17. GEOCHEMISTRY OF BASALTIC ROCKS FROM THE TAG HYDROTHERMAL MOUND (2608 N), MID-ATLANTIC RIDGE1

Susan E. Smith2 and Susan E. Humphris3

  ABSTRACT

Variably altered basalts, hydrothermal clays, and metabasaltic clasts were recovered during the Ocean Drilling Program (ODP) Leg 158 from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound at 2608 N, 4449 W, on the Mid-Atlantic Ridge. These basaltic samples exhibit whole-rock chemical changes resulting from hydrothermal alteration of mid-ocean ridge basalt (MORB) at the edges of the TAG mound and within the stockwork zone. Relatively unaltered basalts are used to constrain the petrogenetic history of the magmas in the TAG region. The whole-rock compositions of basalts have relatively narrow ranges of Mg-numbers of 62.6–64.5, olivine compositions (Fo85.8-Fo86.3), and (La/Sm)cn ratios of 0.66-0.73, but rather variable CaO/Al2O3 ratios of 0.70-0.77 and Na2O contents of 2.29-2.73 wt%. These Leg 158 basaltic whole-rock compositions have major-, trace-, and rare-earth–element characteristics similar to the glasses from this region. Fractional crystallization modeling using pseudoternary projections and liquid lines of descent calculations indicate that the Leg 158 basalts have probably undergone moderate pressure fractionation of olivine, then olivine + plagioclase at 4 to 6 kb, followed by rapid ascent from the upper mantle with very short crustal residence times. Multiple parental melt compositions are indicated by partial melting and fractional crystallization models. Maximum extents of partial melting in the upper mantle are estimated to be between ~11% and ~25% which is a larger variation than near the Kane Transform (MARK) area at ~10 to ~15%. The maximum extents of partial melting along the Mid-Atlantic Ridge show an overall decline from the Azores hotspot region (38N) to the region south of the Hayes Transform (33N), the TAG region (26N), and Mid-Atlantic Ridge near the Kane Transform (MARK) area (23N). Melt production and accumulation processes along the TAG segment, however, provide variability on the local scale.

1Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998. Proc. ODP, Sci. Results, 158: College Station, TX (Ocean Drilling Program).
2Department of Geosciences, University of Houston, Houston, TX 77204-5503, U.S.A. sesmith@uh.edu
3Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.