|
1 Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, 56100 Pisa, Italy (e-mail: cperinelli{at}dst.unipi.it)
2 CNR–IGG, UO di Firenze, via G. La Pira, 4, 50121 Firenze, Italy
3 CNR–IGG, UO di Roma, p.le Aldo Moro 5, 00185 Roma, Italy
4 Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira, 4, 50121 Firenze, Italy
5 Dipartimento di Scienze della Terra, Università di Ferrara, via Saragat 1, 44100 Ferrara, Italy
6 INGV Sezione di Sismologia e Tettonofisica, via di Vigna Murata 605, 00143 Roma, Italy
Magma generation in the Ross Sea system is related to partial melting of strongly metasomatized mantle sources where amphibole most probably plays a crucial role. In this context, metasomatism induced by a mela-nephelinite melt in lithospheric mantle of the Mt. Melbourne Volcanic Province (northern Victoria Land (NVL), Antarctica) was investigated experimentally studying the effects of melt interaction with lherzolite at 1.5–2.0 GPa and T=975–1300 °C, and wehrlite at 1.0 GPa and T=1050–1250 °C. The experiments were designed to induce melt infiltration into the ultramafic rocks. The observed modifications in minerals are compared with those found in mantle xenoliths from NVL. The effects of metasomatic modifications are evaluated on the basis of run temperature, distance from the infiltrating melt and the diffusion rates of chemical components. Both in lherzolite and wehrlite, clinopyroxene exhibits large compositional variations ranging from primary diopside to high-Mg–Cr–(Na) augitic and omphacitic clinopyroxenes in lherzolite, and to low-Mg and high-Ti–Al–Fe–Na augites in wehrlite. Olivine (in wehrlite) and spinel (in lherzolite) are also compositionally modified: the former shows enrichment in Fe and the latter displays a higher Cr/(Cr+Al) ratio. The systematic variations in mineral compositions imply modifications of the chemistry of the infiltrating melt as recorded by the glass veinlets and patches observed in some charges. In experiments involving wehrlite paragenesis, the glass composition approaches that of melt patches associated with both amphibole-free and amphibole-bearing natural samples, and is related to olivine + clinopyroxene crystallization coupled with primary clinopyroxene dissolution at the contact between the metasomatizing melt and the solid matrix. Even if amphibole crystallization was not attained in the experiments, we were able to explain the occurrence of amphibole in the natural system considering that in this case a hot metasomatizing melt infiltrates a cooler matrix.
