The Andean continental arc is built upon the thickest crust on Earth, whose eruption products reflect varying degrees of crustal assimilation. In order to robustly model magma evolution and assimilation at subduction zones such as the Andes, the compositions of parental magmas feeding crustal magma reservoirs need to be defined. Here we present new olivine and clinopyroxene oxygen isotope data from rare mafic volcanic rocks erupted at the margins of the giant Altiplano-Puna Magma Body (APMB) of the Altiplano-Puna Volcanic Complex, Central Andes. Existing olivine and pyroxene δ18O values for the Central Andes are highly variable and potentially not representative of sub-arc parental compositions. However, new olivine (n = 6) and clinopyroxene (n = 12) δ18O values of six Central Andean volcanoes presented here display a narrow range, with averages at 6.0‰ ± 0.2 (2σ S.D.) and 6.7‰ ± 0.3 (2σ S.D.), consistent with a common history for the investigated minerals. These data allow us to estimate the δ18O values of sub-arc, parental melts to ca. 7.0‰ ± 0.2 (2σ S.D.). Parental melts feeding the APMB and associated volcanic centres are postulated to form in the felsic continental crust following assimilation of up to 28% high-δ18O basement rocks by mantle-derived magmas.
Eruption products of frontal arc volcanoes usually exhibit heterogeneous chemical and isotopic compositions because parental magmas are compositionally modified by incorporation of continental crust either at their source via subducted sedimentary material or by crustal contamination during subsequent ascent through the crust (e.g.1). A classic example of a volcanic arc, with near ubiquitous geochemical features of continental crust in its erupted products, is that of the Central Andes, which is associated with the thickest crust on Earth (70–74 km, ref. 2). The great thickness and compositional heterogeneity of the crust through which magmas must pass en route to the surface increases the likelihood for primitive magma compositions to be modified by crustal overprinting, yet knowledge of the primitive end-member is required in order to make robust models of subduction-related element fluxes. Oxygen isotopes allow robust modelling of crustal recycling in subduction zones as i) they undergo minimal fractionation at mantle temperatures, ii) there is a strong contrast between the δ18O values of mantle-derived magma and crustal rocks, and iii) the end-members involved have very similar O contents (e.g.3,4,5). In order to utilise oxygen isotopes to assess the magnitude of crustal material assimilated by evolved magmas along the Central Andean arc, it is necessary to know the δ18O value of the parent magma, which itself may be compositionally modified by assimilation in the deep crust. However, the oxygen isotope compositions of mafic magmas in the Central Andes are not well constrained and the existing δ18O values, including those obtained on olivine and pyroxene phenocrysts (Fig. 1), are highly variable (Supplementary Table S1). Existing oxygen isotope data obtained by conventional and laser fluorination analysis of olivine6,7,8 and pyroxene6,8,9,10 from Central Andean volcanoes have δ18O values ranging from 5.0 to 8.3‰ (average = 6.7‰, n = 19) and 5.5 to 8.7‰ (average = 6.2‰, n = 39), respectively (see Supplementary Tables S2 and S3). In contrast, typical mantle rocks show very limited variations in their δ18O values (e.g. olivine = 5.2 ± 0.3‰; clinopyroxene = 5.6 ± 0.4‰; ref. 11). The spread in the Central Andean literature data suggests that the oxygen isotope ratios of some of these lavas were modified by various processes post-dating the formation of the parental melt (e.g. extensive fractionation, late-stage assimilation, mixing of isotopically diverse magmas, or alteration). The challenge, therefore, is to constrain the parental melt value, before extensive fractionation or late-stage assimilation has taken place. We aim to achieve this goal by analysing the δ18O values of single minerals from rare, weakly differentiated lavas with relatively low silica contents (SiO2 = 54.6 to 57.2 wt%12) and relatively low Sr and high Nd isotope ratios (87Sr/86Sr = 0.70554 to 0.70669; 143Nd/144Nd = 0.51234 to 0.51251; ref. 12). The samples selected for this study are from six individual volcanoes (La Poruña, San Pedro, Paniri, La Poruñita, Palpana and Chela), which were active at different times and are all located around the western margin of the Altiplano-Puna Magma Body (APMB) melt anomaly in the Altiplano-Puna Volcanic Complex (Fig. 1). Based on their radiogenic isotope compositions, our samples have experienced limited degrees of crustal modification (e.g. assimilation) and are therefore ideally suited to obtaining the parental δ18O values locked in early-formed crystals. The crystal-focused approach we employ here offers critical new insights that have until now been under-explored due to the limitations inherent in whole-rock geochemical approaches (e.g. the susceptibility of whole-rock samples to secondary alteration and the fact that the δ18O values of whole-rock samples represent averages of the various phases that constitute the sample, cf.1). Our results therefore contribute to filling the gaps in our knowledge of subduction related parental magma compositions feeding the largest continental magma system on Earth.
