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