serpentinite
Serpentines are hydrous magnesium-rich silicate minerals that are typically colored gray, green, or white. (left - serpentinized rock with chrysotile) Hydration of olivines to serpentine and magnetite consumes water and produces a significant amount of heat in an exothermic reaction sufficient heat to elevate rock temperatures about 260 ºC, which can generate non-volcanic hydrothermal vents. The Lost City Hydrothermal Field (LCHF) on the Atlantis Fracture Zone near the Mid-Atlantic Ridge is an example of serpentinization-derived hydrothermal activity. Serpentinization may be among the earliest forms of metabolic energy on Earth. Serpentinization causes volume expansion and concomitant decrease in specific gravity (from 3.3 to 2.7 g/cm3) of the hydrothermally altered rocks. Metasomatic alterations convert olivines and pyroxenes to assemblages that depend upon composition of rock and fluids, temperature, and pressure: serpentine, brucite, magnetite, antigorite (at temperatures greater than 600°C), lizardite, and chrysotile, talc, magnesian chlorites, rare awaruite (Ni3Fe), and native iron. Serpentinites are soft, oily looking green to black fine-grained rock that is often highly sheared, breaking into scaly fragments with smooth shiny faces that tend to have translucent edges. Soapstones are rocks composed of the mineral talc and can be mistaken for serpentinite. Soapstone, however, is usually light green to grey in colour and is soft enough to be scratched by a fingernail, whereas serpentinites can only be scratched by a knife. Serpentinites are associated with subduction zones and are formed by the action of high pressure and heat upon hornblende schists or mafic/ultramafic igneous rocks, such as peridotite, gabbro, or basalt. Serpentinites are mostly located in areas where orogeny has sealed off of an ocean basin. The Coast Ranges in California comprise primarily slices of oceanic crust that have been faulted and folded along the coastline. The rock is relatively weak, so mountains containing high levels of serpentinite are prone to erosion and to recurrent landslides. |
| links: images: diagram: conceptual model for serpentinization of peridotitic source and subsequent hydrothermal dolomitization and exhalative block shale formation; schematic cross section of the Mariana forearc showing generalized structural relationship of serpentinite mud volcanoes to faulting in the outer half of the forearc; hand-samples: polished serpentinite, serpentinite, 2, 3, 4, 5, 6, 7; serpentinite with calcite; serpentinite compared to basalt; serpentinite with antigorite; serpentinite, Staten Island; close-ups: serpentinite with talc veins; serpentinite with asbestos vein; serpentinite; serpentinite, the Lizard, Cornwall; light brown serpentinite with dark brown veins, and light to dark green serpentinite with brown veins (marble-like); fabric in a piece of cut serpentinite, from a quarry in Chiesa di Valmalenco; formations: light-colored steatite (at base) overlain by a thin band of black metasediments, and cut by thick bands of green serpentinite (formed as a closely packed, matrix-free breccia in rocks probably originally laid down as a sequence of komatiite lava flows), Dunrossness Spilitic Group near Cunningsburgh; ophiolites represented as serpentinite with inclusions of high-pressure metamorphics with blocks of metasilicite- schist quartzite, and intercalation of green schist (high pressure metaophiolites are related to a subduction complex); serpentinite boulder, and close-up; S/C fabric in serpentinite, Coast Range, Central California; serpentinite of the Great Valley complex, Oakland Hills, associated with mercury deposits in the San Francisco Bay region, and with gold deposits in the Sierra Nevada foothills; serpentinite altered to asbestos with veins and nodules of talc, and intensely sheared serpentinites, hanzburgite, dunite and pyroxenite in Del Puerto Canyon; serpentinite, Marin county, and serpentinite boulder; Serpentinite Melange Zone, and block of sheeted dyke complex in Povorotninsky serpentinite melange, Late Mesozoic Thrust-Fold Belt of North-East Russia; serpentinized peroditites, France; submarine serpentinized peridotites; the top of the Western Massif has extensive, low-lying exposures of basement rock, which is probably serpentinite, the product of serpentinization reactions of the type that create hydrothermal systems like the Lost City; Staten Island; serpentinite; close-up of zoned serpentinite found SW of the corner of 43rd Street and Sixth Avenue in Manhattan, NY; Rainbow mine in ultramafic (talc-serpentinite) belt of Vermont; Henneke Series mollisol based on serpentinite; thin-sections: serpentinite fsu; serpentinite; olivine; partial alteration of olivine in a dunite to serpentine group minerals, ppl; peridotite with olivine crystal completely altered to serpentine group minerals, poikilitically enclosed by an unlatered clinopyroxene; peridotite with olivine crystal undergoing alteration to serpentine along fractures; relict olivine crystals in a serpentinite, ppl; totally serpentinized peridotite with pseudomorphs of olivuine and asbestiform crystals of the serpentine mineral chrysotile occupy the centre of the fiel of view (red-brown, opaque material is the iron-rich clay mixture iddingsite, and green material may be bowlingite (chlorite-rich)); serpentinization; serpentine minerals; serpentinization-induced microfracturing; webpages: The Lost City 2005; The Lost City: Serpentinization; NOAA Ocean Explorer Gallery; Serpentinite Locations on Cal Poly Land; Le Massif du Chenaillet; Newly Discovered Ophiolite Scrap in the Hartland Formation of Midtown Manhattan. |
