Previous   |    Next

doi:10.2204/iodp.proc.309312.205.2009

Discussion and conclusions

The assemblage of anhydrite; quartz; and calcite, pyrite, sphalerite, and chalcopyrite is widely interpreted to result from hydrothermal alteration of basalts (e.g., Alt et al., 1996). Anhydrite potentially indicates the rapid mixing of hydrothermal fluids and seawater (e.g., Bethke, 1996). Quartz clasts suspended in anhydrite (Fig. F5C) is a structure that is reminiscent of discharge zones of hydrothermal vents (Delaney et al., 1987; Saccocia and Gillis, 1995). The enclosure of needlelike iron oxides in quartz (Fig. F5F) and vugs of quartz + chlorite in otherwise unaltered basalt (Fig. F5B) also reflect a disequilibrium system such as that found in high-temperature hydrothermal systems. Lastly, lenses of sphene require high-pH fluids to mobilize and concentrate titanium (Fig. F4A).

The local presence of potassium-rich alkalai feldspars (in some cases rich in SrO and/or BaO) is also indicative of hydrothermal alteration. A mineralized breccia from Core 309-1256D-122R also contained some alkalai feldspar (J.C. Alt, pers. comm., 2008), but otherwise, potassium-rich feldspars are not widely reported in Hole 1256D or in ocean crustal rocks in general. Microstructural relationships provide some information about the relative timing of the alteration that led to the K-feldspars. For example, the feldspar phenocryst in Figure F6A is crosscut by the chilled margin that has a dominantly chloritic composition and does not have elevated potassium concentrations. In another example, in Figure F6C, a potassium-rich feldspar clast is suspended in and appears to have reacted with the surrounding chilled margin. In other words, the potassium alteration must have occurred before (or during) the formation of the chilled margin. Although K-feldspars could signify aluminum mobility, such as that reported in lower temperature (~220°C) alteration of hyaloclastites (e.g., Zierenberg et al., 1995), they could equally reflect cation mobility, and such low temperatures did not likely prevail in the intrusive center where the chilled margin crosscutting relationships developed.

Many of the hydrothermal microstructures and minerals, such as the actinolite-chlorite-albite assemblage in veins and vugs (e.g., Fig. F5), are found in more generally hydrothermally altered basalt, away from dike margins, and likely grew over a range of time scales and temperatures. In contrast, the Expedition 312 structural geology team preferred the interpretation that some of the microstructural crosscutting relationships formed roughly simultaneous with dike emplacement, a process inferred in modern ridge settings (Delaney et al., 1998; Curewitz and Karson, 1998). Several of the chilled margin phases and microstructures are not found in other parts of Hole 1256D. Anhydrite, for example, is reported in cores recovered from the lavas and the upper portion of the SDC but is otherwise not thought to be widespread in the SDC (see the “Expedition 309/312 summary” chapter). The microstructure of the stretched lenses of sphene is unique to chilled margins and likely required relatively high temperature ductile deformation, such as that found along a cooling dike margin. In Hole 1256D, K-feldspars are only found in the host rock adjacent to or within the chilled margins, and the potassium alteration occurred prior to the more widespread alteration. Crosscutting relationships in scanning electron microscopy between altered clasts and grains of secondary minerals and the surrounding chilled margins certainly leaves open the hypothesis of synintrusion hydrothermal alteration. Further investigation of the Hole 1256D chilled margins using in situ isotopic analysis and more detailed and high-resolution mineralogy could potentially test this hypothesis further, and future drilling and submersible investigations of other ocean crustal localities could help determine the geologic record of intrusion-related hydrothermal fluid flow.

Top of page   |    Previous   |    Next