Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

Olivine + clinopyroxene ± amphibole cumulates have been widely documented in island arc settings and may constitute a significant portion of the lowermost arc crust. Because of the low melting temperature of amphibole (∼1100°C), such cumulates could melt during intrusion of primary mantle magmas. We have experimentally (piston-cylinder, 0·5-1·0 GPa, 1200-1350°C, Pt-graphite capsules) investigated the melting behaviour of a model amphibole-olivine-clinopyroxene rock, to assess the possible role of such cumulates in island arc magma genesis. Initial melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and are Al2O3-rich. With increasing melt fraction (T > 1190°C at 1·0 GPa), the melts become ultra-calcic while remaining strongly nepheline-normative, and are saturated with olivine and clinopyroxene. The experimental melts have strong compositional similarities to natural nepheline-normative ultra-calcic melt inclusions and lavas exclusively found in arc settings. The experimentally derived phase relations show that such natural melt compositions originate by melting according to the reaction amphibole + clinopyroxene = melt + olivine in the arc crust. Pargasitic amphibole is the key phase in this process, as it lowers melting temperatures and imposes the nepheline-normative signature. Ultra-calcic nepheline-normative melt inclusions are tracers of magma-rock interaction (assimilative recycling) in the arc crus

Procesos petrogenéticos y condiciones físicas pre-eruptivas responsables de la erupción reciente del volcán Tutupaca (Tacna)

El volcán Tutupaca (17°01’ S, 70°21’ O) es considerado como uno de los 7 volcanes históricamente activos del Perú. Está ubicado en la parte sur del arco volcánico peruano, en el extremo norte del departamento de Tacna. En los últimos siglos ha presentado actividad eruptiva, que ha sido descrita en las crónicas de Zamácola y Jauregui (1804), quienes reportaron periodos eruptivos entre 1787 y 1802 AD. Recientes estudios vulcanológicos confirman estas fuentes históricas: el volcán experimentó una importante erupción explosiva, que originó un depósito de avalancha de escombros y una secuencia piroclástica. Este evento ha sido datado en 220 ± 30 años AP (Manrique, 2013; Valderrama et al., 2013). En este trabajo se ha caracterizado petrológicamente los productos eruptivos recientes de este volcán, describiendo de manera detallada su mineralogía con la finalidad de determinar las condiciones físicas pre-eruptivas (presión, temperatura, fugacidad del oxígeno, wt.% H2O); así como los procesos petrogenéticos responsables de las variaciones geoquímicas observadas. Para constreñir estos parámetros se utilizan diferentes geotermómetros y geobarómetros desarrollados por diferentes autores y que asocian la composición química de los minerales con los parámetros físicos que prevalecieron al momento de su cristalización. Entre los termo-barómetros utilizados podemos citar el termómetro anfibol – plagioclasa (Holland y Blundy, 1994) y el barómetro de Al-in-amphibole (Médard et al., 2013)

Genèse de magmas riches en calcium dans les zones de subduction et sous les rides médio-océaniques : approche expérimentale.