Study area and sample selection
Common volcanic products in the Central Andes include stratovolcanoes and extensive ignimbrite deposits, but several monogenetic volcanoes of mafic character also exist12,13. Within the volcano-tectonic ignimbrite province of the Altiplano-Puna Volcanic Complex14, the location of mafic volcanism is largely confined to the borders of the partial melt anomaly termed the Altiplano-Puna Magma Body (APMB12; Fig. 1). The APMB is the largest known zone of partial melting in the continental crust throughout the world, with an estimated melt volume of 500,000 km3 and spanning a region of ca. 200 km in diameter15,16 (Fig. 1). Based on geophysical surveys, this anomaly, located in the upper crust, shows an increasing melt fraction from its margin (ca. 4 vol%) to its centre (up to 25 vol%) (e.g.16,17,18). In this region, volcanoes outside the limits of the APMB are composed of lava that is more primitive than the volcanoes situated directly above the APMB12.
The volcanoes included in this study comprise, in order of increasing eruption age, La Poruña, San Pedro, Paniri, La Poruñita, Palpana and Chela, all of which are situated within the Altiplano-Puna Volcanic Complex but peripheral to the proposed APMB reservoir19 (Fig. 1). In this region, the ascending parental basaltic-andesite magma is thought to have avoided significant contamination by evolved melts from the APMB as demonstrated by the lowest 87Sr/86Sr and highest 143Nd/144Nd being towards the borders of the large felsic body12,20. The studied volcanoes (Fig. 1), together with the other Pliocene to Quaternary andesitic-to-dacitic stratovolcanoes, dacitic domes and monogenetic cones, overlie Miocene rhyodacitic-to-rhyolitic ignimbrite sheets21.
La Poruña (21°53′S; 68°30′W) is a well-preserved 140 m high scoria cone 100 ka in age22 situated on the west flank of the 6000 m San Pedro stratovolcano complex (21°53′S; 68°24′W). La Poruña is composed of pyroclastic material and an extensive basaltic-andesite to andesite lava flow that extends up to 8 km to the south-west of the main vent, whereas San Pedro is a composite stratovolcano formed by two superimposed coalescent cones21. The entire La Poruña volcano represents a monogenetic, relatively small to medium volume and short-lived singular eruption, whose magmatic evolution has been described as a two-stage evolutionary process involving minor assimilation and fractionation, followed by selective assimilation during turbulent ascent22. In contrast, San Pedro is a >100 km2 andesitic-to-dacitic volcanic field, with a long-lived (from ca. 510 ka to present) but episodic eruptive centre, whose recent mafic activity (<160 ka) is genetically similar to La Poruña22. Paniri (22°03′S; 68°14′W) is a stratovolcano constructed during four separate stages between 1.4 Ma to 100 ka, whose most primitive activity is represented by isolated basaltic-andesite to andesite lava flows erupted at ca. 400 kyr ago23. La Poruñita (21°17′S; 68°15′W), situated in the northernmost part of the projected APMB, is a scoria cone ca. 600 ka in age of about 700 m in diameter24, similar in shape and composition to La Poruña12. Palpana (21°32′S; 68°31′W) is a conical stratovolcano built up of mafic andesite lava flows. The summit of the volcanic edifice has a crater morphology (dimensions 1.8 km by 1.3 km) that is truncated by the last-erupted dome21. Chela volcano (21°24′S; 68°30′W) is very similar in shape and composition to Palpana. The shape and relatively monotonous composition have been related to rapid construction of the volcanic edifices at ca. 4.1 Ma for Chela and ca. 3.8 Ma for Palpana, followed by restricted erosion and limited duration of magmatic differentiation24.
