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.

Plagioclase in Yellow Dog peridotite (plagioclase-bearing lherzolite)

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

 Orthopyroxene at upper left (gray), enclosing plagioclase laths; clinopyroxene (magenta) at lower right; olivine at lower left, upper right, and top center. Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 mm (2X objective).

This is an unusually rich concentration of plagioclase (center) for the Yellow Dog peridotite. Most of the plagioclase in my samples is scattered as single laths, perhaps two or three small grains in a typical frame this size.  Older geology texts rule out ANY plagioclase if a rock is to be called "peridotite", but the IUGS classification allows up to 10 percent, in which case they are called "plagioclase-bearing ____". However, the consultant's report states that the plagioclase can average 25 to 30 percent "over significant intervals". In order to accommodate this, the consultant uses a maximum of 30% plagioclase for peridotite. The report defines rocks with between 10% and 30% plagioclase as "feldspathic peridotite".  (This report is on the Web. You can get it in PDF format by Googling "Eagle Deposit Geology".) Under the IUGS classification, these rocks would either be olivine gabbro, olivine norite, or olivine gabbronorite.

Talc in weathered Newdale dunite

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

In the above XPL image, a large talc clot (center) is surrounded by weathered olivine grains. The solid black grains at top center, bottom center, and bottom left are chromite. Linear patterns in the lower part of the talc suggest that this part of the talc is partially-altered tremolite/actinolite. Below is a PPL image of the same area at the same scale, showing clearly the chromite and the serpentine meshwork in the dunite. Newdale, Yancey County, western North Carolina. Imaged area 2.7 mm x 4 mm.

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

Talc in pyroxene in Webster dunite

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

A vein of talc (lower left to upper right) bisects two large grains of orthopyroxene, surrounded by olivine, in dunite. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. XPL. Imaged area 2.7 mm x 4 mm.

Deformation bands in olivine

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

In this image of dunite, several large olivine grains (center, top left, top right, bottom right) show deformation banding. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, Western north Carolina. XPL. Imaged area 2.7 mm x 4 mm.

Twin Sisters dunite - cataclasis

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

This image illustrates the cataclastic texture described by Ragan (1963)*. The large subhedral olivine grain is inferred to have formed in the "primary"phase, during which the relatively intact igneous body was emplaced in the crust in the solid state. Subsequent strain resulted in the deformation banding seen in many of the grains, the strain lamellae in the large grain, and the fracturing that produced the mosaic of small grains. Later recrystallization of some small grains caused the strain features to be eliminated, as in the tight cluster at the lower right, where each of the grains shows a uniform interference color. Cascade Mountains, Whatcom County, northwestern Washington. XPL. Imaged area 2.7 mm x 4 mm. Link to Dan McShane's Twin Sisters page.


*D.M. Ragan (1963), Emplacement of the Twin Sisters Dunite, Washington, American Journal of Science, v. 261, p. 549-565.

Twin Sisters dunite - full thin section

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

Thin section of Twin Sisters dunite showing strained olivine and pyroxene grains with deformation banding. Cascade Mountains, Whatcom County, northwestern Washington. Macrophotograph, XPL. Imaged area 21 mm x 40 mm. Link to Dan McShane's Twin Sisters page.

Pseudomorphs after pyroxene in ancient (1.2 Ga) metaperidotite

Click on the image to enlarge.      Sample: Jeff Chiarenzelli; Photo: Dan Snyder

A small body of highly-metamorphosed rocks jn the Adirondack lowlands has been interpreted as metaperidotite because of its CIPW normative mineralogy (harzburgite) and because pseudomorphs after ferromagnesian minerals have been found there (Chiarenzelli et al., 2011)*. The protolith has been dated at ~1.2 Ga using detrital zircons. In this image, two pseudomorphs after pyroxene, now probably consisting of an amphibole mineral, can be seen at left center and top center. Pyrites Complex, St. Lawrence County, northern New York. XPL. Imaged area 1.3 mm x 2 mm.

Thanks to Dr. Jeff Chiarenzelli, of St. Lawrence U., for the loan of this and many other thin sections.

* Jeff Chiarenzelli, Marian Lupulescu, Eric Thern, and Brian Cousens (2011), Tectonic implications of the discovery of a Shawinigan ophiolite (Pyrites Complex) in the Adirondack lowlasnds, Geosphere, 2011;7;333-356

Samani - the town that loves peridotites

Samani, a small coastal town on the island of Hokkaido, Japan, has made the most of its proximity to the Horoman peridotite body, which includes Mt. Apoi, site of a natural park devoted to "learning about the earth's transformation from peridotite". The town square in Samani (pictured) has been an open-air petrological museum, displaying a wide variety of locally-occurring peridotites.

Unfortunately, Samani experienced a 3.7-meter wave during the tsunami of March 11, 2011. I don't know more than that about the fate of the town, but since the town square was only few tens of meters from the shoreline, I fear the museum may no longer exist. I've e-mailed the town authorities and am awaiting a reply.

