E. Heath et al., LONG MAGMA RESIDENCE TIMES AT AN ISLAND ARE VOLCANO (SOUFRIERE, ST. VINCENT) IN THE LESSER ANTILLES - EVIDENCE FROM U-238 TH-230 ISOCHRON DATING, Earth and planetary science letters, 160(1-2), 1998, pp. 49-63
High-precision (TIMS) measurements of U and Th isotope concentrations
have been determined on whole rocks and mineral separates from Soufrie
re volcano on St. Vincent in the Lesser Antilles island are. The whole
rocks display relatively constant U/Th ratios (similar to 0.5), and a
re characterised by excess U-238 relative to Th-230 which is attribute
d to the addition of U-rich fluids from the subducting slab to the man
tle source of the magmas. Mineral isochrons for four recently erupted
(<4 ka) rocks yield ages of 46-77 ka and the combined data have an age
of 58 +/- 7 ka with an initial (Th-230/Th-230) ratio of 1.05. Mixing
of young magmas, or magma-cumulate mixing, are unlikely to be responsi
ble for these isochronous relationships. Contamination of young, pheno
crystic magmas with previously crystallised magma could produce the is
ochrons, in which case the ages provide maximum and minimum ages, resp
ectively, for these two components. However, no lavas with (Th-230/Th-
230) ratios of 1.054 have been found and there is no petrographic evid
ence for mixing or phenocryst-magma disequilibrium. Our preferred inte
rpretation is that the isochrons reflect long magma residence times wi
thin the are crust. Following initial crystallisation, due to heat los
s to the wall rocks, the magmas were maintained at a temperature close
to their liquidus while Th-230-ingrowth occurred in both the magma an
d the crystals. Maintenance of a relatively constant temperature requi
res good insulation by cumulate layers and probably heating from below
by influxes of fresh magma which either did not mix with the resident
magma or else had an essentially indistinguishable composition. Subtr
action of the residence times from the similar to 90 ka inferred to ha
ve elapsed since fluid addition to the mantle wedge leaves similar to
30 ka for transfer through the wedge. Long residence times may typify
more evolved lavas that develop in thicker are crust whereas tholeiiti
c are lavas are inferred to have trivial crustal residence times but s
imilar transfer times through the mantle wedge. (C) 1998 Elsevier Scie
nce B.V. All rights reserved.