Serpentinized Lherzolite

The name is derived from the Lherz Massif, an alpine peridotite complex (also known as orogenic lherzolite complex). A lherzolite is an ultrabasic igneous rock dominated by essential Olivine and clinopyroxene and orthopyroxene in equal proportions. Accessory minerals include plagioclase, spinel, garnet, ilmenite, chromite and magnetite. Lherzolites are a peridotite and the main component of the upper mantle. Their aluminous phases change with pressure, with plagioclase present at low pressures, spinel at intermediate pressure and garnet at high pressure. Metasomatism of lherzolite in the mantle can produce accessory micas and amphiboles. Lherzolite is common as a mantle xenolith within mantle-derived magmas and is present in the mantle sequence of ophiolites. Mantle lherzolite usually contains Cr-diopside as the clinopyroxene and enstatite as the orthopyroxene. Lherzolite can also form as cumulates within layered intrusions.

Plagioclase can occur in lherzolites and other peridotites that crystallize at relatively shallow depths (20 – 30 km). At greater depth plagioclase is unstable and is replaced by spinel through the reaction:

CaAl₂Si₂O₈ (plg) + Mg₂SiO₄ (ol) = 2MgSiO₃ (opx) + CaMgSi₂O₆ (cpx) + MgAl₂O₄ (sp)

At approximately 90 km depth, spinel decompose and pyrope garnet becomes the stable aluminous phase.

MgAl₂O₄ (sp) + 1.55 Mg₂Si₂O₆ (opx) + 0.45 CaMgSi₂O₆ (cpx) = 0.85 Mg₃Al₂Si₃O₁₂ (garnet) + 0.15 Ca₃Al₂Si₃O₁₂ (garnet) + Mg₂SiO₄ (ol)

Garnet lherzolite is a major constituent of the Earth's upper mantle (extending to 300 km depth).

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Olivine and pyroxene (pale brown) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene (low interference colors) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene (low interference colors) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. XPL image, 2x (Field of view = 7mm)
Olivine crystals rimmed by serpentine + magnetite (dark) veins. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene (pale brown) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene (low interference colors) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. XPL image, 2x (Field of view = 7mm)
Olivine and pyroxene (pale brown) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene (low interference colors) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene (pale brown) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. PPL image, 2x (Field of view = 7mm)
Olivine and pyroxene (pale brown) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene (low interference colors) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene (pale brown) crystals. Olivine crystals are rimmed by serpentine + magnetite (dark) veins. PPL image, 2x (Field of view = 7mm)
Garnet with kelifitic rims (made by fibrous minerals). PPL image, 2x (Field of view = 7mm)	Garnet with kelifitic rims (made by fibrous minerals). PPL image, 2x (Field of view = 7mm)	Garnet with kelifitic rims (made by fibrous minerals). PPL image, 2x (Field of view = 7mm)
Garnet (isotropic) with kelifitic rims (made by fibrous minerals). XPL image, 2x (Field of view = 7mm)	Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. PPL image, 10x (Field of view = 2mm)	Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)
Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. PPL image, 10x (Field of view = 2mm)	Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)
Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)
Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Mesh texture: olivine remnant "islands" rimmed by serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)