Intermediate and deep ocean current circulation in the Mozambique Channel: New insights from ferromanganese crust Nd isotopes

The Mozambique Channel plays a key role in the exchange of water masses between the Indian and Atlantic Oceans, which include the North Atlantic Deep Water (NADW) inflow from the south and the North Indian Deep Water (NIDW), an aged form of the NADW spreading poleward from the northern and equatorial Indian Ocean basin. Several authors assume that the Davie Ridge acts as a topographic barrier to the northward advection of NADW, which would therefore be absent in the Comoros Basin. Other studies suggest that the NADW flows from the south of the Mozambique Channel to the Comoros Basin, indicating that the Davie Ridge may not currently constitute a blocking topographic barrier to deep water mass circulation. To address this question, we studied ferromanganese (Fe, Mn) crusts collected over 2000 km in the Mozambique Channel, from the Agulhas Plateau to the Glorieuses Islands. Neodymium (Nd) isotope compositions (εNd) of surface scrapings range between εNd = −10.1 above the Agulhas Plateau, which might reflect the NADW inflow, and more radiogenic values between εNd = −8.0 and − 8.2 in the Glorieuses area, highlighting the NIDW influence. However, value of εNd = −9.4 measured north of the Davie Ridge cannot be explained by the sole influence of the NIDW and therefore highlights the advection of the NADW northeast of the Comoros Basin. We estimate that the contribution of the NADW through the channel is up to 68% in the Agulhas Plateau and 60% north of the Davie Ridge. These findings are consistent with previous hydrographic studies and suggest that the Davie Ridge does not currently act as topographic barrier to deep currents

The temporal evolution of back-arc magmas from the Auca Mahuida shield volcano (Payenia Volcanic Province, Argentina)

In order to better constrain the temporal volcanic activity of the back-arc context in Payenia Volcanic Province (PVP, Argentina),we present newK?Ar dating, petrographic data,major and trace elements from23 samples collected on the Auca Mahuida shield volcano. Our new data, coupled with published data, show that this volcano was built from about 1.8 to 1.0 Ma during five volcanic phases, and that Auca Mahuida magmas were extracted from, at least, two slightly different OIB-type mantle sources with a low partial melting rate. The first one, containing more garnet, was located deeper in the mantle, while the second contains more spinel and was thus shallower. The high-MgO basalts (or primitive basalts) and the low-MgO basalts (or evolved basalts), produced from the deeper and shallower lherzolite mantle sources, respectively, are found within each volcanic phase, suggesting that both magmatic reservoirs were sampled during the 1 Myr lifetime of the Auca Mahuida volcano. However, a slight increase of the proportion of low MgO basalts, as well as of magmas sampled from the shallowest source, can be observed through time. Similar overall petrological characteristics found in the Pleistocene?Holocene basaltic rocks from Los Volcanes and Auca Mahuida volcano suggest that they originated from the same magmatic source. Consequently, it can be proposed that the thermal asthenospheric anomaly is probably still present beneath the PVP. Finally, our data further support the hypothesis that the injection of hot asthenosphere with an OIB mantle source signature, which was triggered by the steepening of the Nazca subducting plate, induced the production of a large volume of lavas within the PVP since 2 Ma.Fil: Pallares, Carlos. Centre National de la Recherche Scientifique; FranciaFil: Quidelleur, Xavier. Centre National de la Recherche Scientifique; FranciaFil: Gillot, Pierre-Yves. Centre National de la Recherche Scientifique; FranciaFil: Kluska, Jean-Michel. TOTAL SA; FranciaFil: Tchilinguirian, Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Sarda, Philippe. Centre National de la Recherche Scientifique; Franci

VLTI/PIONIER survey of disks around post-AGB binaries. Dust sublimation physics rules

