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

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

Mar 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.1233v&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 6, pp.1233-1241

Computer Science

19

Scientific paper

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

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