrams, suggest that the metasomatizing agent was an ephemeral alkali-rich carbonatite. These metasomatic reactions appear to have involved an increase in oxygen fugacity from lherzolites (−0.56 to 1.29Δlog units) to wehrlites (up to 2.68Δlog units), with a concomitant decrease in temperature (lherzolites: 908–993°C; wehrlites: 816–893°C). Geobarometric indicators point to equilibration in the range of 7–12 kbar, above the carbonate stability field. An experimentally determined carbonatite composition reacting with a harzburgitic–lherzolitic protolith was successfully balanced with the observed secondary paragenesis allowing calculation of the trace element content of the metasomatizing carbonatitic fluid/s. These modelling procedures were subsequently used to estimate a set of Kd cpx/carb values for mantle depths, which compare well with (scarce) data from both natural occurrences and experimental runs. The origin of the CO2 and H2O in the metasomatizing melts or fluids is problematic. The melts or fluids may have originated from recycled material in or, most probably, below the asthenospheric mantle, brought down in a subducting slab at a former convergent plate margin.

Carbonatite Metasomatism of the Oceanic Upper Mantle: Evidence from Clinopyroxenes and Glasses in Ultramafic Xenoliths of Grande Comore, Indian Ocean

material in or, most probably, below the asthenospheric mantle, Petrological and geochemical data are presented for a suite of brought down in a subducting slab at a former convergent plate ultramafic xenoliths (lherzolites and wehrlites) from La Grille margin. volcano, Grande Comore island. These xenoliths have peculiar textural features characterized by reaction rims or glassy patches, where clinopyroxene grows at the expense of orthopyroxene. The glasses vary from very strongly silica-oversaturated to strongly undersaturated varieties. They are generally homogeneous within a single xenolith, are extremely alkali rich, with Na2O contents (up

U–Th–Pa–Ra systematics for the Grande Comore volcanics: melting processes in an upwelling plume

High precision TIMS Pa-U disequilibrium data have been obtained on a series of recent samples from the Karthala and La Grille volcanoes, Grande Comore. We use this data together with U–Th–Ra and Sr isotope, major and trace element data to constrain melting-induced fractionation of the U series nuclides. ( 231Pa/ 235U) activity ratios range between 1.7 and 1.9 for the historical Karthala volcanics and are around 2.0 for two recent La Grille samples. While the Karthala magmas could have been generated in the upwelling asthenosphere, the La Grille lavas are thought to involve some lithospheric melting. The Karthala data is best explained by a model of slowly upwelling mantle during 230Th and 231Pa grow in from their parent nuclides. Pb isotope data is consistent with a recent fractionation of 230Th/ 232Th induced by a slow melting process in the source of the Karthala. The upwelling velocity of the mantle plume is constrained to be a few cm per year. The La Grille data can be explained by interaction of the Karthala melts with the lithosphere. The degree of melting of the lithosphere still needs to be small to preserve the U-series fractionation. The higher 231Pa/ 235U in the La Grille lavas probably reflects a higher cpx/garnet mode than for the Karthala.

Ra–Th–Sr isotope systematics in Grande Comore Island: a case study of plume–lithosphere interaction

Here, we present new mass spectrometry measurements of U–Th–Ra disequilibria and Sr isotopes for historical volcanics from the Karthala and La Grille volcanoes in Grande Comore Island, Comores Archipelago. Alkali basalts from the Karthala are characterised by large 230Th (33–47%) and 226Ra excesses (21–53%), and radiogenic Sr compositions (0.7034–0.7041). In contrast, La Grille basanites have less radiogenic 87Sr/ 86Sr (∼0.7032) with less 226Ra excesses (21%) but similar 230Th– 238U disequilibria (45%). The Karthala samples display much more scatter in the Ra–Th isochron diagram than in the Th–U isochron diagram, suggesting a decoupling of the two parent–daughter systems. Correlations between the Sr isotopes and U–Th–Ra disequilibria suggest a mixing relationship between La Grille and Karthala sources. La Grille basanites are shown to result from partial melting of the old metasomatised oceanic lithosphere beneath the archipelago whereas Karthala lavas are derived from the Comore plume. The presence of amphibole in the lithosphere is responsible for Ra–Ba, Ba–Th and Ra–Th fractionation during melting such that D Ba > D Th > D Ra. The partitioning of these elements clearly differs from partial melting of garnet–lherzolite where D Th > D Ra > D Ba. Part of the variations in the 226Ra excesses is attributed to radioactive decay between the source and the surface. Short residence times (<1000 year) preclude the presence of a large magma chamber and may be an argument for rapid segregation of the melts through cracks or veins during melt transport.

