Upper Mantle Mush Zones beneath Low Melt Flux Ocean Island Volcanoes: Insights from Isla Floreana, Galápagos

Abstract

The physicochemical characteristics of sub-volcanic magma storage regions have important implications for magma system dynamics and pre-eruptive behaviour. The architecture of magma storage regions located directly above high buoyancy flux mantle plumes (such as Kīlauea, Hawai’i and Fernandina, Galápagos) are relatively well understood. However, far fewer constraints exist on the nature of magma storage beneath ocean island volcanoes that are distal to the main zone of mantle upwelling or above low buoyancy flux plumes, despite these systems representing a substantial proportion of ocean island volcanism globally. To address this, we present a detailed petrological study of Isla Floreana in the Galápagos Archipelago, which lies at the periphery of the upwelling mantle plume and is thus characterised by an extremely low flux of magma into the lithosphere. Detailed in situ major and trace element analyses of crystal phases within exhumed cumulate xenoliths, lavas and scoria deposits, indicate that the erupted crystal cargo is dominated by disaggregated crystal-rich material (i.e., mush or wall rock). Trace element disequilibria between cumulus phases and erupted melts, as well as trace element zoning within the xenolithic clinopyroxenes, reveals that reactive porous flow (previously identified beneath mid-ocean ridges) is an important process of melt transport within crystal-rich magma storage regions. In addition, application of three petrological barometers reveal that the Floreana mush zones are located in the upper mantle, at a depth of 23.7 ± 5.1 km. Our barometric results are compared to recent studies of high melt flux volcanoes in the western Galápagos, and other ocean island volcanoes worldwide, and demonstrate that the flux of magma from the underlying mantle source represents a first-order control on the depth and physical characteristics of magma storage.

Geochemical Constraints on the Structure of the Earth's Deep Mantle and the Origin of the LLSVPs

“…The extreme radiogenic isotope composition of Floreana (that is, elevated 206 Pb/ 204 Pb and 187 Os/ 188 Os ratios) is used to define the FLO mantle end-member, which displays similar 206 Pb/ 204 Pb signatures to the global HIMU mantle (Gibson et al, 2016;Harpp, Geist, et al, 2014). As a result, the FLO mantle is hypothesized to represent recycled Archean oceanic crust (Gibson et al, 2016;Harpp, Geist, et al, 2014), although the absence of evidence for a pyroxene-rich component in the mantle source of the Floreana lavas casts doubt on the recycled crustal origin (Gleeson, Gibson, & Stock, 2020;Vidito et al, 2013).…”

“…Taken at face value, the Ca content (and to a lesser extent the Ni content) of olivines in basalts from Floreana in the southern Galápagos, which originates from melting of LLSVP material with highly radiogenic Pb isotope signatures (Harpp & Weis, 2020), suggests that there is a notable contribution from melts from a pyroxenitic source component (Gleeson, Gibson, & Stock, 2020;Harpp, Geist, et al, 2014;Vidito et al, 2013). However, the low Ca contents and moderately high Ni contents observed in some of the Floreana olivines can instead be explained by chemical modification in a cumulate mush (Gleeson, Gibson, & Stock, 2020). As such, there is no significant evidence in the olivine minor element systematics of the Floreana basalts to indicate that there is a substantial contribution of melts from a pyroxenitic source component (Figure 3).…”

Constraints on the behaviour and content of volatiles in Galápagos magmas from melt inclusions and nominally anhydrous minerals

“…analyses (Gleeson et al, 2020a). The melt inclusions and embayments have a similar range of FeOt contents to the Floreana whole-rock (~ 8 -10.5 wt%) and are in Mg-Fe disequilibrium with their olivine hosts (i.e., melt inclusion glass Mg#, where Mg# = Mg/(Mg + Fet) molar, are typically lower than expected based on the composition of the host olivines; Supplementary Information).…”

“…This manuscript represents a pre-print of a manuscript that has undergone peer-review and has been accepted for publication with Geochimica et Cosmochimica Acta. Gleeson et al (2020a) where EPMA spots were located as close as possible to the SIMS pits. For glass analyses, spots were also placed close to the SIMS pits and the EPMA was calibrated with appropriate mineral and metal standards (see Gleeson and Gibson, 2019 for details).…”

“…This manuscript represents a pre-print of a manuscript that has undergone peer-review and has been accepted for publication with Geochimica et Cosmochimica Acta. (Gleeson et al, 2020a;Ubide et al, 2014;Wieser et al, 2019), detailed petrological characterisation is required to understand the relationship between the erupted crystal cargo and carrier melt before…”