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

Abstract

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 CO 2 in the shrinkage bubbles. On the other hand, the glass of the groundmass is CO 2 -poor and contains up to 0.70 wt.% of dissolved H 2 O, as estimated by infrared spectra. It is inferred that CO 2 was released at every stage of evolution of the lamproite magma (CO 2 -rich shrinkage bubbles), whereas H 2 O was retained for longer in the liquid. At Gaussberg, CO 2 seems to have a major role at relatively high pressure where it favoured the crystallization of H 2 O-poor microphenocrysts; the uprise of the magma to the surface decreased the solubility of CO 2 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, olivinebearing magma batch assimilated them during its rise to the surface.

Fluid inclusions in diamonds from the Diavik mine, Canada and the evolution of diamond-forming fluids

“…Alternatively, Brey et al (1991) and Girnis et al (1995Girnis et al ( , 2005 advocate the formation of kimberlites as saturated melts that degas at sub-crustal pressures. Similarly, Salvioli-Mariani et al (2004) show a CO 2 release from lamproitic magmas at shallow pressures. Water and Cl may also be lost during degassing (Kamenetsky et al, 2004;Maas et al, 2005).…”

“…Kimberlites have lower volatile contents, but CO 2 , preserved in calcite and dolomite, accounts for an average of about 6 wt% and may reach levels of 18 wt% (Price et al, 2000), and water in serpentine, phlogopite, and secondary minerals may reach 4-8 wt% (Mitchell, 1986). Lamproites commonly contain about 5 wt% of volatiles (CO 2 and H 2 O) but may contain up to 15 wt% (Wannamaker et al, 2000;Rao et al, 2004;Salvioli-Mariani et al, 2004).…”

Cryptic metasomatic agent measured in situ in Variscan mantle rocks: Melt inclusions in garnet of eclogite, Granulitgebirge, Germany

“…Clinopyroxene can trap inclusions (e.g. Salvioli-Mariani, Mattioli, Renzulli, & Serri, 2002;Schiano, Clocchiatti, Boivin, & Medard, 2004;Veksler, Nielsen, & Sokolov, 1998) and their absence can be explained by recrystallization of the clinopyroxene to smaller sized grains, which caused the obliteration of the inclusions. This is also supported by the size difference between garnet, generally larger, and clinopyroxene (see Figure 3c,d).…”

Magmatic Evolution and Ascent History of the Aries Micaceous Kimberlite, Central Kimberley Basin, Western Australia: Evidence from Zoned Phlogopite Phenocrysts, and UV Laser 40Ar/39Ar Analysis of Phlogopite–Biotite

“…Trends toward Na-rich, Al-poor, peralkaline residual liquids are common to several lamproites that have been studied, e.g. Gaussberg in Antarctica (Salvioli-Mariani et al, 2004), and these may provide an analogy for the petrogenesis of OPRK at Aries. Two samples of OPRK from the north extension lobe have whole-rock K/Na ratios of <1, which is quite different from samples of MPK (Table 5), and this might have had an influence on the crystallization of richterite in the OPRK (cf.…”

“…Two samples of OPRK from the north extension lobe have whole-rock K/Na ratios of <1, which is quite different from samples of MPK (Table 5), and this might have had an influence on the crystallization of richterite in the OPRK (cf. Salvioli-Mariani et al, 2004).…”