Olivine- and pyroxene-phyric lava and scoria are ubiquitous at La Poruña, San Pedro, Paniri, La Poruñita, Palpana and Chela and vary from basaltic-andesite to andesite in composition, with whole-rock elemental and Sr and Nd isotope compositions that range from e.g., SiO2 = 54.6 to 62.9 wt%, MgO = 1.6 to 6.1 wt%, Sr = 389 to 885 ppm, Cr = 5 to 625 ppm, 87Sr/86Sr = 0.705541(10) to 0.707656(10), and 143Nd/144Nd = 0.512337(12) to 0.512513(50) (see12,22,23). Recent work on these volcanoes utilised whole-rock elemental and Sr and Nd isotope data to construct an evolutionary model, in which limited magmatic differentiation occurred at mid-upper crustal levels12,22. Lavas of these selected volcanoes may thus represent the composition of parental magmas feeding volcanism within the Altiplano-Puna Volcanic Complex, as the magmas feeding these mafic eruptions largely escaped assimilation of APMB felsic melts during ascent12. In this study, we focussed on sample material containing suitable mafic mineral phases for single mineral oxygen isotope analysis.
Basaltic-andesite lavas from La Poruña contain ca. 30 vol.% phenocrysts (plagioclase > olivine > clinopyroxene > orthopyroxene) and Fe-Ti oxides set in a microlite-rich groundmass of plagioclase and pyroxene and a small percentage of remaining glass. Olivine (up to 2.5 mm in size; ≤12 vol.%) textures include subhedral crystals, embayments and skeletal textures. Clinopyroxene is the most common pyroxene phase and occurs as euhedral to subhedral individual crystals (up to 2 mm; ≤10 vol.%) or as reaction rims on orthopyroxene phenocrysts. Besides occurring individually, clinopyroxene crystals occur as glomerocrysts with plagioclase, olivine and orthopyroxene (Fig. 2).
Basaltic-andesites from San Pedro are porphyritic (ca. 15 vol.% phenocrysts), glassy and fine-grained, with plagioclase > olivine > clinopyroxene > orthopyroxene as phenocrysts. Olivine (up to 2 mm; ≤10 vol%) appears frequently embayed, resorbed, or with reaction rims of pyroxene (Fig. 2). Clinopyroxene appears as euhedral to subhedral phenocrysts (up to 1.5 mm; ≤8 vol.%). Clinopyroxene phenocrysts occur individually or as crystal clots.
Basaltic-andesites from La Poruñita are vesicular and fine-grained, containing olivine > clinopyroxene = orthopyroxene as phenocrysts (≤15 vol.%). These phenocrysts are set in a hypocrystalline groundmass, which is made of microlites of plagioclase, interstitial glass, clinopyroxene and orthopyroxene. Olivine morphologies are varied, including subhedral and resorbed crystals (up to 0.5 mm in size; ≤7 vol.%). Most of these phenocrysts show embayments and oxide inclusions. Clinopyroxene appears as euhedral to subhedral individual phenocrysts (up to 1.5 mm; ≤4 vol.%).
Basaltic-andesites from Paniri are plagioclase-olivine-pyroxene-phyric lavas. These contain ca. 30 vol.% phenocrysts of plagioclase > olivine = clinopyroxene = orthopyroxene. Olivine and pyroxene frequently form glomerocrysts with plagioclase. Clinopyroxene (up to 1.5 mm in size; ≤8 vol.%) also appears individually as euhedral to subhedral crystals.
