Eruption Recurrence Rates and Compositional Variability of Discrete Lava Flows on the S-EPR from 238U-230Th-226Ra- 210Pb-232Th

Quantification of the absolute ages and geochemistry of individual seafloor lava flows provides important constraints on the magmatic processes responsible for building the oceanic crust. Here we present new ²³⁸U-²³⁰Th-²²⁶Ra-²¹⁰Pb radioactive disequilibrium age constraints (decadal to millennial time scale) for 3 mid-ocean ridge lava flows at 17° 26'S on the East Pacific Rise (EPR): Aldo-Kihi, Rehu-Marka, and a neighboring unnamed flow. Our continuing study using high-resolution surveys and manned-submersible sampling (NAUDUR, 1993, and STOWA, 1991, expeditions) has previously shown that Aldo-Kihi is compositionally variable, is probably one of the youngest axial lavas in the 17° -19° S region, and was most likely erupted from a series of fissures extending >18 km along the ridge axis (Sinton et al., JGR, in revision). Rehu Marka has a more trace element enriched and evolved composition. The strongest age constraints in our U-series data set are from the ²¹⁰Pb-²²⁶Ra (half-life = 22.3 yrs) and ²²⁶Ra-²³⁰Th (half life = 1600 yrs) systems. ²¹⁰Pb-²²⁶Ra disequilibrium (as 5-7% Pb deficits) is common in lavas from our S-EPR study area and slightly lower than disequilibria we have measured in lavas erupted in 1991 and 1992 at 9° 50'N EPR. Although we are still developing our understanding of how this disequilibrium arises in MORB (e.g., how the radioactive "clock" is set for this isotope pair) a number of features of our preliminary data support the idea that these lavas are very young and that geologically observed contact relationships in the field separate the products of chronologically distinguishable eruptions. Also, the extent of ²²⁶Ra-²¹⁰Pb disequilibrium in 3 Aldo-Kihi samples compared to that observed at 9° 50'N indicates that the Aldo-Kihi lava probably erupted within the last 10-20 yrs, and the higher but still <1 (²¹⁰Pb/²²⁶Ra) activity ratio in a lava sampled near to but outside the boundaries of Aldo-Kihi indicates it is slightly older, but probably only by a decade or so. Although the older lava's major element composition is very similar to Aldo-Kihi, it has distinct U-Th-Ra chemistry, indicating it is from a different parental magma. Finally, the compositionally very distinct Rehu Marka flow just to the north has no ²²⁶Ra-²¹⁰Pb disequilibrium, indicating it is likely older than the maximum resolvable age with this method (100-120 years). An age estimate (about 750 yrs) of the latter can be made from its ²²⁶Ra excess. Together, these preliminary age constraints provide insight into eruption recurrence rates and the processes that allow for preservation of compositional variability within proximally located (in space and time) lava flows along this magmatically robust segment of the EPR.