THE ORIGIN OF CRATONIC PERIDOTITES - A MAJOR-ELEMENT APPROACH

Cratons formed in tectonic events that occulted at levels extending to as much as several hundred kilometers within the upper mantle, clearl y deeper than the thicknesses of Phanerozoic oceanic plates. These eve nts were primarily Archean, but Re depletion ages for the peridotites indicate that some extended into the Proterozoic. The peridotites that comprise craton roots are spinel facies to depths of similar to 100 k m. The section between 100 and 200 km is coarse garnet peridotite with equilibration temperatures ranging upward to similar to 1100 degrees C. The peridotites of deepest origin commonly are deformed, metasomati cally enriched in Ti and Fe, and record temperatures up to 15000C. The peridotites that form the principal mass of the craton root are buoya nt because their Mg/(Mg+Fe) is greater than that of circumcratonic per idotites, and because they contain less garnet-forming Al. Most also a re characterized by being orthopyroxene-rich relative to oceanic perid otites. The low-temperature peridotites have provided our best estimat es of the bulk composition of the craton mantle, although they have no t escaped late-stage metasomatism. Balancing bulk analyses for Fe and Ca against the probe analyses and modes for these elements shows that, in some cases, there has been significant introduction from the kimbe rlite during eruption. Cratonic peridotites are depleted in most magma phile elements, but their compositional variations are not well charac terized by depletion models, even with allowance for metasomatism duri ng eruption. Plots of Fe vs. Si: for example, have a negative trend-op posite from that expected if these elements were jointly concentrated in melt rather than in the residue. Negative Fe/Si could be the produc t of a mixing or unmixing process involving olivine and orthopyroxene, or it could be the product of a metasomatic reaction of olivine with melt to form orthopyroxene. Kelemen and Hart (1996) have observed that there will be enrichment of Ni in orthopyroxene in the event that it is generated by melt-rock reaction, but their interpretation is not un ique. Similar enrichments can be generated by cooling assemblages of o rthopyroxene and olivine that originated with variable modal proportio ns (Herzberg, 1998). An excellent positive correlation of Ni in opx wi th modal opx is found for Premier peridotites, which is the result eit her of cooling or melt-rock reaction. Those from Kimberley, however, s how a flat trend. The opx-rich and opx-poor peridotites at Kimberley a ppear to have equilibrated following cooling to ambient conditions. Th e origins of cratonic peridotites remain an imperfectly understood, fi rst-order geologic problem.