On the nature and origin of garnet in highly-refractory Archean lithospheric mantle: constraints from garnet exsolved in Kaapvaal craton orthopyroxenes

Abstract

The widespread occurrence of pyrope garnet in Archean lithospheric mantle remains one of the 'holy grails' of mantle petrology. Most garnets found in peridotitic mantle equilibrated with incompatible-trace-element enriched melts or fluids and are the products of metasomatism. Less common are macroscopic intergrowths of pyrope garnet formed by exsolution from orthopyroxene. Spectacular examples of these are preserved in both mantle xenoliths and large, isolated crystals (megacrysts) from the Kaapvaal craton of southern Africa, and provide direct evidence that some garnet inthe sub-continental lithospheric mantle formed initially by isochemical rather than metasomatic processes. The orthopyroxene hosts are enstatites and fully equilibrated with their exsolved phases (low-Cr pyrope garnet ± Cr-diopside). Significantly, P-T estimates of the postexsolution orthopyroxenes plot along an unperturbed conductive Kaapvaal craton geotherm and reveal that they were entrained from a large continuous depth interval (85 to 175 km). They therefore represent snapshots of processes operating throughout almost the entire thickness of the sub-cratonic lithosphericmantle.New rare-earth element (REE) analyses show that the exsolved garnets occupy the full spectrum recorded by garnets in mantle peridotites and also diamond inclusions. A key finding is that a few low-temperature exsolved garnets, derived from depths of ∼90 km, are more depleted in light rare-earth elements (LREEs) than previously observed in any other mantle sample. Importantly, the REE patterns of these strongly LREE-depleted garnets resemble the hypothetical composition proposed for pre-metasomatic garnets that are thought to pre-date major enrichment events in the sub-continental lithospheric mantle, including those associated with diamond formation. The recalculated compositions of pre-exsolution orthopyroxenes have higher Al2O3 and CaO contents than their post-exsolution counterparts and most probably formed as shallow residues of large amounts of adiabatic decompression melting in the spinel-stability field. It is inferred that exsolution of garnet from Kaapvaal orthopyroxenes may have been widespread, and perhaps accompanied cratonization at ∼2.9 to 2.75 Ga. Such a process would considerably increase the density and stability of the continental lithosphere.

An exsolution origin for Archean mantle garnet

“…Similar exsolved samples have previously been described from the SCLM (Aoki et al, 1980, Dawson, 2004, Dawson et al, 1980, Gibson, 2017, but what is significant about BP002 is the visible physical disintegration of the megacryst into the main body of the xenolith. Several observations suggest that garnet and orthopyroxene throughout the granular portion of sample BP002 likely originated from structural break-up of exsolved orthopyroxene-garnet megacrysts: the presence of strained orthopyroxene amongst the relatively unstrained olivine and garnet; the occurrence of garnet inclusions in orthopyroxene; and the identical compositions of the spatial association of garnet and orthopyroxene more generally seen in granular peridotites (Cox et al, 1987, Saltzer et al, 2001).…”

“…Their model is supported by the spatial relationship between garnet and orthopyroxene (Saltzer et al, 2001) and by experimental phase equilibria (Canil, 1991, Canil, 1992. Examples of garnet exsolution in Cr-rich orthopyroxene megacrysts have been described from several cratons (Aoki et al, 1980, Dawson et al, 1980, Dawson, 2004, Gibson, 2017, Wasch et al, 2009. However, thus far no direct link between these rare exsolved garnet and the distinct grains that occur in granular harzburgite has been convincingly demonstrated.…”

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

“…This roughly coincides with the depth range where cratonic lithosphere displays the highest seismic velocities, in most cases, in southern Africa and elsewhere. Gibson () inferred that the garnet exsolution probably occurred during the lithospheric thickening, possibly during the Archean cratonization; further evidence is needed to establish the timing and mechanisms unambiguously. Seismic data relating to the amplitude and lateral variability of the sub‐Moho increase in with depth can help by providing new observational constraints on the composition, with broader inferences on the evolution of the cratons.…”

“…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, ). Pressure and temperature estimates of Gibson (), obtained from the mantle xenoliths and megacrysts from the Kaapvaal Craton, revealed that the orthopyroxenes with exsolved garnet were entrained from a broad, 85–175 km depth interval. This roughly coincides with the depth range where cratonic lithosphere displays the highest seismic velocities, in most cases, in southern Africa and elsewhere.…”

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.…”