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Summary. The East Pacific Rise at 12-15' S is topographically smooth with a crestal horst or linear volcanic peak marking the present axis of spreading. The Galapagos Rise at 14-17's is topographically rough with a possible central graben marking the extinct spreading axis. The seafloor spreading magnetic anomalies on the East Pacific Rise are of low amplitude, but fracture-zone anomalies at 13-14's have amplitudes of up to 1250 nT. Anomalies of this amplitude at the magnetic equator must be formed within the fracture zone by some combination of block reversal boundaries, anomalously-high magnetic intensities, and/or anomalously-large thicknesses of the magnetic layers within the fracture zone. Magnetization and majorelement chemical analyses of basalts dredged from four locales along the fracture zone indicate that the large magnetic-anomaly amplitudes are caused by the high iron and titanium content of these ferrobasalts. The magneticanomaly profdes from the Galapagos Rise and its fracture-zone system are of normal amplitude and are extremely difficult to correlate internally or with the geomagnetic timescale. Eighty-one heat-flow measurements indicate that the values measured are controlled by sediment thickness. Where the thickness of the sediment blanket is greater than loom, high heat flow is measured and possibly is representative of the total heat transfer at the seafloor. Where the sediment thickness is less than loom, seawater circulation in the oceanic crust is thought to remove most of the heat convectively; thus causing low conductive heat-flow values to be measured by the usual heat-flow apparatus. The heat loss by convective processes is probably a function also of topographic roughness and sediment permeability.

Summary The spatial pattern of cooling near a spreading ridge crest was investigated with a suite of 71 precisely-navigated heat-flow stations on the Galapagos spreading centre, East Pacific, near 86"W longitude. Stations are on crust less than 1.0 My old on which bathymetry and sediment distribution are well known. Values vary from near zero to greater than 30 HFU (10-6 cal cm-2 s-1). The average over the entire region is significantly less than that predicted by theoretical conduction models of a cooling lithosphere. We observe a regular variation of the heat-flow pattern with a wavelength of 6 & 1 km approximately normal to the ridge crest. Heat-flow maxima are characteristically located near faults and local topographic highs. The locations of fields of small sediment mounds, apparently hydrothermal vents, are also restricted to these faulted, elevated areas of high heat flow. Near-axis, bottom water temperature anomalies of several hundredths "C were detected. The low average, the low minima in the heat-flow pattern, and the water temperature anomalies suggest that hydrothermal circulation accounts for approximately 80 per cent of the geothermal heat released near the ridge crest. We conclude that the hydrothermal circulation pattern is controlled by one or more of the following physical properties of the system : highly developed cellular convection, discrete zones of high permeability, variation in the strength of heat sources near the base of the crust, or bottom topography. Our results imply that heat-flow studies near active oceanic ridges will be of most value if they are sufficiently detailed and well navigated to define the systematic small-scale variations that appear to be caused by hydrothermal circulation.

Summary The axis of the Galapagos spreading centre has a high but variable heat flow. A distinct local minimum in the heat-flow field exists in a region 4–6 My old on the northern flank of the spreading centre. This minimum is unlikely to have been produced from refraction of heat flux due to either topographic and sediment thickness irregularities or to the transient effect of sediment loading. We suggest that the oceanic crust has an ‘effective’ permeability to fluid flow and that hydrothermal circulation causes much of the heat loss to arise through mass transport. This mechanism also lowers the conductive heat flow from beneath the sediments causing a regional minimum in the observed heat flux. We consider two different models for the control of the ‘effective’ permeability. The first invokes chemical reactions to open and close the cracks which give rise to the ‘effective’ permeability. In the second, no reactions are invoked and the oceanic crust never completely loses its permeability. The latter model implies that in areas of rough topography with outcropping basement highs the regional heat flow cannot be determined solely by conductive measurements.

Summary Geochemical constraints based upon compositions and mechanisms of formation and eruption of island arc volcanic rocks strongly suggest hydrous conditions in the region of primary magma generation. The source of most of the water is probably dehydration of the oceanic crust during underthrusting. Chemical reactions involving dehydration of representative oceanic crustal assemblages absorb enough heat to possibly negate any frictional heat which may be generated along the Benioff-Wadati zone in the upper 75 to 100 km of the underthrust zone. Calculations of thermodynamic parameters of relevant reactions at elevated pressures and temperatures show that N 50cal of heat are absorbed per gramme of oceanic crust dehydrated. The result of this heat absorption is suppression of isotherms not only in the oceanic crustal section of the downgoing slab but also in the overlying mantle wedge. Therefore, low heat fiow measured from the trenches to the volcanic arcs may be caused in part by the heat-sink effect of dehydration in the downgoing slab. The water released by dehydration either remains in the slab promoting its melting and the generation of andesitic primary magmas and/or escapes into the overlying wedge of mantle promoting its melting and the generation of the basaltic or andesitic primary magmas. The major thermal effect of the released water is to cool the wedge overlying the slab. The progressive geochemical changes in arc volcanism both in space and time may be controlled by the thermal conditions at the downgoing slab-peridotite wedge interface.