THE MAGMA BODY AT KILAUEA-IKI LAVA LAKE - POTENTIAL INSIGHTS INTO MIDOCEAN RIDGE MAGMA CHAMBERS

Although geological and geophysical measurements at active ridges have provided indirect samples and remote images and ophiolites have provi ded fossil evidence, the lack of direct sampling of in situ physical a nd chemical properties severly hinders our understanding of subsurface magmatic systems at mid-ocean ridges. In contrast with the ridge sett ing, substantial information has been gained from direct sampling of K ilauea Iki lava lake. We reanalyze time series physical and petrologic al data from both in situ sampling and remote seismic and electromagne tic imaging of this lava lake in order to gain insights into the evolu tion of an enclosed basaltic magma body with many characteristics simi lar to a mid-ocean ridge magma chamber. Although the analogy is imperf ect, our analysis of observations from Kilauea Iki suggests that at a mid-ocean ridge (1) a fine structure exists at the top of the magma bo dy in the form of horizontal segregation veins (i.e., sills), (2) sust ained convection within the magma chamber is highly unlikely, (3) a '' dry-out'' zone above the magma body and below the deepest penetration of hydrothermal circulation may exist, (4) with high magma supply (e.g ., the East Pacific Rise), where quasi steady state melt zones are pre sent, the magma chamber must be completely replenished very frequently (at least every 300-600 years), and (5) the onset of rigid behavior o f the basaltic partial melt does not happen at a unique ''critical'' c rystal fraction. These results also show that significant differences may occur in interpretation of the rheology of a magmatic system depen ding upon the type of petrologic, seismic, or drilling data analyzed.