Gilles Chazot

Determination of partition coefficients between apatite, clinopyroxene, amphibole, and melt in natural spinel lherzolites from Yemen: Implications for wet melting of the lithospheric mantle

Plio-Quaternary basaltic volcanic fields along the south coast of Yemen contain mantle xenoliths of anhydrous and metasomatized amphibole-(± apatite) bearing spinel lherzolites. Partial melting of clinopyroxene and/or amphibole in a closed system produced silicate glass from which new clinopyroxene, olivine, and spinel have crystallized. Partition coefficients between mineral phases in the matrix and particularly those (apatite, clinopyroxene, amphibole) formed during the metasomatic event, and between glass and the new clinopyroxene crystallized from it have been calculated by analyzing trace element composition with an ion microprobe. Partition coefficients between apatite and clinopyroxene or amphibole are large (e.g., ≫ 1) for the REEs, Ba, Sr, Y, and Hf and confirm that apatite, when present in the mantle, is an important repository for these elements. On the other hand, partitioning between amphibole and clinopyroxene is very close to unity for most of the analyzed trace elements, except for Ba, Nb, Zr, and Hf. Trace element ratios such as Zr/Nb, Ba/Zr, or Ba/Nb can thus be used in basaltic rocks to assess the presence of amphibole in their mantle sources. Partition coefficients between clinopyroxene and melt are within the range of published values for most of the analyzed elements, with some low values for Zr (e.g., 0.030–0.353) and near unity for the HREE. Using partition coefficients between amphibole and clinopyroxene and Depx/melt from the literature, we calculate the partitioning of trace elements between amphibole and a basaltic melt. The Damp/melt obtained are lower than most of the published values, but are in good agreement with recent values measured in experiments at 1.5 GPa and 1100°C. The partition coefficient for Nb between amphibole and melt is always low (<0.20), which indicates that residual amphibole during partial melting in hydrated mantle (amphibole-bearing peridotite) cannot account for Nb depletion in arc magmas.

Oxygen isotopic composition of hydrous and anhydrous mantle peridotites

Oxygen isotope ratios, determined using the laser fluorination technique, are reported for minerals from anhydrous and hydrous (i.e., amphibole-bearing) spinel lherzolites from Yemen, as well as from hydrous spinel lherzolites and amphibole megacrysts from Nunivak Island, Alaska. Oxygen isotopic compositions of olivine vary from 5.1–5.4%c and of pyroxene from 5.5–6.0%c and no systematic difference exists between minerals in hydrous and anhydrous lherzolites. The oxygen isotopic composition of the amphibole in the peridotites and of the amphibole megacrysts is also very homogeneous and varies from δ18O = 5.3−5.6%o. These results indicate that the metasomatic minerals in the lherzolites are in oxygen isotopic equilibrium with the peridotitic minerals. The only isotopic disequilibria are observed in minerals which have grown in melt-pockets formed by partial melting of amphibole. The homogeneity of the oxygen isotopic ratios of mantle minerals in this study indicate that the fluids circulating in the mantle and precipitating amphibole or mica had the same oxygen isotopic compositions as the mantle protolith or that the fluids had been buffered by the isotopic composition of the olivine, the most abundant mineral, during percolation through the peridotites.

Metasomatism of the shallow mantle beneath Yemen by the Afar plume—Implications for mantle plumes, flood volcanism, and intraplate volcanism

Amphibole :t apatite-bearing lherzolite xenoliths from Yemen have Sr, Nd, and Pb isotope ratios that are the Saille as those of Oligocene flood and Quatemary intraplate basalts in Yemen, and also the Afar plume, which is genetically linked with fuis volcanism. The xenoliths have minerai, chemical, and isotopic characteristics consistent with enrichment of shallow mantle by carbonatitic melts and hydrous fluids from the Afar plume during or shortly after Oligocene flood volcanism. Separation of carbonatitic melts and hydrous fluids from mantle plumes may affect the composition of erupted flood basalts and is consistent with an origin of sncb plumes by recycling of oceanic lithosphere. Mantle plumes appear to have volatile fluxes that are large enough to affect solidus temperatures, both within plumes and the overlying lithospheric mande, during the generation of large igneous provinces. The fertile lithosphere formed by plumederived melts and fluids is an important potential source for intraplate volcanism that cao be exploited by melting during later extension or erosional unloading of the lithosphere. peridotites and Saille anhydrous peridotites from Ataq have isotope ratios identical to those of melts of the Cenozoic Afar plume and chemical signatures that are suggestive of recent metasomatism by carbonatitic melts and hydrous fluids derived from gamet facies mantle. RESULTS Trace Elements

