Eighty samples of submarine basaltic lava were sampled 

 from an 8 km segment of the floor and walls of the inner 

 rift valley of the Mid-Atlantic Ridge during project Famous. 

 The samples were collected from outcrops and talus slopes by 

 the three submersibles: Alvin, Archimede, and Cyana at 

 water depths of about 2,600 m. 



The early formed mineral content of the pillow lavas' 

 glassy margins enables classification of the rocks into 5 types: 

 (1) olivine basalt, (2) picritic basalt, (3) plagioclase-olivine- 

 pyroxene basalt, (4) aphyric basalt, and (5) plagioclase-rich 

 basalt. Chemical and mineralogical study indicates that at least 

 4 types are directly interrelated and that types (1) and (2) are 

 higher-temperature, primitive lavas and types (3) and (4) 

 are lower-temperature, differentiated lavas derived from the 

 primitive ones by crystal-liquid differentiation. The plagioclase- 

 rich basalts also have a chemical composition of their glass 

 comparable to that of the most differentiated basalts (types 3 

 and 4) but they differ in their greater amount of early formed 

 plagioclase (12-35%). 



In general, the mineralogical variation across the rift 

 valley shows an asymmetrical distribution of the major basalt 

 types. Despite the mineralogical diversity of the early formed 

 crystals, the chemistry of the basalt glasses indicates a sym- 

 metrical and a gradual compositional change across the rift 

 valley. Based primarily on their chemistry, the rock types 1 and 

 2 occupy an axial zone 1.1 km wide and make up the central 

 volcanic hills. Differentiated lavas (types 3, 4) occupy the 

 margins and walls of the inner rift valley and also occur near 

 the center of the rift valley between the central hills. 



FeO/MgO ratios of olivine and coexisting melt indicate 

 that the average temperature of eruption was 40° C higher 

 for the primitive melts (types 1 and 2). Aside from major ele- 

 ments trends, the higher temperature character of the primitive 

 basalts is shown by their common content of chrome spinel. 



The thickness of manganese oxide and palagonite on 

 glassy lava provide an estimate of age. In a general fashion, 

 the relative age of the various volcanic events follow the 

 compositional zoning observed in the explored area. Most of 

 the youngest samples are olivine basalt of the axial hills. Most 

 older samples occur in the margins of the rift valley (West 

 and N.E. part of explored area), but are significantly younger 

 than the spreading age of the crust on which they are erupted. 

 Intermediate lava types occur mainly east of the rift valley 

 axis and in other areas where plagioclase — olivine — pyroxene 

 basalt and aphyric basalt are present. 



The above relations indicate that the diverse lava types 

 were erupted from a shallow, zoned magma chamber from fis- 

 sures distributed over the width of the inner rift valley and 

 elongate parallel to it. Differentiation was accomplished by 

 cooling and crystalization of plagioclase, olivine, and clino- 

 pyroxene toward the margins of the chamber. The centrally 

 located hills were built by the piling up of frequent eruption 

 of mainly primitive lavas, which also are the youngest flows. 

 In contrast, smaller and less frequent eruptions of more dif- 

 ferentiated lavas were exposed on both sides of the rift valley 

 axis. 



Galapagos Spreading Center 



Scientists from Oregon State University, Woods Hole 

 Oceanographic Institution, Scripps Institution of Oceanog- 



raphy, Massachusetts Institute of Technology, Stanford Uni- 

 versity, and the U.S. Geological Survey are participating in a 

 program to study hydrothermal circulation processes on the 

 Galapagos Spreading Center (fig. 28). The research submer- 

 sible Alvin will be used during February and March 1977 to 

 do very detailed sampling of the hydrothermal water as it is 

 discharged from the ocean crust. These detailed samples will 

 enable the scientists to accurately determine the chemical com- 

 position, temperature, and rate of discharge of the hydrother- 

 mal fluids. 



Previous oceanographic research in the area northeast 

 of the Galapagos Islands had indicated that hydrothermal cir- 

 culation was occurring along the Galapagos Spreading Center. 

 In June and July of 1976, scientists in the IDOE sponsored 

 program sampled bottom water temperatures and chemistry, 

 and collected sediment samples in the area to pinpoint the most 

 likely spots for hydrothermal activity. Two acoustic beacons 

 were left on the seafloor to help scientists to relocate them- 

 selves during the diving program. 



Directly over the zone of crustal generation along the 

 spreading center scientists found bottom water temperature 

 anomalies as high as 0.25°C. As normal seawater circulates 

 down into the ocean crust in hydrothermal systems, it is gradu- 

 ally heated and driven upward. Where it discharges at the sea 

 floor, it raises the temperature of near bottom water. During its 

 transit, the seawater reacts chemically with the basalt of the 

 ocean crust. Laboratory studies have shown that magnesium 

 is removed from seawater; calcium and manganese are added 

 to seawater during basalt-seawater reactions. In the area of 

 high bottom water temperatures over the Galapagos spreading 

 Center, the bottom water showed low concentrations of mag- 

 nesium and high concentrations of calcium and manganese. 

 This would be expected if a source of seawater that had 

 reacted with basalt were present at the spreading center. 



Thick crusts of manganese-rich material were recovered 

 from the surface of small sediment mounds to the south of the 

 spreading center. The mounds have apparently been the site 

 of hydrothermal discharge, with the manganese precipitating as 

 the hydrothermal fluids mix with seawater. On one of the 

 mounds, the measured heat flow exceeded 30 h.f.u. Surpris- 

 ingly, there were no recorded bottom water temperature anom- 

 alies over these mounds. 



The use of the submersible will allow scientists to ex- 

 amine one of the remaining unanswered questions related to 

 this process, the precise chemical composition and temperature 

 of the fluid as it discharges at the sea floor. Samples obtained 

 from a surface ship have undergone mixing between the hydro- 

 thermal fluids and bottom waters. Because there is no way to 

 determine the amount of mixing that has occurred, there is 

 no method of calculating the initial chemical compositions or 

 temperature of the hydrothermal fluids. These two initial 

 values arc important in evaluating the significance of this proc- 

 ess in modifying the chemical composition of seawater. 



Nazca Plate 



The Nazca Plate lies in the southeastern Pacific parallel 

 to the western boundary of the metallogenic province of the 

 Andes. This project was initiated in 1971 as a cooperative 

 research effort between the University of Hawaii's Institute of 

 Geophysics (HIG) and Oregon State University (OSU) with 



47 