CaO-rich, Al2O3-poor ultracalcic primitive melts occur in all geodynamic environments. Some of them do not present a "garnet signature" and should originate at P < 3 GPa. They are subdivided into a "nepheline-normative” alkaline-rich, silica-poor group uniquely found in arcs and in "hypersthene-normative” melts associated with tholeiitic basalts (MORB, OIT, IAT). The high CaO contents (to 19.0 wt%) and CaO/Al2O3 ratios (to 1.8) exclude an origin from fertile lherzolites. Experimental investigation of the liquidus of a hypersthene-normative and a nepheline-normative ultracalcic melt results in olivine+clinopyroxene saturation at distinct pressure-temperature conditions. Our results in combination with melting experiments from the literature suggest that hypersthene-normative melts result from melting of a refractory olivine+clinopyroxene±orthopyroxene source (either refractory lherzolite or cumulate wehrlite) in the upper mantle and their presence indicate elevated (~1400 °C) mantle temperatures. Contrasting, nepheline-normative ultracalcic melts form from crustal wehrlitic sources at lower temperatures (~1200 °C). To account for the high alkaline and low silica contents, and the relatively low temperatures, source wehrlites must have contained amphibole. Melting experiments on amphibole-wehrlites that frequently occurs at the base of the arc crust show that melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and become ultracalcic with increasing melt fraction, with strong compositional similarities to natural nepheline-normative ultracalcic melts. Such melts should thus originate by melting according to amphibole+clinopyroxene = melt+olivine in the arc crust at temperatures near 1200 °C. They reflect interactions between high-Mg basalts and crustal amphibole-bearing cumulates that are likely to melt during intrusion of hot high-Mg basalts, due to the characteristically low melting temperature of amphibole (~1100 °C).Des liquides ultracalciques primitifs (riches en CaO, pauvres en Al2O3) ont été observés dans la plupart des contextes géodynamiques. Les liquides ultracalciques à néphéline normative (uniquement observés dans les arcs) et les liquides ultracalciques à hypersthène normatif (associés à des basaltes tholeiitiques) ne présentent pas de "signature du grenat" et sont donc générés à P < 3 GPa. Les teneurs élevées en CaO jusqu'à 19,0 %) et les rapports CaO/Al2O3 élevés (jusqu'à 1,8) ne peuvent pas être obtenus par fusion de lherzolites fertiles. L'étude des relations de phases au liquidus de deux liquides ultracalciques (à néphéline et à hypersthène normatifs) montre qu'ils sont en équilibre avec olivine et clinopyroxène pour des conditions P/T très différentes. La comparaison de nos résultats et d'expériences de fusion publiées indique que les liquides à hypersthène normatif proviennent de la fusion d'une source réfractaire à olivine+clinopyroxène±orthopyroxène (lherzolite réfractaire ou cumulat wehrlitique) dans le manteau supérieur. Leur présence traduit des températures très élevées (~1400 °C). Les liquides ultracalciques à néphéline normative résultent de la fusion de wehrlites crustales pour des températures plus basses (1200 °C). Des expériences de fusion de wehrlites à amphibole (fréquemment observées en base de croûte d'arc) montrent que la composition des liquides est contrôlée par la déstabilisation de l'amphibole ; ils sont à néphéline normative et deviennent ultracalciques par augmentation du taux de fusion. Ces liquides sont très similaires aux liquides ultracalciques à néphéline normative naturels, qui sont probablement produits selon la réaction amphibole+clinopyroxène = liquide+olivine. La présence d'amphibole permet d'expliquer les teneurs élevées en alcalins et faibles en silice et les faibles températures de fusion. Ces liquides sont des témoins des interactions entre les basaltes magnésiens d'arc et les cumulats crustaux à amphibole

Experimental melting of phlogopite-bearing mantle at 1 GPa: Implications for potassic magmatism

International audienceWe have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000–1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts (Φ=1.4Φ=1.4 to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050–1075 °C, while phlogopite-breakdown occurs at 1150–1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite–lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite–harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from ∼3.9 wt.%∼3.9 wt.% in lherzolite to ∼6.7 wt.%∼6.7 wt.% in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5–52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field

Petrologic imaging of silicic to intermediate magma chambers through amphibole barometry

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Petrologic imaging of the magma reservoirs that feed large silicic eruptions

International audienceLarge silicic explosive eruptions are one of Nature's most hazardous phenomena. Very few have been witnessed and unravelling the complex conditions that lead to these eruptions remains a difficult task because the characteristics of the feeding magma reservoirs are still insufficiently constrained from geophysical imaging. Here we show that a barometer based on the composition of amphibole in equilibrium with plagioclase and biotite, common minerals in the eruptive products of large silicic eruptions, records the thickness and depth of magma reservoirs with uncertainties of 0.8 and 2.7 km, respectively. Pressures are given by the equation: P (MPa) = 892 · VIAl + 101, where VIAl is the octahedral aluminum content of the amphibole. With the example of a Miocene Turkish ignimbrite, we show that reservoirs feeding large silicic eruptions can be pancake-shaped. Our new barometer, valid between 650 and 950 °C, can be used in combination with volcanological and geophysical data to infer the size, shape and depth of magma reservoirs and may serve as a tool for monitoring future activity. This temperature-independent barometer is also applicable to any volcanic or plutonic rock containing amphibole + plagioclase + biotite and is in excellent agreement with previously published temperature-dependent barometers within their calibration range