Experimental Determination of Oxygen-isotope Fractionations Between CO_2 Vapour and Soda-melilite Melt

Oxygen-isotope measurements of igneous rocks and minerals can constrain their origin and evolution, but they are interpretable only in light of known ^(18)O/^(16)O fractionations among co-existing silicate melt, minerals, and magmatic volatiles. There have been many experimental studies of mineral-mineral and mineral-fluid fractionations but few involving silicate melt, particularly basic and ultrabasic melts relevant to study of mantle geochemistry and basaltic volcanism (Muehlenbachs & Kushiro, 1974). Consequently, fractionations involving such melts are generally estimated based on the systematics of differences in δ^(18)O between phenocrysts and co-existing glass or groundmass. Estimates of this type, although useful, are insufficient for interpreting the usually subtle oxygen isotope variations observed in many basaltic suites. Therefore, we have undertaken a series of experiments examining oxygen-isotope fractionations involving basic silicate melts. We report here measurements of fractionations between the CO_2 and soda-melilite melt. This composition has been used previously as an analog for basic silicate melts

Experimental determination of oxygen isotope fractionations between CO_2 vapor and soda-melilite melt

We report results of experiments constraining oxygen isotope fractionations between CO_2 vapor and Na-rich melilitic melt at 1 bar and 1250 and 1400°C. The fractionation factor constrained by bracketed experiments, 1000.lnα_(CO2-Na melilitic melt), is 2.65±0.25 ‰ (±2σ; n=92) at 1250°C and 2.16±0.16 ‰ (2σ; n=16) at 1400°C. These values are independent of Na content over the range investigated (7.5 to 13.0 wt. % Na_2O). We combine these data with the known reduced partition function ratio of CO2 to obtain an equation describing the reduced partition function ratio of Na-rich melilite melt as a function of temperature. We also fit previously measured CO_2-melt or -glass fractionations to obtain temperature-dependent reduced partition function ratios for all experimentally studied melts and glasses (including silica, rhyolite, albite, anorthite, Na-rich melilite, and basalt). The systematics of these data suggest that reduced partition function ratios of silicate melts can be approximated either by using the Garlick index (a measure of the polymerization of the melt) or by describing melts as mixtures of normative minerals or equivalent melt compositions. These systematics suggest oxygen isotope fractionation between basalt and olivine at 1300°C of approximately 0.4 to 0.5‰, consistent with most (but not all) basalt glass-olivine fractionations measured in terrestrial and lunar basalts

Écosystèmes tropicaux: Actes du 2ème colloque de restitution du programme de recherche

Le Ministère chargé de l’environnement a lancé en 1999 le programme de recherche « écosystèmes tropicaux » avec l’appui du GIP ECOFOR. Ce programme vise à une meilleure connaissance des écosystèmes appliquée à la gestion et la conservation dans les départements, pays et les territoires d’outre-mer et leur environnement régional. Son objectif principal est de promouvoir une recherche finalisée sur le thème de la diversité biologique et de son utilisation dans le cadre des écosystèmes tropicaux dans les départements, pays et territoires d’outre-mer. Il s’agit de mettre au point les outils et les méthodes qui permettront aux pouvoirs publics d’optimiser les stratégies de préservation du patrimoine naturel et de sa biodiversité, en accord avec les usages des populations locales, dans une perspective de développement durable

The evolution of magma during continental rifting: New constraints from the isotopic and trace element signatures of silicic magmas from Ethiopian volcanoes

Magma plays a vital role in the break-up of continental lithosphere. However, significant uncertainty remains about how magma-crust interactions and melt evolution vary during the development of a rift system. Ethiopia captures the transition from continental rifting to incipient sea-floor spreading and has witnessed the eruption of large volumes of silicic volcanic rocks across the region over ∼45 Ma. The petrogenesis of these silicic rocks sheds light on the role of magmatism in rift development, by providing information on crustal interactions, melt fluxes and magmatic differentiation. We report new trace element and Sr–Nd–O isotopic data for volcanic rocks, glasses and minerals along and across active segments of the Main Ethiopian (MER) and Afar Rifts. Most δ18Odata for mineral and glass separates from these active rift zones fall within the bounds of modelled fractional crystallization trajectories from basaltic parent magmas (i.e., 5.5–6.5h) with scant evidence for assimilation of Pan-African Precambrian crustal material (δ18Oof 7–18h). Radiogenic isotopes (εNd= 0.92–6.52; 87Sr/86Sr = 0.7037–0.7072) and incompatible trace element ratios (Rb/Nb 100 km3), and estimate that crystal cumulates fill at least 16–30% of the volume generated by crustal extension under the axial volcanoes of the MER and Manda Hararo Rift Segment (MHRS) of Afar. At Erta Ale only ∼1% of the volume generated due to rift extension is filled by cumulates, supporting previous seismic evidence for a greater role of plate stretching in mature rifts at the onset of sea-floor spreading. We infer that ∼45 Ma of magmatism has left little fusible Pan-African material to be assimilated beneath the magmatic segments and the active segments are predominantly composed of magmatic cumulates with δ18O indistinguishable from mantle-derived melts. We predict that the δ18Oof silicic magmas should converge to mantle values as the rift continues to evolve. Although current data are limited, a comparison with ∼30 Ma ignimbrites (with δ18Oup to 8.9h) supports this inference, evidencing greater crustal assimilation during initial stages of rifting and at times of heightened magmatic flux