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Published August 1984 | public
Journal Article

¹⁸O/¹⁶O and chemical relationships in K-rich volcanic rocks from Australia, East Africa, Antarctica, and San Venanzo-Cupaello, Italy


18O/16O analyses were made on a set of leucite-bearing igneous rocks from a variety of localities around the world. The samples chosen for study all have relatively primitive characteristics, such as low SiO2 contents (42–52 wt.%). Whole-rock δ18O values from the 10–15 m.y. old Australian leucitites range from +7.1 to +11.0; however, none of these values represents the original δ18O of the erupted lava, as every sample has undergone some subsolidus18O enrichment. The δ18O values of the primary magmas (≈ +6.5) can be accurately calculated from the δ18O of clinopyroxene mineral separates. However, the leucite mineral separates (pure, stoichiometric KA1Si2O6) have δ18O = +8.8to+10.6, and clearly have not retained their primary igneous oxygen isotopic compositions. The whole-rock δ18O values exhibit a positive correlation with H2O+ contents, and extrapolation to a plausible water content for such a subaerially erupted lava ( <0.5 wt.% H2O) gives a range of primary δ18O values of +5.8 to +6.8. The much less hydrated Bufumbira, Africa and Gaussberg, Antarctica samples have δ18O = +6.7to+7.9and+6.1to+7.0, respectively. These have probably undergone only minor subsolidus18O enrichments. The San Venanzo, Italy, samples are even less hydrated, so their extremely high δ18O values of +10.8 to +12.0 are definitely magmatic values. Those data confirm our earlier conclusions that the leucite-bearing magmas erupted in north-central Italy are unique in having much higher δ18O values than potassic rocks from southern Italy or anywhere else in the world. The northward increase in δ18O in the leucite-bearing rocks of Italy is thus clearly a manifestation of progressively greater interaction between mantle-derived magmas (δ18O = +5.5to+7.5) and the high-18O sedimentary and metasedimentary rocks of the Italian continental crust (δ18O = +15to+25), which were strongly heated during the episode of Tuscan anatectic granitic magmatism.

Additional Information

This research was supported by National Science Foundation grants EAR-76-21310 and EAR-78-16874, a cooperative Italy-United States C.N.R.-N.S.F. grant No. 80.00955.05 and Centro Studi per la Geocronologia e Geochimica delle Formazioni Recenti (C.N.R.). We wish to thank B. di Sabatino and M. Preite Martinez for their help in the field work in the San Venanzo area.

Additional details

August 22, 2023
October 24, 2023