Mineralogy of the Laetolil Footprint Tuff: A comparison with possible volcanic sources from the Crater Highlands and Gregory Rift

• First detailed mineralogical study of the Laetolil Footprint Tuff 7.
• Difference in magnetite composition from Laetoli and Sadiman.
• Documentation of carbonate–silicate melt inclusions in magnetite from Laetoli.
• Sadiman volcano is not a source for Laetolil Footprint Tuff.

Sadiman volcano in northern Tanzania was postulated as a source of the Upper Laetolil Tuff 7 renowned for its 3.66 Ma old footprints of Australopithecus afarensis as both localities show similarities in terms of their mineralogy. Despite this widely accepted view, Laetoli and Sadiman differ in some key features, particularly in the absence of melilite at Sadiman, and in their magnetite composition. Magnetite from these localities shows significant differences in Mg, Al and Ti contents. Compositions of major, minor and accessory minerals and the occurrence of carbonate–silicate melt inclusions in magnetite from Laetoli indicate their formation from a fractionated carbonate- and melilite-bearing nephelinitic melt. This differs from the melt from which the Sadiman rocks formed. We propose that the nephelinitic Mosonik volcano, located to the north-east from Laetoli, could be a potential source for the Upper Laetolil Footprint Tuff 7.