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| Titel |
Veined pyroxenite xenoliths in Ugandan kamafugites: mantle or magma? Using in situ techniques for 87Sr/86Sr-isotopes and trace elements as tools |
| VerfasserIn |
Klemens Link, Simone Tommasini, Eleonora Braschi, Sandro Conticelli, Erasmus Barifaijo, John V. Tiberindwa, Stephen F. Foley |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042888
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| Zusammenfassung |
| The genesis of pyroxenite nodules in Ugandan kamafugites and their possible genetic
relationships is a matter of debate. In earlier studies the pyroxenites were considered either as
xenoliths from pervasively metasomatized peridotite mantle (Lloyd, 1981) or as
distinct paragenesises occurring as veins within the peridotitic mantle (Harte et al.,
1993). In both cases the xenoliths would represent mantle material that was at least
partly involved as source material for the kamafugite melts. A third alternative
could be that they represent cumulates of the lavas. In any case, the nodules provide
important information for understanding the generation of ultrapotassic lavas and for
characterizing the rift-related lithosphere mantle as part of the initial continental rift
process. Originally the ultrapotassic kamafugites were considered to be single stage
partial melts of pervasively metasomatized mantle but new geochemical studies
indicate a multistage development (Rosenthal et al., 2009). Nd, Hf and Os isotopes
point to mixing between components derived from metasomatically influenced
peridotite and mica-pyroxenite. In-situ investigation of the Sr-isotope and trace
element compositions of individual minerals in a number of xenoliths allows us to
constrain their genesis and relation to the host lavas. The nodules appear to originate by
near-liquidus crystallization of melts derived from enriched peridotite within the cratonic
lithosphere mantle. They later partially remelted to form one source of the potassium-rich
kamafugites.
Sr-isotopes from different domains within single mineral grains in the nodules and host
lavas are used to trace the nodules’ role as a potential source to lavas, and trace element
measurements are used to support the conclusions. Rb/Sr- measurements from the biotites to
constrain the time between nodule crystallization and eruption of the Quaternary lavas to
about 3.3 Ma. This also suggests a significant increase of the geothermal gradient beneath the
preceding rift within that time.
Structures on microscopic scale indicate at least two different generations of mineral
growth clearly related to multiphase magmatic events forming the nodules. Rare composite
samples allow a correlation between the older and younger parageneses, demonstrating
reaction between the older matrix pyroxenite and the younger, high-Ti melt. The relatively
low (~0,13wt%) Cr2O3-contents together with the high LREE concentrations measured in
the oldest observed clinopyroxenes (La~12,4 x PRIMA with La/Lu~21) as well as the lack
of any other characteristic mineral relicts argue against a pervasively overprinted peridotite
mantle.
Comparable 87Sr/86Sr- values close to bulk earth values as well as similar 143Nd/144Nd-
ratios in the nodules (0,512480-0,5122573) and the lavas (average: 0,512551) support a
genetic link between the kamafugites and the nodules as suggested by experiments (Lloyd et
al. 1985). Low radiogenic 87Sr/86Sr ratios in Rb-free clinopyroxene and perovskite
(0,704459-0,704487) constrain initial values for the source whereas slightly more radiogenic
values from cogenetic Rb-bearing biotites (0,704754- 0,704762) are the result of radioactive
decay after mineral growth. The majority of the kamafugite 87Sr/86Sr values lie
between the two end-members (0,704624- 0,704717). Additionally considering
microscale structures showing melting processes we conclude that the nodules
represent one source and that the intermediate 87Sr/86Sr values of the lavas reflect the
melting of differing proportions of biotite and clinopyroxene in the source region. |
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