Evidence of crustal contamination at Hekla volcano from230Th-238U disequilibria

129 CRYSTAL CANNIBALISATION, CRUSTAL CONTAMINATION AND THE PETROGENESIS OF ARC MAGMAS*

U-Th SERIES ISOTOPE SYSTEMATICS AND TRACE ELEMENT GEOCHEMISTRY OF VOLCANIC ROCKS FROM SANTORINI

D. M. PYLE and M. IVANOVICH (Dept of Earth Sciences, cambridge University, CB2 3EQ, UK and UraniumSeries Disequilibrium Section, NPD, Harwell Laboratory, Oxon OXII ORA, UK)

A. SNEYERS, J. HERTOGEN, J.P.P. HUIJSMANS, M.BARTON. (University of Leuven, B-3030 Leuven, Belgium)

The detailed petrogenesis of a major pyroclastic rhyodacite and a subsequent lava shield has been studied, using short-lived daughters of the 238U and 232Th decay series. Reworking, or 'cannibalisation', of older cumulate material is a widespread phenomenon. Andesitic blebs and clots of gabbroic cumulate material (Olivine Fo70-7%, Plagioclase An88-gg) crystallised soon before eruption at 8Oka from a source with (23°Th/232Th)%2. During eruption these were incorporated into a rhyodacite melt with (23°Th/232Th)%O.6. Magnetites and ilmenites, now coexisting in the rhyodacite, crystallised independently from these respective end-members. The later lava shield also contains crystal populations from two sources of distinct Th-isotopic compositions, inferred to have been (i) a basic crystal-rich mush and (ii) a dacite magma. The magma intruded the mush and the two intermingled prior to eruption, but the crystals retained the isotopic signatures of the parental materials. Both the acid and basic end-members are mutually-contaminated versions of the material found in the earlier rhyodacite which has a crystallisation age indistinquishable from that of the lava shield. Most eruptions on Santorini involve ode or more magmas of distinct Th-isotopic compositions. Invariably, more basic materials show little U-enrichment and have initial (23°Th/232Th) ratios >0.9. More silicic magmas have distinctly lower initial (23°Th/232Th), variable degrees of U-enrichment, and point to a crustal influence greater than previously recognised. * This work is a part of UKAEA Underlying Research Prograrmme.

Whole rock U-Th series isotope data are presented for lavas from two eruption centers (Micro Profitis Ilias/MPI/ and Skaros) of the Santorini complex, Aegean Sea, Greece. Volcanic activity on Santorini mainly occurred between |00 ka and Recent. Previous studies have indicated that eruption centers show temporal geochemical differences that reflect depletion of the upper mantle source regions. Petrogenetic trends for both centers are dominated by fractional crystallization with superimposed processes of magma mixing and assimilation. Episodic replenishment of magma chambers is deduced from cyclic compositional variations observed in stratigraphic sequences. Trace element data indicate that the basaltic component involved in Santorini volcanism is rather depleted in LIL-elements. High precision 87Sr/86Sr data (Wyers, 1987) show that assimilation was more important for the ~ I than for Skaros lavas. (230Th/238U) alpha activity ratios of lavas from both centers are close to equilibrium (0.97-1.05); it is argued that measured (230Th/232Th) ratios represent initial ratios. In general, samples from one volcanic center are characterized by nearly constant (230Th/232Th); it suggests that reservoirs were homogeneous with respect to (230Th/232Th) Samples from Skaros have (230Th/232Th) between 0.92 and 0.97. One sample has (230/232) of 1.04 and (238/232) of 0.95; it also shows abundant evidence for magma mixing. The disturbance of U-Th equilibrium by mixing has already been demonstrated by Pyle et a h (1988). Samples from ~ I show systematically lower ~30/232Th) ratios : 0.82-0.86. This dichotomy may be related to :(I) assimilation of a crustal component especially for MPI; (2) time dependent geochemical differences among source regions; (3) a combination of these two through mixing processes.

EVIDENCE OF CRUSTAL CONTAMINATION AT HEKLA VOLCANO FROM 23°Th- -7"3suDISEQUILIBRIA O. SIGMARSSON, C. HEMOND and M. CONDOMINES (Centre de Recherche Volcanologique,Universit6 Blaise Pascal ,5 rue Kessler, 63038 Clermont - Fen'and, France). The volcanic products of Hekla, Iceland, consist of basaltic lavas, intermediate lavas or tephra and dacitic to rhyolitic tephra. The most violent eruptions begin with an acid plinian phase, followed by emission of basaltic andesites. The Th content ranges from 1.l ppm in the basahs to 11 ppm in the rhyolites and the Q3°Th / 238U) ratio is higher than 1 (average 1.10). The Th / U and (231rl'h / 232Th) ratios are the same in the basahs and the basaltic andesites (3.2 and 1.05 resp.). On the contrary, Th / U ratios are higher (3.6) and (23°Th / 232Th) ratios lower (0.97) in the rhyolites than in the basaltic andesites. When plotted versus Th content, Th / U and (23°Th / 232Th) ratios show good positive and negative correlation respectively. These correlations are best explained by mixing between basaltic andesite magma and silicic malts, with higher Th / U and lower (23°Th / 232Th) ratios, fomaed by partial melting of the basaltic crust in radioactive equilibrium. On the other hand, Sr, Nd and probably O isotopic ratios are constant in Hekla products which is easily explained if the basaltic crust beneath the volcano is relatively young and if it has not been strongly affected by hydrothermal alteration. We envisage the following model to account for the rock compositions found at Hekla: At the base of the crust basaltic magmas are stored and form basaltic andesites by crystal fractionation which in turn are injected into a magma chamber in the crest. There the basaltic andesites can become contaminated by a crustal derived rhyolitic melt The amount of contamination increase with increasing time between eruptions. Based on data from Hekla, we think that crustal assimilation may play an important role in the genesis of silicic rocks in Iceland.

U-Th-Ta FRACI'IONATION IN MAGMA ~OURY3ES OF THE ITALIAN K-RICH VOLCANISM. CONSTRAINTS FROM DISTRIBUTION COEFFICIENTS AND Th-U D£SEQUILIBRIUM STUDIES. and C. FLEHOC (Laboratoire de G4oehimie Compar4e et Systematique, U.P.M.C., 4 Flace Jussieu, 75252 Paris Cedex 05). A systematic study of trace elements characteristics and Th-U disequilibrium of primary magmas of the recent potassic volcHnism of Latium, Campania and Eolian islands Provinces (Italy) has been performed. The magma sources are characterized by strong Th-Ta fractionations with increasing Th/Ta ratios from the south (IU) to the north (50) of the whole potassic province. The variations of this ratio are eorrelat~J to the variations of Sr and Nd isotopic ratios. Strong variations of the Th/U ratio occur in some eogenetie lava series. They are due to the interaction of the magma bodies under differentiation with leaching crustal flulds and allow to measure the initial Th/U ratio of each Province. The magma sources are thus characterized by relatively low Th/U ratios (2.5~ Th/U <3.5) due to recent Th-U fractionations probably occuring when these sources have been isolated from a common primot~Jial m~untle source. Fractionat ion models using measured distribution coefficients of Ll, Th and Ta are tested and the existence of a m~ior and unusual differentiation process of the primordial mm]tle with very strong enrichments of Th, U i :~nd associated elements (but Ta). i.~=d i s c u s s ~ /