Research ArticleLong-term storage of subduction-related volatiles in Northern Victoria Land lithospheric mantle: Insight from olivine-hosted melt inclusions from McMurdo basic lavas (Antarctica)
Introduction
Volatile elements (H-C-O-F-Cl-S) deeply affect the mantle rheology and play a fundamental role in the genesis of basic magmas. Besides contributing to lower the mantle viscosity and its solidus and facilitating convective mechanisms, they drive the onset and extent of partial melting processes, ultimately affecting the composition of primary melts (Green 1973). At shallower levels, the volatiles content constraint the P-T condition of crystallization and magmatic fractionation of primary melts, ultimately determining the eruptive behavior of volcanic systems (Mollo et al., 2015; Giacomoni et al. 2018; Lanzafame et al. 2020). The prograde metamorphism of hydrated oceanic lithosphere triggers the segregation and migration of volatiles into the supra-subductive mantle wedge (Abers et al., 2006). In arc settings, part of these volatile components will be emitted in the atmosphere through volcanism, although a substantial portion could be dragged into the mantle transition zone (410–670 km) and/or at the core-mantle boundary at about 2900 km of depth. This extremely deep recycled volatile component could return to the surface through mid-ocean ridge (MORB), oceanic-island (OIB) and continental rift (Tucker et al. 2019).
Rheological and petrological studies have contributed in highlighting the complex interlink between the volatile circulation (primary and/or recycled) and plate tectonics since convection and simple non-Newtonian mantle behavior are not sufficient to provide the required toroidal motion for plate tectonic (Bercovici and Karato 2003; Abers et al., 2006). On the other hand, the recycling of volatiles (e.g., H2O and CO2) and their migration through intergranular boundaries seems to be necessary to achieve a self-lubricating state for mantle convection (Bercovici and Karato 2003). In this respect, the recycling of volatiles and the quantification of the global volatile cycle are key aspects for understanding the overall magmatic and thermal evolution of the planet. Moreover, the volatile fluxes from the mantle to the exosphere modulate Earth's atmosphere and climate on short and long-time scales and are critical to maintain the planet habitable (Dasgupta and Hirschmann, 2010; Dasgupta et al., 2010).
Although the role of recycled volatiles in subductive settings have been widely investigated (Zellmer et al., 2014), their long-term storage, effects on mantle convection and melting, as well as their role in the petrogenesis of intraplate and continental rift settings is still largely unknown. Petrological and experimental studies have revealed the importance of hydrated and/or carbonated solidus for the generation of alkaline magmas (Pilet et al. 2008). Schilling et al. (1980) were among the first to propose that “hot spots” could be also considered “wet spots”, highlighting the importance of water in the punctual decompressional melting processes (e.g., Green 1973). In addition to H2O, the presence of CO2 and the coexistence of mantle heterogeneities and recycled crustal lithologies at mantle depth, may contribute to the melting anomalies observed in intraplate settings.
The complex tectonic processes responsible for the formation of continents, such as multiple accretions, contribute to the generation of intraplate magmatism by creating chemical heterogeneities in the lithospheric and sub-lithospheric mantle. Volatile-rich, metasomatized and/or eclogitic materials may develop gravitational instabilities that cause the higher density materials to sink into the convective mantle by lithospheric thinning, delamination or drip processes, introducing easily fusible material in the upper mantle (Elkins-Tanton 2007; Dasgupta et al., 2010).
