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

Abstract

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 SiO 2 contents (42–52 wt.%). Whole-rock δ 18O 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 δ 18O of the erupted lava, as every sample has undergone some subsolidus 18O enrichment. The δ 18O values of the primary magmas (≈ +6.5) can be accurately calculated from the δ 18O of clinopyroxene mineral separates. However, the leucite mineral separates (pure, stoichiometric KA1Si 2O 6) have δ 18O = +8.8to+10.6 , and clearly have not retained their primary igneous oxygen isotopic compositions. The whole-rock δ 18O values exhibit a positive correlation with H 2O + contents, and extrapolation to a plausible water content for such a subaerially erupted lava ( <0.5 wt.% H 2O) gives a range of primary δ 18O values of +5.8 to +6.8. The much less hydrated Bufumbira, Africa and Gaussberg, Antarctica samples have δ 18O = +6.7to+7.9and+6.1to+7.0 , respectively. These have probably undergone only minor subsolidus 18O enrichments. The San Venanzo, Italy, samples are even less hydrated, so their extremely high δ 18O 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 δ 18O values than potassic rocks from southern Italy or anywhere else in the world. The northward increase in δ 18O in the leucite-bearing rocks of Italy is thus clearly a manifestation of progressively greater interaction between mantle-derived magmas ( δ 18O = +5.5to+7.5 ) and the high- 18O sedimentary and metasedimentary rocks of the Italian continental crust ( δ 18O = +15to+25 ), which were strongly heated during the episode of Tuscan anatectic granitic magmatism.