Extended Data Fig. 4 | Structure of the lithospheric keel beneath the
Daldyn Kimberlite Field, Siberian craton, Russia^60 ,^62. A pronounced highly
depleted layer extends about 140–190 km depth, and is progressively
melt-metasomatized towards its base. This is overlain by a less depleted layer
that still has very magnesian olivine but shows a strong trend towards
decreasing XMgOliv (MgO/(MgO+FeO) in olivine) with depth and a marked kink
near 140 km depth. Chromite is most abundant and most Cr-rich around
170–180 km. The highly depleted root may have extended to about 220–230 km
depth. The nature of the stratification, whether primary or metasomatically
overprinted, was evaluated using subsidiary data including the distribution of
chromites, and profiles of whole-rock Al 2 O 3 (estimated from Cr and/or Y
contents of garnets) and XMgOliv calculated from garnet data^89. The colour key
shows rock types based on major- and trace-element patterns in garnet
xenocrysts (see Methods). Harzburgites are defined as having mineral
assemblages of olivine+opx+garnet ± chromite. Depleted lherzolites have
minor clinopyroxene and depleted trace-element patterns. Depleted/
metasomatized lherzolites contain minor clinopyroxene but have
metasomatically enriched trace-element signatures. Fertile lherzolites contain
abundant clinopyroxene and have been highly enriched in trace elements by
(usually carbonatitic) metasomatism. Melt-metasomatism results in a rapid
decrease in XMgOliv, and increases in Zr and Ti, ascribed to percolation of mafic
melts and related fluids. LAB, lithosphere–asthenosphere boundary. %Cr 2 O 3
and %TiO 2 indicate weight per cent of these oxides in chromite.