Yellow Dog peridotite - digital mosaic.

Click on image to enlarge         Photo © Daniel R. Snyder

Yellow Dog peridotite. Digital mosaic of four 2x-objective images. In this image: rounded and fractured olivine crystals with serpentine meshwork and stringers of magnetite along fractures, pyroxene (yellow), plagioclase (white/gray), and a little mica (red). Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 5.4 mm x 7.8 mm.

Yellow Dog peridotite (plagioclase-bearing lherzolite)

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

The Yellow Dog peridotite, thought to be approximately 1.1 Ga in age, is composed  of partly serpentinized peridotite, containing 40 to 50 percent olivine (1/2 to 2/3 serpentinitzed) and 30 percent pyroxenes consisting of subequal amounts of orthopyroxene and clinopyroxene. (Klasner, et al., 1979)*. In addition, it contains 5 to10 percent plagioclase, which would classify it as a "plagioclase-bearing lherzolite" according to the IUGS criteria.

The peridotite body was first described by William Morris in his 1977 M.S. thesis**. He named it the "Yellow Dog Plains peridotite". The surface outcrop is roughly oval-shaped in horizontal cross-section. Doug Hull's comment below is valid.  Appendix C (Geology) to the mining development permit request is now on the Web. You can get it in PDF format by Googling "Eagle Deposit Geology". This report includes cross-sections, at irregular horizontal intervals and at 50-meter vertical intervals, of what is referred to as the "Eagle Deposit". These diagrams justifiy calling it a dike.The report states that the "intrusion at the surface"  extends 480 meters in length and is 100 meters wide at its thickest point. The peridotite intrudes metasediments that are at least 1.9 Ga in age.  

The image above shows two heavily fractured olivine macrocrysts, which were most likely carried upward by magma originating in the upper mantle. A serpentine meshwork fills most of the fractures, and stringers of magnetite occupy the larger fractures in the crystal on the right. Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 mm.

Below is an optical scan of a fractured surface of a hand sample of Yellow Dog peridotite. 2400 dpi optical scan, Imaged area 17 mm x 17 mm. 

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

*John S. Klasner, David. W. Snider, W. F. Cannon, and John F. Slack (1979), The Yellow Dog Peridotite and a Possible Buried Igneous Complex of Lower Keweenawan Age in the Northern Peninsula of Michigan, Geological Survey Division, Michigan Department of Natural Resources, 38 p.

**Morris, William J., (1977) Geochemistry and Origin of the Yellow Dog Plains Peridotite, Marquette County, Northern Michigan, unpublished master's thesis, Michigan State University, 82p. 

Mantle lherzolite xenolith in basalt - full thin section.

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

Spinel lherzolite xenolith, San Carlos Indian Reservation, Arizona. Lherzolite is at left; brightly colored grains are olivine, gray and brown grains are pyroxene.  Felty-textured basalt is at right. XPL macrophotograph. Imaged area 21 mm x 34 mm. Click on the image to enlarge. It's a lot more interesting close up!

"Holly-leaf" spinel pattern in mantle lherzolite xenolith

Click on image to enlarge          Photo © Daniel R. Snyder

Light green olivine , olive-green enstatite (top; bottom center), and a spinel grain with "holly-leaf" stringers extending from the main mass. Cr-diopside lherzolite xenolith. San Carlos Indian Reservation, Arizona. Reflected-light photomicrograph of polished section. Imaged area 2.7 mm x 4 mm.

Mantle xenolith - reflected-light photomicrograph

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

Light green olivine , olive-green enstatite, and emerald-green chromian diopside in a Cr-diopside lherzolite xenolith. I'm leaning toward calling the curved, dark grain in the center spinel. This is, after all, a spinel lherzolite, and in such rocks spinel is usually near pyroxenes. San Carlos Indian Reservation, Arizona. Reflected-light photomicrograph of polished section. Imaged area 2.7 mm x 4 mm.

