Description: Vein smectites with large Rb/Sr enrichments from extensively altered basaltic oceanic crust in Deep Sea Drilling Project hole 417A in the western Atlantic define a highly constrained Rb/Sr isochron age of 108 +/- 3 m.y. This age is identical to a less well constrained age of 108 +/- 17 m.y. for vein smectites with lower Rb/Sr enrichments from adjacent hole 418A and to the 108 m.y. age of crust formation derived by paleontological and magnetic anomaly correlation. Reasonable agreement exists between the 87Sr/86Sr ratio of vein calcites from both sites and the seawater 87Sr/86Sr ratio at the time. Pervasive low-temperature alteration in the contrasting environments of sites 417 and 418 appears to be coeval and essentially coincident with basement formation. Alteration may be used to advantage in determining ages of old oceanic crust.

Description: 87Sr/a6Sr ratios, Sr, K, Rb and Cs contents and the petrology of basalts and secondary phases recovered from deep basement drilling at DSDP Sites 332B, 417A, 417D and 418A show that the oceanic crust alters in four distinct stages characterized by formation of palagonite, smectite and calcite (Stages I, II and III, respectively). Stage IV represents the final compaction of the crust, including a dehydration of the crust without major chemical changes. Isotopic age determinations by Hart and Staudigel (1978, doi:10.1029/GL005i012p01009) and Richardson et al. (doi:10.1029/JB085iB12p07195), show that, at least for Atlantic-type crustal regimes, Stage I and II last for a maximum of 3 m.y., and Stage III lasts beyond Stage I and II, but is probably completed in significantly less than 10 m.y. Stage IV is long-lived and may still be active at 100 m.y. Stages I and II, the phase of halmyrolysis, include geochemically significant interactions between seawater and basalt for the upper 500 m of layer II and involve volumes of seawater containing a large basaltic component. Stage III solutions show evidence of less seawater-basalt interaction, at least to depths of 500 m; calcites deposited from these solutions have Sr isotopic ratios close to seawater values; but also have very low Sr/Ca ratios indicating a large basalt Ca component in the solutions. Smectite formation is the result of the interaction of seawater and basalt. The initial 87Sr/a6Sr ratios of smectites represent the Sr isotopic composition of the solution when the smectite is being formed. Thereafter, alkalies may be continuously added to interlayer positions in the smectite in order of decreasing hydration energy (Cs is more enriched than Rb, Rb more than K). The later-formed carbonates have very low alkali concentrations, and 87Sr/86Sr ratios identical to contemporaneous seawater. Therefore, since the alkali concentrations in a whole rock sample are affected by different alteration processes, the alkali concentrations alone are not reliable indicators of the degree of alteration.

Description: Ocean crustal carbon uptake during seafloor alteration at DSDP Sites 417A, 417D, and 418A exceeds the estimated loss of carbon during magmatic ridge outgassing. If these sites are representative for oceanic crust in general, 2.2-2.9 x 10**12 moles of carbon are removed from the oceans per year as a net flux of carbon between the oceanic crust and seawater. Although most of this carbon occurs as calcium carbonate, this ocean crustal carbonate probably cannot be considered part of the marine calcium carbonate sink since much of the Ca in these carbonates must be derived from basalt alteration that is not balanced by a concomitant uptake of seawater Mg. Our present estimate cannot be satisfactorily applied to global carbon budgets, because of uncertainties in the bulk Ca/Mg budget of ocean floor alteration and because of the uniqueness of our estimate. Yet, our data document that the formation of ocean crust provides a significant sink for carbon that should be included in models of the global cycling of carbon. Furthermore, magmatic outgassing during ocean crust emplacement and seafloor basalt alteration may provide a buffering mechanism for atmospheric carbon.