Recycling Of Volatiles In The Subcontinental Lithospheric Mantle Beneath Coyhaique (Chilean Patagonia) Revealed By Noble Gas Isotopes (He-Ne-Ar)

NOVAIS-RODRIGUES, E.; JALOWITZKI, T.; SUMINO H.; CONCEIÇÃO R.V.; SCHILLING M.E.; GERVASONI F.; TELES, L.; BERTOTTO G.W.; KLEMME, S.; BERNDT, J.; BUSSWEILER, Y.

Congreso; 50° Congresso Brasileiro de Geologia Geologia e Sociedade: Construindo pontes para um planeta sustentável; 2021

Noble gases are highly incompatible, chemically inert elements, and they vary largely both in abundance and in isotopic compositions according to their geochemical reservoirs. Therefore, noble gases are very important geodynamic tracers to Earth?s mantle processes. In Patagonia continental backarc, ultramafic mantle xenoliths hosted by alkaline basalts bring significant information about chemical heterogeneities of the subcontinental lithospheric mantle (SCLM). Such heterogeneities might be due to material influx (e.g. water, sediment, altered oceanic crust) from the subduction zone in the southeastern Pacific Ocean. Xenoliths studied here were collected from the Balmaceda flood basalts (59-54 Ma), approximately ~320 km east of Chile Trench and ~30 km southeast of the city of Coyhaique. Aiming to evaluate the subduction effects on noble gas isotopes of Coyhaique spinel-lherzolites, we analyzed He, Ne, and Ar isotopic ratios of five selected whole-rock samples using the crushing extraction method. We performed a larger number of strokes (100, 500, 1000, and 2000 times) in order to avoid cosmical ray and/or weathering effects from the mineral lattice. The helium concentrations [4He] of all studied samples show a narrow range between 4.1 and 67.1 × 10-9 cm3 STP/g. Here, we report the first strongly radiogenic 3He/4He ratios for mantle xenoliths from Patagonia (0.20-2.52 RA, 1σ uncertainties) [RA= (3He/4He)air = 1,39 x 10-6]. The sample PM25-06 shows extreme radiogenic 3He/4He ratios, varying from 0.20 ± 0.01 to 0.44 ± 0.02 RA. The least radiogenic analyzed sample is PM25-08, with 3He/4He ratios varying from 1.52 ± 0.08 to 1.90 ± 0.12 RA. The sample PM25-02 shows the highest 3He/4He ratio (2.52 RA). However, this data was obtained by 100 times strokes and might reflect some cosmogenic contribution due to the slightly lower helium concentration ([4He] = 4.4 × 10-9 cm3 STP/g) compared with the other lherzolites ([4He] = 5.0-8.3 × 10-9 cm3 STP/g). Moreover, the narrow range of 3He/4He isotope ratios obtained in the next number of strokes for this sample (1.49-1.61 RA) corroborate with this assumption. The helium results presented here are significantly radiogenic when compared with mantle xenoliths from Cascades volcanic arc that are associated with 4He-rich melt or fluid interaction (3He/4He = 4.14-4.77 RA). Our samples are also equivalentor slightly more radiogenic than basaltic andesite phenocrysts (olivine and clinopyroxene, 3He/4He = 4.40-6.84 RA) and water/gas of hot springs (3He/4He = 1.44-4.82 RA) from Andean Southern Volcanic Zone. Similarly, the radiogenic Chile Ridge MORB glasses from show 3He/4He ratios ranging from 3.51 to 4.39 RA. The neon isotope ratios (21Ne/22Ne = 0.0299-0.0308; 20Ne/22Ne = 9.56-9.64) are more nucleogenic than the worldwide arc-related lavas mean (21Ne/22Ne = 0.0309 ± 0.0043; 20Ne/22Ne = 9.89 ± 0.24) and the crustalneon trend, considering those samples that differ from air within 1σ uncertainty. The 40Ar/36Ar ratios indicate atmospheric contamination, ranging from 325 to 551 (40Ar/36Arair = 296). Based on these new data, we conclude that the strongly radiogenic/nucleogenic signatures of Coyhaique spinel-lherzolites attest the effective recycling of volatiles into the mantle wedge through subducting sediments from Andean continental crust and/or subducting oceanic crust beneath Patagonia.