Orthopyroxene and clinopyroxene (diopside) in lherzolite

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

Pale yellow grains are pyroxenes. Lower pyroxene grain is orthopyroxene, upper grain (actually several contiguous smaller grains) is clinopyroxene. When the stage was rotated, the orthopyroxene grain showed parallel extinction, the clinopyroxene grain - inclined extinction. However, there is also a morphological difference in this case: the orthopyroxene grain has a more regular, parallel cleavage pattern, while the clinopyroxene  has shorter, curving, and kinked cleavage cracks. Bright-colored grains are olivine, black areas are grains at extinction and voids. Cr-diopside lherzolite mantle xenolith. San Carlos Indian Reservation, Arizona. XPL. Imaged area 1.3 mm x 2 mm.

Lherzolite xenolith in basalt

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

Felty material at top is basalt, bright colored grains are olivine, large pale yellow grain is orthopyroxene. Spinel Lherzolite mantle xenolith in basalt, San Carlos Indian Reservation, Arizona. XPL. Imaged area 1.3 mm x 2 mm.

Mantle lherzolite xenolith in basalt - closeup of hand sample

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

Closeup of hand sample of mantle peridotite shown in previous post. Basalt has enclosed a small block of peridotite (bottom center) as well as the larger block (top), of which only a portion is included in the hand sample. Basalt is gray, olivine is light green, orthopyroxene is olive green, clinopyroxene is emeral green, spinel is dark gray-brown. Note that there is almost no reaction zone between the peridotite and the basalt. San Carlos Indian Reservation, Arizona. Scale in centimeters.

In the 1960's, amid speculation as to the composition of the mantle, Ringwood (1966)* developed the concept of a non-specific olivine-pyroxene rock which he named "pyrolite". To avoid the ongoing debate about mineral composition, pyrolite was to be defined in chemical terms, such that a material with the agreed-upon bulk chemistry would meet the requirements of density (as inferred from seismic data) and of minor elements necessary to conform to the composition of magmas that were thought to be derived from partial melting of the mantle. Different pyrolite models were proposed by several workers, and synthetic pyrolite was produced. Ringwood (1986)* produced a diagram of the mineral assemblages, densities, and phase transformations displayed by pyrolite from a depth of 100 kilometers to 850 kilometers.

Although Ringwood was widely respected and had a number of colleagues and followers, he also had detractors. One of these was Don Anderson, who refuted the concept of pyrolite on several grounds. His main objections were that the pyrolite model did not  adequately address trace elements and isotopes, or evidence for mantle heterogeneity. He also pointed out that materials of made up of different proportions of major elements could satisfy the density requirements, and criticized supporters of the pyrolite model for making ad hoc changes to the definitions as they went along. Anderson favored a dominant role for eclogite in the mantle. His main concessions were to acknowledge that "the mantle between about 800 km and 2600 km appears to be relatively homogeneous" and that "it appears that MgO and SiO2 in approximately equal molar proportions are implied for the lower mantle" (Anderson, 1989)*.

A.E. Ringwood died in 1993. Don L. Anderson published The New Theory of the Earth in 2007**. It repeats verbatim much of the of the earlier edition, but without most of the invective against the pyrolite advocates.


*Anderson, Don L., (1989), Theory of the Earth, Blackwell, 353 p.
**Anderson, Don L., (2007), New Theory of the Earth, Cambridge University Press, 384 p.
*Ringwood, A.E., (1966), Mineralogy of the mantle, in P. M. Hurley (ed.), Advances in the Earth Sciences, MIT Press.
*Ringwood, A.E., (1986), Constitution and evolution of the mantle, Proceedings of the 4th International Kimberlite Conference, v. 2.  Geological Society of Australia.

Mantle lherzolite xenolith in basalt - hand sample.

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

Granular lherzolite xenolith in basalt. Green masses are peridotite (lherzolite), gray mass is basalt. San Carlos Indian Reservation, Arizona. Scale in centimeters.