Andesites from Palpana contain ca. 30 vol.% phenocrysts of plagioclase > clinopyroxene > orthopyroxene and minor olivine set in a hypocrystalline groundmass of glass, plagioclase and pyroxene. Clinopyroxene (up to 2.5 mm; ≤15 vol.%) occurs as subhedral to euhedral tabular crystals (Fig. 2).
Basaltic-andesites from Chela are composed of ca. 25 vol.% phenocrysts (plagioclase > olivine» clinopyroxene > orthopyroxene) set in a glassy groundmass. Olivine generally appears as subhedral crystals in a seriate crystal size distribution as phenocryst and microlites (up to 2 mm; ≤10 vol.%; Fig. 2). Glomerocrysts (olivine-pyroxene) and pyroxene reaction rims on olivine are common. Orthopyroxene and clinopyroxene occur only rarely as glomerocrysts and microlites.
Oxygen isotope data
We determined the δ18O values of (i) olivine from lavas from three volcanoes whose erupted products contain large (≥2 mm) olivine phenocrysts (La Poruña, San Pedro, and Chela) and (ii) clinopyroxene from five volcanoes whose erupted products contain large (≥1.5 mm) pyroxene phenocrysts (La Poruña, San Pedro, Paniri, La Poruñita and Palpana) (Table 1). All minerals were individually selected under a binocular microscope before analysis and were visually free of inclusions or alteration. Laser Fluorination (LF, see Methods) analysis of olivine gave δ18O values of 5.7 to 6.2‰ for La Poruña (n = 4), 6.2‰ for San Pedro (n = 1), and 5.8‰ for Chela (n = 1). These values overlap the higher values obtained for olivine from mantle-derived basalts (cf. up to 6.3‰25; Fig. 3). Individual clinopyroxene crystals analysed by LF for La Poruña, San Pedro, La Poruñita and Palpana volcanoes gave average δ18O values of 6.4 to 7.2‰ (n = 5), 6.7 to 6.9‰ (n = 2), 7.0‰ (n = 1) and 6.3 to 7.0‰ (n = 2), respectively. The clinopyroxene crystals analysed here have higher δ18O values than mantle-derived pyroxene (cf. up to 6.5‰, after25; Fig. 3).
Our olivine and pyroxene δ18O values display substantially narrower ranges than the available data for the Central Andes (Fig. 3). Published olivine δ18O values6,7,8,9,10 tend to have either relatively high (>6.5‰) or mantle-like δ18O values. Notably, our olivine oxygen isotope data from La Poruña, San Pedro and Chela volcanoes have among the lowest δ18O values (δ18O = 5.7% to 6.2‰) with respect to all olivine data reported thus far for the Central Andes (cf. Parinacota6; Cerro Galán7; San Pedro8; Fig. 4). Our clinopyroxene data (δ18O = 6.3% to 7.2‰) overlap with the δ18O values previously obtained for pyroxene from the Central Andes (5.5‰ to 8.7‰6,8,9,10) and are at the higher end of the previously reported data range (excluding one exceptionally high value reported for Toconce volcano8; Fig. 3).
Available whole-rock geochemistry for the studied samples (Table 1) reveal that clinopyroxene-phyric (e.g. PAL-02) and olivine-pyroxene-phyric lavas (e.g. POR-06) have higher SiO2 contents than samples that only contain olivine as phenocrysts (e.g. CHE-03). It is thus possible that pyroxene crystallised at a higher crustal level than olivine and might record late-stage crustal assimilation (cf.26,27,28). We also note that arc lava pyroxenes frequently contain inclusions of plagioclase (which would have higher δ18O values) and/or oxides (lower δ18O values) (e.g. Fig. 2; see also Fig. 4 in Deegan et al.26). Because of the very dark appearance of pyroxene under the binocular microscope, it is both difficult to determine if inclusions are present and what they are. The wider variation of pyroxene δ18O values in this study, compared to olivine may, therefore, be due to either late-stage crustal assimilation or inclusions of various types in the analysed material.
