The origin of the Merensky cyclic unit: Sr‐isotopic and mineralogical evidence for an alternative orthomagmatic model

Mineralogical, Sr‐isotopic and field data on the Critical and Main Zones of the Bushveld Complex indicate that the major input of magma at the Merensky level was fundamentally different to the resident magma. Mineralogical and isotopic data from above and below the Merensky Reef indicate that: the En content of the orthopyroxene and the An content of the plagioclase of the Critical Zone cumulates immediately preceding the Merensky Reef are higher (ca En0.80 and An0.74, respectively) than those in the Main Zone above the Merensky cyclic unit (ca En0.73 and An0.70, respectively); the Main Zone magmas started to precipitate substantial cumulus augite in addition to pigeonite and the new magma was therefore gabbronoritic or gabbroic in character, in contrast to the noritic resident liquid; olivine is rare in the Main Zone and the large influx of magma and the mixing associated with the Merensky cyclic unit resulted in only minor precipitation of chromite (ca 20 mm); this is in contrast to the major layers of chromitite, including the PGE‐rich UG2 (> 1 m), produced by apparently smaller influxes of magma in the upper part of the Upper Critical Zone; and the 87Sr/86Sr initial ratio of the upper part of the Upper Critical Zone (0.7064) is substantially different from that of the Main Zone (0.7064–0.7090) and this change is initiated at the base of, or within, the Merensky cyclic unit. These data indicate that the new magma was fundamentally different, more evolved and hence probably compositionally and thermally denser than the resident magma. This implies that the influxes were of a ‘fountain’ type and involved the input of cool dense ‘gabbroic’ magma with a high Sr‐isotopic initial ratio (ca 0.7090) into the chamber containing warm buoyant ‘noritic’ magma with a lower Sr‐isotopic initial ratio (0.70635). The first fountain of magma entrained and mixed with the resident magma in the chamber and the blended liquid then flowed along the interface between the resident liquid and the crystalline floor. This cooler magma fountain resulted in the evolution of two phase (bronzite + liquid) blobs as well as immiscible sulphide from the overlying warmer liquid. These plunged through the new liquid layer to form the Merensky Reef. The sulphide scavenged sufficient PGE, Ni and Cu from the overlying resident magma to account for the quantity and distribution in the Merensky Reef.