Implications of widespread high-μ volcanism on the Arabian Plate for Afar mantle plume and lithosphere composition

Abstract We report on 55 Nd–Sr–Pb and 23 Hf isotopic compositions for late Miocene to Plio–Quaternary basalts from the Arabian Plate. The sampling profile represents a 2500 km N–S transect along the Red Sea margin from Syria to Yemen. 206Pb/204Pb displays a wide range (from 18.60 to 19.55) from south to north, indicating a pervasive high-μ distribution that questions the previous attribution of this component to the Afar plume. We rather suggest that the high-μ signature resides within the Arabian lithospheric mantle. Compared to basalts from northern and central Arabia (Saudi Arabia to Syria), basalts from southern Arabia (Yemen) display similar ranges for 143Nd/144Nd, 176Hf/177Hf, and 206Pb/204Pb, but are shifted towards compositions more radiogenic in Sr and 208Pb. These distinct time-integrated Rb/Sr and Th/U ratios between north and south are believed to reflect heterogeneous development of the Arabian lithosphere through time and/or result from the thermal effect of the Afar plume on the southern part of the lithosphere.

SILICATE GLASSES IN SPINEL LHERZOLITES FROM YEMEN : ORIGIN AND CHEMICAL COMPOSITION

Abstract Anhydrous and amphibole-bearing spinel lherzolites found in alkali basalts from Southern Yemen contain melt-pockets with silicate glasses from which have crystallized euhedral clinopyroxenes, olivines and spinels. These glasses have variable major element compositions related to variations in the composition of the melting phases. In the hydrous lherzolites, the variations in the major element composition of the glasses are directly correlated to the variations in the composition of residual amphiboles in the melt-pockets. The same observations can be made for the composition of clinopyroxene and glass in the anhydrous lherzolites. The trace element compositions of the residual phases and the glasses are consistent in the different samples and all the observations indicate that the glasses originated by in-situ melting of amphibole or clinopyroxene in the lherzolites. If infiltration of a metasomatic melt or fluid caused the melting event, these data are indicative of very low fluid/rock ratios and attainment of conditions close to equilibrium between the fluid and the solid phases. Surprisingly, Cl-rich apatite, when present, was not melted in these samples, which indicates that apatite can be a residual phase during partial melting at mantle conditions. Comparison of the glasses in the Yemen lherzolites with published chemical compositions for mantle glasses shows a dichotomy between glasses produced by (a) in-situ melting of amphibole or clinopyroxene (± phlogopite), either in closed-system or during reaction with a metasomatic fluid at low fluid/rock ratios, and (b) glasses resulting from interaction between the peridotite and a metasomatic melt or fluid at higher fluid/rock ratios.

Partitioning of phosphorus between olivine, clinopyroxene and silicate glass in a spinel lherzolite xenolith from Yemen

Abstract The melting reaction: amphibole+apatite=olivine+cpx+spinel+liquid is documented in spinel lherzolite xenoliths found in Plio-Quaternary alkali basalts from Yemen (Ataq Volcano). All the phases involved in this reaction, including the melt, have been preserved within millimeter-size glassy melt pockets. In a previous study, ion microprobe analyses performed on these phases allowed determination of trace-element partition coefficients between apatite, clinopyroxene, amphibole and glass. In this study, we focus on the phosphorus distribution between apatite, clinopyroxene, olivine and glass. Silicate/melt partition coefficients for this element (Dsilicate/melt) are presently poorly known and the Yemen mantle xenoliths represent a good opportunity to obtain new data and to discuss the behaviour of phosphorus during mantle melting. Electron microprobe (EMP) analyses of the JK3 sample and two of its melt pockets in particular, yield P2O5 concentrations within the range of 0.75–1.3 wt.% in the glass, 0.07–0.15 wt.% in olivine, 0.035–0.07 wt.% in amphibole and 0.015–0.06 wt.% in clinopyroxene. Silicate/melt partition coefficients derived from EMP data on newly formed melt pocket phases (Dolivine/melt=0.1, Dcpx/melt=0.05), as well as from the inspection of the phosphorus distribution among the matrix minerals, were used in a simple batch-melting calculation. The composition of most natural basalts is well-matched for MOR basalt and an alkali basalt (and OIB) source containing 90 and 250 ppm P, respectively. From these data, and by using relevant Nd source compositions, the model is also able to reproduce the P/Nd ratio of about 70 that is typically observed in natural basalts including MORB and alkali basalts (and OIB). The relatively high phosphorus-content of the mantle silicates studied here, the corresponding new silicate/melt partition coefficients and the high solubility of apatite in basaltic melts strongly suggest that apatite is not required to host phosphorus in most upper-mantle sources of natural basalts.