You probably know that peridotite is the commonest rock on the planet. Starting at the base of the crust and going straight down for 400 kilometers, the Earth's mantle is mostly peridotite as we know it at Earth-surface conditions.  Recall from petrology that there is compelling evidence for a phase change at the 400-km level. Because of the very high pressure at that that depth, olivine is thought to convert to a spinel structure - it's still olivine, (Mg,Fe)2SiO4, but it has a different crystal structure, and it's about 9 percent denser. This high-pressure polymorph of olivine undergoes another phase change at a depth of 670 kilometers. If that's all the peridotite there was, it would still represent 20 percent of the volume of Earth. Beyond the 670-km. level, and down to 2900 kilometers, everything is still mantle, and is believed to consist of other phase transformations of olivine, as well as pyroxenes and garnet, as pressure increases. So all in all the mantle, consisting mainly of peridotites (variously defined) accounts for 80 percent of the volume of our planet.

If you'd like to learn more about the composition and petrology of the mantle, most college petrology books devote a few pages to the subject. Beyond that,  I recommend a very thorough book by A. E. Ringwood, (1975), The Composition and Petrology of the Earth's Mantle, 604 p.,  McGraw-Hill. If your library doesn't have it, you can  get it used online for about $25.00. Ringwood later (1986) also wrote a shorter review of the subject in the Proceedings of the 4th International Kimberlite Conference, published by the Geological Society of Australia. It's only 29 pages and contains some updated references, and is an easier (and faster) read than the 1975 tome.

So, of there's so much peridotite on earth, why does so little of it crop out at the surface? As your petrology book will tell you, the short answer to that is that the rocks that form the upper crust are generated by partial melting of peridotite in the lower crust and upper mantle, which forms magma that works its way upward and solidifies as granites and other intrusive igneous rocks if it cools before it reaches the surface, or flows out on the surface as lavas that cool to form basalts and other extrusive igneous rocks. Since this is only partial melting, what remains behind in the mantle is still peridotite.

How do we know all this if we can't see into, or drill down or sink mineshafts into the mantle? A lot of what we know comes from indirect evidence - mainly seismic wave records, from which rock densities can be inferred. And in the past 50 years, great advances in experimental petrology have allowed scientists to replicate the behavior of rocks and minerals at mantle-like pressures. But in terms of looking at actual mantle-derived peridotites, there are only three ways this can be done. And again, these are listed in most petrology books:

1. Ultramafic xenoliths are carried to the surface by magma rising rapidly through conduits in mantle wall rock.  If a basaltic magma moves through peridotite country rock, it will pluck peridotite blocks from the wall of the conduit and carry them to the surface. Since the peridotite melts at a relatively high temperature, while the basaltic magma stays fluid at a lower temperature, the xenolith can reach the surface without being melted by the magma. The image above shows such a xenolith brought to the surface in an alkaline basalt. The deepest-origin xenoliths, from depths of more than 300 kilometers, are brought up in the magmas of kimberlites.

2. Because the oceanic crust is considerably thinner than the continental crust, mantle rocks are sometimes exposed on the ocean floor along fault zones. Of course, these are samples only of the uppermost mantle.

3. Ophiolites are layered mafic and ultramafic rock sequences, formerly oceanic crust and upper mantle, that were emplaced onto active continental margins. Sometimes transported far inland by low-angle thrust faulting, as in the Appalachians, the peridotite component of such an ophiolite survives as a coherent body of ultramafic rocks. The disadvantages of such ultramafic bodies are that they represent a sampling of only the shallowest part of the mantle, they are typically metamorphosed (recrystallized), so the original fabric and structure are destroyed, and they are highly susceptible to alteration into serpentine, talc, chlorite, and other hydrous secondary minerals. The advantage of these bodies - originally, and still, called Alpine peridotites - is that they are widely distributed throughout the world, and are generally accessible for study. Many of the images in the later pages of this blog are from such exposures.