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 modeling, shows long compositional trends for the ilmenites. These 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 the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite 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/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) 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 melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10–100)/PM with La / Ybn ~ 10–25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (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 of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) 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.
Highlights
We investigate the hypothesis of ilmenite megacryst formation during polybaric fractionation of protokimberlite melts (Ashchepkov et al, 2010, 2012, 2013a) in feeder channels and mantle veins and accompanying metasomatic systems
The highest temperature metasomatites show depths of origin near 4.0–7.5 GPa. This coincides well with the conditions determined by most thermobarometric methods and agrees well with the subcratonic lithospheric mantle (SCLM) thickness determined by seismic methods (O’Reilly et al, 2009) near 270 km
For the development of magmatic system in the mantle before the kimberlite eruption, we suggest a three-stage model (Mitchell, 1986; Le Roex et al, 2003)
Summary
Magnesian ilmenites or picroilmenites occur in all kimberlites of group I (Boyd and Nixon, 1973; Sobolev, 1974; Dawson and Smith, 1977; Gurney et al, 1979; Schulze et al, 1995; Griffin et al, 1997; Kostrovitsky et al, 2004; Wyatt et al, 2004; Amshinsky and Pokhilenko, 1983; and references there in) and more rarely in kimberlite II or orangeites (Mitchell, 1977). Ilmenites occur as discrete rounded nodules up to 7–8 cm, forming up to 30–90 wt % of the heavy mineral fraction in weathered kimberlites. Sometimes they occur in intergrowths with phlogopites, olivines (Ol) (Moore et al, 1992; Robles-Cruz et al, 2009), Cr-poor pyropes (Rodionov et al, 1991; Gurney et al, 1979; Smith et al, 1975; Harte and Gurney, 1975; Schulze et al, 2001) or clino- and orthopyroxenes (Dawson and Reid, 1970; Boyd and Nixon, 1973; Rodionov et al, 1988). Groundmass ilmenites in kimberlites are lower in MgO, NiO and Cr2O3 (Pasteris, 1980; Mitchell, 1986)
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