(Table 2) Chemical composition of gases in basalts from DSDP Leg 65 Holes

Basalts from different structural provinces in the ocean basins, such as mid-ocean ridges, island arcs, and oceanic plateaus, show marked differences in major and minor element composition stemming from differences in magma source. In addition, there are variations even within individual provinces, based on such processes as crystal fractionation, secondary alteration, and hydrothermal alteration. It is also known that hydrothermal processes can cause changes in the gas composition of submarine basalts. For example, Zolotarev et al. (1978) have established that hydrothermal alteration frequently causes an increase in the CO2 content of basalts. If the homogeneity in composition and concentration of organic gases in oceanic basalts is associated with degassing during epimagmatic alteration, it would be interesting to investigate the relative abundance of gas phases in young basalts from midoceanic ridges. This chapter deals with the distribution of organic gases and CO2 in young basalts recovered on Leg 65 from the Gulf of California. Our aim was to establish the relationship between gas composition and degree of alteration.

(Table 1) Heavy-mineral analyses for DSDP Leg 67 Holes from electron microprobe

Heavy and light minerals were examined in 29 samples from Sites 494, 498, 499, 500, and 495 on the Deep Sea Drilling Project Leg 67 Middle America Trench transect; these sites represent lower slope, trench, and oceanic crust environments off Guatemala. All samples are Quaternary except those from Hole 494A (Pliocene) and Hole 498A (Miocene). Heavy-mineral assemblages of the Quaternary sediments are characterized by an immature pyroxene-amphibole suite with small quantities of olivine and epidote. The Miocene sediments yielded an assemblage dominated by epidote and pyroxene but lacking olivine; the absence of olivine is attributed to selective removal of the most unstable components by intrastratal solution. Light-mineral assemblages of all samples are predominantly characterized by volcanic glass and plagioclase feldspar. The feldspar compositions are compatible with andesitic source rocks and frequently exhibit oscillatory zoning. The heavy- and light-mineral associations of these sediments suggest a proximal volcanic source, most probably the Neogene highland volcanic province of Guatemala. Sand-sized components from Site 495 are mainly biogenic skeletons and volcanic glass and, in one instance (Section 495-5-3), euhedral crystals of gypsum.