Hydrothermal anorthitization of plagioclase within the magmatic/hydrothermal transition at mid-ocean ridges: examples from deep sheeted dikes (Hole 504B, Costa Rica Rift) and a sheeted dike root zone (Oman ophiolite)

Autor: Christine Laverne, David A. Vanko
Rok vydání: 1998
Předmět:
Zdroj: Earth and Planetary Science Letters. 162:27-43
ISSN: 0012-821X
Popis: 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 peak 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 temperatures and/or the a Ca / a Na 2 ratios of hydrothermal fluids overlapped and even exceeded those inferred for the reaction zones of many modern black smoker systems. The volume of hydrothermally altered rock at this stage of evolution of the reaction zone was much less than 10%. Stage 1 is manifest in hand specimen by reaction halos adjacent to hornblende veins and by irregular alteration patches. There is no epidote associated with Stage 1. Wallrock reactions involved the replacement of magmatic clinopyroxene by magnesiohornblende and of magmatic plagioclase by hydrothermal plagioclase. These secondary minerals were not in equilibrium. Hydrothermal plagioclase varies in composition from An 54 to An 95 , and has low MgO and FeO Tot . It has distinctive bright cathodoluminescence and a porous appearance. Stage 1 at Hole 504B occurred at pressures between 38 and 85 MPa (0.38 and 0.85 kbar) and temperatures in excess of about 425°C; these P–T conditions closely 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 albite, of all the fresh sheeted dikes that were unaffected during Stage 1. Stage 1 alteration at Hole 504B is unique among oceanic and ophiolitic samples. However, we report one area from the Oman ophiolite that exhibits features similar to Stage 1 at Hole 504B. Early halos around hornblende veins contain secondary hornblende and porous hydrothermal calcic plagioclase (An 55 –An 78 ) with low MgO and FeO Tot . Fluid inclusions in quartz show that some early hydrothermal fluids had 5.6±2.3 wt% NaCl equivalent at temperatures constrained to be above 320° and below 510°C, as well as a saline brine with 31 to 40 wt% equivalent NaCl and similar temperature constraints. Although evidence for Stage 1 alteration of sheeted dikes is far from ubiquitous, it is now identified in an oceanic setting (Hole 504B) and in an ophiolitic example from Oman.
Databáze: OpenAIRE
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