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Subaerially erupted tholeiites at Hole 642E were never exposed to the high-temperature seawater circulation and al­ teration conditions that are found at subaqueous ridges. Alteration of Site 642 rocks is therefore the product of the in­ teraction of rocks and fluids at low temperatures. The alteration mineralogy can thus be used to provide information on the geochemical effects of low temperature circulation of seawater. Rubidium-strontium systematics of leached and unleached tholeiites and underlying, continentally-derived dacites reflect interactions with seawater in fractures and vesicular flow tops. The secondary mineral assemblage in the tholei­ ites consists mainly of smectite, accompanied in a few flows by the assemblage celadonite + calcite (± native Cu). Textural relationships suggest that smectites formed early and that celadonite + calcite, which are at least in part cogenetic, formed later than and partially at the expense of smectite. Smectite precipitation occurred under variable, but generally low, water/rock conditions. The smectites contain much lower concentrations of alkali elements than has been reported in seafloor basalts, and sequentially leached frac­ tions of smectite contain Sr that has not achieved isotopic equilibrium. Sr/Sr results of the leaching experiments suggest that Sr was mostly derived from seawater during early periods of smectite precipitation. The basalt-like Sr/Sr of the most readily exchangeable fraction seems to suggest a late period of exposure to very low water /rock. Smectite for­ mation may have primarily occurred in the interval between the nearly 58-Ma age given by the lower series dacites and the 54.5 ± 0.2 Ma model age given by a celadonite from the top of the tholeiitic section. The 54.5 ± 0.2 Ma Rb-Sr model age may be recording the timing of foundering of the Vriring Plateau. Celadonites precipitated in flows below the top of the tholeiitic section define a Rb-Sr isochron with a slope corresponding to an age of 24.3 ± 0.4 Ma. This isochron may be reflecting mixing effects due to long-term chemical interaction between seawater and basalts, in which case the age provides only a minimum for the timing of late alteration. Alternatively, inferrential arguments can be made that the 24.3 ± 0.4 isochron age reflects the timing of the late Oligocene-early Miocene erosional event that af­ fected the Norwegian-Greenland Sea. Correlation of Sr/Sr and 1/Sr in calcites results in a two-component mixing model for late alteration products. One end-member of the mixing trend is Eocene or younger seawater. Strontium from the nonradiogenic endmember can not, however, have been derived directly from the basalts. Rather, the data suggest that Sr in the calcites is a mixture of Sr derived from seawater and from pre-existing smectites. For Site 642, the reaction involved can be generalized as smec­ tite + seawater ++ celadonite + calcite. The geochemical effects of this reaction include net gains of K and C02 by the secondary mineral assemblage. The gross similarity of the reactions involved in late, low-temperature alteration at Site 642 to those observed in other sea floor basalts suggests that the transfer of K and C02 to the crust during low-tempera­ ture seawater-ocean crust interactions may be significant in calculations of global fluxes. INTRODUCTION The formation and subsequent cooling of ocean crust results in the overprinting of alteration effects of near-ridge, high-tem­ perature seawater-basalt interactions by long-lived, low-temper­ ature alteration. Determining the relative importance of high temperature and ongoing, lower temperature effects is necessary if we are to achieve an understanding of the fluxes between ocean crust and seawater, but multiple generations of alteration have made the complexities of varying hydrothermal conditions difficult to unravel. The early Tertiary tholeiites recovered at Ocean Drilling Pro­ gram (ODP) Site 642 (Fig. 1) were erupted subaerially (Eldholm, Thiede, Taylor et al., 1987) and had almost certainly cooled to ambient temperatures before foundering. The virtually pristine major and trace element characteristics of most flows (Parson et al., Viereck et al., this volume) suggests that the basalts were lit­ tle affected by high-temperature seawater-rock interactions. In 1 Eldholm, O., Thiede, J., Taylor, E., et al., 1989. Proc. ODP, Sci. Results, 104: College Station, TX (Ocean Drilling Program). 2 Lamont-Doherty Geological Observatory, Columbia University, Palisades, NY 10964. 3 Department of Geology, Colorado College, Colorado Springs, CO 80903. 4 L-DGO Contribution No. 4355. the tholeiitic section (upper volcanic series Fig. 2), vesicles that have remained unfilled by secondary minerals predominate (Ship­ board Scientific Party, 1987), suggesting that seawater-rock re­ actions have been of limited extent during postfoundering his­ tory, as might be expected under low-temperature conditions. The virtual isolation of low-temperature alteration in Hole 642E tholeiites can provide a basis for evaluating the impor­ tance of low-temperature seawater-ocean crust interactions. In this paper we report the results of an Sr isotope and alkali ele­ ment study of host rocks and secondary minerals from the vol­ canic section of Hole 642E, and reflect on their implications for the timing of events in the Norwegian-Greenland Sea as well as for the extent of low-temperature seawater/basalt interactions. BACKGROUND As Layer 2 ocean crust is formed and cooled, it is subjected to a variety of alteration events. Workers who have studied alter­ ation assemblages in basalts have proposed highly similar parageneses of secondary minerals (e.g., Bass, 1976; Stakes and Scheidegger, 1981; Staudigel et al., 1981b). Textural evidence that smectites are the earliest clay minerals to form agrees with experimental results (e.g., Hajash, 1975; Bischoff and Dickson, 1975; Seyfried and Bischoff, 1977; Seyfried and Janecky, 1985). At least some smectite formation is associated with the circula­ tion of seawater at elevated temperature, and the fluids involved in the reactions are at relatively low pH and are oxygen-poor.

Rb-Sr Systematics of Site 642 Volcanic Rocks and Alteration Minerals