Two-phase separation and fracturing in mid-ocean ridge gabbros at temperatures greater than 700°C

Details Two-phase separation and fracturing in mid-ocean ridge gabbros at temperatures greater than 700°C Two-phase separation and fracturing in mid-ocean ridge gabbros at temperatures greater than 700°C

May 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987e%26psl..83...53k&link_type=abstract

Earth and Planetary Science Letters, Volume 83, Issue 1-4, p. 53-66.

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49

Scientific paper

Microthermometric analyses of fluid inclusions on a suite of hydrothermally altered gabbros recovered just south of the eastern intersection of the Kane Fracture Zone and the Mid-Atlantic Ridge, record the highest homogenization temperatures yet reported for mid-ocean ridge hydrothermal systems. Fluid salinities in the high temperature inclusions are more than ten times that of seawater. Multiple generations of fluid inclusions entrapped along healed microfractures exhibit three distinct temperature-compositional groups. We interpret these populations as having been trapped during three separate fracturing events.
The earliest episode of brittle failure in the gabbros is represented by coplanar, conjugate vapor-dominated and brine-dominated fluid inclusion arrays in primary apatite. Vapor-dominated inclusions exhibit apparent homogenization temperatures of 400°C and contain equivalent salinities of 1-2 wt.% NaCl. These inclusions are interspersed with liquid-dominated, sulfide-bearing inclusions containing salinities of 50 wt.% NaCl equivalent. These high salinity inclusions remain unhomogenized at temperatures greater than 700°C.
Compositional and phase relationships of the fluid inclusions may be accounted for by two-phase separation of a fluid under 1000-1200 bars pressure. These pressures require that fluid entrapment occurred under a significant lithostatic component and indicate a minimum entrapment depth of 2 km below the axial valley floor. This depth corresponds to a minimum tectonic uplift of 3 km, in order to emplace the samples at the 3100 m recovery depth. The microfracture networks within magmatic apatites represent fluid flow paths for either highly modified, deeply penetrating seawater or a late stage magmatic aqueous fluid. The inclusions may have formed close to the brittle-ductile transition zone adjacent to an active magma chamber.
Following collapse of the high temperature front, lower temperature fluids of definite seawater origin circulated through the open fracture networks, pervasively altering portions of the gabbros. This stage is represented by low-to-moderate (1-7 wt.% NaCl equivalent) salinity inclusions in plagioclase, apatite, epidote, and augite, which homogenize at temperatures of approximately 200-300°C and 400°C. Formation of hydrous mineral assemblages, under greenschist to lower amphibolite facies conditions, resulted in sealing of the vein system and may have resulted in modification of seawater salinities by as much as a factor of two. During or following these later stages of hydrothermal activity the gabbros were emplaced high on the axial walls by differential uplift attending formation of the flanking mountains.

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