Harzburgite is a coarse-grained, dark-colored ultramafic plutonic rock. It has mainly olivine, orthopyroxene, and a small amount of clinopyroxene.
Ultramafic rocks are low in silica (< 45 wt. %) and have more than 90% mafic minerals. Mafic minerals are dark-colored minerals high in magnesium and iron.
We may also call these rocks ultrabasic. This term refers to any rock that is extremely low in silica. However, it doesn’t have to be high in magnesium or iron minerals like ultramafic ones.
Harzburgite is a variety of peridotites. Peridotites are coarse-grained, ultramafic rocks with at least 40% olivine. These rocks from the Earth’s upper mantle are rich in magnesium and have considerable iron.
Harry Rosenbusch, a German petrographer, applied the name harzburgite in 1887. He named these rocks after the Harz Mountains of Germany. Before, this rock was known as saxonite.
_(16601144800).jpg){kind=link}
Description and appearance
Harzburgites have a coarse-grained texture. Their color is dark green to greenish-black rock when fresh. Usually, they will have a speckled appearance with olive green crystals and dark to black orthopyroxene.
Also, they may have a few dark clinopyroxene crystals and interstitial whitish plagioclase.
On the other hand, weathered, serpentinized, or altered harzburgites will have a reddish-brown, brownish, brownish-black, dark gray, or black surface. Some specimens may appear dark bluish-gray or blue-gray.
Usually, olivine easily undergoes weathering and hydrothermal alteration (hot fluid alteration) compared to pyroxene. Therefore, even with a little alteration, this rock may change colors.
Composition
Let us now look at chemical and mineral composition.
1. Harzburgite chemical composition
Chemically, harzburgite is an ultrabasic rock. Ultrabasic rocks have less than 45% silica content by weight.
Also, this rock is high in magnesium and has a substantial amount of iron. However, it is low in sodium and potassium.
According to data from Le Maitre 1976 from 206 sample analyses, the average percentage weight chemical composition of harzburgite is SiO2: 43.72%, TiO2: 0.28%, Al2O3: 2.57%, Fe2O3: 6.00%, FeO: 7.09%, MnO: 0.16%, MgO:36.34%, CaO:3.18%, Na2O: 0.34%, K2O: 0.15%, P2O5: 0.14%.
2. Mineral composition
Harzburgite is an ultramafic rock. It has mainly olivine and orthopyroxene with minor to no clinopyroxene. Orthopyroxene is usually magnesium-rich enstatite or bronzite.
Accessories minerals include spinel (picotite), garnet, ilmenite, magnetite, chromite, or chrome diopside. Also, it may have interstitial plagioclase, usually labradorite.
Sometimes, this rock may have micas and amphiboles. These minerals result from metasomatism in the mantle.
Narrowing down to its classification under peridotites, harzburgite is one of the pyroxene peridotites together. The others are wehrlite and lherzolite. These rocks have 40 to 90% olivine and more than 5% orthopyroxene. Other peridotites are kimberlite, dunite, hornblende and pyroxene hornblende peridotites.
Lastly, on the olivine, orthopyroxene, and clinopyroxene ternary diagram, harzburgite is a plutonic rock with between 40-90% olivine, more than 5% orthopyroxene, and less than 5% clinopyroxene.
How does it form?
Harzburgites form in the upper mantle from the partial melting and extraction of basaltic magma from lherzolite. Lherzolite is a pyroxene rich peridotite.
Partial melting of lherzolite will deplete clinopyroxene since it is the first to melt. This leaves a residue rock rich in olivine and orthopyroxene, transforming lherzolite to harzburgite.
If more melting occurs, it will deplete most or all the pyroxene, forming dunite rock.
Also, accumulation of olivine and orthopyroxene in the magma chamber during fractional crystallization may form harzburgite. This occurs at high pressure where olivine and low calcium pyroxenes like bronzite and enstatite can crystallize together, forming harzburgite.
Where is Harzburgite found?
Most harzburgites occur in ophiolites. However, they can occur as xenoliths in rocks derived from mantle magma. Also, some are part of large layered igneous intrusions.
Ophiolites are part of Earth’s crust and underlying mantle (oceanic or continental), uplifted, exposed, and usually placed on the continental crust.
Examples of ophiolites with harzburgite include Trinity ophiolite and Coast Range ophiolite in California, USA, and Semail Ophiolite (Wadi Tayin Massif) in Oman.
Others are Troodos Ophiolite in Cyprus, Bay of Islands Ophiolite in Newfoundland, Canada, and Mount Tavai ultrabasic massif in Malaysia.
Also, ophiolites from mountain buildings (continental-continental collision) may have these rocks. The older name for this ophiolite is alpine peridotite massif.
Such may be called orogenic lherzolites. Examples are Lherz Massif in France, Lanzo Massif in North Italy, and Horoman Massif in Japan.
Lastly, layered igneous intrusions like the Stillwater igneous complex of Montana, USA, and the Bushveld Igneous Complex in South Africa have harzburgites.
Frequently Asked Questions (FAQs)
You can distinguish dunite from harzburgite by looking at their weathered or serpentinized surfaces. Dunite will have rough projections on the surface, not harzburgite. These projections are from orthopyroxene.
Yes. Garnet harzburgite is one of the xenoliths in kimberlites that bear diamonds. Other xenoliths are eclogite or garnet pyroxenite.
References
- Le Maitre, R. W. (1976). The chemical variability of some common igneous rocks. Journal of Petrology, 17(4), 589–598. https://doi.org/10.1093/petrology/17.4.589