IMMISCIBLE SEPARATION OF METALLIFEROUS FE TI-OXIDE MELTS FROM FRACTIONATING ALKALI BASALT - P-T-F(O-2) CONDITIONS AND 2-LIQUID ELEMENTAL PARTITIONING/

Globules of iron-dominated (59-69 wt% FeOtot) and titanium-dominated ( 43.5 wt% TiO2) oxide melts have been detected in igneous xenoliths fro m Pliocene-to-Pleistocene alkali basalts of the Western Carpathians. F luid inclusion and mineral composition data indicate immiscible separa tion of the high-iron-oxide melt (HIM) at magmatic temperatures. The H IM separation occurred during clinopyroxene (augite) accumulation in a n alkali trachybasalt and continued during crystallization of amphibol e (kaersutite) and K-feldspar (anorthoclase), the latter coexisting wi th trachyte and alkalic rhyolite residual melts. Some HIM was also exp elled from sub-alkalic rhyolite (70-77% SiO2), coexisting with An(27-4 5) plagioclase and quartz in granitic (tonalite-trondhjemite) xenolith s. Oxygen fugacities during HIM separation range from -1.4 to +0.6 log units around the QFM buffer. A close genetic relationship between HIM -hosted xenoliths and mantle-derived basaltic magma is documented by m ineral delta(18)O values ranging from 4.9 to 5.9 parts per thousand, V -SMOW. delta D values of gabbroic kaersutite between -61 and -86 parts per thousand, V-SMOW are in agreement with a presumed primary magmati c water source. Most trace elements, except Li, Rb and Cs, have prefer entially partitioned into the HIM. The HIM/Si-melt partition coefficie nts for transition elements (Sc, V, Cr, Co, Ni) and base metals (Zn, C u, Mo) are between 2-160, resulting in extreme enrichment in the HIM. La and Ce also concentrate in the silicic melt, whereas Tb-Tm in the H IM. Hence, the immiscible separation causes REE fractionation and prod uces residual silicic melt enriched in LREE and depleted in HREE. The weak fractionation among Tb-Tm and Yb, Lu can be attributed to recurre nt extraction of the HIM from the magmatic system, while flat HREE cho ndrite-normalized patterns are interpreted to indicate no or little lo ss of the HIM.