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| Titel |
Combining experimental and simulation studies to constrain the mechanism of metasomatic vein formation |
| VerfasserIn |
Sébastien Pilet, Peter Ulmer, Michael B. Baker, Edward M. Stolper, Othmar Muntener |
| 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 |
250034916
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| Zusammenfassung |
| Metasomatized lithosphere is considered to be a potential source for alkaline lavas observed
in oceanic and continental setting [1, 2]. Experiments on natural amphibole-rich veins have
recently demonstrated that high degree of melting of amphibole-bearing metasomatic veins
with various amount of interaction with surrounding peridotite reproduce key features of the
major and trace element compositions of alkaline lavas observed in intraplate volcanoes [3].
However, this model suggests that many geochemical features of alkaline rocks are directly
inherited from the metasomatic veins itself. The formation of metasomatic veins is
interpreted as the migration, cooling, and fractional crystallization of low degree
melts within the lithospheric mantle which would generate a continuum of phase
assemblages: from anhydrous to hydrous veins plus a cryptic enrichment in the surrounding
peridotite [4]. To constrain this process, two complementary studies have been
undertaken.
(1) A series of high-pressure experiments simulating the liquid line of descent of a
basanitic magma differentiating within continental or mature oceanic lithosphere (1.5 GPa)
has been performed to evaluate whether the evolution of this magma within the lithosphere
was able to produce cumulates similar to metasomatic veins. The cumulates produced in this
series of experiments are characterized by an anhydrous clinopyroxene + olivine
assemblage at high temperature (1250º-1160ºC), while at lower temperature (1130Ë -
980Ë C), hydrous cumulates with dominantly amphibole + minor clinopyroxene,
spinel, ilmenite, titanomagnetite and apatite (1130ºC- 980ºC) are formed. This data
supports the interpretation that anhydrous and hydrous metasomatic veins could be
produced during continuous differentiation processes of primary, hydrous alkaline
magmas at high pressures [4]. However, the comparison between the cumulates
generated from an initial ne-normative liquid (basanite) or from hy-normative initial
composition (hawaiite) [5] indicates that for all hydrous liquids, the different phases
formed upon differentiation are similar even though the proportions of hydrous versus
anhydrous minerals could vary significantly. This suggests that the formation of
amphibole-bearing metasomatic veins observed in the lithospheric mantle could be linked to
the differentiation of initial hydrous liquids ranging from ne-normative to hy-normative in
composition.
(2) To evaluate the trace element composition of amphibole-cumulates, we did Monte
Carlo simulations of metasomatic vein formation in the context of mid-ocean ridges based on
the model proposed by Niu and O’Hara [2]. This model assumes that some liquid produced at
depth at the periphery of a ridge is not collected to form MORB, but generates a modal and
cryptic metasomatic enrichment of the lithospheric mantle. For the calculation, we
assume that initial metasomatic agents represent low-degree melts from peridotitic
sources similar to depleted MORB mantle. The inferred polybaric crystallization
sequence of the low-degree melts was constrained by experiments and the mineral
compositions and modes observed in metasomatic veins. The main conclusion of these
simulations is that the compositions of calculated hydrous cumulates are highly
enriched in incompatible trace elements and that their patterns share many similarities
with the patterns of alkaline OIBs (positive Nb/La, negative Ce/Pb, same slope of
REE).
The combination of fractional crystallisation experiments and Monte Carlo simulations
supports the model proposed by Harte et al. [5] for the formation of metasomatic veins; and
suggests that low degree melts from “normal” mantle following by fractional crystallisation
at high pressure allowed to produce hydrous cumulates suitable to be a source of alkaline
magma observed in intraplate setting.
[1] Lloyd and Bailey (1975) Phys. Chem. of the Earth, 9: 389-416; [2] Niu and O’Hara
(2003) J. Geophys. Res. 108, 2209; [3] Pilet et al. (2008) Science 320, 916-919; [4] Harte et
al. (1993) Philos. Trans. R. Soc. Lond. Ser. A, 342, 1-21Â ; [5] Nekvasil et al. (2004) J. Petrol.,
45: 693–721. |
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