Lherzolite is a coarse-grained ultramafic rock with 40-90% olivine. Ultramafic rocks have less than 45% silica (SiO2) and more than 90% mafic minerals. Mafic minerals are dark-colored minerals rich in iron and magnesium.
Alfred Lacroix, a French geologist and mineralogist, named and described lherzolite in 1894. He named it after Lherz Massif in the French Pyrenees, its type locality.
Lherz Massif is an obducted upper mantle emplaced in a mountain-building belt or orogen. It formed from a continent-continent collision. Older tests will refer to it as an orogenic or alpine peridotite massif.
Besides this rock, Lherz Massif also has other ultramafic rocks like harzburgite and dunite and layers of other rocks like hornblendite, garnet, and spinel pyroxenites.
This rock is a peridotite
Lherzolite is a peridotite, specifically one of the pyroxenes peridotites. Peridotites are ultramafic rocks with at least 40% olivine.
On the other hand, pyroxene peridotites have 40-90% olivine, with orthopyroxene and clinopyroxene more than 5%. Other examples are harzburgite and wehrlite.
Appearance
Lherzolite is a dense, coarse-grained, dark greenish to greenish-black rock. It may be massive or layered, and alteration may change its colors.
On a hand specimen, this rock will have dark green forsterite olivine (iron-rich olivine) and enstatite (orthopyroxene). Clinopyroxene is the bright emerald greenish chrome diopside on hand specimens.
Other minerals are purplish red pyrope garnet and smaller amounts of accessories like spinel.
Composition
Chemically, lherzolite is an ultrabasic, ultramafic rock with less than 45 wt.% silica. It is high in magnesium, has considerable iron, and is low in sodium and potassium.
Ultrabasic rocks have less than 45 wt.% silica. Most are also ultramafic, but not all. Only those with more than 90% of mafic minerals are ultramafic.
According to data from Le Maitre 1976 from 179 samples, the % chemical composition of lherzolite is SiO2: 45.43%, TiO2: 0.45%. Al2O3: 4.39%, Fe2O3: 5.15%, FeO: 7.44%, MnO: 0.17%, MgO: 30.31%, CaO: 5.68%, Na2O: 0.59%, K2O: 0.27% and P2O5: 0.12%.
Mineralogically, lherzolite has 40-90% olivine, considerable but subordinate orthopyroxene, and clinopyroxene (rich in chromium). Other minerals include garnets, aluminum spinel, ilmenite, magnetite, and chromite.
Also, this rock may have plagioclase or pyrope garnet, depending on the depth at which it crystallizes. Those crystallizing at shallower depths of 20-30 km (12-19 mi) will have plagioclase feldspar.
On the other hand, those that crystallize at greater depths of about 90 km (56 mi) will have pyrope garnet.
Also, this rock may have accessory minerals like mica or amphibole. These accessory minerals form from the metasomatism of lherzolite.
Metasomatism occurs when the chemical composition of a rock changes by 1) addition or 2) removal of some chemical components.
Spinel and garnet lherzolite
Lherzolite may have various accessory minerals like spinel, anorthite, phlogopite, or amphibole. If in a considerable amount, you can use them as suffixes.
If any of these minerals is less than 5%, add the accessory name, a hyphen, and the word bearing to the rock name. For instance, you can have spinel-bearing lherzolite.
However, we drop the word bearing if the accessory mineral is more than 5%.
For instance, spinel lherzolite has a considerable amount of spinel. This rock is a major constituent of the upper mantle up to depths of about 300 km (186 mi).
On the other hand, garnet lherzolite has a considerable amount of garnet.
Lastly, plagioclase lherzolite forms at shallower levels in the crust. In such rocks, most olivine is usually altered to serpentine, forming a serpentinite rock.
Occurrence
Lherzolite dominates the upper mantle. Also, it occurs on the lower sections of the ultramafic portion of ophiolites or cumulates in layered intrusions.
Additionally, mantle-derived magmas like basalt and kimberlite will have lherzolite xenoliths. Xenoliths are foreign rock pieces or fragments found in igneous rocks.
Lastly, this rock likely also occurs in the moon’s lower mantle.
Fertile and depleted lherzolite
Partial melting of lherzolite forms basaltic magmas.
A fertile lherzolite is one in which no basaltic melt has been extracted. Such will be rich in the green aluminum oxide spinel and clinopyroxene.
Clinopyroxene will have a small amount of sodium oxide and titanium oxide. This happens as these minerals preferentially melt.
On the other hand, if basaltic melt has been extracted, you have a depleted lherzolite. Such will have less abundant clinopyroxene and chromium-rich spinel. Also, their clinopyroxene will have lower titanium oxide and sodium oxide.
Lastly, extremely depleted lherzolite with only a small amount of orthopyroxene and clinopyroxene is harzburgite or dunite.
References
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- 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