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12 September 2019 à 11 h 00 min - 12 h 00 min

Study of primitive olivine-hosted melt inclusions from the Ecuadorian segment of Northern Volcanic Zone


The contribution of the different geochemical reservoirs (slab-mantle-crust) to arc magma genesis is a long-lasting discussion in the scientific community. The nature of the initial slab component triggering the melting of the mantle wedge is difficult to constrain since arc magmas undergo compositional changes during their crustal residence before eruption. The adakite-signature and high Th/Nb and Th/La of magmas from Ecuador are thought to beinherited from a hydrous siliceous melt slab component or from a lower- to mid-crustal process of mantle melts.

We present new analysis of major, trace and volatile elements of a group of relatively rare melt inclusion (MI) hosted in primitive olivines (Fo_80-90 ) from 5 Ecuadorian volcanoes that sit upon a ~50 km-thick crust. These MIs come from Cotacachi, Cubilche and Sangay volcanoes, as well as Tulabug and Cono de la Virgen (CLV) cones. These MIs display ultrabasic to basic compositions (42.0-51.8 wt.%. SiO_2 ) and fall_in the field of basalts, trackybasalts and tephrite basanites in the TAS diagram. On the basis of trace elements, we recognize two different slab components added to the mantle wedge below the Ecuadorian arc. One that is rich in fluid-mobile (Ba/La: 30-110 and Pb/Ce: 0.1-0.5, B/Nb: up to 22) and poor in fluid-immobile elements (La/Nb: 2.2-4, Th/Nb: up to 0.7) observed everywhere, except for MIs coming from CLV. The other that is relatively enriched in fluid-immobile elements (La/Nb:7.2-15.5, Th/Nb: up to 3.2) and has low fluid-mobile elements contents (Ba/La: 15-36 and Pb/Ce: 0.02-0.1, B/Nb: up to 4.4). Using arc basalt simulator 3 (Kimura et al. 2010), we found that one group of MI (from Cotacachi, Cubilche, Tulabug and Sangay) compositions are consistent with the addition to the mantle wedge of 2-5% of an aqueous fluid-like slab component and a second group (CLV) with the addition of 10-20% of a hydrous siliceous melt. This work reveals that high (La/Yb)_N, Th/Nb and Th/La is a characteristic of some primitive magmas before any crustal interaction affects their composition.

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12 September 2019
11 h 00 min - 12 h 00 min
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