Petrology, mineralogy, and chemistry of basaltic rocks at DSDP Leg 81 Holes

We detail the petrography and mineralogy of 145 basaltic rocks from the top, middle, and base of flow units identified on shipboard along with associated pyroclastic samples. Our account includes representative electron microprobe analyses of primary and secondary minerals; 28 whole-rock major-oxide analyses; 135 whole-rock analyses each for 21 trace elements; 7 whole-rock rare-earth analyses; and 77 whole-rock X-ray-diffraction analyses. These data show generally similar petrography, mineralogy, and chemistry for the basalts from all four sites; they are typically subalkaline and consanguineous with limited evolution along the tholeiite trend. Limited fractionation is indicated by immobile trace elements; some xenocrystic incorporation from more basic material also occurred. Secondary alteration products indicate early subaerial weathering followed by prolonged interaction with seawater, most likely below 150°C at Holes 552, 553A, and 554A. At Hole 555, greenschist alteration affected the deepest rocks (olivine-dolerite) penetrated, at 250-300°C.

(Table 7) Fe**3+/Fe**2+ ratios in selected nontronite samples at DSDP Leg 70 Holes

Since its discovery in 1974 (Klitgord and Mudie, 1974), the Galapagos mounds hydrothermal field has received much attention. Sediment samples were taken during Leg 54 of the Deep Sea Drilling Project (DSDP) and by other expeditions to the area (e.g., Corliss et al., 1978). While a hydrothermal origin for the mounds sediments has been generally accepted, several different theories of origin for the mounds themselves have been proposed (e.g., Corliss et al., 1978; Natland et al., 1979; Williams et al., 1979). One of the aims of DSDP Leg 70 was to return to the mounds field and, using the new hydraulic piston cor er described elsewhere in this volume, to obtain more complete recovery of mounds sediments than had previously been possible. It was our hope that this would help in our understanding of the nature and origin of these deposits. In this chapter, we describe the results of chemical analysis of over 250 sediment samples taken during the course of Leg 70.

Peak areas and heights for minerals of ODP Leg 113-696B (Table 1)

Seismic data acquired over the eastern shelf and margin of the South Orkney microcontinent, Antarctica, have shown a high-amplitude reflection lying at a sub-bottom two-way traveltime (TWT) of 0.5-0.8 s. There appear to be two causes for the reflection which apply in different parts of the shelf. The more widespread cause of the reflection is a break-up unconformity associated with the opening of Jane Basin to the east. This is clearly seen where reflections in the underlying sequence are discordant. In contrast, in Eotvos Basin and the southeastern part of Bouguer Basin, the high-amplitude reflection in places cuts across bedding and is interpreted to be caused by silica diagenesis. A post-cruise analysis of core samples from Site 696 in Eotvos Basin by X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed the presence of a silica diagenetic front at 520-530 mbsf. The position of the unconformity at this site is uncertain, but probably coincides with a change of detrital input near 548 mbsf. Fluctuations of physical properties related to the depth of the diagenetic front are difficult to separate from those related to the variation of detrital composition over the same depth interval. Correlation of the drilling record with the seismic record is difficult but with a synthetic seismogram it is demonstrated that diagenesis is the probable cause of the high-amplitude reflection. In Bouguer Basin at Site 695 the depth of the high-amplitude reflection was not reached by drilling; however, the reflection is probably also caused by silica diagenesis because of the biogenic silica-rich composition of the sediments cored. The estimated temperatures and ages of the sediments at the depths of the high-amplitude reflections at Sites 695 and 696 compare favorably with similar data from other diagenetic fronts of the world. The high-amplitude reflection in Bouguer Basin is commonly of inverse polarity, possibly caused either by interference between reflections from several closely-spaced reflecting layers, such as chert horizons, or by free gas trapped near the diagenetic front.