ALBERT

Description: During DSDP Leg 42A, drilling at site 373A in the central Tyrrhenian Sea penetrated Miocene basaltic basement for 190 m and recovered basalt from up to 450 m below the sea floor. Despite mineralogical and chemical alteration due to hydrothermal processes under the high heat flow regime in the Tyrrhenian marginal basin, petrological analogies to high-Al abyssal basalts can be traced. Two interlayered basalt groups have been recognized: (1) the lower-Ti group (core 5/1-2, core 7, 10), and (2) the higher-Ti group (core 5/3, core 6, core 11, core 12 and outer barrel), respectively. We consider a distinction of two magma types: a more primitive magma leading to the plagioclase-bearing, lower-Ti basalts with affinities to high-Al abyssal tholeiites and a more differentiated magma leading to basalts with higher Ti.

Description: We obtained major and trace element data on 113 samples from basalts drilled during DSDP Legs 69 and 70 in the Costa Rica Rift area. The majority have major and trace element characteristics typical of ocean-ridge tholeiities. Most of the basalts are relatively MgO rich (MgO 〉 8 wt.%) and have Mg values (MgO/MgO + 0.85FeO x 100) of about 53, characteristics that clearly indicate that the various magmas underwent only a small amount of crystal fractionation before being erupted onto the seafloor. According to their normative mineralogies, the rocks are olivine tholeiites. A few samples plot close to the diopside-hypersthene join of the projected basalt tetrahedron. Except for basalts from two thin intervals in Hole 504B, which differ significantly from all the other basalts of the hole, practically no chemical downhole variation could be established. In the two exceptional intervals, both TiO2 and P2O5 contents are markedly enriched among the major oxides. The trace elements in these intervals are distinguished by relatively high contents of magmatophile elements and have flat to enriched chondrite-normalized distribution patterns of light rare earth elements (LREE). Most of the rocks outside these intervals are strongly depleted in large-ionlithophile (LIL) elements and LREE. We offer no satisfactory hypothesis for the origin of these basalts at this time. They might have originated within pockets of mantle materials that were more primitive than the LIL-element-depleted magmas that were the source of the other basalts. A significant change with depth in the type of alteration occurs in the 561 meters of basalt cored in Hole 504B. According to the behavior of such alteration-sensitive species as K2O, H2O-, CO2, S, Tl, and the iron oxidation ratio, the alteration is oxidative in the upper part and nonoxidative or even reducing in the lower part. The oxidative alteration may have resulted from low temperature basalt/seawater interaction, whereas hydrothermal solutions may be responsible for the nonoxidative alteration.