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This dissertation combines mineralogical data and petrographic and field observations with geochemical analysis (major, trace and isotope) to provide new insights into the hydrology and geochemistry of mid-ocean ridge hydrothermal systems. Two study areas were chosen to study two different aspects of hydrothermal circulation: high-temperature on-axis hydrothermal systems were studied using samples from the Hess Deep Rift (Cocos Plate, Equatorial Pacific) and low-temperature off-axis hydrothermal systems were studied at the Troodos Ophiolite in Cyprus. Significant findings include the documentation of a previously unknown warm fluid that pervades the lavas leaching Li from newly formed crust. This finding corroborates a model of broad hydrothermal discharge in the sheeted dikes. In the off-axis low-temperature regime, lateral flow of warm fluid is documented in the lavas, advecting heat from the oceanic lithosphere, with minor geochemical changes to the lavas. The sedimentary cover was found to influence alteration in two ways. The longer an area remains unsedimented allowing the free ingress and egress of seawater, the deeper the enrichment of alkali metals is observed. The maximum enrichment in alkali metals (K, Rb, Cs) however, is similar in both locations. The sedimentary cover can also modify the seawater before it becomes impermeable to fluid flow; early metaliferrous oxide sediments react with seawater, creating a fluid that mobilizes and fractionates the REEs and Y. The fractionation results in negative Ce anomalies, positive Eu anomalies, and negative Y anomalies. Basalts altered under these conditions also lack the ubiquitous Fe-oxides and Fe-oxyhydroxides that are commonly associated with alkali metal uptake. In situ trace element analysis of alteration minerals formed at low-temperature confirmed that secondary phyllosilicates are strongly enriched in alkali metals (K, Rb, Cs and Li), Ba is found in adularia and zeolites, Sr is hosted in carbonates, and no phases were found to be enriched in U. The concentrations of K2O, Rb, Cs (as well as B) are highest in celadonites, whereas Li concentrations are highest in smectites (saponite, Al-saponite, beidellite) and smectite-chlorite mixtures, and much higher than previously reported. Alkalis are also taken up into palagonite, with Li having the highest concentrations, over 1000 ppm in one analysis. Crystal chemical factors were found to be the dominant control on trace element uptake, and for the phyllosilicates no correlation was found between the temperature, age of the crust, texture of the phyllosilicates. In phyllosilicates the K, Rb and Cs are adsorbed as exchange cations, with enrichment (Cs > Rb > K) increasing with decreasing hydration energy, whereas the uptake of Li and B does not correlate with the hydration energy. Lithium concentrations also do not correlate with the Mg content, suggesting substitution of Li for Mg is not the only mechanism of Li uptake into phyllosilicates as has been suggested. / Graduate

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Ocean Engineering; and the Woods Hole Oceanographic Institution), 1991. / Includes bibliographical references (p. 247-253). / Sr, Nd and Pb isotopic analyses of 477 samples representing 30 islands or island groups, 3 seamounts or seamount chains, 2 oceanic ridges and 1 oceanic plateau [for a total of 36 geographic features] are compiled to form a comprehensive oceanic island basalt [OIB] data set. These samples are supplemented by 90 selected mid-ocean ridge basalt [MORB] samples to give adequate representation to MORB as an oceanic basalt end-member. This comprehensive data set is used to infer information about the Earth's mantle. Principal component analysis of the OIB+MORB data set shows that the first three principal components account for 97.5% of the variance of the data. Thus, only four mantle end-member components [EMI, EMII, HIMU and DMM I are required to completely encompass the range of known isotopic values. Each sample is expressed in terms of percentages of the four mantle components, assuming linear mixing. There is significant correlation between location and isotopic signature within geographic features, but not between them, so discrimination analysis of the viability of separating the oceanic islands into those lying inside and outside Hart's (1984, 1988) DUPAL belt is performed on the feature level and yields positive results. A "continuous layer model" is applied to the mantle component percentage data to solve for the spherical harmonic coefficients using approximation methods. Only the degrees 0-5 coefficients can be solved for since there are only 36 features. The EMI and HIMU percentage data sets must be filtered to avoid aliasing. Due to the nature of the data, the coefficients must be solved for using singular value decomposition [SVD], versus the least squares method. The F-test provides an objective way to estimate the number of singular values to retain when solving with SVD. With respect to the behavior of geophysics control data sets, only the degree 2 spherical harmonic coefficients for the mantle components can be estimated with a reasonable level of confidence with this method. Applying a "delta-function model" removes the problem of aliasing and simplifies the spherical harmonic coefficient solutions from integration on the globe to summation over the geographic features due to the properties of deltafunctions. With respect to the behavior of geophysics control data sets, at least the degree 2 spherical harmonic coefficients for the mantle components can be estimated with confidence, if not the degrees 3 and 4 as well. Delta-function model solutions are, to some extent, controlled by the nonuniform feature distribution, while the continuous layer model solutions are not. The mantle component amplitude spectra, for both models, show power at all degrees, with no one degree dominating. The DUPAL components [EMI, EMII 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 is inferred to result from topography at the core-mantle boundary. The DUPAL and DMM components, for both models, correlate well [and negatively] at degree 3 with the velocity anomalies of the Clayton-Comer seismic tomography model in the 2500-2900 km depth range [immediately above the core mantle boundary]. The EMII component correlates well [and positively] at degree 5 with the velocity anomalies of the Clayton-Comer model in the 700-1290 km depth range, indicating a subduction related origin. Similar positive correlations for the geoid in the upper lower mantle indicate that subducted slabs extend beyond the 670 km seismic discontinuity and support a whole-mantle convection model. / Lynn A. Oschmann. / Ph.D.