The groundmass assemblage and texture of the >69-m-thick compound flow unit (Unit 1) recovered from Site 989 are consistent with rapid extrusion and cooling of thin flows. We conclude that Unit 1 records multiple breakouts from a distal tube at an abrupt break in the slope resulting in the formation of a lava fan delta. The break in the slope may be related to normal faulting shown in the seismic interpretation of the crust in the vicinity of Site 989 (Fig. 2). We cannot exclude the alternative explanation that Unit 1 represents multiple episodes of magma spillover from a near-vent lava lake; however, the low vesicularity of the unit makes this hypothesis less tenable. Based on the present work, and ⁴⁰Ar-³⁹Ar dating results of Tegner and Duncan (Chap. 6, this volume) linking Site 989 Unit 1 with the younger, more seaward lavas of Site 990, we surmise that lava composing Unit 1 flowed considerable distance from the site of eruption, most likely from the axial rift system east (seaward) of Site 989.

In addition to considerations of the emplacement history of lavas at Site 989, our experimental results provide more general implications for the cooling conditions for glass formation and for the crystallization of tholeiitic basalt. We show that glass formation is strongly dependent on cooling rate and that it is unlikely to occur at cooling rates of <10ºC/s. This control on glass formation, as much as the secondary alteration, may be responsible for the scarcity of glass in subaerial flows from the Northeast Atlantic. Finally, we present experimental evidence for large kinetic delays in the nucleation of olivine at low-moderate cooling rates. The extreme suppression of olivine, relative to plagioclase, crystallization under conditions of dynamic crystallization provides not only an explanation for the prevalence of plagioclase and paucity of olivine in lavas from Site 989, but also may be a factor responsible for the high proportion of plagioclase-phyric basalts recovered from the SDRS of Southeast Greenland rifted margin and described from elsewhere in the Northeast Atlantic magmatic province.