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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.

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We present U–Pb dates from peridotitic pyrope-rich garnet from four mantle xenoliths entrained in a kimberlite from Bultfontein, South Africa. Garnet dates magmatic emplacement due to the high mantle residence temperatures of the source material prior to eruption, which were most likely above the closure temperature for the pyrope U–Pb system. We determine a U–Pb date of 84.0 ± 8.1 Ma for the emplacement of the Bultfontein kimberlite from garnet in our four xenolith samples. The date reproduces previous dates obtained from other mineral-isotope systems (chiefly Rb–Sr in phlogopite). Garnet can be dated despite extremely low concentrations of U (median ∼0.05 μg/g), because concentrations of common Pb are often low or non-detectable. This means that sub-concordant garnets can be dated with moderate precision using very large laser-ablation spots (130 μm) measured by quadrupole inductively coupled plasma – mass spectrometry (LA-Q-ICP-MS). Our strategy demonstrates successful U–Pb dating of a U-poor mineral due to high initial ratios of U to common Pb in some grains, and the wide spread of isotopic compositions of grains on a concordia diagram. In addition, the analytical protocol is not complex and uses widely available analytical methods and strategies. This new methodology has some advantages and disadvantages for dating kimberlite emplacement versus established methods (U-based decay systems in perovskite and zircon, or Rb- or K-based systems in phlogopite). However, this method has unique promise for its potential application to detrital diamond prospecting and, more speculatively, to the dating of pyrope inclusions in diamond.