Picroilmenites in Yakutian kimberlites: variations and genetic models

Abstract

Abstract. Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modelling shows that long compositional trends for the ilmenites are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of picroilmenite's parental magmas was estimated for the three distinct phases of kimberlite activity from Yubileynaya and closely located Aprelskaya pipes showing heating and increase of Fe of mantle peridotites minerals from stage to stage and splitting of the magmatic system in the final stages. High pressure (5.5–7.0 GPa) Cr-bearing Mg-rich ilmenites (Group 1) reflect the conditions of high temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1–10/C1 and have flattened, spoon-like or S- or W-shaped REE patterns with Pb > 1. These result from melting and crystallization in melt – feeding channels in the base of the lithosphere, where high temperature dunite – harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group2) trace the high temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during the melt migration within the channels. Group 2 ilmenites reveal inclined REE enriched patterns (10–100)/C1 with La/Ybn 10–25 similar to those derived from kimberlites, and HFSE peaks (typical megacrysts). A series of similar patterns results from polybaric AFC crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high Cr content (Group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr- rich ilmenites are typical for veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. The highest in TRE ilmenites 1000/C1 have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet – spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn -Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst-forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.