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| Titel |
Picroilmenites in Yakutian kimberlites: variations and genetic models |
| VerfasserIn |
I. V. Ashchepkov, N. V. Alymova, A. M. Logvinova, N. V. Vladykin, S. S. Kuligin, S. I. Mityukhin, H. Downes, Yu. B. Stegnitsky, S. A. Prokopiev, R. F. Salikhov, V. S. Palessky, O. S. Khmel'nikova |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1869-9510
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| Digitales Dokument |
URL |
| Erschienen |
In: Solid Earth ; 5, no. 2 ; Nr. 5, no. 2 (2014-09-02), S.915-938 |
| Datensatznummer |
250115335
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| Publikation (Nr.) |
 copernicus.org/se-5-915-2014.pdf |
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| Zusammenfassung |
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. |
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