Abstract:
The chemistry of mid-ocean ridge basalt (MORB) is studied almost exclusively using samples dredged from active spreading ridges. However, flows erupted at the ridge axis itself eventually make up the lowermost part of the extrusive section, and may not be representative of the entire oceanic crust. In contrast, samples drilled from old oceanic crust will include flows erupted both on- and off-axis, and also can be used to determine whether changes in MORB composition occur over the lifetime of an ocean basin. Despite this, few detailed studies of ancient MORB have been carried out. We have analysed major element compositions of more than 400 fresh volcanic glasses from 35 DSDP-ODP drillsites in the Atlantic and Pacific, which range in age from 10 to 170 Ma. Trace element analyses of the same samples using LA-ICPMS are in progress. In contrast to some previous studies, we find no significant difference in fractionation-corrected major element composition of Mesozoic and zero-age MORB in either the Pacific or the Atlantic. There is no indication that EMORB are concentrated in the upper parts of the oceanic crust, as would be expected if they were preferentially erupted off-axis. Instead, the youngest lavas (upper 100m) at a given drillsite tend to have relatively homogenous compositions, which could be explained if larger-volume flows flow further from the axis. If this is the case, then sampling MORB only from active spreading ridge axes may not give an entirely accurate picture of the average composition of the oceanic crust. It also means that direct comparison of lavas from slow- and fast-spreading ridges, which differ in axial topography and average flow volumes, will not be straightforward.