Context.Post-asymptotic giant branch (pAGB) binaries are surrounded by circumbinary disks of gas and dust that are similar toprotoplanetary disks found around young stars.Aims.We aim to understand the structure of these disks and identify the physical phenomena at play in their very inner regions. Wewant to understand the disk-binary interaction and to further investigate the comparison with protoplanetary disks.Methods.We conducted an interferometric snapshot survey of 23 post-AGB binaries in the near-infrared (H-band) usingVLTI/PIONIER. We fit the multi-wavelength visibilities and closure phases with purely geometrical models with an increasingcomplexity (including two point-sources, an azimuthally modulated ring, and an over-resolved flux) in order to retrieve the sizes,temperatures, and flux ratios of the different components.Results.All sources are resolved and the different components contributing to theH-band flux are dissected. The environment ofthese targets is very complex: 13/23 targets need models with thirteen or more parameters to fit the data. We find that the inner diskrims follow and extend the size-luminosity relation established for disks around young stars with an offset toward larger sizes. Themeasured temperature of the near-infrared circumstellar emission of post-AGB binaries is lower (Tsub∼1200 K) than for young stars,which is probably due to a different dust mineralogy and/or gas density in the dust sublimation region.Conclusions.The dusty inner rims of the circumbinary disks around post-AGB binaries are ruled by dust sublimation physics.Additionally a significant amount of the circumstellarH-band flux is over-resolved (more than 10% of the non-stellar flux is over-resolved in 14 targets). This hints that a source of unknown origin, either a disk structure or outflow. The amount of over-resolvedflux is larger than around young stars. Due to the complexity of these targets, interferometric imaging is a necessary tool to reveal theinteracting inner regions in a model-independent way.status: accepte

Volcanic rock alterations of the Kwanza Basin, offshore Angola - Insights from an integrated petrological, geochemical and numerical approach.

18 pagesInternational audienceThe Lower Cretaceous presalt section of the Kwanza Basin (Angola) is in the spotlight following the discoveries of petroleum systems in this basin, and more generally in the South Atlantic. These systems are mostly composed of continental carbonates in close association with volcanic rocks. This work is focused on the study of an offshore Kwanza presalt volcanic sequence characterized as Valanginian trachytic subaerial lava flows. A detailed petrological analysis of the altered trachyte in association with fluid inclusion microthermometry was conducted in order to depict the initial mineralogy (albite, sanidine, titanomagnetite) and obtain a paragenetic sequence (quartz, siderite, kaolinite, calcite). Thermodynamic equilibrium modelling of the trachytes alteration by meteoric fluids, over a range of temperatures (25 °C–200 °C) and CO2 partial pressure (pCO2: 0.01 mbar to 100 bar), were performed with PHREEQC, and compared to the observed paragenetic sequence. Some numerical simulations reflect the observed paragenesis. As a result, the pCO2 is constrained by the occurrence of siderite (from 0.1 bar at 50 °C to 30 bar at 125 °C) and kaolinite (from 0.2 bar at 50 °C to 1.2 bar at 125 °C). The simulations emphasize the need for a high pCO2 in the hydrothermal system, to achieve the observed trachyte transformation. After reaching equilibrium with the trachytes, the simulated fluids highlight a mid-alkaline to near neutral pH with high Fe, HCO3+CO3, and alkali concentrations. The palaeofluids could have evolved from Ca- and Mg-rich to Ca- and Mg-poor with increasing temperature. Inversely, Si concentrations are positively correlated with increasing temperatures. This methodology, integrating a petrological approach and numerical simulations, proves to be a powerful tool leading to better understanding of the proxies (pCO2, temperature, redox conditions) controlling paragenesis. To push further, these simulations are also a step toward improved understanding of palaeofluid evolutions in presalt systems and better prediction of reservoir quality

