HYDROTHERMAL ANORTHITIZATION OF PLAGIOCLASE WITHIN THE MAGMATIC HYDROTHERMAL TRANSITION AT MIDOCEAN RIDGES - EXAMPLES FROM DEEP SHEETED DIKES (HOLE 504B, COSTA-RICA RIFT) AND A SHEETED DIKE ROOT-ZONE (OMAN OPHIOLITE)/

The unique petrological record from the deep sheeted dikes of ODP Hole 504B gives insight into the magmatic/hydrothermal transition and the hydrothermal reaction zone at mid-ocean ridges. A high-temperature pea k hydrothermal event at Hole 504B, defined as Stage 1, corresponds to an early stage in the evolution of an axial reaction zone, and perhaps to an initial high-temperature phase of black smoker activity. The te mperatures and/or the a(Ca)/a(Na)(2) ratios of hydrothermal fluids ove rlapped and even exceeded those inferred for the reaction zones of man y modem black smoker systems. The volume of hydrothermally altered roc k at this stage of evolution of the reaction zone was much less than 1 0%. Stage 1 is manifest in hand specimen by reaction halos adjacent to hornblende veins and by irregular alteration patches. There is no epi dote associated with Stage 1. Wallrock reactions involved the replacem ent of magmatic clinopyroxene by magnesiohornblende and of magmatic pl agioclase by hydrothermal plagioclase. These secondary minerals were n ot in equilibrium. Hydrothermal plagioclase varies in composition from An(54) to An(95), and has low MgO and FeOTot. It has distinctive brig ht cathodoluminescence and a porous appearance. Stage 1 at Hole 504B o ccurred at pressures between 38 and 85 MPa (0.38 and 0.85 kbar) and te mperatures in excess of about 425 degrees C; these P-T conditions clos ely approach the two-phase field for seawater. Stage 1 is overprinted by retrograde alteration that caused further replacement within halos and patches as well as 10-40% alteration, mostly to chlorite and albit e, of all the fresh sheeted dikes that were unaffected during Stage 1. Stage 1 alteration at Hole 504B is unique among oceanic and ophioliti c samples. However, we report one area from the Oman ophiolite that ex hibits features similar to Stage 1 at Hole 504B. Early halos around ho rnblende veins contain secondary hornblende and porous hydrothermal ca lcic plagioclase (An(55)-An(78)) with low MgO and FeOTot. Fluid inclus ions in quartz show that some early hydrothermal fluids had 5.6 +/- 2. 3 wt% NaCl equivalent at temperatures constrained to be above 320 degr ees and below 510 degrees C, as well as a saline brine with 31 to 40 w t% equivalent NaCl and similar temperature constraints. Although evide nce for Stage 1 alteration of sheeted dikes is far from ubiquitous, it is now identified in an oceanic setting (Hole 504B) and in an ophioli tic example from Oman. (C) 1998 Elsevier Science B.V. All rights reser ved.