226Ra- 230Th disequilibrium in axial and off-axis mid-ocean ridge basalts

Abstract

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