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.

Pyroxene grains and pseudomorphs in Josephine serpentinized peridotite.

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

In this specimen,  A few very small remnants of olivine (bright-colored grains) can be seen around the image, but almost all of the olivine has been altered to serpentine. I collected this specimen on the border between the Josephine peridotite and the Onion Camp complex of Yule et al. (2006)*. I decided to call it Josephine peridotite for two reasons: 1. There is little evidence of shearing, and 2. Many large pyroxene grains still persist, and large serpentine pseudomorphs after pyroxene are distributed throughout the specimen. The large gray pyroxene grain at the left side of the image above still appears to be mostly pyroxene, while the broken grain in the center has been altered to bastite-texture serpentine. Hourglass-texture serpentine, probably an alteration product of olivine, occupies the rest of the image. Klamath Mountains, Josephine County, southwest Oregon, XPL. Imaged area 2.7 mm x 4 mm.

In the image below, the white spots on the large hand specimen are reflections from the surfaces of platy pseudomorphs after pyroxene; gray-green grains are pyroxene. Note that there appears to be more remnant pyroxene than pseudomorphosed pyroxene. The brown mesostatis is serpentine. The surface of the small hand specimen has been rough polished, showing dark grains of pyroxene. Lighter gray areas are serpentinized pyroxene grains (pseudomorphs). Scale is in centimeters.

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

*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.

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.