E-MORB glasses reveal a metasomatized mantle source of nascent oceanic crust in the Guaymas Basin, Gulf of California

The formation of nascent ocean basins represents the beginning of the periodic Wilson cycle, and the geochemical characteristics of nascent oceanic crust are crucial for understanding how an ocean initially opens. The Gulf of California, which hosts young (<6 Ma) and active oceanic spreading centers (Lizarralde et al., 2007), is a prime example of a continental margin that has undergone oblique rifting and records the early stages of seafloor spreading. However, the mantle source composition of the nascent oceanic crust in the central part of the Gulf has not been comprehensively investigated yet. Here, we present major and trace element contents as well as B-Sr isotope compositions for basaltic glass samples from off-axis sills drilled by the International Ocean Discovery Program (IODP) Expedition 385 at Sites U1547 and U1548 in the intrusive sill-riddled Guaymas Basin. These glassy samples represent tholeiites and predominantly showtrace element patterns akin to enriched mid-ocean ridge basalts (E-MORBs), but with distinctive enrichments in Ba and K, as well as depletions in Nb, Ta, and Ti. They also have high B contents (3.07–3.67 ppm) with enriched Sr isotopes (⁸⁷Sr/⁸⁶Sr = 0.7032–0.7037) and heavy B isotopes (δ¹¹B = -5.52‰ to 1.20‰), showing that the nascent oceanic crust in the Guaymas Basin might be generated through partial melting of a depleted MORB mantle (DMM) source, which has been metasomatized by melts from subducted slab materials including partially dehydrated sediment and altered oceanic crust components. Additionally, magmas in the Gulf of California show a systematic decline in their enrichment in fluid-mobile elements (Ba) and depletion in fluid-immobile elements (Nb, Ta, and Ti) from the northern (e.g., Isla San Luis volcanic center) to the central part (Guaymas Basin) and southward to the mouth (e.g., Alarcón Basin) of the Gulf. This suggests that the enriched (recycled) components in their mantle source were gradually extracted and exhausted during Gulf opening and oceanic crustal accretion that advanced in a northward direction. Our results indicate that the Guaymas Basin magmas were derived from a mantle that was fertilized by subduction components. The subduction signature is different from nascent ocean basins that evolved from intraplate rifting, such as the Red Sea, corroborating the Gulf opening as a process that started in response to long-term oblique convergence at the eastern Pacific plate margin without any influence from a mantle plume.