The nature and origin of geochemical variation in Mid-Atlantic Ridge basalts from the Central North Atlantic

Details The nature and origin of geochemical variation in Mid-Atlantic Ridge basalts from the Central North Atlantic The nature and origin of geochemical variation in Mid-Atlantic Ridge basalts from the Central North Atlantic

Oct 1978

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1978gecoa..42.1501w&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 42, Issue 10, pp.1501-1516

Computer Science

47

Scientific paper

Seventy-two basalts from 58 dredge stations located along the Mid-Atlantic Ridge from 29°N to 59°N have been analyzed for 87 Sr / 86 Sr and for K, Rb, Cc, Sr and Ba. The Sr-isotope profile along the ridge has three distinct maxima, one coinciding with the Azores platform (0.70345), one at 45°N (0.70340) and the third at 35°N, in the vicinity of the Oceanographer Fracture Zone. Basalts from ridge segments between 29°N and 33°N, and 49°N and 59°N have 87 Sr / 86 Sr ratios typical of `normal' mid-ocean ridge basalts (0.70230-0.70280). Profiles of K, Rb, Cs, Sr, Bz, Rb/Sr and Ba/Sr are similar to the 87 Sr / 86 Sr profile, but Rb/K, Cs/K and Ba/K show broad maxima between 35°N and 45°N. These variations result from chemical and isotopic heterogeneity in the mantle, and are interpreted as caused by a mantle plume beneath the Azores which mixes with the LIL-element-depleted asthenosphere. Additional plumes may exist beneath 45°N and 35°N. Compared to the LIL-element-depleted asthenosphere, the Azores mantle plume is 10 to 30 times enriched in LIL elements with very small (>> 0.1) bulk crystal/melt partition coefficients (Rb, Cs, Ba, La). Mildly incompatible elements (0.1 < D < 1) (Sr, Sm, Yb) are only 0.8-3 times enriched. These, observations suggest that LIL element differences between these two mantle reservoirs resulted from processes involving solid-liquid equilibria and not vapor-solid or vapor-liquid equilibria. Isotope systematics indicate that neither mantle reservoir remained a closed system since the formation of the Earth, but it is not possible to determine the time at which heterogeneity first developed.

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