Textural and petrologic study of the subvolcanic mafic and ultramafic ejecta of Piton de la Fournaise as a key to unravel the shallow magmatic crystallization processes at La Réunion Island (Indian Ocean).

The Réunion Island consists of two juxtaposed volcanic massifs, the oldest Piton des Neiges (in the north-west) and the youngest, presently active Piton de la Fournaise (in the south-east). A third, poorly known buried volcano, named Les Alizés was mainly defined through exploratory drilling and constitutes the basement of Piton de la Fournaise (Lénat et al., 2012). The construction of Piton de la Fournaise sensu stricto started at about 400-450 ka. Its lavas are mainly transitional basalts defining a differentiation trend towards mugearites, intermediate in character between the alkaline and tholeiitic series, both aphyric (“cotectic basalts”) and olivine-rich (“oceanites”). They slightly differ from Les Alizés aphyric basalts (“abnormal group”) that belong to the more alkalic (K2O-rich) series, with higher MgO and lower CaO and SiO2 contents. The moderate chemical heterogeneity has been since long attributed to a combination of slight mantle source heterogeneity and influence of clinopyroxene fractionation at mantle/underplating depth. Magmas stored at crustal depths and erupted at the summit area are quite homogeneous and result from the hybridization process of “tholeiitic” and “alkaline” end members (Di Muro et al., 2014; Boudoire et al., 2018; Vlastelic et al., 2018). The present Ph.D. work deals with a petrological study of mafic and ultramafic ejecta of the “Bellecombe Ash Member” (BAM, ≈ 5.5-3.0 ka), in order to unravel the pre-eruptive conditions of basalt crystallization and possibly matching the various recognized petrographic groups of the subvolcanic igneous rocks with the whole extrusives (Piton de la Fournaise, Piton des Neiges, Les Alizés) combining mineralogy, petrographic texture and geochemistry. Large caldera forming events such as that of the Bellecombe Ash Member, the largest explosive activity produced by Piton de la Fournaise involve, as ejecta, a large fraction of the crustal magmatic plumbing system and allowed to i) access to a rare snapshot of the magmatic processes (cumulate vs. liquid composition; volatile exsolution and percolation; melt and rock assimilation) occurring below one of the most active basaltic volcanoes of the world, and ii) study the magmas driving these rare but very hazardous geological events. The petrological study of more than one hundred samples, representative of the ejecta within the Bellecombe Ash Member, emphasizes the presence of many subvolcanic mafic and ultramafic igneous rocks: dunites, wherlites, ophitic gabbros, sub-ophitic gabbros, poikilitic gabbros, doleritic gabbros, micro-monzogabbros and porphyrogabbros. Cumulate cognates sensu stricto are present, often with quenched basaltic glass entrapped interstitially and in minerals (melt inclusions), with no disequilibrium textures recognized in the subvolcanic crystal frameworks. Other subvolcanic clasts (e.g. doleritic gabbros) may simply represent slowly-cooled equivalents (with no crystal-liquid fractionation) of the erupted cotectic basalts. Chemistry of both minerals (olivine, clinopyroxene, feldspar, oxides and phlogopite) and interstitial glass from the subvolcanic ejecta has been performed by EMPA. It is worth to note that porphyrogabbros emphasize a wide range of feldspars, as interstitial crystals between the large olivine phenocrysts, represented by plagioclase, anorthoclase and sanidine, defining a complete miscibility feldspar curve. Olivines of each mafic and ultramafic lithotype ejecta show different forsterite contents and the whole compositional range is from Fo69 (sub-ophitic gabbros) to Fo90 (dunite), with an overall range in CaO contents from 0.05 to 0.4 wt.%. A fluid inclusion chemical and thermobarometric study (CO2, H2O and noble gases) was made on selected olivine, clinopyroxene, and feldspar crystals. Noble gases and CO2 content in fluid inclusions of the studied ejecta reveal variable concentrations and elemental ratios (e.g. He/Ar*), indicating, for the whole mafic and ultramafic suite of the Piton de la Fournaise magmatic plumbing system a wide range of depths, from the underplating zone to the submarine base of the edifice and various extent of magmatic degassing. The above barometric data have been also confirmed by fluid inclusion study by Raman spectroscopy analyses. A comprehensive geochemical comparison among the analysed mafic and ultramafic ejecta (major and trace elements by ICP-OES-MS) and the whole compositional spectrum of the Piton de la Fournaise and Les Alizés volcanic rocks available from literature was also carried out. Major and trace elements geochemistry allowed to confirm the three fractional crystallization trends already defined by the literature, namely