Serpentinized Trinity peridotite

Click on image to enlarge.   Specimen: Michael Davis   Image: Dan Snyder

Optical scan of sawed surface of a small hand specimen of serpentinized Trinity peridotite. A 6-mm-thick slice of this specimen crumbled easily with the fingers, and thus the specimen was too fragile to use for a thin section. Olivine has largely been altered to yellowish-gray chrysotile serpentine, which also occupies some of the smaller fractures. Chrysotile has a waxy luster which, although not apparent in this image, shows up well on the hand specimen.  Dark linear features are earlier-formed fracture fillings of the antigorite or lizardite varieties of serpentine. Dark, rounded masses are relict or pseudomorphosed pyroxene. According to Quick (1981)*, the Trinity peridotite is highly serpentinized, except where glaciation has exposed large outcrops of relatively unserpentinized peridotite.  Eastern Klamath Mountains, northern California.  Imaged area 4.75 cm by 5.54 cm.

Excerpt from USGS Open File of02-490 s1

Hess (1989)* states the areal extent of the Trinity peridotite incorrectly as 3,700 sq. km. He cites Quick (1981) as the source. Quick, however does not give a figure for the areal extent of the peridotite outcrop. He states only that "Contiguous outcrops of ultramafic rocks occur over an area about 50 km wide and 75 km long." (Quick, 1981)**. It appears that Hess multiplied 50 by 75 and got 3,750, then dropped the 50 to avoid the impression of spurious accuracy. A glance at the map shows that this figure doesn't account for the various plutons lying within the contiguous area of peridotite, and that the actual areal extent of the peridotite itself is more like 1,500 sq. km.  I did a point count on graph paper, which resulted in an area of 1,237 sq. km.



* Hess, Paul C., 1989, Origins of Igneous Rocks, p. 80.

**Quick, James E., 1981. Petrology and petrogenesis of the Trinity Peridotite, an upper mantle diapir in the Eastern Klamath Mountains, northern California. Journal of Geophysical Research, v. 86, No. B12, p. 11,838.


Many thanks to Professor Michael Davis, of the University of California at Riverside, for the specimen.

The fibrous structure of chrysotile, the asbestiform variety of serpentine, can be seen clearly a thin chip of the mineral at 100x (10x objective) in a microscope. The dark strips at the top and bottom of the chip are remnants of the fracture wall to which it was anchored, and the dark horizontal strip in the center may be a cross-section of an antigorite vein occupying the center of the fracture. Plane polarized light. Length of chip ~0.5 mm.

Rounded olivine grains in peridotite of the Los Pinos pluton.

Click on image to enlarge.   Sample: Michael Davis   Image: Dan Snyder

   Rounded olivine grains surrounded by amphibole. The Los Pinos pluton is one of several gabbroic plutons intruding the Peninsular Ranges batholith in the southernmost part of California. "Olivine and amphibole...are the dominant phases and form up to 88% of the rock." (Walawender, 1976)*. Walawender describes the petrogenesis of the peridotite as an example of Bowen's classic reaction series. Olivine was first mineral to solidify from melt, along with minor plagioclase (not shown in this image). Outer parts of olivine grains were resorbed by melt as pyroxene (not shown) solidified. Amphibole solidfied last and occupied interstices between other minerals. Red material in large grain at center is iddingsite, occupying part of an irregular fracture.  Los Pinos Mountain, Peninsular ranges, San Diego County, California. XPL. Imaged area 2.7mm by 4mm.

See post of December 17, 2012 for a macrophotograph of the entire thin section.

Many thanks to Professor Michael Davis, of the University of California at Riverside, for the specimen.

Click on image to enlarge.   Sample: Michael Davis   Image: Dan Snyder

   PPL image of same area as above. Dark smudges in amphibole are made up of "aligned, rod-like, opaque minerals" (Walawender, 1976)*.


* Walawender, M. J, 1976. Petrology and Emplacement of the Los Pinos Pluton, southern California. Canadian Journal of Earth Sciences, v. 13, pp. 1288-1300.

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.

Second alteration cycle - serpentine altering to anthophyllite (Presque Isle peridotite)

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

The grain shown in this image, originally olivine, was altered to serpentine, most of which has been altered in turn to anthophyllite. That is, I thought it was anthophyllite, but then I read Michael Lewan's (1972)* M. S. thesis, in which he reported NO anthophyllite in the three samples of Presque Isle peridotite for which he carried out modal analysis. However, all three samples contained small amounts of chlorite (7.3, 7.1, and 5.4 percent volume. I've never seen chlorite that looks like this, but maybe... If you know, please send me a comment.

Tiny red or orange anthophyllite(?) crystals occupy most of the former olivine grain, except for the two remaining areas of serpentine, gray-green in the XPL image above (lower left and upper right). The image below, also XPL, shows most of the anthophyllite(?) crystals (those aligned from upper left to lower right, or vice versa) at extinction - as extinct as they get. The serpentine areas show more clearly in this view, while the anthophyllite(?) crystals that are "at extinction" have dark outlines of varying thickness.

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

In the PPL image below, the serpentine areas are relatively featureless, while elsewhere the elongated crystals of anthophyllite(?) show distinct outlines. Yellow-orange color in PPL image is due to iron-oxide staining from hematite. Black spots are magnetite. Presque Isle peridotite, Marquette County, northern Michigan. Imaged area 0.5 mm x 0.8 mm.

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

*Lewan, Michael D., 1972, Metasomatism and Weathering of the Presque Isle Serpentinized Peridotite, Marquette, Michigan, unpublished M.S. thesis, Michigan Technological University.

Presque Isle peridotite - full thin section

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

This image shows the texture of the peridotite, in which olivine and pyroxene have been almost entirely pseudomorphed by serpentine, much of which has been replaced in turn by amphiboles. Light-colored areas are carbonate veins. Marquette County, northern Michigan. XPL Macrophotograph. Imaged area 25 mm x 41 mm.

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.