423 



FUTURE DIRECTIONS IN OCEAN SCIENCES 85 



Off-Ridge Processes 



Off-ridge processes can be studied to determine how Earth 

 functioned in the past and whether there are additional temporal 

 variability and forcing functions that will not be discovered by 

 studying the geological present. The seafloor contains a record of 

 the creation of the oceanic lithosphere. In addition, important 

 questions concern changes in the oceanic crust as it ages. The 

 older oceanic crust also contains information concerning mantle 

 convection and composition. Several important investigative themes 

 can be identified. 



Chemical and Isotopic Record of Mantle Convection 



Because the mantle is overlain by the crust, it is not possible 

 at present to sample the suboceanic mantle directly, except in 

 tectonically anomalous areas (e.g., oceanic fracture zones). The 

 basalts that are derived from the mantle, however, are indirect 

 mantle samples that have been modified by partial melting and 

 partial crystallization. Because the oceanic crust is thin and its 

 composition is similar to the magma, and because the spreading 

 center provides a relatively permeable and free pathway to the 

 surface, ocean ridges are the sites where the magma is least modi- 

 fied. Thus ocean ridge basalts typically provide the least adulter- 

 ated record of mantle composition and temperature. Mapping 

 crustal composition can provide quantitative information about 

 the size, distribution, and composition of mantle reservoirs and 

 the efficiency of convective stirring. This information is a record 

 and an opportunity to map indirectly the composition and tem- 

 perature of the mantle. 



Variation of Melt Production (Convective Vigor) Through Time 



There is strong evidence that plate separation rates and basal- 

 tic magma production rates along ridges and within plates are not 

 constant. For example, a 50 to 75 percent increase in the rate of 

 formation of oceanic crust and a doubling in the production rate 

 of basaltic magmas between 120 and 80 million years ago (Figure 

 3-2; Larson, 1991) has been documented. The changes may be due 

 to a large m.antle-derived super plume that may have lifted off the 

 core-mantle boundary and have been responsible for increased seafloor 

 spreading and large-scale oceanic plateau production (e.g., Ontong 

 Java and Kerguelen plateaus). It has been suggested that the super 