Mantle sources and magma‐continental crust interactions during early Red Sea‐Gulf of Aden rifting in southern Yemen: Elemental and Sr, Nd, Pb isotope evidence

Large-scale magmatic activity, ranging from late Oligocene to Quaternary, is associated with the Red Sea-Gulf of Aden rifting throughout the Arabian passive margin. The Southern Yemen area represents the southernmost extremity of this magmatic range, facing the Afar area, and provides a means of studying the magmatic records of early stages of rifting (30–16 Ma) in a plume-related context. We investigate major and trace elements, and Nd, Sr and Pb isotopes of a bimodal series of transitional affinity consisting of (1) thick olivine-basalt traps overlain by ignimbritic rhyolites, (2) basaltic, rhyolitic, trachytic and peralkaline dykes with a prevailing N120–140° E orientation, and (3) gabbroic, syenitic and granitic plutons. Major and some trace element variations from basalts to felsic rocks are consistent with low-pressure fractional crystallization. Mass balance calculations using major elements suggest the fractionation of clinopyroxene (6–9%), olivine (∼6%), plagioclase (42–43%), magnetite (∼12%), apatite (1–2%) ± alkali feldspar (16%). However, LILE (large ion lithophile element) enrichment and high initial 87Sr/86Sr ratios (up to 0.7074 and 0.710 in rhyolites and pantellerites, respectively) require the felsic rocks to be generated through significant crustal assimilation. Nd and Sr isotopic ratios of the rhyolitic traps can be reproduced by bulk mixing between magmas similar to the underlying basaltic unit and the Arabian Proterozoic basement. On the other hand, an assimilation-fractional crystallization process is required to account for the isotopic diversity of the rhyolitic and peralkaline dykes. Rhyolites can be derived from a basaltic liquid by a moderate fractionation rate (F = 0.47) and a high crustal assimilation rate (R = 0.45), whereas the pantellerites require more significant fractionation rate (F = 0.07) and a very low assimilation rate (R = 0.05). Elemental and isotopic signatures of the basalts do not support a significant crustal contribution in their formation and their isotopic diversity (87Sr/86Sr from 0.7034 to 0.7051, 143Nd/144Nd from 0.512676 to 0.513045 and 206Pb/204Pb from 17.96 to 18.66) mainly reflects mantle source heterogeneities. Sr-Nd-Pb isotopic data are consistent with a binary mixing between depleted and enriched source regions. The depleted end-member corresponds to an asthenospheric reservoir approaching that producing mid-ocean ridge basalts at Gulf of Aden/Red Sea spreading centers. The enriched reservoir, intermediate between enriched mantle I and II end-members, is supposed to be located within old subcontinental lithosphere related to the Pan-African orogenic events. Unlike the modern volcanics from Afar, no HIMU (high U/Pb ratio) signature has been recognized in our sampling. This rules out any significant chemical influence of the Afar plume upon early rift-related volcanism in Southern Yemen and suggests a continental rift initiation of passive type. However, one can suspect that the Afar plume may have supplied the excess heat required to produce so voluminous traps and to trigger melting in the lithospheric mantle, making the distinction between passive and active rifting more ambiguous.

Unraveling climatic changes from intraprofile variation in oxygen and hydrogen isotopic composition of goethite and kaolinite in laterites: an integrated study from Yaou, French Guiana

An integrated study of O and H isotopes in the lateritic profile of Yaou, French Guiana, was undertaken to investigate the usefulness of stable isotopes as tracers of climatic changes in continental environments. The studied profile is composed of a 27 m thick saprolite, mostly developed in the past under wet-and-dry tropical climate in association with a duricrust, overlain by a 3 m thick yellow latosol formed more recently under present equatorial hot and humid climate. δ18O–δD values determined for weathering goethite (pseudomorphs after pyrite) and kaolinite (microcrystalline clay groundmass) throughout the 30 m deep profile reflect formation temperatures consistent with present (25°C) and realistic past climatic temperatures (20°C–30°C), indicating that weathering minerals formed in isotopic equilibrium with their genetic environment and were not subjected to significant isotope exchange after formation. A distinct shift downward (2‰ for δ18O, 15‰ for δD) from low to high δ18O–δD values occurs around 20 m depth in the saprolite. It is interpreted as recording the change from the past tropical to the present equatorial climate. Goethite and kaolinite in the 5–10 m thick saprolite interval immediately above the active basement weathering front are in isotopic equilibrium with modern water and must have formed under present equatorial-humid conditions. In contrast, goethite and kaolinite found higher up in the saprolite and in the duricrust formed in the past under tropical wet and dry climate from waters distinctly depleted in 18O and D relative to modern water. The marked depletion of paleo-meteoric water at Yaou most likely reflects a more contrasted or “monsoonal” character of the ancient tropical climate. The present study shows that ancient weathering minerals in lateritic profiles preserve their δ18O–δD values and carry a time signal. The time signal is best expressed in minerals formed rapidly at the weathering front and not subjected to post-formational remobilization, such as the goethite pseudomorphs after pyrite occurring at Yaou. Groundmass kaolinite is more susceptible to partial remobilization, through successive dissolution-precipitation reactions, which may obliterate the paleoclimatic signal. Unraveling the climatic record carried by weathering minerals in old soil systems is greatly enhanced by studying both oxygen and hydrogen isotopic compositions.