It is possible to estimate the δ18O values of the equilibrium melt by using mineral-melt fractionation factors appropriate for basaltic-andesite (SiO2 average 55.8 wt% among our samples; Table 1). These are calculated to be Δolivine-melt = −1.3 and Δpyroxene-melt = −0.7, using the silica-based equations in Bindeman et al.29. Olivine with δ18O values of 5.7 to 6.2‰, therefore, crystallised from magma having a δ18O value of 7.0 to 7.5‰ (average = 7.3‰ ± 0.17, n = 6). Clinopyroxene with δ18O values between 6.3 and 7.2‰ similarly calculates to magma δ18O values of 7.0 to 7.9‰ (average = 7.4‰ ± 0.29, n = 12). These magma δ18O values are within error of each other but are up to 2.0‰ higher than the accepted values for normal mid-ocean ridge basalts (N-MORB) (δ18O = 5.4‰ to 5.8‰30) and MORB glass (δ18O = 5.4 to 5.8‰31). They are also higher than previously reported δ18O values from mantle-derived rocks in subduction zones elsewhere (e.g. δ18O ≤ 6.3‰5,26,31,32). Given that our samples have relatively high SiO2 contents and Mg numbers that range from 54.6 to 57.2 wt% and 49 to 61, respectively (Table 1), they are unlikely to represent primary or primitive mantle-derived magmas. Indeed, the O-isotope data presented here suggest assimilation of e.g. high-δ18O felsic continental crust resulting in an 18O-enriched parental magma.
The high calculated melt δ18O values presented here cannot be explained by closed-system Rayleigh fractionation (see calculated curve in Fig. 5) as this would only increase primitive δ18O values by 0.2 to 0.3‰ (e.g.29). Pre-Mesozoic felsic metamorphic and plutonic complexes form the Central Andean basement of northern Chile at ca. 18°S to 25°S have δ18O values that range between 6.4‰ to 11.8‰33. If it is assumed that the mantle-derived magma had a δ18O value of 5.7‰ (e.g.34), a minimum of approximately 21% assimilation of local crust with a δ18O of 11.8‰ would be required to reach a magma value of 7.0‰, using simple mass balance calculations (X = [δ18Ofinal − δ18Oinitial]/[δ18Oassimilant − δ18Oinitial], where X is the amount of contamination as a fraction) and assuming equal oxygen content for all end-members. This estimated degree of assimilation agrees well with the estimates based on radiogenic isotope and trace element modelling using data from the same samples, which require about 12 to 28% assimilation (see Supplementary Information), in broad agreement with recent findings for the studied volcanoes (cf. ~13% to 23%20). Binary mixing modelling shows that our data are best explained by interaction between primitive mantle-derived melt and continental crust with high 87Sr/86Sr ratios (>0.714) and δ18O values of 11.8‰ to 19.5‰ (Fig. 5), which is not unreasonable for felsic crust in the whole Central Andean region given that e.g. Damm et al.33 reported δ18O values up to 15.2‰ for Precambrian basement rocks from northern Argentina. The isotope modelling so far assumes simple mixing, which probably approximates behaviour in a deep crustal hot zone, but models involving AFC would likely require greater overall assimilation for the same result, because high-δ18O material is removed in the cumulates. Notably, the steady increase in SiO2 with no change in δ18O value at ca. 7.0‰ (Fig. 5) is consistent with parental magmas that underwent closed-system fractional crystallisation after an initial stage of crustal assimilation by mantle-derived magmas.
