Rb-Sr systematics of volcanic rocks and alteration minerals of ODP Hole 104-642E

Subaerially erupted tholeiites at Hole 642E were never exposed to the high-temperature seawater circulation and alteration conditions that are found at subaqueous ridges. Alteration of Site 642 rocks is therefore the product of the interaction 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 tholeiites 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 fractions of smectite contain Sr that has not achieved isotopic equilibrium. 87Sr/86Sr results of the leaching experiments suggest that Sr was mostly derived from seawater during early periods of smectite precipitation. The basalt-like 87Sr/86Sr of the most readily exchangeable fraction seems to suggest a late period of exposure to very low water /rock. Smectite formation 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 Voring 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 affected the Norwegian-Greenland Sea. Correlation of 87Sr/86Sr 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 smectite + seawater ++ celadonite + calcite. The geochemical effects of this reaction include net gains of K and CO2 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-temperature seawater-ocean crust interactions may be significant in calculations of global fluxes.

(Table 1) K, Rb and Sr concentrations and Sr isotopic composition of sediments and igneous rocks from DSDP Hole 30-286

To evaluate the possible contribution of ocean floor sediments during the genesis of the volcanism of Vanuatu (New Hebrides) active margin, we have determined the balance of Sr isotopes and K, Rb and Sr contents for the stratigraphic column of site 286 (leg 30, DSDP). This site is located on the oceanic plate that will be subducted. Analyses have been performed on sedimentary and igneous rocks, before and after acid leaching. The Sr isotopic data do not support the occurrence of some continental component in arc magmas of this active margin which is really intraoceanic. It is demonstrated that the d'Entrecasteaux fracture zone results from the intense fracturing of typical oceanic crust. The analyses of the volcanogenic components of the sediments show a change in the source of volcanoclastic detritus from the Loyalty islands in the Eocene to the volcanic arc of Vanuatu (New Hebrides) during Pliocene and Quaternary times. The determined balance of Sr isotopes and of K, Rb, Sr contents, may be used for calculation of multicomponent melting mixing models for the origin of Vanuatu arc magmas, but we emphasize that in these models the Sr isotopes cannot be considered as an appropriated tracer of sediment contribution.