Bertrand Moine

Open-system processes in the genesis of silica-oversaturated alkaline rocks of the Rallier-du-Baty Peninsula, Kerguelen Archipelago (Indian Ocean)

Abstract The Rallier-du-Baty Peninsula forms the southwestern part of the Kerguelen Archipelago (Indian Ocean), whose magmatic activity is related to the long-lived 115-Ma Kerguelen plume. The peninsula is mostly made of alkaline rocks constituting two well-defined ring complexes. This paper focuses on the northern ring complex, which is not yet known. Recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6.2 to 4.9 Ma, and two later volcanic systems. 40Ar/39Ar dating of a trachytic ignimbrite linked to the Dome Carva volcano complex yields an age of 26±3 Ka. This represents the last major eruptive event on the Kerguelen Archipelago. The volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas. The volume of erupted felsic magma is by far the larger, and is represented by abundant pyroclastic deposits and lava flows. Boulders of plutonic rocks are found to the northwest of Dome Carva, and represent intermediate rocks (i.e. monzogabbros and monzonites) that are not present at the surface. Basic rocks are mostly trachybasalts and trachyandesites, while true basalts are scarce. Their mineralogy consists chiefly of plagioclase, olivine, diopside and oxides. Sieve-textured plagioclase is common, as well as corroded olivine and diopside phenocrysts. Peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks. They consist of abundant sanidine, augite and magnetite phenocrysts and interstitial quartz, aegerinic pyroxenes and Na-amphiboles. Ring dykes of quartz-poor alkali feldspar syenites display the same mineralogy, except hornblende is common and replaces diopside. Hornblende is particularly abundant in intermediate monzogabbros. Major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of MgO, CaO, P2O5, TiO2, FeO, Ba, Sr and Ni from basic to felsic rocks. However, the scattering of the data from the basic rocks indicates that other processes have operated. The overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control. An increase of incompatible elements from trachyte to rhyolite is observed. The felsic lavas display an increase of 87Sr/86Sr(i) without any significant variations in the Nd isotopic composition. The genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation, mixing between different basic magmas and probable assimilation of Ba-rich oceanic crust. Major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent. Discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution. The overall evolution from trachyte to rhyolite is thought to be controlled by crystal fractionation. High 87Sr/86Sr(i) of the trachytes is interpreted to reflect interaction with an ocean-derived component, probably during assimilation of hydrothermally altered oceanic crust. Boulders of amphibole-bearing monzonites and monzogabbros found to the northwest of Dome Carva are thought to represent intermediate magma composition that formed at depths but did not erupt.

Incompatible trace element and isotopic (D/H) characteristics of amphibole- and phlogopite-bearing ultramafic to mafic xenoliths from Kerguelen Islands (TAAF, South Indian Ocean)

Alkali basalts from the Kerguelen Islands have entrained numerous phlogopite- and amphibole-bearing ultramafic to mafic xenoliths. These are subdivided into mantle harzburgites, dunites and associated composite xenoliths that represent mantle wall-rock (Type-I) and high pressure (10–15 kbar) segregates (Type II). A lamprophyric dyke containing phlogopite megacrysts has been also studied. Chemical compositions of amphiboles and phlogopites from both xenolith types are similar to those recognized in many ultramafic and mafic volatile-bearing xenoliths from kimberlites and alkali basalts and in peridotites and pyroxenites from orogenic lherzolite massifs. Interstitial amphibole and phlogopite in harzburgites and dunites probably formed during diffuse percolation of highly alkaline basic silicate melt within the upper mantle (porous flow). Evidence from composite xenoliths suggest that similar mantle melts migrated through a network of dykes generated by hydraulic fracturing in the Kerguelen upper mantle. The lamprophyre is the surface expression of this highly alkaline magmatic activity. The δD values of −92 to −61‰ SMOW for mica and amphibole of Type I and Type II xenoliths and of the phlogopite megacrysts are within the accepted mantle range. Calculated δD-H 2 O values in equilibrium with amphiboles and micas have a bimodal distribution (- 65 ± 5‰ and −83 ± 5‰) indicating that the percolating fluids were isotopically heterogeneous. The ubiquity of the highly alkaline magmatic activity is probably related to the late intraplate activity of the Kerguelen mantle plume.

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.

The Neogene to Recent Rallier-du-Baty nested ring complex, Kerguelen Archipelago (TAAF, Indian Ocean): stratigraphy revisited, implications for cauldron subsidence mechanisms