The Process of Plume-Lithosphere Interactionsin the Ocean Basins-the Caseof Grande Comore

Grande comore island is formed by two contemporaneously active volcanoes, karthala and la grille. previous work has shown that la grille lavas are alkalic low-degree melts of a metasomatized lithospheric mantle source containing amphibole. this study presents major, trace element and sr–nd–pb isotope analyses of a detailed sample suite from karthala. the lavas are alkali basalts displaying trace element and isotopic variations that indicate mixing between the deep mantle plume source and the lithospheric mantle source of la grille. this study delineates how the process of plume–lithosphere interaction occurs in grande comore, characterizes the composition of the metasomatized lithospheric mantle, and identifies its origin. lithosphere-derived la grille lavas show similar trace element patterns to plume-related karthala lavas, but lower trace element contents for comparable degrees of partial melting. addition of a few percent of plume melt to anhydrous depleted lithospheric mantle generates a mantle composition that can serve as the source of la grille lavas. this plume-related metasomatism occurs pervasively at the base of the lithospheric mantle, lowers its solidus temperature, and causes it to be a major contaminant of plume melts. karthala lavas are shown to be mixtures of plume melts with high-degree melts of the lithospheric mantle. to the extent that this process is valid in other locations, it implies that isotopic variations of other oceanic islands may reflect contamination of nearly uniform plume melts with lithospheric melts.

The Process of Plume-Lithosphere Interactions in the Ocean Basins--the Case of Grande Comore

Grande Comore Island is formed by two contemporaneously active INTRODUCTION volcanoes, Karthala and La Grille. Previous work has shown that Most individual oceanic islands display significant isotopic La Grille lavas are alkalic low-degree melts of a metasomatized variations in erupted basalts, and the cause of these lithospheric mantle source containing amphibole. This study presents variations is still a matter of great debate. There is major, trace element and Sr–Nd–Pb isotope analyses of a detailed still no consensus as to whether they reflect intrinsic sample suite from Karthala. The lavas are alkali basalts displaying heterogeneity of the deep mantle source, mixtures of the trace element and isotopic variations that indicate mixing between plume source with entrained mantle, or contamination the deep mantle plume source and the lithospheric mantle source of of plume-derived melts by shallow lithospheric mantle La Grille. This study delineates how the process of plume–lithosphere or crust. Recently there has been increased recognition interaction occurs in Grande Comore, characterizes the composition of the significance of lithospheric contributions to the of the metasomatized lithospheric mantle, and identifies its origin. isotopic diversity found in oceanic islands, especially those Lithosphere-derived La Grille lavas show similar trace element located near continents and on oceanic plateaux (Gerlach patterns to plume-related Karthala lavas, but lower trace element et al., 1988; Hoernle et al., 1991; Barling et al., 1994; contents for comparable degrees of partial melting. Addition of a Class et al., 1996; Milner & le Roex, 1996). The possibility few percent of plume melt to anhydrous depleted lithospheric mantle of lithospheric contributions to Hawaiian tholeiitic lavas generates a mantle composition that can serve as the source of La has been widely discussed [e.g. Tatsumoto (1978), StauGrille lavas. This plume-related metasomatism occurs pervasively digel et al. (1984), Chen & Frey (1985), Hegner et al. at the base of the lithospheric mantle, lowers its solidus temperature, (1986), Stille et al. (1986), Kurz & Kammer (1991), Eggins and causes it to be a major contaminant of plume melts. Karthala (1992), Eiler et al. (1996), Hauri et al. (1996), and references lavas are shown to be mixtures of plume melts with high-degree therein], although these studies disagree on the identity melts of the lithospheric mantle. To the extent that this process is of the lithospheric component. Nevertheless, models of valid in other locations, it implies that isotopic variations of other lithospheric contributions to plume lavas have met reoceanic islands may reflect contamination of nearly uniform plume sistance on theoretical, geophysical, and geochemical melts with lithospheric melts. grounds. Significant melting of anhydrous lithosphere as

