INVESTIGADORES
BERTOTTO Gustavo Walter
congresos y reuniones científicas
Título:
Mantle metasomatism recorded by adakite melts in phlogopite-bearing spinel-lherzolites from los gemelos volcano, Argentinean Patagonia: EPMA mineral chemistry and geothermobarometry
Autor/es:
SANTOS, A.; JALOWITZKI, T.; ORIHASHI, Y.; HIRATA D.; MASSAFERRO, G.I.; ALRIC, V.; GERVASONI F.; BERTOTTO G.W.; SCHILLING M.E.; MAZZUCCHELLI M.; GIOVANARDI TOMMASO
Reunión:
Congreso; 50° Congresso Brasileiro de Geologia Geologia e Sociedade: Construindo pontes para um planeta sustentável; 2021
Resumen:
Sixteen spinel-lherzolite xenoliths within Pleistocene alkali basalts exposed at Los Gemelos volcano (0.54-0.34 Ma), in the Canquel plateau, Argentinean Patagonia, represent samples of the underlyinglithospheric mantle. Microprobe analysis show that olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), spinel (sp), and phlogopite (phlog) are compositionally homogeneous within each sample, and even between cores and rims. Olivine is forsteritic (Fo90-91), with Mg# of 90.4?91.6, high NiO (0.38?0.42 wt.%) but low CaO (0.02?0.05 wt.%) contents. Orthopyroxene corresponds to Mg-enstatite (Mg# = 91.0?92.3, En90.0-91.3 Fs7.6-8.9 Wo0.7-1.0), with low CaO (0.34?0.51 wt.%), Al2O3 (1.88?2.97 wt.%), and TiO2 contents (< 0.1 wt.%). Clinopyroxenes (En47.4-49.5 Fs2.9-3.9 Wo47.5-49.1) are Cr-diopside (Cr# = 12.5?25.0) and they are characterized by higher Mg# values (92.5?94.5) than the coexisting orthopyroxene and olivine. They show high CaO contents (21.43?23.27 wt.%) and Ca/Al ratios (4.78?8.54), coupled with moderate to high Cr2O3 (0.78?1.74 wt.%) contents, but low Al2O3 (2.45?4.13 wt.%), Na2O (0.59?1.12 wt.%), and TiO2 (0.03?0.20) contents.The cpx show negative correlation between Mg# vs. Al2O3 contents, as expected for variable degrees of melt extraction. However, positive correlation between Mg# vs. CaO contents, and negative correlation between CaO and Al2O3 contents, could be explained by subsolidus re-equilibrium after the partial melting process. The aluminum phases of lherzolites are aluminum (two samples) and chromium spinels (fourteen samples), which show large compositional variations in Mg# (64.8?75.8), Cr2O3 (18.6%?35.7%) and Cr# (20.4?41.8), with low TiO2 (0.02?0.16 wt.%) contents. The studied samples fall in the olivine (Fo vs. NiO) and olivine?spinel (Fo vs. Cr# sp) mantle arrays. The hydrated mineral identified in two samples (LG-08 and LG-12) is classified as phlogopite that contains narrow ranges of Mg# (93.1?93.2), TiO2 content (2.19?2.34 wt.%), and Na/(K+Na) ratio of ~0.15 in atomic proportions, as observed in metasomatic phlogopites in peridotites. Mineral core average compositions were used for geothermobarometric calculations. Equilibrium temperatures for Los Gemelos lherzolites were estimated from seven geothermometers by PTEXL spreadsheet, including two-pyroxene, Ca-in-opx, and Al-in-opx. The thermometers gave a variable range of equilibrium temperature (772?1024 ºC). The pressure estimates using the Ca-in-olivine barometer range from 1.6 to 2.2 GPa, suggesting depths of equilibration between 52 to 73 km. Abundant metasomatic veins of SiO2-rich melts are observed in six samples. They have trachyandesite composition (SiO2 = 58.44?61.53 wt.% and alkalis = 8.05?10.26 wt.%), with high Al2O3 (16.34?17.65 wt.%), Na2O (4.31?5.70 wt.%), K2O (3.63?4.72 wt.%), CaO (5.21?5.52 wt.%), TiO2 (2.08?2.78 wt.%), and low MgO = 1.29?2.55 wt.% contents. Based on SiO2 and Na2O vs. K2O diagrams, the veins show banakite composition with shoshonite affinity. Except by the higher TiO2 contents, these features are similar to high SiO2 (≥56 wt.%), Al2O3 (≥15 wt.%), and low MgO (< 3 wt.%) worldwide adakite composition, as well as with Miocene adakites from Patagonia and Holocene adakites from Andean Austral Volcanic Zone. Furthermore, they are similar with metasomatic glass veins from Cerro del Fraile mantle xenoliths.