gabbro- wherlite- and dunite-fractionations which well agree with the erupted basalts in the Réunion Island in the last 500 ka. A wide range of textures in these ejecta results from variable P-T gradients in the subvolcanic environment of crystallization. In particular the CO2-H2O fluid inclusion and thermobarometric studies carried out by Raman spectroscopy on selected single crystals (olivine and clinopyroxene) permitted to better understand the depths of crystallization of the samples. The study of the mafic and ultramafic subvolcanic igneous rocks of Piton de la Fournaise entrapped in the BAM eruption allow to target the transcrustal plumbing system still active during large caldera forming events. Compositions of the interstitial glass and minerals in some subvolcanic ejecta has been also compared with small volumes of evolved liquids (trachytes) detected as pumiceous components in the products of the Bellecombe Ash Member caldera-forming eruption and the old Piton des Neiges volcano. In general, the textural and petrologic study of the subvolcanic ejecta of Piton de la Fournaise provides new insights on the magmatic plumbing system and the subvolcanic crystallization processes at La Réunion Island. Some peculiarities on the processes of crystallization in the magmatic plumbing system were pointed out, with respect previous knowledges. Some ejecta defined as porphyrogabbros could represent oceanite magmas stored at crustal levels where, the interstitial basaltic melts supporting the abundant olivine crystals gave rise to a sanidine- and phlogopite-bearing micro-cryptocrystalline groundmass. This micro-cryptocrystalline groundmass could be really considered cogenetic with trachytic liquid, which may be erupted as glassy pumice if the undercooling would be high enough to preclude groundmass crystallization. The overall textural and geochemical data of the investigated mafic-ultramafic ejecta, do not only reflect different mode of subvolcanic crystallization (cumulate sensu stricto vs. slowly-cooled equivalents of the steady state basalts which are represented by the doleritic gabbro and some of the subophitic gabbro ejecta) but also (i) the interaction between basaltic liquids and cumulates already formed at crustal level of the Piton de la Fournaise plumbing system and (ii) the possibility that small pockets of liquids may also evolve to trachytic compositions. The compositional range from basalt to basaltic trachyandesite of the interstitial glass found in the intrusive clasts, indicate that the melt was entrapped within the crystal frameworks and quenched during the eruption. This likely suggests the clast were disrupted from the wall rocks and transported very fast to the surface, mostly as cognates. If not entrapped interstitially during in situ crystallization, silicate liquid could be also expelled from the crystal frameworks, forming separated melt pockets or larger body, e.g. slightly differentiated sills. Fluid inclusion barometry (Raman analyses) and noble gases and CO2 from fluid inclusion crushing extraction and mass spectrometer analyses indicate the dunites are the intrusive ejecta from the deepest levels, approximately at the underplating levels of formation (~13 km b.s.l.). Wherlites should have crystallized close to the submarine base of the edifice (~6 km b.s.l.), whereas the mafic subvolcanic samples are even shallower. Nevertheless, a large composite intrusive ejecta (B108) emphasizes the coexistence of mesoscale ultramafic-mafic patchy microdomains (wherlite to plagioclase-wherlite to troctolite to gabbro) at intermediate crustal level of crystallization, below 0.3 GPa according to the liquidus curves of the La Reunion magmas (Fisk et al., 1988). Textures in this composite ejecta can be the result of basaltic melts interacting with wherlitic cumulates at relatively low pressure, leading to the instability of clinopyroxene and high dissolution of olivines (now highly rounded), with a high loss of temperature of the system. In this way at the same pressure, but lower temperature, plagioclase is the liquidus phase and therefore wherlitic cumulates interacting with basaltic melts at relatively low pressure can produce plagioclase-wherlite and troctolite patchy microdomains at the mesoscale. Similar interactions between batches of basaltic magmas and cumulates already present in the magmatic plumbing system are also highlighted by other wherlites (i.e. B28 sample) coupled with olivine dissolution/reaction leading also to clinopyroxene. Finally, the presence of loose quartz xenocrysts in products at the Langevin Caldera, possibly with aqueous inclusions with low-salinity and low CO2 content and an enrichment of some trace elements (e.g. Li, Al, Sr) at their rims could suggest the involvement of the hydrothermal system of Piton del la Fournaise during explosive eruptions of the volcano.