Magma evolution along detachment fault-affected ridge segments: Petrology and geochemistry of MORBs from the Tianxiu segment, Carlsberg Ridge

Abstract

The interaction between mid-ocean ridge magmatism and tectonic movement is well-documented. Spreading rates, for instance, influence magma generation, while magmatic processes can, in turn, shape the structural features of the ridge, such as parallel normal fault systems and symmetrical oceanic detachment faults. However, it remains unclear whether oceanic detachment faults directly impact the evolution of magma beneath the ridge. Here, we present new petrological and geochemical data of basalts from the Tianxiu segment of the Carlsberg Ridge, an asymmetrical ridge featuring detachment faults and an active hydrothermal system. All Tianxiu basalts are classified as depleted mid-ocean ridge basalts, with consistent Sr-Nd-Pb isotopic compositions indicative of a depleted peridotite-dominated mantle source. Corrected major element data indicates that the Tianxiu segment has undergone lower degrees of melting compared to adjacent symmetrical segments (62.5–63.5°E). Crystallization pressure for the Tianxiu basalts, calculated at 3.96 kbar, is similar to that found at the TAG segment (4.03 kbar), but significantly higher than that of the adjacent symmetrical ridge segments (2.28 kbar). Pressure estimates from clinopyroxene in Tianxiu samples ranged from 0.21 to 8.71 kbar, with an average of 5 kbar, suggesting that crystallization primarily occurs within the lithospheric mantle. These results point to a magma system characterized by a narrow melting interval, with lower degrees of melting and higher crystallization pressures. We conclude that the detachment fault and associated hydrothermal system at the Tianxiu segment alter the thermal structure of the shallow lithosphere, thereby affecting the magma generation process. This study enhances our understanding of the relationship between oceanic detachment faults and mid-ocean ridge magmatism.