We propose, therefore, a model of magmatic evolution for the Altiplano-Puna Volcanic Complex where mantle-derived (primitive) magmas are injected into the felsic continental crust. Upon stagnation, these mantle-derived magmas assimilated basement rocks with high-δ18O values and highly radiogenic Sr isotope ratios to form a parental magma with a δ18O value of ca. 7.0‰ (Fig. 5). Building on the model of González-Maurel et al.12 for the western boundary of the Altiplano-Puna Volcanic Complex, parental melts ascended to mid to upper crustal storage levels, where they stalled, differentiated and fractionated13, avoiding significant further contamination by e.g. felsic melts derived from the APMB as these mafic melts by-passed the molten APMB body. At these crustal levels, olivine and subsequent clinopyroxene crystallisation occurred, which is consistent with recent thermobarometric estimations performed in Quaternary lavas from the southwestern border of the Altiplano-Puna Volcanic Complex28.
In conclusion, volcanic rocks from the most mafic volcanoes at the western border of the Altiplano-Puna Volcanic Complex of the Central Andes have the lowest reported δ18O values of 5.7 to 6.2‰ (average = 6.0‰, n = 6) for olivine, whereas clinopyroxene yielded higher δ18O values of 6.3 to 7.2‰ (average = 6.7‰, n = 12). These mineral data are consistent with crystallisation from a magma of the same O-isotope composition, allowing a robust δ18O estimate of 7.0‰ for the sub-arc, parental magma of the APMB and associated volcanic centres in the Altiplano-Puna Volcanic Complex. This composition may be representative of parental magmas in the wider Central Andean region.
Sample selection and preparation
In this study we analysed crystals from the least silicic materials identified at La Poruña, San Pedro, Paniri, La Poruñita, Palpana and Chela volcanoes. These volcanoes have among the least evolved baseline Sr and Nd isotopic compositions thus far reported for the western boundary of the Altiplano-Puna Volcanic Complex province12. Pristine inclusion-free olivine and pyroxene crystals were hand-picked under a binocular microscope.
Oxygen isotope analysis by laser fluorination
Olivine and pyroxene grains visibly free of alteration or inclusions were selected by hand-picking under a binocular microscope. Laser fluorination (LF) analyses were then carried out at the Department of Geological Sciences, University of Cape Town (UCT), South Africa. The oxygen isotope results are reported in standard δ-notation relative to V-SMOW (Vienna Standard Mean Ocean Water), where δ = [(18O/16O)sample/(18O/16O)V-SMOW − 1]* 1000. Full analytical details of the laser fluorination method employed at UCT are given in Harris and Vogeli35. Measured values of the UCT in-house standard MON GT (Monastery garnet, δ18O = 5.38‰) were used to normalise the raw data and correct for drift in the reference gas. The δ18O value of MON GT was established by cross-calibration with the UWG-2 garnet standard of Valley et al.36 and San Carlos olivine. The long-term average difference in δ18O values of duplicates of MON GT is 0.15‰, which corresponds to a 2σ S.D. value of 0.15‰. Laser fluorination data are given in Table 1. All analyses gave gas pressures of O2 that were consistent with ~100% conversion of mineral to O2.
The authors declare that all relevant data are available within the article and its supplementary information files.
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We thank the two anonymous reviewers for their constructive suggestions and comments that helped to significantly improve this manuscript, and Christoph Beier for editorial handling. This work was supported by the Comisión Nacional de Investigación Científica y Tecnológica (CONICYT-PCHA/Doctorado Nacional/2015-21150403 to OG-M), the Swedish Research Council (VRT, grant number 621-2013-5628 to FMD), the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) in cooperation with the National Research Foundation (NRF) of South Africa (grant number SA2015-6212 to FMD and CH) and NRF Incentive Funding for Rated Researchers (grant number 119128 to PLR).
The authors declare no competing interests.
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González-Maurel, O., Deegan, F.M., le Roux, P. et al. Constraining the sub-arc, parental magma composition for the giant Altiplano-Puna Volcanic Complex, northern Chile. Sci Rep 10, 6864 (2020). https://doi.org/10.1038/s41598-020-63454-1