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 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.

Unique Amphibole Bearing Mantle Column Beneath the Leningrad Kimberlite Pipe, West Ukukit Field, NE Yakutia

“…The TiO2-Cr2O3 plot shows the joint decrease with the TiO2. The Cr rich varieties are typical for the high-temperature mantle metasomatites compositions reacted with the protokimberlitic melts [10,11,43]. There are two levels of the Al2O3 concentrations possibly corresponding to 2 magmatic impulses (Figure 8).…”

“…In Svetlana pipe, most of the compositions are plotting outside from the kimberlite trend. The TiO2 axis is pressure-dependent [43]. The TiO2-Cr2O3 plot shows the joint decrease with the TiO2.…”

Aillikites and Alkali Ultramafic Lamprophyres of the Beloziminsky Alkaline Ultrabasic-Carbonatite Massif: Possible Origin and Relations with Ore Deposits

“…The Cu-Zn ores as well as Ta-Nb, Th-U and REE specialization and high MnO content evidences about the complex origin of the ore components in BZM. High alkalis, carbonates, high H 2 O associated with Ba, Sr, U, Pb, Zn, Cu, Au, Ag are often considered to be a subduction-related fluid metasomatism [64,74,76,77,111,112]. However, the rare earths and elements such as Nb, Ta, Zr, Hf, Ti, are not commonly attributed to subduction [3,8,14,74,76] though the remobilization of the subducted crustal sediments can be a possible source of HFSE and other elements [2,76].…”

“…However, the rare earths and elements such as Nb, Ta, Zr, Hf, Ti, are not commonly attributed to subduction [3,8,14,74,76] though the remobilization of the subducted crustal sediments can be a possible source of HFSE and other elements [2,76]. All these elements are concentrated in the perovskites and may be the result of re-melting of specific metasomatic fronts, which occur around essentially carbonatitic melts such as protokimberlites [112]. The source of the phosphorus is also debatable [89,91,113].…”

“…An additional source can be considered the transfer of some components (LREE, LILE) by fluxes of volatiles, or melts that are saturated in volatiles [77,107,108,111]. One of the effective mechanisms of concentration of ore components, including rare-metals and radioactive elements, is multi-stage small degrees melting of carbonated mantle and crystallization with the formation of a metasomatic front, abundant in carbonates, perovskite, apatite, along with ilmenite and mica [112]. Such metasomatites are suggested to be associated with protokimberlite melts, which are essentially carbonatitic in final stage near 4.0 GPa [30,100].…”