Geochemical Characteristics of the Lavas from the “Adventive Cones” of Piton de La Fournaise Volcano (La Reunion Island)

Abstract:

Piton de la Fournaise, the youngest volcano of La Réunion Island, is renowned for being frequently active. Whereas the current activity is mainly located within the “Enclos Fouqué” caldera, ~100 strombolian cones lie on the volcano’s flanks, thought to date from ~300 years to a few thousand years. Our study focuses on these “adventive cones”, by studying bulk-rock major and trace element compositions, mineral phase compositions and olivine-hosted melt inclusions. The Piton de la Fournaise lavas (younger than ~450 ka) have been subdivided into three compositional groups (see attached figure, and Lénat et al. 2012). Almost all recent and historical lavas belong to two of these groups: “cotectic basalts” and “olivine-rich basalts”, marked by a constant CaO/Al₂O₃ ratio of ~0.8, and MgO content ranging from 5 to 30 wt % reflecting different degrees of olivine accumulation. The third group, called here “mid-alkaline basalts”, corresponds to compositions commonly encountered for the “adventive cones”. It mainly consists of magnesian basalts at 7.55 - 10.24 wt% MgO and CaO/Al₂O₃ values down to 0.55. At constant MgO content, this group shows higher alkali content and a relative deficiency in Ca compared to the historic basalts. The “adventive cones” lavas usually contain magnesian olivines (Fo > 86). Such crystals are not at the equilibrium with their host lava, raising thus the question of the recycling processes. According to Bureau et al. (1998; 1999), magnesian olivines come from deep storage levels. The specific geochemistry of the “adventive cones” lavas is attributed either to a high-pressure fractionation of a clinopyroxene-rich assemblage (Albarède et al. 1997), or to an assimilation process involving wehrlite-gabbro cumulates (e.g. Salaün et al. 2010). Our new data show that the source of these magmas is chemically homogeneous to that of current magmas. However, their ascent clearly bypasses the current lava reservoirs, especially the shallow magma chamber.