RA-226 TH-230 DISEQUILIBRIUM IN AXIAL AND OFF-AXIS MIDOCEAN RIDGE BASALTS

We describe Ra-226-Th-230 disequilibrium in mid-ocean ridge basalt (MORB) glasses from the Juan de Fuca, Gorda, and East Pacific ridges. These first mass spectrometric measurements of Ra-226 in MORB glasses at sub-picogram abundance levels confirm the large excesses over Th-230 determined by radon-emanation techniques and alpha spectrometry. All off-axis MORB glasses have Ra-226-Th-230 and U-234-U-238 in secular equilibrium. This suggests that magmatic processes, not secondary post-eruption alteration, generate U-238-series disequilibrium in these MORB. Least evolved, N-MORB from axial valleys have (Ra-226 /Th-230) between 2.2-2.3. Differentiated and enriched E-type MORB have consistently low (Ra-226 /Th-230) ratios compared with N-MORB from the same ridge sections. Ra-Th fractionation may be less pronounced, or magma residence-transit periods may be long for differentiated MORB. Also, E-MORB may be generated by different melt extraction volumes and rates. Estimated Ra-226-Th-230 ages for N-MORB agree with location on and off ridge segments, and with Th-U model ages. These preliminary results show that Ra-226-Th-230 disequilibrium could be used to quantify volcanic episodicity at ocean ridges.