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.

Serpentinization is a processes whereby rock (usually ultramafic) is changed, with the addition of water into the crystal structure of the minerals found within the rock. A common example is the serpentinization of Peridotite (or dunite) into serpentinite (the metamorphic equivalent). Serpentinization is a geological low-temperature metamorphic process involving heat and water in which low-silica mafic and ultramafic rocks are oxidized and hydrolyzed with water into serpentinite. Peridotite at and near the seafloor is converted to serpentine, brucite, magnetite, and other minerals. In the process large amounts of water are absorbed into the rock increasing the volume and destroying the structure.
Serpentinization is accompanied by release of heat (it is an exothermic reaction); the heat released by a complete Serpentinization reaction of 1 Kg of peridotites is 250 Joules, enough to bring the temperature of 1 Liter of Water up by 50°C degrees under normal temperature and pressure conditions.

Serpentinization is also accompanied by a change in the physical properties of the rock; the volume increase by up 30 % with concomitant decrease in density (a non Serpentinized peridotites has a density of 3300Kg/m3 whereas a serpentinite has a density of approximately 2600 Kg/m3. In parallel, the speed of propagation of seismic waves decrease during Serpentinization: from nearly 8Km/s in non serpentinized rocks, to 5.5Km/s in serpentinites. The production of magnetite during Serpentinization is accompanied by a increase of the rock’s magnetic susceptibility; finally, serpentine is less tolerant to deformation than olivine and pyroxenes; 15-20% of serpentine in peridotites is enough to decrease the rock’s tolerance to deformation and can specifically facilitate the motion on the faults.

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
Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. PPL image, 2x (Field of view = 7mm)
Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. PPL image, 2x (Field of view = 7mm)
Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. PPL image, 2x (Field of view = 7mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. PPL image, 2x (Field of view = 7mm)
Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 2x (Field of view = 7mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. PPL image, 2x (Field of view = 7mm)	Garnet with kelifitic rims (made by fibrous Amphiboles). PPL image, 2x (Field of view = 7mm)
Garnet with kelifitic rims (made by fibrous Amphiboles). PPL image, 2x (Field of view = 7mm)	Garnet with kelifitic rims (made by fibrous Amphiboles). PPL image, 2x (Field of view = 7mm)	Garnet with kelifitic rims (made by fibrous Amphiboles). XPL image, 2x (Field of view = 7mm)
Olivine crystals, rimmed by Serpentine + magnetite veins. PPL image, 10x (Field of view = 2mm)	Olivine crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Olivine crystals, rimmed by Serpentine + magnetite veins. PPL image, 10x (Field of view = 2mm)
Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Olivine crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Olivine crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)
Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)
Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)	Olivine and pyroxene crystals, rimmed by Serpentine + magnetite veins. XPL image, 10x (Field of view = 2mm)
Olivine crystals partially alterated by serpentine. PPL image, 2x (Field of view = 7mm)	Olivine crystals partially alterated by serpentine. XPL image, 2x (Field of view = 7mm)	Olivine crystals partially alterated by serpentine. PPL image, 2x (Field of view = 7mm)
Olivine crystals partially alterated by serpentine. XPL image, 2x (Field of view = 7mm)	Olivine crystals partially alterated by serpentine. PPL image, 2x (Field of view = 7mm)	Olivine crystals partially alterated by serpentine. PPL image, 2x (Field of view = 7mm)
Olivine crystals partially alterated by serpentine. XPL image, 2x (Field of view = 7mm)	Olivine crystals partially alterated by serpentine. PPL image, 2x (Field of view = 7mm)	Olivine crystals partially alterated by serpentine. XPL image, 2x (Field of view = 7mm)
Olivine crystals partially alterated by serpentine. PPL image, 2x (Field of view = 7mm)	Olivine crystals partially alterated by serpentine. XPL image, 2x (Field of view = 7mm)	Olivine crystals partially alterated by serpentine. XPPL image, 2x (Field of view = 7mm)