Epidosite

Epidosite: A low to medium-temperature metasomatic rock mostly formed by the alteration of basaltic rocks, but also other basic to acid igneous rocks; principally composed of epidote and quartz, with minor chlorite and amphibole. The term epidosite (épidotite in its original French form) was coined by Cordier in 1868.

Epidosites form in various geological settings, most notably in sub-seafloor hydrothermal systems (Fig.1) where large quantities of chemically modified seawater react with and replace the basalts, gabbros and rarer plagiogranites that make up the oceanic crust (Seyfried et al., 1988). Reactions of downwelling seawater with fresh basalts creates Ca-depleted, Mg- and Na- enriched "spilite" alteration (albite + chlorite + hematite + titanite ± augite ± epidote ± quartz ± calcite). The fluid in turn becomes enriched in Ca and depleted in Mg and Na. This chemically evolved, upwelling fluid can create Ca-enriched, Mg- and Na-depleted "epidosite" alteration (epidote + quartz + titanite + hematite). Epidosites are thought to represent deep segments of major fluid up-flow zones in vertically extensive hydrothermal convection cells under active submarine spreading ridges or in other submarine extensional structures. The ascending hydrothermal fluid eventually vents on the seafloor, forming black-smoker-type volcanogenic massive sulphide deposits.

epidosite2020(1).png

Fig.1: Conceptual models of water–rock interaction beneath fast spreading ridges. Modified from Gilgen, S. A., Diamond, L. W., & Mercolli, I. (2016).

Sheeted dikes in Cyprus, which were sometimes more or less greenish in color. It is mineral epidote that gives them this bright green tone. From Sand Atlas.

Epidosite from Løkken ophiolite in Norway. From Sand Atlas.

Epidosite from Texas Creek, Colorado, USA. From Dexter Perkins.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Gilgen, S. A., Diamond, L. W., & Mercolli, I. (2016). Sub-seafloor epidosite alteration: Timing, depth and stratigraphic distribution in the Semail ophiolite, Oman. Lithos, 260, 191-210.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 10x (Field of view = 2mm)

Photo
Epitote (gree-yellow) and quartz (colorless) crystals. Epidosite from Campiglia Marittima, Tuscany, Italy. PPL image, 2x (Field of view = 7mm)	Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Epitote (gree-yellow) and quartz (colorless) crystals. Epidosite from Campiglia Marittima, Tuscany, Italy. PPL image, 2x (Field of view = 7mm)
Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Epitote (gree-yellow) and quartz (colorless) crystals. Epidosite from Campiglia Marittima, Tuscany, Italy. PPL image, 2x (Field of view = 7mm)	Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)
Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Epitote (gree-yellow) and quartz (colorless) crystals. Epidosite from Campiglia Marittima, Tuscany, Italy. PPL image, 2x (Field of view = 7mm)
Epitote (gree-yellow) and quartz (colorless) crystals. Epidosite from Campiglia Marittima, Tuscany, Italy. PPL image, 2x (Field of view = 7mm)	Epitote (gree-yellow) and quartz (colorless) crystals. Epidosite from Campiglia Marittima, Tuscany, Italy. PPL image, 2x (Field of view = 7mm)	Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)
Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Epitote (high birefringence) and quartz (grey birefringence) crystals . Epidosite from Campiglia Marittima, Tuscany, Italy. XPL image, 10x (Field of view = 2mm)