Global isotopic signatures of oceanic island basalts

Officially, support for graduate study in the MIT/WHOI Joint Program inOceanography was provided by the US Navy under the CIVINS program.

Sr, Nd and Pb isotopic analyses of 477 samples representing 30 islands orisland groups, 3 seamounts or seamount chains, 2 oceanic ridges and 1 oceanicplateau [for a total of 36 geographic features] are compiled to form acomprehensive oceanic island basalt [OIB] data set. These samples aresupplemented by 90 selected mid-ocean ridge basalt [MORB] samples to giveadequate representation to MORB as an oceanic basaJt end-member. Thiscomprehensive data set is used to infer information about the Earth's mantle.Principal component analysis of the OIB+MORB data set shows that the firstthree principal components account for 97.5% of the variance of the data. Thus.only four mantle end-member components [EMI, EMII, HIMU and DMM] arerequired to completely encompass the range of known isotopic values. Eachsample is expressed in terms of percentages of the four mantle components,assuming linear mixing. There is significant correlation between location andisotopic signature within geographic features, but not between them, sodiscrimination analysis of the viability of separating the oceanic islands into thoselying inside and outside Hart's (1984, 1988) DUPAL belt is performed on thefeature level and yields positive results.A "continuous layer model" is applied to the mantle component percentagedata to solve for the spherical harmonic coefficients using approximationmethods. Only the degrees 0-5 coefficients can be solved for since there are only36 features. The EMI and HIMU percentage data sets must be filtered to avoidaliasing. Due to the nature of the data, the coefficients must be solved for usingsingular value decomposition [SVD], versus the least squares method. The F-testprovides an objective way to estimate the number of singular values to retainwhen solving with SVD. With respect to the behavior of geophysics control data sets, only the degree 2 spherical harmonic coefficients for the mantle componentscan be estimated with a reasonable level of confidence with this method.Applying a "delta-function model" removes the problem of aliasing andsimplifies the spherical harmonic coefficient solutions from integration on theglobe to summation over the geographic features due to the properties of deltafunctions.With respect to the behavior of geophysics control data sets, at leastthe degree 2 spherical harmonic coefficients for the mantle components can beestimated with confidence, if not the degrees 3 and 4 as well. Delta-functionmodel solutions are, to some extent, controlled by the nonuniform featuredistribution, while the continuous layer model solutions are not.The mantle component amplitude spectra, for both models, show power atall degrees, with no one degree dominating. The DUPAL components [EMI,EMil and HIMU], for both models, correlate well with the degree 2 geoid,indicating a deep origin for the components since the degrees 2-3 geoid isin ferred to result from topography at the core-mantle boundary. The DUPALand DMM components, for both models, correlate well [and negati vely! at degree3 with the velocity anomalies of the Clayton-Comer seismic tomography modelin the 2500-2900 km depth range [immediately above the core-mantle boundary].The EMil component correlates well [and positively] at degree 5 with thevelocity anomalies of the Clayton-Comer model in the 700-1200 km depth range,indicating a subduction related origin. Similar positive correlations for the geoidin the upper lower mantle indicate that subducted slabs extend beyond the 670km seismic discontinuity and support a whole-mantle convection model.

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1991