Garnet 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).

lherzolite2017.jpg

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



lherzolitegranato.jpg

Garnet Lherzolite: Garnet (pink), olivine and orthopyroxene (deep green), Cr-diopside (Emerald green). From James St. John.





garnet(peridotite).jpg

Garnet Lherzolite: Garnet (pink), olivine (deep green), Cr-diopside (Emerald green). From Mindat

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
lherzolite(grt)(1).jpg

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(2).jpg

Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(grt)(3).jpg

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(4).jpg

Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(grt)(5).jpg

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(6).jpg

Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(grt)(8).jpg

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(9).jpg

Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(grt)(10).jpg

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(11).jpg

Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(grt)(12).jpg

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(13).jpg

Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(grt)(14).jpg

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(15).jpg

Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(grt)(16).jpg

Olivine, chromite and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 20x (Field of view = 1mm)
lherzolite(grt)(17).jpg

Olivine, chromite and garnet (isotropic) crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 20x (Field of view = 1mm)
lherzolite(grt)(18).jpg

Olivine and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(19).jpg

Olivine and garnet crystals with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(grt)(23).jpg

Garnet crystals in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
lherzolite(grt)(24).jpg

Garnet crystals in a serpentinized Garnet Lherzolite. XPL image. 20x (Field of view = 7mm)
lherzolite(grt)(25).jpg

Garnet crystals in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
lherzolite(grt)(26).jpg

Garnet crystals in a serpentinized Garnet Lherzolite. XPL image. 20x (Field of view = 7mm)
lherzolite(grt)(27).jpg

Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
lherzolite(grt)(28).jpg

Garnet crystal in a serpentinized Garnet Lherzolite. XPL image. 20x (Field of view = 7mm)
lherzolite(grt)(29).jpg

Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
lherzolite(grt)(30).jpg

Garnet crystal in a serpentinized Garnet Lherzolite. XPL image. 20x (Field of view = 7mm)
lherzolite(grt)(31).jpg

Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
lherzolite(grt)(35).jpg

Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
lherzolite(grt)(36).jpg

Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
lherzolite(grt)(37).jpg

Garnet crystal in a serpentinized Garnet Lherzolite. PPL image. 20x (Field of view = 7mm)
lherzolite(9).jpg

Orthopyroxene (gray), Clinopyroxene and Olivine. XPL image. 10x (Field of view = 2mm)
lherzolite(4).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(5).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(15).jpg

Olivine and spinel crystals. PPL image. 2x (Field of view = 7mm)
lherzolite(2).jpg

Olivine and spinel crystals. PPL image. 2x (Field of view = 7mm)
lherzolite(6).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 2x (Field of view = 7mm)
lherzolite(7).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 2x (Field of view = 7mm)
lherzolite(8).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 2x (Field of view = 7mm)
lherzolite(3).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(10).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 2x (Field of view = 7mm)
lherzolite(11).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 2x (Field of view = 7mm)
lherzolite(12).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(13).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). PPL image. 10x (Field of view = 2mm)
lherzolite(14).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)
lherzolite(16).jpg

Garnet with kelifitic decomposition rims (consisting of fibrous pyroxene). XPL image. 10x (Field of view = 2mm)