The Trans-Antarctic Rift represents the most extended continental break up on the Earth. Notwithstanding the origin of the related magmatism and the nature of its mantle sources are still poorly understood, especially if the focus is addressed to the volatiles species. To this aim the petrologic features of the basic lavas are compared with those of olivine-hosted melt inclusions (MI) and the volatile contents of these latter are compared with those analyzed in amphibole from mantle xenoliths of the same area. Three distinct localities provided olivine-phyric basalts, while in two other localities amphibole-bearing mantle xenoliths are found. Lavas are olivine-phyric (up to 15 %vol) with minor clinopyroxene and plagioclase in a glassy to microcristalline plagioclase-dominated groundmass; opaque minerals are mostly magnetites and subordinately ilmenites. Samples are basanites (42.41-44.8 SiO2 wt%; 3,11-6,19 Na2O+K2O wt%) and basalts (44.91-48.73 SiO2 wt%; 2.81-4.55 Na2O+K2O wt%). Olivine-hosted melt inclusions (MI) were analized for major element and volatiles (H2O. CO2, S, F, and Cl) after HT (1300°C) and HP (6 kbar) homogenization at Reensealer Polytecnic Institute (New York-USA). Volatile content in MI and amphiboles has been determined by SIMS at Woods Hole Oceanographic Institute (Massachusets-USA) and at the IGG-CNR laborary of Pavia. MI are compositionally comparable to host lavas but show a wider variability. Two compositionally distinct populations can be identified, sometimes coexisting in the same sample: a high-Fe-Ti-K and a low-Fe-Ti-K groups. The H2O content ranges from 0.70 wt% to 2.64 wt% and CO2 from 25 ppm to 341 ppm (H2O/(H2O+CO2)~ 1, mol%). At comparable H2O contents few samples show a remarkable higher CO2 values (1322 ppm to 3905 ppm) with a H2O/(H2O+CO2) down to 0.88. F and Cl content varies from 1386 to 10 ppm and from 1336 to 38 ppm respectively. Concentration of volatiles show a good correlation with alkalies, especially with K2O. Assuming a partial melting degree able to reproduce the major element composition of basanites and basalts (from 3 to 7%) the volatile content of the mantle source was calculated. Accordingly, the estimated amount of volatiles is as follows: 790-1850 ppm of H2O, 120-273 ppm of CO2, 42-97 ppm of F and 40-93 ppm of Cl. These estimates were then compared with the volatile content measured in mantle amphibole and in NAMs from the two localities nearby. In order to obtain the water content measured in MI, these latter appear enriched in Cl with respect to the content observed in the amphiboles. This disagreement can be due to: i) mantle amphiboles are not representative of those present in the magma sources, ii) phlogopite may be also present in the magma source, although it is absent in the xenoliths. In both cases this study indcate an anomalous K and Cl enrichment in the lithospheric mantle domain of NVL.
Volatile content in olivine melt inclusions from Cenozoic basic lavas of Northern Victoria Land (Antarctica)
L OTTOLINI
2015
Abstract
The Trans-Antarctic Rift represents the most extended continental break up on the Earth. Notwithstanding the origin of the related magmatism and the nature of its mantle sources are still poorly understood, especially if the focus is addressed to the volatiles species. To this aim the petrologic features of the basic lavas are compared with those of olivine-hosted melt inclusions (MI) and the volatile contents of these latter are compared with those analyzed in amphibole from mantle xenoliths of the same area. Three distinct localities provided olivine-phyric basalts, while in two other localities amphibole-bearing mantle xenoliths are found. Lavas are olivine-phyric (up to 15 %vol) with minor clinopyroxene and plagioclase in a glassy to microcristalline plagioclase-dominated groundmass; opaque minerals are mostly magnetites and subordinately ilmenites. Samples are basanites (42.41-44.8 SiO2 wt%; 3,11-6,19 Na2O+K2O wt%) and basalts (44.91-48.73 SiO2 wt%; 2.81-4.55 Na2O+K2O wt%). Olivine-hosted melt inclusions (MI) were analized for major element and volatiles (H2O. CO2, S, F, and Cl) after HT (1300°C) and HP (6 kbar) homogenization at Reensealer Polytecnic Institute (New York-USA). Volatile content in MI and amphiboles has been determined by SIMS at Woods Hole Oceanographic Institute (Massachusets-USA) and at the IGG-CNR laborary of Pavia. MI are compositionally comparable to host lavas but show a wider variability. Two compositionally distinct populations can be identified, sometimes coexisting in the same sample: a high-Fe-Ti-K and a low-Fe-Ti-K groups. The H2O content ranges from 0.70 wt% to 2.64 wt% and CO2 from 25 ppm to 341 ppm (H2O/(H2O+CO2)~ 1, mol%). At comparable H2O contents few samples show a remarkable higher CO2 values (1322 ppm to 3905 ppm) with a H2O/(H2O+CO2) down to 0.88. F and Cl content varies from 1386 to 10 ppm and from 1336 to 38 ppm respectively. Concentration of volatiles show a good correlation with alkalies, especially with K2O. Assuming a partial melting degree able to reproduce the major element composition of basanites and basalts (from 3 to 7%) the volatile content of the mantle source was calculated. Accordingly, the estimated amount of volatiles is as follows: 790-1850 ppm of H2O, 120-273 ppm of CO2, 42-97 ppm of F and 40-93 ppm of Cl. These estimates were then compared with the volatile content measured in mantle amphibole and in NAMs from the two localities nearby. In order to obtain the water content measured in MI, these latter appear enriched in Cl with respect to the content observed in the amphiboles. This disagreement can be due to: i) mantle amphiboles are not representative of those present in the magma sources, ii) phlogopite may be also present in the magma source, although it is absent in the xenoliths. In both cases this study indcate an anomalous K and Cl enrichment in the lithospheric mantle domain of NVL.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