Magnesite in dunite

Click on the image to enlarge.           Photo: Dan Snyder

A cluster of magnesite (MgCO3) grains in Newdale dunite. Magnesite commonly occurs where peridotites are permeated by carbonate-rich fluids.  Light gray, darker gray, and pale yellow grains at center and left are magnesite. Dark gray grains are at partial extinction; pale yellow grains probably contain a small amount of iron carbonate (siderite) in solid solution. The partially extinct (medium gray) grain at the upper edge of the cluster gives an idea of grain size. Colored grains at bottom, upper left, and right are olivine. Newdale, Yancey County, North Carolina. XPL. Imaged area 2.7 mm x 4 mm. Link to Carolina Geological Society 1997 Guidebook (PDF).

Anthophyllite in dunite

Click on image to enlarge.            Photo: Dan Snyder

Anthophyllite in Newdale dunite. Irregularly shaped grains are olivine; long, straight grains are anthophyllite. Newdale, Yancey County, North Carolina. XPL. Imaged area 1.3 x 2 mm. Link to Carolina Geological Society 1997 Guidebook (PDF)

Thanks to Dr. Samuel Swanson of the University of Georgia for directing me to this exposure.

Webster-Addie ultramafic body - sheared dunite, full thin section

Click on the image to enlarge. Click twice to enlarge more- it's a lot more 

interesting close up.      Photo: Dan Snyder.

Tectonized, sheared dunite. Upper 2/3 of image is composed mainly of polygonal olivine grains (more saturated colors) with scattered pyroxene grains (grays and pale yellows), especially at top left; lower 1/3 is composed of sheared talc clots with crushed and elongated grains of olivine and pyroxene. By itself, this thin section probably wouldn't fit the conventional definition of dunite - too much pyroxene - but it was part of a more extensive exposure that is clearly dunite. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. XPL. Imaged area 22 mm x 39 mm.

Pyroxene in dunite

Click on the image to enlarge.           Photo: Dan Snyder

A large orthopyroxene grain in dunite. According to the IUGS classification, a dunite is a rock made up of ferromagnesian minerals, mainly olivine and pyroxene, of which more than 90% is olivine (discounting accessory minerals). In practice, serpentine inferred to be derived from olivine is counted as olivine. Although a pyroxene grain dominates this photograph, there are only a few such grains in the sample.  Webster-Addie ultramafic body, Jackson County, North Carolina. Reflected-light photomicrograph of polished block. Ordinary light. Imaged area 3.3 mm x 4 mm.

Olivine weathering microtexture in dunite

Click on the image to enlarge.      Sample:Michael Velbel; Photo: Dan Snyder

Characteristic diamond-shaped etch pits in olivine. The etch pit field is adjacent to a microfracture in the olivine, through which fluid has permeated, creating the etch pits. Orange areas are iron-oxide mineral stains. Webster-Addie ultramafic body, Jackson County, western North Carolina. Plane-polarized light (PPL). Magnification 400X.

Thanks to Dr. Michael Velbel, Michigan State U., for the loan of this and many other thin sections.

Etch pits in olivine - Webster-Addie ultramafic body

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

The dark specks that appear to be mineral grains or polishing grit are actually microscopic etch pits  along fractures in and around this olivine grain. These fractures provided access to fluids that developed weathering textures in the olivine. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. XPL. Imaged area 0.5 mm x 0.8 mm.

The etch pits stand out more clearly in this PPL image of the same feature (same scale as above):

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

Weathering microtexture in olivine.

Click on the image to enlarge. Sample: Michael Velbel; photo: Dan Snyder

Coalescing funnel-shaped etch pits forming serrated edges in olivine (light gray). Darker gray at top and right is serpentine. Webster-Addie ultramafic body, Jackson County, western North Carolina. Scanning electron microscope (SEM) image of polished thin section (backscattered electron image), magnification 650x.

Thanks to Dr. Michael Velbel, Michigan State U., for the sample, and thanks to Dr. Velbel and NASA for the SEM time. Thanks also to E. Danielewicz for assistance with electron microscopy.

Pyroxene lamellae, amphibole in dunite

Click on image to enlarge.           Photo: Dan Snyder

Exsolution lamellae in pyroxene (pale yellow grains at left and bottom, gray grain at right); ampohibole mineral (grains with diagonal cleavage patterns: center, top center); olivine (brightly colored grains at  center and top center). Webster-Addie utlramafic body, western North Carolina. XPL. Imaged area 1.3 mm x 2 mm.

Thanks to Dr. Michael Velbel, Michigan State U., for the loan of this and many other thin sections.

Polyhedral texture in dunite - Scanning Electron Microscope Image

Click on the image to enlarge.  Sample: Michael Velbel; photo: Dan Snyder

Granular (recrystallized) olivine, fractured dunite fragment, Webster-Addie ultramafic body, Jackson County, western North Carolina. Scanning electron microscope (SEM) image (secondary electron image). Magnification 40x.

Thanks to Dr. Michael Velbel, Michigan State U., for the sample, and thanks to Dr. Velbel and NASA for the SEM time. Thanks also to E. Danielewicz for assistance with the SEM.

Polyhedral texture (mosaic texture) in dunite

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

Classic polyhedral mosaic in dunite. This texture results from recrystallization in the solid state, and is common in metamorphic rocks composed mainly of one mineral, such as dunite. Grains tend to have fairly straight sides, and many grain boundaries intersect in triple junctions at approximately 120˚ angles. Webster-Addie ultramafic body, Blue Ridge Mountains, Jackson County, western North Carolina. Macrophotograph, XPL. Imaged area 8 mm x 14 mm.