Geochemical and isotopic (Sr, Nd, Pb) evidence for plume–lithosphere interactions in the genesis of Grande Comore magmas (Indian Ocean)

Three groups are distinguished among the Grande Comore lavas: Old Karthala, Recent Karthala and La Grille. Major and trace element data allow a clear distinction between Karthala saturated to slightly undersaturated lavas (shield-stage) and La Grille strongly undersaturated primary melts (post-shield stage). All these lavas are strongly enriched in incompatible elements, with relative enrichments in Ba, Nb, La and relative depletions in Rb, K, Sr, Sm. Low HREE contents and strongly fractionated REE patterns indicate that garnet is residual in their sources. La Grille lavas exhibit the highest and most variable degrees of incompatible element enrichment, the strongest relative K depletion and a wide range of the most incompatible element ratios. These lavas are low-degree melts of an amphibole-bearing source. Ratios of the most incompatible elements are homogeneous among Karthala lavas. They were produced by slightly higher degrees of melting, with much smaller amounts of residual garnet and amphibole, probably at shallower depth, than La Grille lavas. La Grille lavas exhibit the most radiogenic 143Nd/ 144Nd (0.51283–0.51287) and the least radiogenic 87Sr/ 86Sr (0.70315–0.70336), 206Pb/ 204Pb (18.98–19.31) and 208Pb/ 204Pb (38.80–39.13). Old Karthala lavas display the most radiogenic 206Pb/ 204Pb (19.47–19.86) of the data set and intermediate 87Sr/ 86Sr and 143Nd/ 144Nd. Recent Karthala lavas exhibit the most radiogenic 87Sr/ 86Sr (0.70362–0.70396), the least radiogenic 143Nd/ 144Nd (0.51265–0.51272) and intermediate 206Pb/ 204Pb. Karthala lavas having `LoNd compositions' require the mixing of closely associated EM I and HIMU components. Recent Karthala lavas reflect a stronger contribution of the EM I component whereas Old Karthala lavas are the most enriched in the HIMU component and contain a additional DM component. La Grille lavas reflect the significant involvement of a DM component, in addition to HIMU and EM I components. A strong coupling between isotopes and incompatible element ratios is observed among La Grille lavas with the most isotopically depleted samples exhibiting the most enriched trace element signature. The geochemical and isotope variability recorded in Grande Comore lavas is a consequence of small and variable degrees of melting of the heterogeneous underlying mantle. We propose that the EM I and HIMU components are closely associated within the mantle plume and that the DM component is located in the oceanic lithosphere. Karthala lavas, corresponding to shallower, higher-degree melts, give the best estimate of the plume composition and must have been produced close to the plume axis. La Grille low-degree melts require lower temperatures for their genesis and must have been generated at the periphery of the plume. The two volcanoes belong to two distinct magmatic systems. The atypical `LoNd' signature of the Grande Comore lavas, contrasting with the Indian Ocean Dupal signature, may be explained by the specific location of the Comores Archipelago, at the junction of two distinct oceanic crusts, in the transition zone between two distinct mantle domains.

Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy

Trace element relationships of near-primary alkalic lavas from La Grille volcano, Grande Comore, in the Indian Ocean, as well as those of the Honolulu volcanic series, Oahu, Hawaii, show that their sources contain amphibole and/or phlogopite. Small amounts of each mineral (2% amphibole in the source of La Grille and 0.5% phlogopite plus some amphibole in the source of the Honolulu volcanics) and a range of absolute degrees of partial melting from ∼ 1 to ∼ 5% for both series are consistent with the observed trace element variation. Amphibole and phlogopite are not stable at the temperatures of convecting upper mantle or upwelling thermal plumes from the deep mantle; however, they are stable at pressure-temperature conditions of the oceanic lithospheric mantle. Therefore, the presence of amphibole and/or phlogopite in the magma source region of volcanics is strong evidence for lithospheric melting, and we conclude that the La Grille and the Honolulu series formed by melting of the oceanic lithospheric mantle.The identification of amphibole ± phlogopite in the source region of both series implies that the metasomatism by fluids or volatile-rich melts occurred prior to melting. The presence of hydrous phases results in a lower solidus temperature of the lithospheric mantle, which can be reached by conductive heating by the thermal plumes. Isotope ratios of the La Grille and the Honolulu series display a restricted range in composition and represent compositional end-members for each island. Larger isotopic variations in shield lavas, represented by the contemporaneous Karthala volcano on Grande Comore and the older Koolau series on Oahu, reflect interaction of the upwelling thermal plumes with the lithospheric mantle rather than the heterogeneity of deep-seated mantle plume sources or entrainment of mantle material in the rising plume. Literature OsSr isotope ratio covariations constrain the process of plume-lithosphere interaction as occurring through mixing of plume melts and low-degree melts from the metasomatized oceanic lithospheric mantle.The characterization of the lithospheric mantle signature allows the isotopic composition of the deep mantle plume components to be identified, and mixing relationships show that the Karthala and Koolau plume end-members have nearly uniform isotopic compositions. Based on independent arguments, isotopic variations on Heard and Easter islands have been shown to be a result of mixing between deep plume sources having distinct isotopic compositions with lithosphere or shallow asthenospheric mantle. To the extent that these case studies are representative of oceanic island volcanism, they indicate that interaction with oceanic lithospheric mantle plays an important role in the compositions of lavas erupted during the shield-building stage of plume magmatism, and that isotopic compositions of deep mantle plume sources are nearly uniform on the scale that they are sampled by melting.

The Geochemistry of Lavas from the Gomores Archipelago, Western Indian Ocean: Petrogenesis and Mantle Source Region Characteristics

New mineral and bulk-rock analyses, as well as Nd, Sr and Pb isotope compositions are presented for lavas from Grande Comore, Moheli and Mayotte, thru of the four main islands of the Comores Archipelago in the western Indian Ocean, and these data an used to evaluate the petrogenesis, evolution and mantle source region characteristics of Comorean lavas. The typically silica-undersaturated, alkaline lavas from all three islands can be grouped into two distinct types: La Grille-type (LGT) lavas, which display strong relative depletions in K, and Karthala-type (KT) lavas, which do not. With the exception of the lavas erupted by La Grille volcano on Grande Comore, which exhibit the petrographic and geochemical characteristics expected of primary mantle-derived magmas, all Comorean lavas analysed have experienced compositional modifications after they segregated from their source regions. Much of this variation can be explained quantitatively by fractional crystallization processes dominated by the fractionation ofolivine and clinopyroxene. Semi-quantitative modelling shows that the consistent and fundamental difference in composition between K-depleted LGT lavas and normal KT lavas can be attributed to partial melting processes, provided amphibole is a residual mantle phase after extraction of LGT magmas at low degrees of melting. Low absolute abundances of the heavy rare earth elements in LGT magmas are interpreted to reflect partial melting within the garnet stability field In contrast, KT magmas, which do not show relative K depletions, are considered to be the products of somewhat larger degrees of partial melting of an amphibolefree source at comparatively shallower depths. Whereas the Nd and Sr isotopic compositions of Comorean lavas (which show a significant range: 87Sr/86Sr = 0.70319–0.70393; 143Nd/Nd = 0.51263–0.51288) bear evidence for a time-averaged depletion in incompatible elements, the high incompatible element abundances of the lavas are interpreted to reflect the effects of a recent mantle enrichment event. At depths well within the garnet stability field this mantle enrichment is interpreted to have taken the form of modal metasomatism with the introduction of amphibole (giving rise to the source of LGT magmas), whereas cryptic metasomatism took place at shallower levels (giving rise to the source of KT lavas). The Nd, Sr and Pb isotope signature of the majority of Comorean lavas (both LGT and KT) is proposed to be the result of predominant4 contributions from a somewhat heterogeneous source4 4 4 presentative of the ambient sub-Comorean mantle, comprising a mixture between a HIMU component and a component on the depleted portion of the mantle array (possibly the source of Indian Ocean MORB), with only limited contributions from an EM I plume component. The lavas erupted by Karthala volcano (the youngest Comorean lavas), however, have significantly different isotopic compositions from all other Comorean lavas (lower 143Nd/144Nd and higher 87Sr/86Sr), suggesting increased contributions from the EM I component.

