30. LOW PRESSURE EXPERIMENTAL CONSTRAINTS ON THE EVOLUTION OF BASALTIC LAVAS FROM SITE 917, SOUTHEAST GREENLAND CONTINENTAL MARGIN1

P. Thy,2 C.E. Lesher,2 and M.S. Fram2

ABSTRACT

One-atmosphere melting experiments conducted on lavas recovered from Site 917 on the southeast Greenland Margin define the liquid line of descent for primitive basalts for the continental volcanic succession. The most primitive aphyric lava from the Lower Series, with 9.1 wt% MgO, defines the crystallization sequence of olivine (1247 5C), plagioclase (1219 5C), followed by augite (1182 5C). A sample from the Upper Series with 10.7 wt% MgO shows a similar crystallization order with plagioclase and olivine appearing at 1228 5C and augite crystallizing at 1182 5C. The liquid lines of descent are in close agreement with the compositional relationships for the primitive members of the Lower and Upper Series and the oceanic succession drilled at Sites 915 and 918, but contrast sharply with those of differentiated lavas from the Middle Series of Site 917 modified by crustal contamination. Some differentiated lavas from the Lower Series indicate excessive olivine fractionation due to suppressed plagioclase crystallization at elevated fH2O. Consideration of Fe-Mg exchange equilibrium for high-MgO aphyric lavas and picrites recovered at Site 917 confirms the accumulative nature of the picrites and is used to calculate melts in equilibrium with typical mantle source. We show that major element compositions for these calculated primary melts (15–18 wt% MgO) are consistent with previous estimates for rifted margin basalts in the North Atlantic. The production of more Fe- and Ti-rich primary melts with time suggests that the efficiency of melt segregation and potential temperature increased during production of the volcanic succession at Site 917.

1Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998. Proc. ODP, Sci. Results,152: College Station, TX (Ocean Drilling Program).
2Department of Geology, University of California, Davis, CA 95616, U.S.A. thy@geology.ucdavis.edu