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| Titel |
Metasomatic Enrichment of the Lithosphere and its Potential Implications for the Formation of Oceanic and Continental Alkaline Magmas |
| VerfasserIn |
S. Pilet, M. B. Baker, E. M. Stolper, O. Müntener |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023308
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| Zusammenfassung |
| The generation of oceanic and continental intra-plate magmas implied that the source of these
magmas was enriched regarding the primitive mantle [1]. However, the nature and origin of
the mantle components that melt below oceanic islands are still in debate. A hypothesis
proposed that the enriched components which melt below oceanic islands correspond to
recycled oceanic crust [2]. Alternatives suggest that these components could correspond to
metasomatized continental or oceanic lithosphere [3-5]. However, if these two hypotheses are
frequently presented as mutually exclusive, we suggest that theses hypotheses could be
complementary.
The fact that oceanic crust produces silica-saturated partial melts seems in agrement with
the implication of this material in the generation of tholeiitic (i.e., hy- and qtz- normative)
magmas from large oceanic islands and continental lava flows as proposed by [6]; however
this fact makes it difficult to envision oceanic crust as a major component in the generation of
alkaline (i.e., ne-normative) magmas. Experiments on metasomatic veins (hornblendites) and
their dehydrated equivalents demonstrate that high-degree melting of these veins followed by
variable amounts of interaction of the liquid with surrounding mantle can reproduce key
features of the major- and trace-element compositions of alkaline magmas [7]. We suggest
two scenarios for the production of alkaline magmas by melting metasomatized
lithosphere: (i) the metasomatized lithosphere experiences a thermal perturbation or
decompression and thereby melts in situ; or (ii) the metasomatized lithosphere is
recycled into the convecting mantle by subduction or delamination and melts during
later upwelling (e.g., in a plume). In continental alkaline magmas, the presence of
amphibole xenocrysts compositionally similar to amphibole in metasomatic veins
is consistent with the “in situ” hypothesis. While such veins may play a role in
alkaline magmas for some oceanic islands and seamounts, long residence times of the
metasomatized sources are required to explain the range of isotopic ratios observed in some
OIBs. The recycling scenario could isolate metasomatic veins for times sufficient for
ingrowth of extreme isotopic ratios such as those observed in alkaline OIBs from
Polynesia.
Since our hypothesis implies that alkaline magmas are typically produced by high degrees
of melting of the metasomatic veins, the process responsible for the metasomatic enrichment
of the lithosphere is an important component of our model. The metasomatic veins are
generally interpreted as cumulates formed during the ascent and differentiation of low-degree
melts derived from volatile-rich peridotite within the lithosphere. To evaluate whether
hydrous cumulates are suitable as sources for alkaline magmas, we did Monte Carlo
simulations of metasomatic vein formation. These simulations indicate that (i) hydrous
cumulates produced by such processes have trace-element patterns suitable to be potential
sources for alkalines magmas observed in continental volcanoes, in oceanic islands, or
seamounst; (ii) the trace-element pattern of these model cumulates could account for the
trace-element patterns of HIMU to EM magmas simply by varying the proportion of
residual metasomatic liquid trapped in the surroundings of the veins (e.g. cryptic
metasomatism); and (iii) the model cumulates can account for the range of isotopic
composition observed in OIBs if the amph |
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