Along the MAR near Iceland and the Azores, major element indices of the degree of mantle melting (Na/Mg in lavas and pyroxene content in peridotites) suggest an unusually high degree of melting, if one assumes constant source composition. In contrast, trace element indices (high La/Sm or K/Ti) from the same regions, interpreted in the same way, indicate a small degree of melting. This apparent paradox is easily resolved; the mantle source composition is not constant along the ridge (e.g., Schilling, 1973). This is borne out by radiogenic isotope ratios, which indicate a long-term enrichment in incompatible elements (such as La and K) in the mantle source where the degree of melting is large (e.g., Hart et al., 1973). Enriched areas with apparent high degrees of melting areas have been interpreted as "hotspots," in accord with the notion that high temperature and chemical enrichment are correlated in the mantle. However, because this correlation between temperature and enrichment is poorly understood and may vary from place to place, there is debate over their relative importance in controlling igneous crustal thickness, crustal composition, axial depth, and geoid height.
Work in the 14° to 16°N region along the MAR can provide constraints for deconvolving the effects of temperature and composition on mantle melting. There is a substantial gradient over 150 km along the ridge, from geochemically "normal" MORB in the north (moderately high Na/Mg and low La/Sm) to strongly "enriched" MORB in the south (low Na/Mg and high La/Sm) (Fig. F3), there is a large gradient in crustal thickness, increasing away from the fracture zone. One hypothesis holds that "enriched" basalts are derived by partial melting of veins that comprise a few percent of the volume of the source region. Drill core will provide a sample that permits determination of the proportion of volumetrically minor veins in the peridotite, and isotope measurements on these veins may place constraints on the original composition of these veins prior to decompression melting.
Another goal of drilling will be characterization of hydrothermal alteration of mantle peridotite and plutonic rocks to quantify chemical changes associated with alteration of peridotite at a variety of temperatures. Systematic geochemical studies of samples with a variety of different extents and types of alteration is necessary to discriminate between trace element features retained from igneous processes vs. those that are dominantly imposed during open system alteration. It is now recognized that a large proportion of slow-spreading lithosphere is composed of serpentinized peridotite, which is eventually subducted, but the composition of this geochemical reservoir is poorly characterized and understood. Also, as for melt transport veins, discussed above, continuous core can be used for detailed studies of the size/frequency and spatial distribution statistics of alteration veins, providing important information on the mechanisms of vein formation and fluid transport (e.g., Kelemen et al., 1998a; Magde et al., 1995).
A variety of recent observations on slow-spreading ridges including the MAR suggests that the crust in these settings is a complicated mixture of gabbroic plutons and partially serpentinized peridotite (review in Cannat, 1996). Mantle peridotite is known to crop out along both flanks of the MAR from at least 14°40' to 15°40'N (Fig. F2). In some cases, lava flows lie directly over mantle peridotite without intervening gabbroic "lower crust." Thus, this region is "magma-starved," an end-member compared to the "robust" East Pacific Rise.
Surprisingly, seismic surveys of regions of slow-spreading ridges with abundant peridotite outcrops generally yield significant crustal thicknesses, if crust is defined as material with a seismic P-wave velocity of <8 km/s. This is true, for example, for the MAR just north of the 15°20'N FZ, within the proposed drilling area (Fig. F4) (R. Detrick and J. Collins, pers. comm., 1998). In general, seismic data have been used to determine an average crustal thickness of 6 to 7 km for oceanic crust formed far from mantle hotspots, independent of spreading rate (e.g., White et al., 1992). This paradox represents a first-order problem in studies of the global ridge system.
If possible, it will be very important to develop a geophysical technique for distinguishing between partially serpentinized peridotite and plutonic gabbroic rocks, even where these have the same seismic velocity and density (e.g., Christensen and Salisbury, 1975; Miller and Christensen, 1997). Obtaining extensive drill core of altered mantle peridotite from well below the surface weathering horizon in the 15°N area, together with prior geophysical characterization of this area and downhole logging, will be a first step in resolving this problem. Physical properties of the samples measured in the laboratory (remnant magnetization, density, seismic velocities and attenuation, and electrical conductivity) can be compared with geophysical data in order to calibrate the large-scale surface techniques used worldwide. Some of these physical properties are likely to be scale dependent, so that in addition to downhole geophysical logging, we suggest that a second ship be used, at a later time, to conduct seismic and electrical conductivity experiments using downhole instruments and seafloor sources. A combination of lithologic observations on core and geophysical measurements made at true seismic wavelengths can then be used to seek out features in the geophysical signals that are characteristic of partially serpentinized peridotite and truly measurable in the field.
Although serpentinized peridotite may comprise a significant proportion of slow-spreading lithosphere, extending up to the seafloor, regional geophysical surveys show a systematic alternation of normal and reversed magnetized seafloor correlated with crustal age, just as in fast-spreading volcanic Pacific crust. Although our drilling leg will not focus on this problem, we will obtain substantial data on the magnetic properties of serpentinized peridotite, which will aid in interpretation of magnetic data for crust formed at slow-spreading ridges.
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