An exsolution origin for Archean mantle garnet

Abstract

It is well established that the cratonic sub-continental lithospheric mantle (C-SCLM) represents a residue of extensively melted peridotite. The widespread occurrence of garnet in C-SCLM remains a paradox because experiments show that it should be exhausted beyond c. 20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene, however, the few documented examples of garnet exsolution in cratonic samples are exotic and do not afford a direct link to garnet in granular harzburgite. We report crystallographic, petrographic and chemical data for an exceptionally wellpreserved orthopyroxene megacryst containing garnet lamellae, juxtaposed against granular harzburgite. Garnet lamellae are homogenously distributed within the host orthopyroxene and occur at an orientation that is unrelated to orthopyroxene cleavage, strongly indicating that they formed by exsolution. Garnet lamellae are sub-calcic Crpyrope and the orthopyroxene host is high-Mg enstatite, these phases equilibrated at 4.4GPa, 975oC. The reconstructed precursor is a high-Al enstatite that formed at higher pressure and temperature conditions of c. 6GPa and 1750oC. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. We hypothesize that high-Al enstatite was a common phase in the C-SCLM and that exsolution during cooling and stabilization of the C-SCLM could be the origin of most sub-calcic garnets in depleted peridotites.

Metamorphism-facilitated faulting in deforming orthopyroxene: Implications for global intermediate-depth seismicity

“…The LBS temperatures are also similar to the conditions for ductile shear zone formation in the oceanic lithospheric mantle before subduction, supporting the notion that seismogenic faulting tends to nucleate in preexisting shear zones in the lithospheric mantle that are ductile at geological timescales. Finally, the Grt exsolution microstructure observed in our partially metamorphosed Opx samples is compatible with microstructures in many upper-mantle harzburgitic and lherzolitic xenoliths ( 37 , 52 – 54 ). Fine-grained Grt-rich reaction products form thin layers along Opx grain boundaries ( 38 ) and in cleavage and kink bands ( 52 ).…”

“…Fine-grained Grt-rich reaction products form thin layers along Opx grain boundaries ( 38 ) and in cleavage and kink bands ( 52 ). Grt lamellae within Opx grains are preferentially parallel to {001}, {011}, and {0-11} planes of the host Opx ( 53 ). Based on the observed microstructures, it has been argued that shear stress enhances Grt exsolution in lithospheric mantle ( 52 , 55 ).…”

A Plethora of Epigenetic Minerals Reveals a Multistage Metasomatic Overprint of a Mantle Orthopyroxenite from the Udachnaya Kimberlite

“…It should be noted that long before the crystallization of the first newly formed minerals of external origin, the orthopyroxenite system had experienced the solid solution breakdown of the high-T orthopyroxene precursor with the formation of exsolved clinopyroxene and magnesio-chromite, as indicated by the numerous morphologically oriented inclusions of these minerals in enstatite. Clinopyroxene, magnesio-chromite and vein-like garnet grains, which are distributed in subordinate amounts around the enstatite crystals, seem to be of exsolution origin as well, as documented in some global orthopyroxenite occurrences [65,66]. Later, the orthopyroxenite served as a natural substrate for crystallization of various minerals that are united by the fact that they are epigenetic with respect to the host rock and crystallized from passing liquids.…”

Shear‐Wave Velocity Structure of Southern Africa's Lithosphere: Variations in the Thickness and Composition of Cratons and Their Effect on Topography

“…The presence of the aluminous phase (garnet) increases shear velocities (Faul & Jackson, 2005;Schutt & Lesher, 2010;Stixrude & Lithgow-Bertelloni, 2005). Even though there may be little spinel left in the highly depleted uppermost mantle of many cratons, which would reduce the impact of the spinel peridotite-garnet peridotite transformation itself, the appearance of garnet by means of exsolution from mantle orthopyroxene has been inferred in many studies, in particular in southern Africa (e.g., Bell et al, 2005;Simon et al, 2003;Tomlinson et al, 2017). The occurrence of the exsolution and the distribution of garnet depends on the cooling and metasomatic history of the lithosphere (e.g., Saltzer et al, 2001).…”