Initial rifting to seafloor spreading: geochemical and petrologic variations in basalts from the Cocos-Nazca Spreading Center

The westward propagating Cocos-Nazca (C-N) spreading center, in the eastern equatorial Pacific, rifts into 0.5 Ma crust accreted at the East Pacific Rise (EPR). The C-N rift offers a rare opportunity to study the magmatic and tectonic evolution of a mid-ocean ridge from initial rifting (west of Hess Deep rift) to full magmatic spreading. In April-May 2018, aboard the R/V Sally Ride (Leg 1806), we collected rock samples (66 dredges) along the axis of the first 9 segments ( 360 km) of the C-N spreading center, the adjacent EPR, and the Dietz Volcanic Ridge (southern boundary of the Galapagos microplate). We also collected on- and off-axis bathymetry, gravity and magnetic data (see abstract by Smith et al.). Initial major and trace element analyses of basalt glasses show that C-N lavas exhibit both long-wavelength (inter-segment) trends as well as significant intra-segment variability. With distance eastward from the rift tip, C-N lavas generally increase in Mg# ( 55 to 65) and decrease in fractionation-corrected FeO (Fe8 from 12.2 to 9.8), while remaining relatively constant in fractionation-corrected Na₂O (Na8 2.6). Ratios of highly incompatible to moderately incompatible trace elements generally decrease from west to east (e.g., Ba/Zr from 0.15 to 0.075), but with significant intra-segment variability, particularly among samples recovered closest to the rift tip. Phenocryst abundances on average increase with distance eastward from the rift tip, and the most phenocryst-rich samples (>20%) often occur at or near segment ends. Together, geochemical and petrologic variations along the C-N rift suggest a complex interplay among variations in melting parameters, source composition, and magma evolution both within and across the nine sampled segments. Finally, samples recovered from the shallow Dietz Volcanic Ridge are moderately enriched (e.g., Ba/Zr 0.4), supporting earlier speculation that this feature is proximal to a discrete 'mini-hotspot' that may have played a role in stabilizing the location of this microplate boundary through time.