THERMAL STRUCTURE OF THE MANTLE BENEATH THE EQUATORIAL MID-ATLANTIC RIDGE - INFERENCES FROM THE SPATIAL VARIATION OF DREDGED BASALT GLASS COMPOSITIONS

We report on the major element composition of basaltic glasses from th e Mid-Atlantic Ridge transecting the equatorial mega-fracture zones fr om 7 degrees S to 5 degrees N (65 stations, 10-20 km sampling interval s, 3.5 - 5 lan water depth range). Many of the basaltic glasses are Na 2O, SiO2, and MgO rich, similar to other basalt glasses erupted along the deepest regions of the midocean ridge system, suggesting melt gene ration by relatively low degrees of partial melting at rather shallow depth in the upper mantle. Along the ridge axis, the compositional var iations show regular and systematic long-wavelength trends with a majo r discontinuity at the complex St. Paul transform fault, just south of St. Peter and Paul islets. A corresponding long-wavelength trend in u pper mantle potential temperature, mean pressure, and degree of meltin g and crustal thickness variation is inferred using parameterized petr ologic decompression melting models. A 600-km-long, nearly linear nega tive gradient in these parameters is apparent from the Charcot fractur e zone (FZ) to the St. Paul FZ. Over the length of this gradient, the upper mantle potential temperature drops by about 70 degrees C, the me an degree of partial melting changes from 7% to 10%, and the inferred crustal thickness varies between 3 and 6 lan. The gradient along the r idge axis is unaffected by the mega-transform fault offsets, implying that a broad (approximately 2000 km wide across-axis and 600 km long a long-axis) cold zone is present in the upper mantle just south of the equator. At the discontinuity across the complex St. Paul transform fa ult, the gradients in inferred potential temperature, mean degree of p artial melting, and crustal thickness abruptly change sign, respective ly increasing by 80 degrees C, rising from 7% to 10%, and changing fro m 3 to 6 km. The discontinuity is clearly related to the Sierra Leone plume affecting the Mid-Atlantic Ridge around 1.7 degrees N, as also e vident from Pb, Nd, and Sr isotopic variations previously reported on the same glasses (Schilling et al., 1994) and the K2O variation report ed here. The cause of the petrologically inferred cold zone and large gradient in the upper mantle south of St. Peter and Paul islets remain s more speculative. On the basis of a passive mantle upwelling flow mo del (Phipps Morgan and Forsyth, 1988) applied to the specific geometry of the equatorial Atlantic, we reject the simplest hypothesis that th e cold zone is produced by the compounding cooling effect caused by th e very long and densely distributed transform fault offsets in the equ atorial Atlantic. The result of this test remains paradoxical in view that good correlations exist between segment length, maximum along-rid ge axis relief per segment, mean segment depth, and per segment averag e bulk compositions of the erupted basalts, and corresponding mean deg rees of melting. Other possible causes for the gradational cold zone a re briefly explored. These include the evolutionary history of the reg ion with respect to adjacent continental mantle, lithosphere age, thic kness, and temperature, and tectonic mode of opening of the Atlantic, as well as large-scale convective motion associated with continental d ispersion. No definite conclusions can be reached. However, we emphasi ze that the petrologically inferred upper mantle thermal structure in the equatorial Atlantic is quite robust and independent of the petrolo gic decompression melting models considered and their underlying detai led assumptions. Large seismic S wave velocity variations are predicte d over the 0-150 km depth range of the upper mantle, based on the repo rted correlation of Nag with S wave velocity reported by Yan et al. (1 989). Thus detailed seismic tomographic mapping could be used to test further the cold upper mantle zone hypothesis for the equatorial Atlan tic.