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Michael D., 1972, Metasomatism and Weathering of the Presque Isle Serpentinized Peridotite, Marquette, Michigan, unpublished M.S. thesis, Michigan Technological University.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

Presque Isle peridotite

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

Peridotite (thoroughly serpentinized). Pseudomorphs of serpentine after olivine (dark green) and anthophyllite after pyroxenes (linear patterns of brown, orange, and yellow) retain the shapes of original crystals. I've seen the Presque Isle peridotite described as "highly deformed". This doesn't look highly deformed to me. A rock body whose protolith is a billion-plus years old and still preserves the original crystal shapes seems to me to hardly be deformed at all.

Michael Lewan's (1972)* M.S. thesis reports on three samples ranging from 54.9 percent to 59.2 percent serpentine by volume. Olivine plus augite account for another 15.7 to 21.2 percent total, about evenly divided. Lewan also found 4.4 percent to 7.8 percent anorthite in his three samples, but I've looked at several thin sections and haven't seen any. Presque Isle Park, City of Marquette, Marquette County, northern Michigan. XPL. Imaged area 1.3 x 2 mm. Link to photo of outcrop.

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Michael D., 1972, Metasomatism and Weathering of the Presque Isle Serpentinized Peridotite, Marquette, Michigan, unpublished M.S. thesis, Michigan Technological University.