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Magmatic evolution of the Gaussberg lamproite (Antarctica): volatile content and glass composition

¹ Dipartimento di Scienze della Terra, Università di Parma, Parco Area delle Scienze 157/A, I-43100 Parma, Italy
² Dipartimento di Fisica and Istituto Nazionale per la Fisica della Materia, Università di Parma, Parco Area delle Scienze 7/A, I-43100 Parma, Italy

^* E-mail: salvioli{at}unipr.it

The lamproite of Gaussberg is an ultrapotassic rock where leucite, olivine and clinopyroxene microphenocrysts occur in a glass-rich groundmass, containing microliths of leucite, clinopyroxene, apatite, phlogopite and rare K-richterite.

Abundant silicate melt inclusions occur in olivine, leucite and, rarely, in clinopyroxene microphenocrysts. Raman investigations on melt inclusions showed the presence of pure CO₂ in the shrinkage bubbles. On the other hand, the glass of the groundmass is CO₂-poor and contains up to 0.70 wt.% of dissolved H₂O, as estimated by infrared spectra. It is inferred that CO₂ was released at every stage of evolution of the lamproite magma (CO₂-rich shrinkage bubbles), whereas H₂O was retained for longer in the liquid. At Gaussberg, CO₂ seems to have a major role at relatively high pressure where it favoured the crystallization of H₂O-poor microphenocrysts; the uprise of the magma to the surface decreased the solubility of CO₂ and caused a relative increase in water activity. As a consequence, phlogopite and K-richterite appeared in the groundmass.

The glass composition of both the groundmass and melt inclusions suggests different evolutions for the residual liquids of the investigated samples. Sample G886 shows the typical evolution of a lamproite magma, where the residual liquid evolves toward peralkaline and Na-rich composition and crystallizes K-richterite in the latest stage. Sample G895 derives from mixing/mingling of different batches of magma; effectively glasses from melt inclusions in leucite and clinopyroxene are more alkaline than those found in early crystallized olivine. Leucite and clinopyroxene crystallized early from a relatively more alkaline batch of lamproite magma and, successively, a less alkaline, olivine-bearing magma batch assimilated them during its rise to the surface.

KEYWORDS: volatile components, silicate melt inclusions, glass composition, residual liquids, lamproite, Gaussberg, Antarctica, Australia

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