201D Seismology

61 Route 9W - PO Box 1000

Palisades

NY

10964-8000

US

Phone: 

(845) 365-8460

Fax: 

(845) 365-8150

collier@ldeo.columbia.edu

Basaltic magmatism is a fascinating process with relevance to a wide range of "big picture" topics in Earth and planetary science. Basaltic lavas and mantle residues reveal complex patterns of geochemical variability at virtually all spatial scales, even within the relatively "simple" basaltic magma systems found at ocean ridges. In my dissertation research, I have examined geochemical variability documented in large ocean ridge data compilations (such as the PetDB global compilation of mid-ocean ridge basalts), with the goal of evaluating the comparative ability of different petrologic hypotheses to simultaneously account for both spatial and statistical patterns of variability.

Among the key geologic results from this research is the argument that simultaneous crystallization and cation-exchange reactions with surrounding mantle rock (which co-author Peter Kelemen and I term "reactive crystallization") may represent a common and important process experienced by magmas within the thermal boundary layer beneath mid-ocean ridges. We predict that the liquid line of descent resulting from reactive crystallization will be very different from that expected for fractional crystallization.

Therefore, "fractionation-corrected" MORB variability could be largely caused by sample-to-sample variations in the relative extents of reactive vs fractional crystallization. Variations in melt transport velocity through the thermal boundary layer, in turn, are predicted to modulate the relative proportion of reactive and fractional crystallization. Reactive crystallization may obscure the compositional effects of, e.g., variations in mantle source properties or deep melt transport within the melting region, which are the currently favored interpretations of "fractionation-corrected" MORB variability. Most recently, we have examined the statistical variability within the mantle section of the Oman ophiolite, among the most spatially extensive surface exposures of ocean ridge processed mantle worldwide.

We have created a preliminary map of intermediate scale mantle compositional variability, combining statistical characterization of large detrital spinel data sets with corresponding drainage basin morphologies extracted from digital elevation data. Our data define multiple spatially coherent, compositionally distinctive ~20-100 km^2 regions, representing perhaps the first direct mapping of compositional mantle domains at this length scale worldwide. We interpret our observations as the consequence of regionally distinctive internal proportions of different mantle lithologies (e.g., dunite versus harzburgite), in turn reflecting the organization of focused melt transport at ocean ridges into ``channel-rich" and ``channel-poor" zones. Some variability within the mapped region could additionally reflect igneous impregnation, variable depletion during melting, or post-ocean ridge magmatism associated with ophiolite emplacement.