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.