Extensive CO2 degassing in the upper mantle beneath oceanic basaltic volcanoes: First insights from Piton de la Fournaise volcano (La Réunion Island)

Abstract

In spite of its major role on the atmospheric volatile budget, climate, and tracking magmatic transfers, mantle (CO 2 ) degassing below volcanoes is still poorly understood. Most of the studies on this scientific topic lack constraint on the CO 2 concentration of primary melts, the depth at which it starts degassing, and the extent of this process in the mantle. In this study of Piton de la Fournaise (PdF) volcano, we couple geochemistry of low solubility gases (He, Ar, CO 2 , d 13 C) in fluid inclusions (FIs) and petro-chemistry of magmatic inclusions on a set of olivine and clinopyroxene crystals from basalts and ultramafic enclaves.We constrain basaltic melt degassing at PdF over a large pressure range (from 4 GPa up to the surface). Based on CO 2 -He-Ar systematics, we infer that extensive degassing occurs already in the upper mantle (4-1 GPa) and it is favored by multiple steps of magma ponding and differentiation up to the mantle-crust underplating depth (0.4 GPa). Thus, we calculate that basaltic melts injected at crustal depth (<0.4 GPa) have already exsolved $94 ± 5 wt% of their primary CO 2 content in accordance with (1) the evolved and degassed signature of erupted lavas and (2) the weakness of inter-eruptive gas emissions in the active area bearing low-temperature vapor-dominated fumaroles. Our results at PdF strongly contrast with previous findings on other ocean island volcanoes having a higher magma production rate and faster magma ascent, like Kilauea (Hawaii), whose basalts experience only limited extent of differentiation and degassing. We propose that extensive degassing already in the upper mantle can be a common process for many volcanoes of the Earth and is tightly dependent on the dynamics of magma ascent and differentiation across multiple ponding zones.Based on the modeling developed in this study, we propose a new estimation of the CO 2 content (up to 3.5 ± 1.4 wt%) in primary basaltic melts at PdF leading to a carbon content in the mantle source of 716 ± 525 ppm. This new estimation is considerably higher than the few previous calculations performed for Ocean Island Basalts (OIB) systems. Another implication of this work involves the possible bias between the d 13 C measured in volcanic gas emissions (<À6‰) and that of primary vapour phase (À0.5 ± 0.5‰) constrained in this work. This bias would confirm the early step of extensive CO 2 degassing within the upper mantle and could represent an alternative for the hypotheses of carbon recycling or mantle heterogeneity in support of the low d 13 C signature of some mantle reservoirs. This study bears significant implications on the global budget of volcanic ⇑ Corresponding author at: Observatoire Volcanologique du Piton de la Fournaise ((G. Boudoire). volatile emissions, chiefly regarding the contribution of past and future emissions of volcanic CO 2 to climate dynamics, and on volcanic gas monitoring.

Gas Geochemistry at Grande Comore and Mayotte Volcanic Islands (Comoros Archipelago), Indian Ocean

“…The 4 He/ 40 Ar* values measured in Karthala (only one reliable value) and Mayotte gases vary in a narrow range (1.2–1.7), falling within that typical of fertile mantle ( 4 He/ 40 Ar* = 1–5; Marty, 2012) and magmatic values from other geodynamic settings (e.g., Boudoire, Rizzo, et al., 2018; Bräuer et al., 2011; Paonita et al., 2012; Rizzo et al., 2019). In magmatic environments, this ratio varies during melts degassing and is indicative of relative entrapment pressures (e.g., Boudoire, Rizzo, et al., 2018; Paonita et al., 2012). Focusing on Mayotte gases for which 4 He/ 40 Ar* is available for different gas emissions and surveys dates, we do not notice systematic variations.…”

“…Gas geochemistry of fumarolic fields at Karthala (Grande Comore), and bubbling gases at Mayotte fall within the typical range of MORB‐type mantle source. Compared with La Reunion data set (Boudoire, Finizola, et al., 2018; Boudoire, Rizzo, et al., 2018; Liuzzo et al., 2015), the Comoros islands data set shows a CH 4 enrichment, and a variable degree of air contamination.…”

Melt inclusion constraints on mantle carbon heterogeneity within an individual mantle plume and at the global scale

“…An alternative, but complementary, approach to constraining CO 2 recycling efficiency is to estimate the amount of carbon within recycled mantle domains where they are sampled at ocean islands. CO 2 -noble gas systematics of fluid inclusions in material erupted in Hawaii and La Réunion Island indicate their mantle sources are carbon rich (Trull et al, 1993;Boudoire et al, 2018), which is further supported by models that couple CO 2 emission rates with magma supply rates at Hawaii (Anderson and Poland, 2017) and high CO 2 contents of primitive melts (Tucker et al, 2019). However, both Hawaii and La Réunion are underlain by mantle plumes transporting primordial (high 3 He/ 4 He) mantle, which may contain high concentrations of carbon (Miller et al, 2019).…”

Recommendations and Protocols for the Use of the Isotope Ratio Infrared Spectrometer (Delta Ray) to Measure Stable Isotopes from CO2: An Application to Volcanic Emissions at Mount Etna and Stromboli (Sicily, Italy)

“…Without the dilution system, the analysis of Exetainer vials appears compromised and requires the use of other more traditional techniques such as the IRMS. A similar recommendation is also available for gas released by crystal crushing whose CO 2 concentrations are generally <2000 ppm in a 16 mL glass tube [37,38].…”