Kelyphite texture

Kelyphite is a petrographical textural term referred to a fine-grained, fibrous intergrowth of multiple phases such as pyroxene, spinel and amphibole, typically developed forming a rim (or a corona) surrounding garnet. Coronae surrounding garnet in peridotite were first described by Müller (1846), Delesse (1850), Hochstetter (1854) and Becke (1882). In 1882, Schrauf coined the name Kelyphite from the Greek word κελνφος, meaning shell, to describe such coronae at Krems (Bohemia). Although the name had been first defined and used for coronae between garnet and olivine in peridotites it was later used to describe also coronae around garnet in eclogite and granulite, the nature of which is however much different.

Schrauf (1882, 1884) thought that kelyphite was a single phase, whereas Becke (1882), Doelter and Hussak (1884) ascertained that it represents a mixture of minerals, including pyroxenes, spinel, and occasionally amphibole and chlorite. Later, many authors identified orthopyroxene (Opx), spinel (Sp), clinopyroxene (Cpx) and amphibole (Am) in kelyphite and most of them emphasized the common existence of two concentric coronae: (i) an outer Opx-rich spinel-free (or spinel-poor) corona on the olivine side (this thin corona develops only where olivine is present), and (ii) an inner spinel-rich corona on the garnet side. This zonal and concentric structure seems to be universal in garnet peridotites.

Kelyphites can be divided into two types or categories according to their host rock lithologies:

1) Kelyphites developed in garnet peridotites, in which olivine is an excess phase. The typical mineral assemblage of the first type is spinel + orthopyroxene + clinopyroxene. Ca-amphibole is a common accessory phase and may occur locally in place of clinopyroxene. In garnet peridotites, spinel-bearing kelyphites around garnet are the main evidence for retrograde metamorphism. They record the transition from high-pressure garnet peridotite to low-pressure spinel peridotite. Therefore, their understanding is important to constrain the exhumation processes that affected high-pressure mantle rocks. Holland (1896), Sederholm (1916) and, principally, Hezner (1903) carried out detailed studies of this metamorphic reaction.

The first elegant interpretation of these coronae was proposed by Fermor (1912) who considered them as resulting from the breakdown of garnet during decompression. In the years 1960-1980, experiments were performed on the Ol + Gt = Opx + Sp equilibrium (e.g. MacGregor, 1964/65; Kushiro and Yoder, 1966; Green and Ringwood, 1967). They demonstrated that this type of kelyphite is not simply a breakdown product of garnet but should be regarded to be a reaction product between garnet and olivine, and that this reaction takes place in the mantle (transition between garnet and spinel peridotites) at around 20 kbar. Since then, kelyphitisation of garnet into a spinel-bearing corona has regularly been interpreted as the effect of retrogression from high-P to low-P conditions; it is considered as the main trace of peridotite exhumation. Most of the kelyphite-forming reactions range between the two extremes:

Garnet + olivine → orthopyroxene + spinel + clinopyroxene

and

Garnet + olivine + H₂O → orthopyroxene + spinel + amphibole.

This explains strong variations in the relative abundance of amphibole and clinopyroxene from one kelyphite to another. These variations were certainly correlated with local fluctuations of PH₂O.

2) Kelyphites developed in olivine-free mafic rocks such as eclogites, garnet pyroxenites or garnet granulites. The mineral assemblage of the second type is typically Opx + An + Sp with or without Cpx and Ca-amphibole. The origin of this assemblage may be ascribed to another reaction:

Garnet → orthopyroxene + clinopyroxene + anorthite + spinel

Bibliography

• Godard, G. (2001). Eclogites and their geodynamic interpretation: a history. Journal of Geodynamics, 32(1-2), 165-203.
• Godard, G., & Martin, S. (2000). Petrogenesis of kelyphites in garnet peridotites: a case study from the Ulten zone, Italian Alps. Journal of Geodynamics, 30(1-2), 117-145.
• Obata, M. (2011). Kelyphite and symplectite: textural and mineralogical diversities and universality, and a new dynamic view of their structural formation. New frontiers in tectonic research-general problems, sedimentary basins and island arcs. InTech, 93-122.
• Obata, M., & Ozawa, K. (2011). Topotaxic relationships between spinel and pyroxene in kelyphite after garnet in mantle-derived peridotites and their implications to reaction mechanism and kinetics. Mineralogy and Petrology, 101(3-4), 217-224.

