B Isotope Evidence for Subduction Input to Arc Magmatic Sources: A Case Study of Lavas from Paniri, Northern Chile

Boron (B) is an ideal geochemical tracer for the hydration of the overlying mantle wedge at subduction zones (e.g., 1,2). The boron isotope composition of magmas, expressed with the standard notation δ11B = ((11B/10B(sample)/11B/10B(reference))-1) x 100, is useful in detecting and quantifying the exchange process between the slab and the mantle due to boron showing strong affinity for silicate melts and aqueous fluids (e.g., 2). The mountain building process witnessed today at the Andes is a good modern example of oceanic-continental convergence, providing a modern-day analogue that can be used to interpret older converging mountain belts around the world.

This study presents new whole-rock boron isotope data from all 7 eruptive units (~1.390±0.290 Ma to 150±6 ka) of Paniri volcano (3), a Pleistocene age stratovolcano forming part of the San Pedro-Linzor Volcanic Chain (SPLVC), Central Andes. Volcanoes within the SPLVC are characterised by relatively high 87Sr/86Sr values (0.7063-0.7094) within the Central Andes (<0.708) and this is attributed to the contamination of the primary magma during its ascent through the thick continental crust (3,4). Though 87Sr/86Sr values at Paniri are low (0.7062 to 0.708) compared to other volcanoes within the SPLVC, with the exception of San Pedro and La Poruña (0.7057 to 0.7066), they still suggest high degrees of crustal contamination.

Our preliminary results show relatively low δ11B values (ranging between -6.11‰ and +0.23‰) and B concentrations that range between 20 to 50 ppm at Paniri. The units showing the lowest δ11B values (Malku and Las Negras) of -6.11‰ and -2.56‰, are dated at >1.3 Ma and 402 ± 46 ka respectively (3). These relatively low δ11B values at Paniri are consistent with observations suggesting that such more negative δ11B values require a magma source that is depleted in δ11B values such as the mantle (MORB, δ11B = -7.1±0.9‰) or Palaeozoic Andean crust (δ11B = -8.9‰)(5). Considering the low 87Sr/86Sr ratios of these units (0.7062), their eruption could be related to magma escape during a mafic recharge event within the volcanic chain (4).

(1) de Hoog, J., C.M. and Savov, I.P., 2018. Boron isotopes as a tracer of subduction zone processes. Boron Isotopes, pp.217-247.

(2) Marschall, 2018. Boron isotopes, pp.189-215.

(3) Godoy, et al, 2018. Journal of South American Earth Sciences, 84, pp.184-200.

(4) González-Maurel, et al., 2019. Lithos, 346, p.105162.

(5) Rosner et al., 2003. Geochemistry, Geophysics,