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Abstract:
Kaersutite to pargasite phenocrysts from Tertiary alkali basalts (Rhon, Central European Province, Germany) yield new high-precision Ar-40/Ar-39 ages of 24.0-24.1 Ma. Major and trace element compositions demonstrate that these high-TiO2 (4-7 wt%) amphiboles are in equilibrium with their host rock. Chemically, these amphibole phenocrysts resemble amphibole from magmatic veins in upper mantle rocks but differ from disseminated amphibole from peridotite. Most amphiboles have similar isotope characteristics to their alkaline basaltic host rocks (Sr-87/Sr-86(24) = 0.7035-0.736, epsilon Nd-24 = +3.8-+4.0, Pb-206/Pb-204(24) = 19.21-19.37, Pb-207/Pb-204(24) = 15.58-15.62, Pb-208/Pb-204(24) = 38.95-39.16), but two samples show contrasting isotopic compositions (epsilon Nd-24 = -4.0 and -2.9; Pb-206/Pb-204(24) = 17.08 and 18.11; Pb-207/Pb-204(24) = 15.51 and 15.58; Pb-208/Pb-204(24) = 37.41 and 37.99), indicating involvement of an ancient crust-derived component during melting. The O isotopic composition of the amphibole phenocrysts ranges from 5.4 to 7.5 parts per thousand, reflecting O isotope heterogeneity of the upper mantle sources. The contrasting isotopic composition of amphibole and host rock pairs furthermore indicates that phenocrysts record the early stages of the volcanic history of the Rhon volcanic field on a regional scale and at a different depth within the lithospheric mantle. Temperature and pressure estimates range from 1,010 and 1,080 degrees C and 0.7 and 1.0 GPa and are compatible with the experimental results on the stability of amphibole in alkaline rocks derived from the upper mantle. Geochemistry of major and trace elements and isotopic compositions shows that igneous amphibole from alkali basalts may preserve isotope heterogeneities suggesting that they sample heterogeneous upper mantle lithologies on a small scale.
