Spinel 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).

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Diagram for Ultramafic rocks. Lherzolite field in blue. From IUGS



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Spinel lherzolite from Mt. Leura, Victoria, Australia. black minerals are Spinel. From James St. John.



Bibliography



• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore
• Eric A.K.Middlemost (1985): Magmas and Magmatic Rocks. Longman, London
• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore


Photo
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Orthopyroxene (pale green), Olivine and Clinopyroxene (colorless) and Spinel (brown) in a Spinel Lherzolite from Pyrenees. PPL image. 2x (Field of view = 7mm)
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Orthopyroxene (pale green), Olivine and Clinopyroxene (colorless) and Spinel (brown) in a Spinel Lherzolite from Pyrenees. PPL image. 2x (Field of view = 7mm)
lherzolite99(4).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(5).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(6).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(7).jpg

Orthopyroxene (pale green), Olivine and Clinopyroxene (colorless) and Spinel (brown) in a Spinel Lherzolite from Pyrenees. PPL image. 2x (Field of view = 7mm)
lherzolite99(8).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(9).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(10).jpg

Orthopyroxene (pale green), Olivine and Clinopyroxene (colorless) and Spinel (brown) in a Spinel Lherzolite from Pyrenees. PPL image. 2x (Field of view = 7mm)
lherzolite99(11).jpg

Orthopyroxene (pale green), Olivine and Clinopyroxene (colorless) and Spinel (brown) in a Spinel Lherzolite from Pyrenees. PPL image. 2x (Field of view = 7mm)
lherzolite99(12).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(13).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(14).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(15).jpg

Orthopyroxene (weak Interference colors), Olivine and Clinopyroxene (high Interference colors) and spinel (isotropic) in a Spinel Lherzolite from Pyrenees. XPL image. 2x (Field of view = 7mm)
lherzolite99(16).jpg

Orthopyroxene (pale green), Olivine and Clinopyroxene (colorless) and Spinel (brown) in a Spinel Lherzolite from Pyrenees. PPL image. 2x (Field of view = 7mm)
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Vermicular green-spinel crystals in a Spinel Lherzolite. PPL image. 10x (Field of view = 2mm)
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Vermicular green-spinel crystals in a Spinel Lherzolite. PPL image. 10x (Field of view = 2mm)
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Vermicular green-spinel crystals (isotropic) in a Spinel Lherzolite. XPL image. 10x (Field of view = 2mm)
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Vermicular green-spinel crystals in a Spinel Lherzolite. PPL image. 10x (Field of view = 2mm)
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Vermicular green-spinel crystals (isotropic) in a Spinel Lherzolite. XPL image. 10x (Field of view = 2mm)
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Vermicular green-spinel crystals in a Spinel Lherzolite. PPL image. 10x (Field of view = 2mm)
lherzolite(spinello)(8).jpg

Vermicular green-spinel crystals (isotropic) in a Spinel Lherzolite. XPL image. 10x (Field of view = 2mm)
lherzolite(spinello)(10).jpg

Vermicular green-spinel crystals in a Spinel Lherzolite. PPL image. 10x (Field of view = 2mm)
lherzolite(spinello)(11).jpg

Vermicular green-spinel crystals in a Spinel Lherzolite. PPL image. 10x (Field of view = 2mm)
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Vermicular green-spinel crystals (isotropic) in a Spinel Lherzolite. XPL image. 10x (Field of view = 2mm)