Lower Oceanic Crust from the Ultraslow-Spreading Gakkel Ridge, Arctic Ocean

Résumé fourni par la source

Abstract The composition of the lower oceanic crust is controlled by melt differentiation, melt-rock reaction processes, and mantle source heterogeneity. Although the gabbroic lower oceanic crust forms a substantial, sometimes even dominant portion of the bulk oceanic crust, its global compositional variability remains poorly constrained. Here, we present a petrographic and geochemical characterization of gabbroic lithologies from the ultraslow-spreading Gakkel ridge (GKR), in the Arctic Ocean. We compare our data from GKR to the local basalts and a global dataset of gabbroic rocks from mid-ocean ridges. Along the GKR, gabbroic rocks were recovered during the AMORE 2001 expedition from three tectono-magmatic segments: the Western Volcanic Zone (WVZ), the Sparsely Magmatic Zone (SMZ), and the Eastern Volcanic Zone (EVZ). The investigated rocks include troctolites, olivine gabbros, gabbros, gabbronorites, and oxide gabbros with pronounced petrological and geochemical differences, mirroring variations in crustal thickness and basalt chemistry. Mineral major and trace element compositions differ between rock types and along the GKR and suggest that the lower gabbroic crust formed by extensive fractional crystallization of tholeiitic magmas whose composition varies along-axis. Specifically, clinopyroxene and plagioclase from the WVZ and EVZ have compositions akin to Mid-Ocean Ridge Basalts (MORB)-type gabbros formed at the ultra-slow spreading South-West Indian ridge, while SMZ gabbros are characterized by elevated Na2Ocpx and K2Oplg concentrations, lower Anorthite contents, and systematic enrichments in light relative to medium and heavy rare earth element contents. Consistent with the coexisting basalts, the compositional variability of the lower crust along the GKR indicates parental melt compositions ranging from MORB-type melts in the WVZ and SMZ to incompatible elements-enriched (E-) MORB in the SMZ. When compared to the dataset of gabbros from mid-ocean ridges worldwide, the GKR lower crust is consistent with general trends of increasing Na2Ocpx and decreasing FeOplg and Anplg with decreasing spreading rate and melt productivity. Nonetheless, the different tectono-magmatic provinces along the GKR show strong compositional variability that cannot be explained solely by variations in the degree of mantle melting or crustal accretion processes. Rather, our study suggests that mantle source heterogeneity must have played a significant role in controlling the composition of the oceanic crust at Gakkel, an effect likely amplified by the low crustal accretion rate at ultra-slow spreading velocity.

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