Marks, Naomi et al. (2011): High-grade contact metamorphism in the Reykjanes geothermal system; implications for fluid-rock interactions at mid-oceanic ridge spreading centers

ODP 206
ODP 111 504
ODP 137 504
ODP 140 504
ODP 148 504
ODP 206 1256


Marks, Naomi
University of California at Davis, Department of Geology, Davis, CA, United States

Schiffman, Peter

Zierenberg, Robert A.

High-grade contact metamorphism in the Reykjanes geothermal system; implications for fluid-rock interactions at mid-oceanic ridge spreading centers
Geochemistry, Geophysics, Geosystems - G (super 3)
American Geophysical Union and The Geochemical Society, United States
Granoblastic hornfels identified in cuttings from the Reykjanes seawater-dominated hydrothermal system contains secondary pyroxene, anorthite, and hornblendic amphibole in locally equilibrated assemblages. Granoblastic assemblages containing secondary orthopyroxene, olivine, and, locally, cordierite and spinel occur within groups of cuttings that show dominantly greenschist facies hydrothermal alteration. Granoblastic plagioclase ranges continuously in composition from An54 to An96, in contrast with relict igneous plagioclase that ranges from An42 to An80. Typical hydrothermal clinopyroxene compositions range from Wo (sub 49) En (sub 3) Fs (sub 48) to Wo (sub 53) En (sub 30) Fo (sub 17) ; clinopyroxene from the granoblastic grains is less calcic with an average composition of Wo (sub 48) En (sub 27) Fs (sub 25) . The hornfels is interpreted to form during contact metamorphism in response to dike emplacement, resulting in local recrystallization of previously hydrothermally altered basalts. Temperatures of granoblastic recrystallization estimated from the 2-pyroxene geothermometer range from 927 degrees C to 967 degrees C. Redox estimates based on the 2-oxide oxybarometer range from log fO (sub 2) of -13.4 to -15.9. Granoblastic hornfels comprised of clinopyroxene, orthopyroxene, and calcic plagioclase have been described in a number of ancient hydrothermal systems from the conductive boundary layer between the hydrothermal system and the underlying magma source, most notably in Integrated Ocean Drilling Program Hole 1256D, Ocean Drilling Program Hole 504B, and in the Troodos and Oman ophiolites. To our knowledge, this is the first evidence of high-grade contact metamorphism from an active geothermal system and the first description of equilibrated amphibole-absent pyroxene hornfels facies contact metamorphism in any mid-ocean ridge (MOR) hydrothermal system. This contribution describes how these assemblages develop through metamorphic reactions and allows us to predict that higher-temperature assemblages may also be present in MOR systems.
Coverage:Geographic coordinates:
West:-23.3000East: -20.0000

Igneous and metamorphic petrology; alteration; amphibole group; anorthite; Arabian Peninsula; Asia; Atlantic Ocean; chain silicates; clinoamphibole; clinopyroxene; contact metamorphism; Cretaceous; DSDP Site 504; East Pacific; Eh; Equatorial Pacific; feldspar group; fluid phase; framework silicates; fugacity; geochemistry; geothermal systems; granoblastic texture; high-grade metamorphism; hornblende; hornfels; hydrothermal conditions; Integrated Ocean Drilling Program; Leg 206; Mesozoic; metamorphic rocks; metamorphism; mid-ocean ridges; mineral assemblages; North Atlantic; North Pacific; Northeast Pacific; Ocean Drilling Program; ocean floors; ODP Site 1256; Oman; ophiolite; oxygen; Pacific Ocean; plagioclase; pyroxene group; Reykjanes Ridge; silicates; Troodos Ophiolite; water-rock interaction;