Abstract The Kerguelen Archipelago is made up of a stack of thick piles of Tertiary flood basalts intruded by transitional to alkaline igneous centres at various times since 30 Ma ago. In the SW, the Rallier-du-Baty Peninsula is mostly occupied by two silicic ring complexes, each with an average diameter of 15 km, comprising dissected calderas cross-cut by subvolcanic cupolas. Previous radiometric determinations yield ages ranging from 15.4 to 7.4 Ma in the southern centre, and 6.2 to 4.9 Ma in the northern one. The felsic ring dykes were injected by coeval mafic magmas, forming, successively, swarms of early mafic enclaves, disrupted synplutonic cone sheets, and late cone-sheets. After the emplacement and subsequent unroofing of the plutonic ring complexes, abundant and thick trachytic pyroclastic flows and falls were emitted from the younger caldera volcanoes, while hawaiite and mugearite lava flows were erupted from marginal maars and cones. Huge trachyte ignimbritic flows filled the glacial valleys in the central Peninsula, and capped lacustrine deposits and older lava flows, while related pumice falls are widespread throughout the archipelago. This powerful plinian eruption took place after the network of glacial valleys was established, but before the Little Ice Age that occurred during the last centuries. In the south of the peninsula, even younger trachytic formations are exposed, and fumarolic vents are still active. The growth mechanisms of a caldera-related ring complex can be explained as a repetitive sequence of two eruptive episodes. The first episode of hydrofracturing, induced by volatile exsolution within the evolving magma chamber, creates a vertical circular fracture zone, along which highly vesiculated magmas are emitted during explosive eruptive events occurring at the surface in a caldera volcano. It is followed by a second episode of cauldron-subsidence of a crustal block down to the degassed magma chamber, induced by pressure release. Downward movement of the crustal block favours the emplacement at shallow depths within the older caldera-filling formations, of discrete magmatic sheets characterized by a 16-km mean diameter and a 1-km mean thickness, corresponding to an average unit volume of 200 km3. Actually, the estimated volumes of the different igneous episodes within the Rallier-du-Baty nested ring complex vary from 60 to 900 km3, and correspond to the production during 15 Ma of about 2800 ± 850 km3 of new materials and a net crustal growth of about 100 ± 30 × 103 m3 per year.

Evidence for Nb2+ and Ta3+ in silicate melts under highly reducing conditions: A XANES study

Abstract Niobium (Nb) K-edge and tantalum (Ta) LIII-edge XANES spectra were acquired at the part-permillion concentration level in silicate glasses quenched from chondritic melts equilibrated at 5 GPa and under moderately to highly reducing conditions (IW-1, IW-4.5, IW-7.9). Standard materials have also been analyzed for Nb and Ta, and the data were used to construct the calibration curves of E0 (threshold energy) vs. valence. Under moderately reducing conditions our results are consistent with niobium and tantalum being mainly pentavalent in the silicate melts as also suggested by previous studies. We do not exclude that at IW-1, a small fraction of Nb and Ta could be reduced, leading to a mean formal valence slightly lower than five. At IW-4.5, Ta is mainly in the form Ta3+, and at IW-7.9, Ta appears to be Ta1+, whereas Nb is divalent (Nb2+). The possibility for Nb and Ta to be present in reduced forms has implications for the behavior of the two elements during the processes of differentiation on planetary bodies formed in the reduced parts of the early Solar System. Element partitioning is a function of size and valence, and our results show that high field strength elements could be reduced, which could change their chemical affinity. This may also be important for the Earth and Moon formation and early differentiation, as exemplified by the “Nb paradox.”

Incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2

The incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2 has been first determined in the (3 + 1)-dimensional symmetry group Cmcm(α00)00s with modulation vector q = 0.383a*. Unit-cell values are a = 5.062 (1), b = 8.790 (1), c = 12.744 (1) Å. Three orthorhombic components are related by threefold rotation about [001]. Discontinuous crenel functions are used to describe the occupation modulation of Ca and some CO3 groups. The strong displacive modulation of the O atoms in vertexes of such CO3 groups is described using x-harmonics in crenel intervals. The Na, K atoms occupy mixed sites whose occupation modulation is described in two ways using either complementary harmonic functions or crenels. The nyerereite structure has been compared both with the commensurately modulated structure of K-free Na2Ca(CO3)2 and with the widely known incommensurately modulated structure of γ-Na2CO3.

Hydrothermal fluid interaction in basaltic lava units, Kerguelen Archipelago (SW Indian Ocean)

Hydrothermally altered basaltic lava-units in the northern Kerguelen Archipelago contain a wide variety of secondary silicate and carbonate minerals, including zeolites, hydrothermal calcite, dolomite and magnesite, as well as celadonite, orthoclase (adularia) and quartz. Petrography, fluid-inclusion microthermometry, trace-elements geochemistry, Sr isotopes and stable-isotope compositions indicate hydrothermal fluid cells derived from meteoric water interacting with basalts and Rb-rich subvolcanic peralkaline rocks at temperatures ranging from 50 to 200 °C associated with the cooling of the lava pile. The calculated δ 18 O values of meteoric-hydrothermal waters in fossil hydrothermal systems are identical to those in present-day hot springs, suggesting that meteoric recharge was continuous throughout the igneous cooling cycles of the 28–23 Ma older host basalts and the younger 15–5 Ma old peralkaline intrusions. The Kerguelen northern coastline hydrothermal system in the basaltic pile demonstrates that much of the silicate mineralogy and almost all carbonate secondary minerals in altered basalts were derived from meteoric-hydrothermal fluids, rather than products of seawater interaction, or even magmatic-hydrothermal fluids associated with peralkaline intrusions.