Marine Tephrostratigraphy: a Powerful Geological Tool to Better Understand the Eruptive History of Montagne Pelée (Martinique, Lesser Antilles).

Abstract:

In the tropical volcanic island environments like the Lesser Antilles Arc, the reconstruction of the volcanic history in term of frequency, chronology and eruptive dynamics is particularly difficult using on-land data, because of the scarcity of outcrops and the extensive vegetation. Additionally, the high erosion rate and the large volcanic flank-collapse events that are recurrent in this region, both favor the destruction of onshore sedimentary records.

However, tephrostratigraphy of marine deposits can represent a most useful tool to complete these gaps.

Site U1397 of IODP Expedition 340 (R/V JOIDES Resolution, March–April 2012) collected offshore Martinique island, was studied to much better constrain the chronology and the magmatic features of eruptions in the recent eruptive history of Martinique and south Dominica. Pristine fragments of volcanic glasses have been studied for morphological and in situ geochemical analyses, performed respectively by SEM, EPMA and LA-ICP-MS/MS, and dated by the stable isotope stratigraphy.

More than a two hundred of tephra layers have been previously identified shipboard during the visual core descriptions in the first ~ 30 m of the core. Post-cruise microscopic observation suggests that only 75% of the 200 tephra have been properly defined and the remaining 25% appears to have been over recorded.

Four type of volcanic clasts are observed in tephra layers: dense to poorly vesiculated glassy clasts (micropumices), dark scoria clasts, fresh crystals and volcanic lithic fragments.

Major geochemical fingerprints suggest that all tephra layers record the most complete volcanic activity of Montagne Pelée during the last ~120 ka, period before which the volcanic activity was located at Mont Conil. In particularly, a new eruptive phase is evidenced by the co-presence of dark scoria fragments and white micropumices and the greater abundance of mafic minerals such as amphibole. These products, heterogeneous in the textures, are characterized by a bimodal composition, from andesite to rhyolite (60%-78% SiO₂) and might result from magma mixing process. This new volcanic phase, called Grand Rivière event, is dated from 22 to ~32 ka and could be closely linked to the earlier volcanic edifice collapse, occurred at ~35 ka that possibly favored the onset of more mafic magmas.