African Superplume Composition: Insight from the Mozambique Margin

International audienceThe Karoo Large Igneous Province is one of the most extensively studied continental trapp [1, 2 and reference therein]. Nonetheless the nature, composition and origin of some parts of this province, and principally its youngest portions outcropping within the Lebombo province remain unclear [3]. Moreover the relationships between this province, the volcanic Mozambique oblique margin formation and the Gondwana break up are still highly debated. To better understand the composition of the mantle source(s) from which the melts originated and their geodynamic implication from Jurassic times to oceanisation within the Mozambique Channel we studied volcanic and plutonic samples from several drill holes within the South MozambicanCoastal Plain (SMCP) as well as samples from the Movene and the Bumbeni complex areas. Weconducted major, trace elements and radiogenic isotopes (Sr, Nd, Pb) analyses as well as 40Ar/39Ar dating. Our results clearly demonstrate that the SMCP basement is composed of continental lithosphere more or less modified by mantle magmatism as expected in a continental flood basalt setting. Conversely to some previous interpretations no oceanic crust is present within the SMCP which has important implications for the initial Gondwana fit[4].Our results indicate that a plume-type mantle source, more or less influenced by Sub-Continental Lithospheric Mantle and Upper Depleted ambient Mantle, is involved in the formation of all Karoo-related magmatism episodes in the area. The isotope composition of the African mantle plume is highly comparable to the one of the Afar plume suggesting that the thermal anomaly may have been active for about 200 Ma. [1] Cox, (1988) InContinental Flood Basalts, Spinger; [2] Jourdan et al.,(2007), J. Petrol.,48:1043-1077; [3] Melluso et al., (2008), J. South Af. Earth Sci.52:139-151; [4] Thompson et al., (2019) Earth Science Reviews191:26-5

Magma genesis controlled by tectonic styles in the northern part of the Arabia plate during Cenozoic time

<p>Widespread lava fields in the northern part of the Arabian platform are the subject of an open geodynamic debate on the origin of the intraplate volcanism. We present new K–Ar ages and whole-rock geochemical data for lava flows from Syria, which allow us to propose a new model of volcano-tectonic evolution highlighting how tectonics have controlled magma genesis in the region during the last 18 Ma. The Cenozoic Syrian lavas are alkaline and subalkaline rocks, typical of magma erupted in such a continental intraplate context. Basaltic samples from different Syrian volcanic provinces show significant variations in terms of incompatible trace element signatures. Crustal contamination plays a negligible role during magma migration and differentiation, and crystal fractionation is essentially restricted to olivine and clinopyroxene. Our results suggest that the Syrian lavas have been generated by variable degrees of partial melting (<em>c.</em> 1–10%) from different levels of a locally heterogeneous lithospheric mantle. The light/medium rare Earth element (LREE/MREE) ratios not only illustrate how the degree of partial melting has changed spatially and temporally during the last <em>c.</em> 18 Ma, but also indicate that the degree and the style of extensional tectonics has changed through time. We conclude that the Cenozoic Syrian volcanism is a consequence of extensional tectonics, under periodical influence of the north- and eastwards convergence at the Arabia–Eurasia margin, which induces rotational tectonic styles. This controls the partial melting at various depths in the mantle. The volcanism of northern Arabia developed in the framework of the Red Sea rifting and initiated at the same time as the southern Red Sea volcanism. It extends up to historical time, progressively smoothed to the north in a contradictory relation with the compressional/extensional setting of the Arabia–Eurasia margin. </p

A triple-star system with a misaligned and warped circumstellar disk shaped by disk tearing

Young stars are surrounded by a circumstellar disk of gas and dust, within which planet formation can occur. Gravitational forces in multiple star systems can disrupt the disk. Theoretical models predict that if the disk is misaligned with the orbital plane of the stars, the disk should warp and break into precessing rings, a phenomenon known as disk tearing. We present observations of the triple-star system GW Orionis, finding evidence for disk tearing. Our images show an eccentric ring that is misaligned with the orbital planes and the outer disk. The ring casts shadows on a strongly warped intermediate region of the disk. If planets can form within the warped disk, disk tearing could provide a mechanism for forming wide-separation planets on oblique orbits.status: publishe