Selective serpentinization in peridotite (harzburgite), Onion Camp complex, Klamath Mountains, Oregon

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Peridotite (harzburgite), Onion Camp complex, Klamath Mountains, Oregon. Colored grains are olivine and orthopyroxene; black areas with white spots are serpentine. XPL  macrophotograph. Imaged area 24 mm by 42 mm. Photo by Dan Snyder.

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Peridotite (harzburgite), Onion Camp complex, Klamath Mountains, Grains with vivid interference colors are olivine; yellowish gray grains at bootom center and top left are orthopyroxene; dark gray areas with light gray and white polygons are serpentine. XPL  Imaged area 1.3 mm by 2 mm. Photo by Dan Snyder.

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Peridotite (harzburgite), Onion Camp complex, Klamath Mountains, Oregon. Colored grains are olivine; gray grain at center is orthopyroxene; dark gray areas with light gray and white polygons are serpentine. XPL  Imaged area  0.5 mm by 0.8 mm. Photo by Dan Snyder.

Peridotite in advanced stage of serpentinization - Onion Camp complex, Oregon

Click on image to enlarge.         Photo © Daniel R. Snyder

The serpentinized peridotite of the Onion camp complex, which is in contact with the variably serpentinized Josephine peridotite, is described by Yule et al. (2006)* as "highly serpentinized and sheared dunite, harzburgite, and sparse websterite....".  In the center of the image above, a pyroxene grain has been pseudomorphosed, probably by an amphibole mineral, with the chain structure preserved as parallel strands of the replacement mineral. A large serpentine pseudomorph after olivine occupies the center of the left side of the image, and a less obvious pseudomorph is at the upper right. Elsewhere in the image, serpentine is ubiquitous. Klamath Mountains, Josephine County, southwest Oregon. XPL. Digital mosaic, imaged area 3.6 mm x 6 mm.

Click on image to enlarge.          Photo © Daniel R. Snyder

Hand samples of serpentinized peridotite of the Onion Camp complex. Smaller sample on right has been rough polished on cut surface. Note hematite coating on left sides of both samples. Scale in centimeters.


*Yule, J. D., Saleeby, J. B., and Barnes, C. G., 2006, A rift-edge facies of the late Jurassic Rogue-Chetco arc and Josephine ophiolite, Klamath Mountains, Oregon, in Snoke, A. W. and Barnes, C.G., eds., Geological studies in the Klamath Mountain province, Californa and Oregon: A volume in honor of William P. Irwin: Geological Society of America Special Paper 410, p. 53-76.

Buck Creek dunite - serpentinized and chloritized

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Polished slab of partially serpentinized and chloritized dunite. Buck Creek ultramafic body, Blue Ridge Mountains, Clay County, western North Carolina. Optical scan at 1524 pixels/cm (600 dpi). Original width 3400 pixels, reduced by Blogger to 1600 pixels at maximum magnification. Slab dimensions 8 cm x 14.5 cm. Below is a higher-resolution image (6096 pixels/cm.) of the 3.5-cm-by-3.5-cm area outlined in the larger image.

Click on image enlarge. Click again to get really close up.

Josephine peridotite - digital mosaic

Ckick on image to enlarge.          Photo © Daniel R. Snyder

In this image: mostly olivine (bright colors) with incipient serpentinization, and a few large (gray) grains of orthopyroxene. Josephine peridotite, a component of the Josephine ophiolite sequence. Klamath Mountains, Josephine County, southwest Oregon. Digital mosaic of four 2x-objective images. XPL. Imaged area 5.1 mm x 7.6 mm.

Deer Lake peridotite - magnetite traces of ancient olivine crystals

Click on image to enlarge.          Photo © Daniel R. Snyder

The Deer Lake peridotite, thought to be Proterozoioc in age, is composed entirely of secondary minerals. Here, the outlines of two adjacent olivine crystals - and their internal fractures - have been preserved by stringers of magnetite that separated out when the olivine was serpentinized. The gray masses appear to be carbonate, possibly magnesite, throughout which magnetite grains have been disseminated, giving the gray color. Deer et al.* state that magnesite may be formed if serpentinites undergo low-grade metamorphism where carbon dioxide is available. A few orange and red wisps of anthophyllite or tremolite signal the beginning of another round of alteration. Marquette County, northern Michigan. XPL. Imaged area 1.3 mm x 2 mm.

Deer, W. A., Howie, R. A., and Zussman, J., (1992), An Introduction to the Rock-forming Minerals,  2nd edition, Pearson-Prentis Hall, 685 p.