Kelyphites in garnet peridotite
Garnet crystal surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. XPL image, 2x (Field of view = 7mm)
Garnet crystal surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. XPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)
Garnet crystal (isotropic) surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. XPL image, 2x (Field of view = 7mm)	Garnet crystals surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet crystals (isotropic) surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. XPL image, 2x (Field of view = 7mm)
Garnet crystals surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet crystals (isotropic) surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. XPL image, 2x (Field of view = 7mm)	Garnet crystals surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)
Garnet crystal surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by kelyphite (orthopyroxene + clinopyroxene + spinel), in a matrix of fractured and partially serpentinized olivine. Garnet peridotite from Duria, Italian Alps. PPL image, 2x (Field of view = 7mm)
Garnet crystal surrounded by a double corona: thin orthopyroxene corona developing between olivine and garnet (garnet + olivine → orthopyroxene + spinel + clinopyroxene) and fibrous kelyphite (orthopyroxene + clinopyroxene + spinel). Garnet peridotite from Duria, Italian Alps. PPL image, 10x (Field of view = 2mm)	Garnet crystal (isotropic) surrounded by a double corona: thin orthopyroxene corona developing between olivine and garnet (garnet + olivine → orthopyroxene + spinel + clinopyroxene) and fibrous kelyphite (orthopyroxene + clinopyroxene + spinel). Garnet peridotite from Duria, Italian Alps. XPL image, 10x (Field of view = 2mm)	Garnet crystal surrounded by a double corona: thin orthopyroxene corona developing between olivine and garnet (garnet + olivine → orthopyroxene + spinel + clinopyroxene) and fibrous kelyphite (orthopyroxene + clinopyroxene + spinel). Garnet peridotite from Duria, Italian Alps. PPL image, 10x (Field of view = 2mm)
Garnet crystal (isotropic) surrounded by a double corona: thin orthopyroxene corona developing between olivine and garnet (garnet + olivine → orthopyroxene + spinel + clinopyroxene) and fibrous kelyphite (orthopyroxene + clinopyroxene + spinel). Garnet peridotite from Duria, Italian Alps. XPL image, 10x (Field of view = 2mm)	Garnet crystal surrounded by a double corona: thin orthopyroxene corona developing between olivine and garnet (garnet + olivine → orthopyroxene + spinel + clinopyroxene) and fibrous kelyphite (orthopyroxene + clinopyroxene + spinel). Garnet peridotite from Duria, Italian Alps. PPL image, 10x (Field of view = 2mm)	Garnet crystal (isotropic) surrounded by a double corona: thin orthopyroxene corona developing between olivine and garnet (garnet + olivine → orthopyroxene + spinel + clinopyroxene) and fibrous kelyphite (orthopyroxene + clinopyroxene + spinel). Garnet peridotite from Duria, Italian Alps. XPL image, 10x (Field of view = 2mm)

Kelyphites in Eclogite
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 2x (Field of view = 7mm)
Garnet crystals with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. XPL image, 10x (Field of view = 2mm)
Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 10x (Field of view = 2mm)
Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (orange) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. XPL image, 10x (Field of view = 2mm)	Garnet crystal with plagioclase + spinel + amphibole corona, amphibole (brown-green) and omphacite (symplectitic mixture of diopside and albite). Retrograde eclogite from Porto Ottiolu, Sardinia, Italy. PPL image, 10x (Field of view = 2mm)

Kelyphites in Granulite
Garnet (pale beige) with kelifitic corona and biotite crystals. Retrograde granulite. PPL image, 2x (Field of view = 7mm)	Kelifitic mixtrure after garnet (the fibrous, gray material), biotite and clinopyroxene crystals. Retrograde granulite. PPL image, 2x (Field of view = 7mm)	Kelifitic mixtrure after garnet (the fibrous, gray material), biotite and clinopyroxene crystals. Retrograde granulite. XPL image, 2x (Field of view = 7mm)
Garnet (pale beige) with kelifitic corona and biotite crystals. Retrograde granulite. PPL image, 10x (Field of view = 2mm)	Garnet (pale beige) with kelifitic corona and biotite crystals. Retrograde granulite. PPL image, 10x (Field of view = 2mm)	Kelifitic mixtrure after garnet (the fibrous, gray material) and irregular clinopyroxene crystals. Retrograde granulite. PPL image, 10x (Field of view = 2mm)