Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.t32c..07b&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #T32C-07
Other
8121 Dynamics, Convection Currents And Mantle Plumes
Scientific paper
In this study, we have analyzed U-series in lavas from the Azores islands and the nearby Mid-Atlantic Ridge (FAZAR cruise) in an attempt to assess the relative importance of melting processes versus source variations in the context of ridge-hotpsot interaction. The lavas were analyzed for 238U-230Th (Turner et al. 1997, Bourdon et al. 1996) 226Ra-230Th and 235U-231Pa disequilibria by thermal ionisation mass spectrometry. Our results for the historic lavas from the Azores islands show that the 231Pa excess are at the low end of the trend found for other OIB (Pickett et al. 1997 and Bourdon et al. 1998) and fall on a positive correlation in a 231Pa/235U versus 230Th/238U diagram. In contrast, lavas from the nearby Mid-Atlantic ridge are characterized by larger (231Pa/235U) activity ratios for similar and greater (230Th/238U) ratios. There is also a weak correlation between 226Ra/230Th and 231Pa/235U. These data do not indicate a simple mixing trend between an N-MORB and an enriched component in the 231Pa/235U versus 230Th/238U diagram since the MORBs which do not have the most radiogenic isotope signatures compared with the Azores island basalts have some of the largest (230Th/238U) and 231Pa/235U. Clearly, the dynamics of melting must have played a role in generating larger 230Th and 231Pa excesses beneath the Mid-Atlantic ridge. We infer that this must be due to the absence of a lithospheric lid as larger excesses of 230Th and 231Pa can be generated for longer melting columns. Thus, ridge-hotspot interaction cannot imply a simple transfer of melt from the hotspot to the ridge. The 230Th/238U and 226Ra/230Th data across the Azores plateau shows a maximum for the island of Terceira and mimics the depth anomaly which is thought to result from the hotspot. This trend is also consistent with observations of rare gases (M. Moreira pers. comm.) and suggests that it must be related to the presence of deep material. The U-series trend is the reverse of the trend found in Hawaii by Sims et al. (2000) which was attributed to variations in upwelling rates across the rising plume. This observation can be rationalized in the context of an equilibrium melt transport model (Spiegelman and Elliott, 1993) where U-series disequilibria are sensitive to upwelling rates. For slow upwelling rates such as below the Azores, larger 230Th excesses are predicted in the center of the plume. This suggests that the upwelling rate beneath the center of the plume must be of the order of a few cm per year which is an order of magnitude lower than values estimated for Hawaii. Turner et al. 1997, Chem. Geol. 139, 145-164. Bourdon et al. 1996, Earth Planet. Sci. Lett. 142, 175-189. Pickett et al. 1997, Earth Planet. Sci. Lett. 148, 259-271. Sims et al. 1999, Geochim. Cosmochim. Acta. 63, 4119-4138. Spiegelman and Elliot, 1993, Earth Planet. Sci. Lett., 118, 1-20.
Bourdon B. P.
Turner Peter S.
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