Comparisons of ^(18)O/^(16)O and ^(87)Sr/^(86)Sr in volcanic rocks from the Pontine Islands, M. Ernici, and Campania with other areas in Italy

Abstract

New ^(18)O/^(16)O and ^(87)Sr/^(86)Sr data dramatically confirm and extend the systematic regional isotopic and geographic correlations observed in Quaternary volcanic rocks from Italy: (1) The High K Series (HKS) and Low-K Series (LKS) potassic magma source regions are isotopically distinct; the relatively rare, primitive (high-Ca) parent magmas of both suites have δ^(18)O = +5.5 to +7.5, but the LKS parent has a much lower ^(85)Sr/^(86)Sr and slightly lower δ^(18)O than the HKS parent. (2) The more abundant evolved (fractionated) magmas of both suites have a wide range of δ^(18)O (+6.0 to + 13.0), but in the offshore island centers (Ischia, Pontine Is., Procida) these magmas are all much lower in δ^(18)O ( < +8.0) than those from the Italian mainland, as a result of the absence of involvement of high-^(18)O continental crust. (3) At a given center, the ^(18)O-enrichments correlate with major-element changes (i.e., CaO depletion and K_2O and SiO_2 enrichment) attributable to fractional crystallization in crustal magma chambers ( 4 to 13 km depth?); because such ^(18)O enrichments cannot be produced in a closed system, these correlations imply that AFC processes were important, particularly north of Rome where they are enhanced by an increase in the temperature of the crust (due to igneous activity associated with the 0-7 Ma Tuscan anatectic event). Thus, the high δ^(18)O values of the potassic magmas of Italy should not be used as evidence in support of genetic models invoking recent subduction of sedimentary rocks into the source regions of these magmas. (4) On δ^(18)O vs. ^(87)Sr/^(86)Sr diagrams, the LKS and HKS magmas at each volcanic center display separate, steep positive trends indicating mixing with high-^(18)O continental crustal material; this type of mixing had little effect on ^(87)Sr/^(86)Sr in these Sr-rich magmas, but it had a dramatic effect on the δ^(18)O values. (5) Two distinctly different groups of rhyolites and quartz-normative trachytes are observed in the northernmost Pontine Islands, a high-^(18)O group similar to the anatectic Tuscan rhyolites (δ^(18)O > + 10) and a low-^(18)O group similar to the rhyolites of the Aeolian Islands (δ^(18)O = +7 to +7.5). (6) All of the magmas produced in and around Italy during the past five million years can be derived by mixing of(i) a SiO_2-rich continental crustal end member, having ^(87)Sr/^(86)Sr ~ 0. 715 to 0. 735 and δ^(18)O ~ + 10 to + 20, with three distinct mantle end members: (ii) a relatively low-K (LKS), moderate-Sr parent with ^(87)Sr/^(86)Sr ~ 0.706 and δ^(18)O ~ +6, dominant to the south of Rome and perhaps related to an upper mantle source modified by a recent subduction event; (iii) a high-K (HKS), high-Sr parent with ^(87)Sr/^(86)Sr ~ 0.711 and δ^(18)O ~ + 7 dominant in the vicinity of Rome and farther north, and possibly related to older subcontinental mantle modified by a recent metasomatic event; and (iv) a very low-K, low-Sr Tyrrhenian Sea MORB type parent with ^(87)Sr/^(86)Sr~ 0.7025 and δ^(18)O ~ +5.7.

Files

Additional details