40Ar39Ar chronology of tertiary magmatic activity in Southern Yemen during the early Red Sea-Aden rifting

Abstract The early opening of the Red Sea rift was accompanied by magmatism that formed lava flows, plutons and dyke swarms. Some of these formations were sampled in Southern Yemen during a preliminary field survey and were investigated by geochemistry and 40 Ar 39 Ar geochronology. Ten plateau ages and more disturbed age spectra on plagioclase, alkali feldspar, biotite and whole rocks are discussed in detail, and allow us to establish the first chronological succession of magmatic events in this area immediately adjacent to the well-studied Afar region. An approximately 2000-m-thick section from the middle part of the traps gives ages ranging from 28.9 ± 0.1 Ma at the bottom to 26.5 ± 0.8 Ma at the top (lower part of rhyolitic lava flows). These ages are concordant with the oldest precisely dated alkaline lava flows from the harrats (Harrat Hadan) of Saudi Arabia. The majority of the analysed samples from two distinct dyke swarms mainly trending N120–150 °E give ages of around 25.5 Ma and 16–18.5 Ma, respectively. These dyke swarms do not reflect any drastic change in stress regime in this region during this period, but show, in the vicinity of the Gulf of Aden, the existence of volcano-tectonic trends similar to those of the Red Sea Rift in Saudi Arabia. Geochemical analysis (trace element and isotopic ratio) of these samples show a break at 21–22 Ma contemporaneous with a major magmatic event of tholeiitic affinity occurring over nearly 1700 km along the Red Sea Rift in Saudi Arabia.

Genesis of silicic magmas during tertiary continental rifting in Yemen

Abstract The magmatism in Yemen associated with Oligo-Miocene continental rifting in the Red Sea and the Gulf of Aden comprises a bimodal mafic-felsic suite represented by flows, dykes and plutons. This paper reports the chemical and Nd-Sr isotopic compositions of the felsic rocks (trachytes, syenites, rhyolites and granites) associated with the Yemen Trap Series and deals with the controversy in regard to the crustal or mantle origin of A-type magmas generated in tensional contexts. The overall age of the studied rocks ranges from 26.5 to 16 Ma. The major and trace element compositions of the felsic rocks can partly be explained by fractional crystallization from some of the basic magmas. This process was more extensive in the case of the rhyolites and some of the granites than that of the trachytes and the syenite. In addition, the trachytes differ from the rhyolites by featuring a negative Nb anomaly that suggests contamination by continental crust. The trachytes have much lower 143 Nd 144 Nd ratios, and higher 87 Sr 86 Sr ratios than the associated basalts, whereas the rhyolites have much higher 87 Sr 86 Sr ratios, but 143 Nd 144 Nd ratios identical to those of the basalts. The plutons span the whole compositional range of isotopic compositions displayed by the trachytes and the rhyolites. The isotopic compositions of the felsic rocks do not support their genesis by direct melting of continental crust. Rather, these rocks were generated in an open magma system that evolved by a combination of fractional crystallization and crustal contamination. Mixing and AFC processes involving heterogeneous crust are both required to explain the trachytic and rhyolitic compositions where various amounts of crystal fractionation and crustal contamination were involved. The genetic differences between the trachytes and the rhyolites are related to their respective ages. The highest degree of contamination observed in the trachytes occurred contemporaneously with the emplacement of the traps and could have been related to a high thermal gradient in the crust due to massive magma transfer at that time. This stage presumably corresponds to the impingement of the Afar plume head against the lithosphere. The less contaminated and more fractionated rhyolites, which formed later during continental rifting, are consistent with a lower thermal gradient.