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| Titel |
Experimental Evidence for Polybaric Intracrustal Differentiation of Primitive Arc Basalt beneath St. Vincent, Lesser Antilles |
| VerfasserIn |
Jon Blundy, Lena Melekhova, Richard Robertson |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093799
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| Publikation (Nr.) |
 EGU/EGU2014-8872.pdf |
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| Zusammenfassung |
| We present experimental phase equilibria for a primitive, high-Mg basalt from St. Vincent,
Lesser Antilles. Experimental details were presented in Melekhova et al (Nature Geosci,
2013); the objective here is to compare experimental phase compositions to those of erupted
lavas and cumulates from St. Vincent.
Starting material with 4.5 wt% H2O is multiply-saturated with a lherzolite assemblage at
1.3 GPa and 1180 °C, consistent with mantle wedge derivation. Experimental glasses from
our study, in addition to those of Pichavant et al (GCA, 2002) and Pichavant & Macdonald
(CMP 2007) on a similar high-Mg basalt, encompass a compositional range from
high-magnesian basalt to dacite, with a systematic dependence on H2O content, temperature
and pressure. We are able to match the glasses from individual experiments to different lava
types, so as to constrain the differentiation depths at which these magmas could be generated
from a high-Mg parent, as follows:
Composition wt% H2OP (GPa) T (°C)
High-Mg basalt 3.9-4.8 1.45-1.751180-1200
Low-Mg basalt 2.3-4.5 1.0-1.3 1065-1150
High alumina basalt 3.0-4.5 0.4 1050-1080
Basaltic andesite 0.6-4.5 0.7-1.0 1050-1130
Andesite 0.6 1.0 1060-1080
The fact that St. Vincent andesites (and some basaltic andesites) appear to derive from a
low-H2O (0.6 wt%) parent suggest that they are products of partial melting of older, high-Mg
gabbroic rocks, as 0.6 wt% H2O is approximately the amount that can be stored in
amphibole-bearing gabbros. The higher H2O contents of parents for the other lava
compositions is consistent with derivation by crystallization of basalts with H2O contents that
accord with those of olivine-hosted melt inclusions from St. Vincent (Bouvier et al, J Petrol,
2008). The generation of evolved melts both by basalt crystallization and gabbro melting
is consistent with the hot zone concept of Annen et al (J Petrol, 2006) wherein
repeated intrusion of mantle-derived basalt simultaneously crystallize by cooling and
melt country rocks composed of ancestral, solidified basalt. Isotopic data for St.
Vincent (Heath et al, J Petrol, 1998) rule out any involvement of much older sialic
crust.
Although our experimental glasses provide a very good match to erupted lavas, the
compositions of residual minerals do not match those of cumulate xenoliths (Tollan et
al, CMP, 2012), which are abundant on St. Vincent. Therefore cumulates are not
entrained fragments of the source region, but shallow accumulations of crystals
generated by cooling of magmas on their journey through the crust. Thus melt
compositions are a consequence of high pressure, H2O-understaurated phase relations,
whereas cumulates are a consequence of low pressure, typically H2O-saturated,
phase relations. We integrate these findings into a simple polybaric model of magma
differentiation on St. Vincent involving a single, high-Mg, mantle-derived parental basalt. |
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