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Experimental evidence for polybaric differentiation of primitive arc basalt beneath St. Vincent, Lesser Antilles

Melekhova, E.; Blundy, J.; Robertson, R.; Humphreys, M.C.S.

Experimental evidence for polybaric differentiation of primitive arc basalt beneath St. Vincent, Lesser Antilles Thumbnail


E. Melekhova

J. Blundy

R. Robertson


Equilibrium crystallization experiments have been performed on a primitive high-MgO basalt (HMB) from Soufrière, St. Vincent, with three initial H2O contents (0·6, 2·3 and 4·5 wt %), at pressures of 0·4, 0·7, 1·0 and 1·3 GPa and temperatures from 1350 to 950°C. Redox conditions, as determined by µXANES analysis of Fe3+ in experimental glasses, were 1–4 log units above the nickel–nickel oxide (NNO) buffer. The aim of the study was to explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent. An experiment with 4·5 wt % initial H2O is multiply saturated close to its liquidus (1180°C and 1·3 GPa) with a spinel lherzolite assemblage, which is consistent with a primary origin for HMB in the mantle wedge. Multiple saturation of HMB with 2·3 wt % H2O was not observed, but is inferred to occur at pressures >1·3 GPa. The experimental results show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P–T–H2O conditions. Low-magnesian basalts (LMB) can be generated from HMB with 2·3–4·5 wt % H2O at pressures of 1·0–1·3 GPa, corresponding to Moho depths beneath St. Vincent. The CaO contents of LMB are sensitive to differentiation pressure: high-CaO LMB are produced at pressures >0·5 GPa. Basaltic andesites (BA) can be generated at 0·7–1·0 GPa from HMB with 0·6–2·3 wt % H2O. High-alumina basalts (HAB) are produced at mid- to upper-crustal conditions (≤0·4 GPa) by differentiation of HMB with high initial H2O (≥4 wt %) through delay of plagioclase crystallization and dominant fractionation of olivine, clinopyroxene and spinel. St. Vincent andesites could be produced from relatively dry (≤0·6 wt % H2O) HMB only at lower-crustal conditions. This is suggestive of a partial melting origin from precursor HMB that had solidified at depth to produce gabbros with ∼30% hornblende (i.e. ∼0·6 wt % structurally bound H2O). The experimentally determined differentiation conditions are consistent with polybaric differentiation within a hot zone that extends from the Moho and uppermost mantle to the mid- or upper crust. Within the hot zone differentiation occurs by a combination of crystallization of HMB with 2–5 wt % H2O and partial melting of ancestral HMB gabbros. Although the experimental melts provide an excellent match to erupted lava compositions, experimental crystal compositions do not match either phenocrysts or cumulate crystals, as preserved in xenoliths. The failure to reproduce natural crystal compositions suggests that these are formed as differentiated magmas ascend and attain their H2O-saturated liquidi at shallower pressures. Thus there is a disconnect between the high-pressure phase compositions and assemblages that generate liquid compositional diversity and the low-pressure composition and assemblages that occur as phenocrysts and in cumulate xenoliths. This finding lends support to the idea of cryptic fractionation in the generation of arc magmas.


Melekhova, E., Blundy, J., Robertson, R., & Humphreys, M. (2015). Experimental evidence for polybaric differentiation of primitive arc basalt beneath St. Vincent, Lesser Antilles. Journal of Petrology, 56(1), 161-192.

Journal Article Type Article
Acceptance Date Dec 5, 2014
Online Publication Date Jan 1, 2015
Publication Date Jan 1, 2015
Deposit Date Feb 10, 2015
Publicly Available Date Mar 31, 2015
Journal Journal of Petrology
Print ISSN 0022-3530
Electronic ISSN 1460-2415
Publisher Oxford University Press
Peer Reviewed Peer Reviewed
Volume 56
Issue 1
Pages 161-192
Keywords High-alumina basalt, Andesite, Cumulates, Plutonic xenolith, Hot zone.


Accepted Journal Article (3 Mb)

Copyright Statement
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Journal of Petrology following peer review. The version of record Melekhova, E., Blundy, J., Robertson, R. and Humphreys, M.C.S. (2015) 'Experimental evidence for polybaric differentiation of primitive arc basalt beneath St. Vincent, Lesser Antilles.', Journal of petrology., 56 (1): 161-192 is available online at:

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