Melt Inclusions Trapped in Mantle Minerals: A Clue to Identifying Metasomatic Agents in the Upper Mantle Beneath Continental and Oceanic Intraplate Regions

Rock samples derived from the Earth's upper mantle commonly show indirect evidence for chemical modification induced by migrating fluids or melts . Such modi f ica t ion , or 'metasomatism' , can be recognized by the precipitation of exotic minerals such as phlogopite, amphibole, or apatite, and by the overprinting of the bulk compositions of the mantle rocks by a chemical signature involving the enrichment of incompatible elements. However, identification of the metasomatic agents occurring in the upper mantle is still a controversial topic. Here, we study the composition of the metasomatic agents more directly by examining trapped melt inclusions in mantle minerals from ultramafic peridotites from both continental and oceanic intraplate regions. The xenoliths samples come from Society islands (Tahaa , Tahit i) , Canary islands (Lanzarote, Hierro), Kerguelen island (Jeanne d'Arc Peninsula), Comores island (La Grille), New Mexico (Kilbourne Hole), Arizona (San Carlos), Germany (Dreiser Eifel), Italy (Mt Ible'0, France (Massif Central), Mongolia (Dariganga), Russia (Vitim Highland) and Vietnam. They are massive anhydrous spinel-lherzolites and harzburgites, with 60-81% olivine, 10-21% orthopyroxene, 0.5-11% clinopyroxene and 0.5-3% spinel. Textures vary from protogranular to porphyroclastic, often with a transition between the two. The xenoliths contain three types of genetically related inclusions hosted by olivine, clinopyroxene and orthopyroxene: silicate melt inclusions, carbonate-rich inclusions, and CO2 fluid inclusions. The inclusions are secondary in nature and form trails along annealed fracture planes in the sheared peridotites. CO2 inclusions and silicate melt inclusions joined by necks and multiphase inclusions consisting of both CO2-rich fluid and silicate-carbonate melts are visible, indicating a cogenetic relationship between fluid and melt inclusions.

Geology of the Comores Archipelago, Western Indian Ocean

SummaryThe Comores Archipelago comprises four principal islands, lying in a WNW–ESE line at the northern end of the Mozambique Channel. The Islands are almost entirely volcanic and show a simple age progression from Grande Comore, with its active shield volcano Kharthala (2650m), east-south-eastwards through Mohéli and Anjouan, which are quite deeply dissected, to Mayotte, which is in an advanced stage of erosion and subsidence, and is encircled by a barrier reef. There is also evidence of minor volcanic rejuvenation in relatively recent times in Mayotte, and perhaps Anjouan and Mohéli. The lavas of Kharthala, and the neighbouring extinct shield volcano (Massif de la Grille) on Grande Comore are all basaltic, ranging from oceanitic and ankaramitic to aphyric types. Numerous tuff cones occur, especially on the flanks of Massif de la Grille. Basaltic lavas, similar to those of Grande Comore, form the bulk of the dissected volcanic shields of Mohéli and Anjouan, but there is also evidence of differentiation towards alkalic under-saturated end products, both as lavas and as intrusive bodies. Lherzolitic and gabbroic nodules are locally abundant on each of the islands, while xenoliths of sandstone and quartzite have been found on the three westernmost islands. Petrochemical data are presented for a considerable variety of Comores rocks, and the volcanic activity is discussed in terms of the major structural features of the Madagascar-Mozambique region.