Ocean Drilling Program, United States

Equilibrium melting and controlled cooling experiments were undertaken to constrain the crystallization and cooling histories of tholeiitic basalts recovered by the Ocean Drilling Program drilling of Site 989 on the Southeast Greenland continental margin. Isothermal experiments conducted at 1 atm. and at the fayalite-magnetite-quartz buffer using lava sample Section 163-989B-10R-7 yielded the equilibrium appearance sequence with decreasing temperature: olivine at 1184+ or -2 degrees C; plagioclase at 1177 degrees C+ or -5 degrees C; augite at 1167+ or -5 degrees C; and pigeonite at 1113+ or -12 degrees C. In controlled cooling experiments using the same starting composition and cooling rates between 10 degrees C/hr and 2000 degrees C/hr, we find a significant temperature delay in the crystallization of olivine, plagioclase, and augite (relative to the equilibrium appearance temperature); pigeonite does not form under any dynamic crystallization conditions. Olivine exhibits the largest suppression in appearance temperature (e.g., 30 degrees for 10 degrees C/hr and >190 degrees at 100 degrees C/hr), while plagioclase shows the smallest ( approximately 10 degrees C at 10 degrees C/hr; 30 degrees C at 100 degrees C/hr, and approximately 80 degrees C at 1000 degrees C/hr). These results are in marked contrast to those obtained on lunar basalts, which generally show a large suppression of plagioclase crystallization and modest suppression of olivine crystallization with an increased cooling rate. The results we report agree well with the petrography of lavas recovered from Site 989. Furthermore, the textural analysis of run products, representing a large range of cooling rates and quench temperatures (1150 degrees C to 1000 degrees C), provide a framework for evaluating cooling conditions necessary for glass formation, rates of plagioclase growth, and kinetic factors governing plagioclase growth morphology. Specifically, we use these insights to interpret the textural and mineralogical features of the unusual compound flow recovered at Site 989. We concluded from the analysis that this flow most likely records multiple breakouts from a distal tube at an abrupt break in slope, possibly a fault scarp, resulting in the formation of a lava fan delta. This interpretation implies that normal faulting of the oldest lava sequences (lower and, possibly, middle series) preceded eruption of Site 989 lavas.