Petrology of Basalts from Hole 504B, Deep Sea Drilling Project, Leg 83

Abstract

Basalts from Hole 504B, Leg 83, exhibit remarkable uniformity in major and trace element composition throughout the 1075.5 m of basement drilled. The majority of the basalts, Group D', have unusual compositions relative to normal (Type I) mid-ocean ridge basalts (MORB). These basalts have relatively high mg values (0.60-0.70) and CaO abundances (11.7-13.7%; Ca/Al = 0.78-0.89), but exhibit a marked depletion in compatible trace elements (Cr and Ni); moderately incompatible trace elements (Zr, Y, Ti, etc.); and highly incompatible trace elements (Nb, LREE, etc.). Petrographic and compositional data indicate that most of these basalts are evolved, having fractionated significant amounts of plagioclase, olivine, and clinopyroxene. Melting experiments on similar basalt compositions from the upper portion of Hole 504B (Leg 70; Autio and Rhodes, 1983) indicate that the basalts are co-saturated with olivine and plagioclase and often clinopyroxene on the 1-atm. liquidus. Two rarely occurring groups, M' and T, are compositionally distinct from Group D' basalts. Group T is strongly depleted in all magmaphile elements except the highly incompatible ones (Nb, La, etc.), while Group M' has moderate concentrations of both moderately and highly incompatible trace elements and is similar to Type I MORB. Groups M' and T cannot be related to Group D' nor to each other by crystal fractionation, crystal accumulation, or magma mixing. The large differences in magmaphile element ratios (Zr/Nb, La/Yb) among these three chemical groups may be accounted for by complex melting models and/or local heterogeneity of the mantle beneath the Costa Rica Ridge. Xenocrysts and xenoliths of plagioclase and clinopyroxene similar in texture and mineral composition to crystals in coarse-grained basalts from the lower portion of the hole are common in Hole 504B basalts. These suggest that addition of solid components either from conduit or magma chamber walls has occurred and may be a common source of disequilibrium crystals in these basalts. However, mixing of plagioclase-laden depleted melts (similar to the Costa Rica Ridge Zone basalts) with normal MORB magmas could provide an alternate source for some refractory plagioclase crystals found out of equilibrium in many phyric MORB. The uniformity of major element compositions in Hole 504B basalts affords an ideal situation for investigating the effects of alteration on some major and trace elements in oceanic basalts. Alteration observed in whole-rock samples records primarily two events-a high-temperature and a low-temperature phase. High-temperature phases include: chlorite, talc, albite, actinolite, sphene, quartz, and pyrite. The low-temperature phases include smectite (saponite), epistilbite or laumontite, and minor calcite. Laumontite may actually straddle the gap between the low-and high-temperature mineral assemblages. Alteration is restricted primarily to partial replacement of primary phases. Metamorphic grade, in general, increases from the top to the bottom of Hole 504B (Legs 69, 70, and 83)...

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

“…The rocks are unusually depleted in incompatible elements (e.g., TiO 2 , Nb, and Zr), but have incompatible element ratios comparable to normal MORB. The basalts have been interpreted as being either very primitive (Emmermann, 1985;Natland et al, 1983), or the result of multi-stage melting of a normal MORB mantle source followed by moderate extents of crystal fractionation (Autio and Rhodes, 1983;Autio et al, 1989;Kempton et al, 1985). The exceptional uniformity in composition of the basalts has been interpreted to indicate the presence of a steady-state magma chamber beneath the rift axis (Natland et al, 1983).…”

Geochemistry of Lavas from Hole 896A

“…The effects of alteration on the bulk-rock chemistry of pillow basalts have been studied extensively in Hole 504B by Kempton et al (1985). These authors concluded that K and Sr are mobile in the upper pillow lava section where seawater interaction is greatest.…”

“…This procedure lowered the detection limit (detection limit, 0.1 ppm) and increased the precision of the analysis (± 5%). The increased quality of these data is essential in the petrogenetic discussion as both Rb and Nb are incompatible elements that have been used to both classify Hole 504B basalts (Autio and Rhodes, 1983;Kempton et al, 1985) and to establish the nature of the source of oceanic magmas in general (Sun and McDonough, 1989). Before discussing the igneous geochemistry of Hole 896A, the effects of alteration must be evaluated because these effects can often mimic magmatic processes (Hellman et al, 1977).…”

“…In the pillow lavas from Hole 504B, the positive correlation between K 2 O and LOI was interpreted as K mobility during the low temperature hydration of the basalts (Dick, Erzinger, Stokking, et al, Kempton et al (1985). The basalts from Hole 896A have been split into two groups: "Lower" refers to basalts from >340 mbsf, and "Upper" refers to basalts from <340 mbsf.…”

Site 803

“…Above 200 mbsf, the hole was significantly wider than the maximum opening of the density tool caliper. Hence, above this depth the log density measurements average about 1.4 g/cm 3 , compared with core measurements of about 1.6 g/cm 3 (see , 1980), Hole 504B on the Costa Rica Rift (Kempton et al, 1985), and the Nauru Basin (Floyd, 1986;Saunders, 1986). Averages for Manihiki Plateau lavas ("M"; Jackson et al, 1976) and Malaita basement basalts ("Ma"; J.…”