Granoblastic textures within the Reykjanes Geothermal System: Evidence for amphibolite-grade contact metamorphism in an active geothermal system and links to IODP Hole 1256D

N.E. Marks¹*, R.A. Zierenberg¹, & P. Schiffman¹

Corresponding author: nemarks@ucdavis.edu
¹University of California-Davis, Department of Geology, Davis, CA, 95616

Abstract:
The Reykjanes geothermal system is a seawater recharged hydrothermal system that appears to be analogous to seafloor black smoker systems. Fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions imposed by glaciation (Arnorsson, 1995; Fridleifsson et al., 2003). Ice sheets overlying the geothermal system during the Pleistocene increased the hydrostatic pressure and therefore the boiling point, resulting in higher temperatures and more intense alteration at shallower depths (Fridleifsson et al, 2005). The deepest borehole to date in the Reykjanes system is RN-17, which was drilled to a depth of 3082 m. Temperatures in the lower portion of the borehole were never recorded due to an obstruction at 2100 m depth, but are estimated to be approximately 340° C. Epidote, albite, and actinolite are ubiquitous within pillow basalt, hyaloclastite, and in veins, implying that greenschist grade conditions have been attained throughout much of the well below approximately 1200 m; Magnesiohornblende is common below about 2100 m. Intrusive events within the system have produced small, but recognizable contact metamorphic effects characterized by granoblastic hornfels consisting of amphibolite grade assemblages of quartz + anorthite + diopside + magnetite + titanite and plagioclase + orthopyroxene + diopside + olivine + ilmenite + magnetite. These have, in turn, been locally cut by actinolite veins, presumably reflective of the present-day, thermal state of the hydrothermal system at these depths. Based on their bulk composition, we believe the hornfels represent the thermally-recrystallized products of earlier-formed, hydrothermal veins consisting of quartz, epidote, and actinolite. The mineralogy of the granoblastic textured cuttings observed in the Reykjanes Geothermal system resembles that of the Granoblastic Dike domains within the sheeted dikes of the IODP Hole 1256D on the East Pacific Rise as well as those observed in the Oman ophiolite (Nicolas et al., 2008; Coggon et al., 2008; Wilson et al., 2006). This high temperature alteration is easily distinguishable from the adjacent mafic intrusives on the basis of mineral chemistry and texture. Pyroxene compositions, along with Mt-Il geothermometry, imply that hornfelsic recrystallization may have occurred at temperatures as high as 625-675 °C at depths of 2 to 2.5 km. If the intrusive activity occurred during the last glaciation, hydrostatic pressures attending amphibolite metamorphism may have been as high as 300 bars. To our knowledge, this is the first time that texturally-equilibrated, amphibolite grade assemblages have been reported from an active geothermal system.

Keywords:
Granoblastic, alteration, sheeted dikes, Iceland, East Pacific Rise

Contributions to Integration and Synthesis:
The granoblastic textures observed in cuttings from the Reykjanes geothermal system are remarkably similar to those of the Granoblastic Dikes observed in within IODP 1256D. The results of my work on these cuttings provides a useful link between processes occurring in subareial mid-ocean ridge settings (i.e. Iceland), submarine systems (i.e. East Pacific Rise), and ophiolite settings. Research on analogous alteration and recrystalization processes in a subaerial setting, will enrich our understanding of spreading center and ophiolite processes.