Correlation of Hotspot Isotopic Data with Mantle Tomography

Abstract

Isotopic data have been gathered from igneous rocks at areas believed to be hotspots. These data include ratios of ^(208)Pb/^(204)Pb, ^(206)Pb/^(204)Pb, ^(207)Pb/^(204)Pb, ^(87)Sr/^(86)Sr, and ^(143)Nd/^(144)Nd. The lead ratios are typically expressed as deviations from the reference lines given by: (^(207)Pb/^(204)Pb) NHRL = 0.1084 (^(206)Pb/^(204)Pb) +13.491. (^(208)Pb/^(204)Pb) NHRL = 1.209 (^(206)Pb/^(204)Pb) +15.627. The expressions for these deviations are given by: Δ7/4 = [(^(207)Pb/^(204)Pb)-(^(207)Pb/^(204)Pb)NHRL]x100. Δ8/4 = [(^(208)Pb/^(204)Pb)-(^(208)Pb/^(204)Pb)NHRL]x100. For the strontium and neodymium ratios, the following expressions are used: ΔSr = {[(^(87)Sr/^(86)Sr)-0.7]-[(^(87)Sr/^(86)Sr)REF-0.7]}x10^4. ΔNd = {[(^(143)Nd/^(144)Nd)-0.51]-[(^(143)Nd/^(144)Nd)REF-0.51]}x10^4. (^(87)Sr/^(86)Sr)REF = .70368. (^(143)Nd/^(144)Nd)REF = .512907. This is the same system for lead used by Hart (1984). The baseline strontium and neodymium ratios, used above, are the median values of the strontium and neodymium isotopic ratios used in this study. The "standard" for strontium is from the Comores hotspot, and the neodymium "standard" is from St. Helena. All of these isotopes and their parent elements are incompatible, entering melts preferentially, but the important aspects of these elements are their relative incompatibilities. It is expected that melts extracted from primitive mantle material would have higher U/Pb, Th/Pb and Rb/Sr ratios than primitive mantle owing to the greater incompatibility of the parent elements as compared to the daughter elements. Conversely, the higher incompatibility of neodymium as compared to its parent, samarium, would result in a lower Sm/Nd ratio in the melt. Castillo (1988) claimed that the Dupal anomaly correlated well with low velocity regions in the lower mantle. In addition, all degree except 2 and 3 in the tomography were discarded. This study examines the quantitative significance of this correlation with the lower mantle, and, since the correlation is to be done depth by depth, it should be possible to determine the depths at which these correlations occur. Additionally, correlations in the upper mantle and at other degrees will be found and evaluated.