The idea that continental flood basalts and continental rifting magmatism could be somehow linked to volatiles re-introduction in the mantle by previous subductive events was firstly drawn by Cox (1978) for the genesis of Karoo-Ferrar, Parana, Deccan and Columbia River Large Igneous Provinces (LIPs). A supporting evidence was recently provided by elemental and isotopic analyses of volatile in melt inclusions of primary composition (Ivanov et al. 2018; Stefano et al. 2011). To date, intimate relationships between the major magmatic rifting phases and the mobilization of alkali- and volatile-rich melts are worldwide recognized (e.g. Casetta et al. 2019). Subduction-related and intraplate LIPs can be distinguished on the basis of the Nb/La ratio of the erupted lavas. More recent studies on olivine- and cpx-hosted melt inclusions from Siberian Traps (Ivanov et al. 2018), Columbia River (Stefano et al. 2011), Basin and Range, Karoo, and Deccan evidenced that high (although variable) volatiles concentrations are associated to LIPs genesis. The investigation of these melt inclusions (MI) in phenocrysts from lavas of the Cenozoic East African and Rio Grande Rift highlighted the presence of a hydrated/carbonated sub-lithospheric domains in their mantle source, suggesting that the volatile-rich contents were the result of old subductions, namely the Pan-African (600–700 Ma) and the Laramide (80–40 Ma) respectively (Hudgins et al. 2015; Rowe et al. 2015).
The present study will contribute to this fascinating issue by investigating the major and trace element geochemistry and volatile content of olivine-hosted MIs from basaltic and basanitic lavas of the Cenozoic Western Antarctic Rift System (WARS) in Northern Victoria Land. These MIs can potentially preserve the chemical heterogeneities of the magma-forming source, since immediately after formation, they remain isolated from the surrounding uprising magma during differentiation (Danyushevsky et al., 2002). Thus, MI may represent one of the most powerful tools for analyzing the volatile (H2O, CO2, F, Cl and S) content of primary magmas, with the aim of understanding their origin and recycling in the mantle source/s. Their geochemical features will be also-compared with those of the metasomatic melts that were recognized in the Northern Victoria Land (NVL) Sub Continental Lithospheric Mantle (SCLM) thanks to a long series of investigations on ultramafic xenoliths carried onto the surface by alkaline lavas (Coltorti et al. 2004; Pelorosso et al., 2016, Pelorosso et al., 2017; Perinelli et al. 2008). In this respect, and taking into account its complex tectono-magmatic evolution, NVL may represent an almost unique site where studying how subduction-related metasomatism and melt/fluid infiltration can be preserved within the lithospheric and sub-lithospheric mantle for hundreds million years before the production of rift-related alkaline magmas.
Section snippets
Geological outline
The WARS is one of the most extended rift systems on Earth, extending for about 3000 km, from the Ross Sea (Queen Maud Mountains-Northern Victoria Land) to the Weddell Sea (Whitmore-Horlick Mountains) and from 750 to 1000 km in width. It is the result of a complex geological process initiated about 550 Ma ago when East Antarctica was still part of the active Paleo-Pacific margin of Gondwana. The tectonic blocks constituting the West Antarctica were joined along the active margin with the
Sampling sites and analytical methods
Samples were collected during the XX (2004/2005) and XXVII (2011/2012) Italian Antarctic Expeditions at the Mario Zucchelli Base (Terra Nova Bay) and funded by Programma Nazionale Ricerca Antartide (PNRA). Olivine-phyric lavas come from three different NVL localities (Fig. 1), namely Shield Nunatak (74° 24′ 664″ S – 164° 30′ 511″E), Eldridge Bluff (73° 24′ 664′ S – 164° 40′ 833″ E) and Handler Ridge (72° 30′ 733″ S – 164° 40′ 833″ E). Shield Nunatak and Eldridge Bluff are both located in the
Petrography of lavas and MIs
All samples have a porphyritic texture with porphyritic indexes (P.I., defined as the percentage of phenocrysts with respect to the entire area of the thin section) in the range 10–15% for Handler Ridge and 20–30% for Eldridge Bluff and Shield Nunatak. All lavas have a common mineral assemblage made up of olivine (10–30%), clinopyroxene (5–15%), plagioclase (2–15%) and dispersed magnetite (2–7%), embedded in a glassy to hyalophilitic matrix. Rare apatite micro-phenocrysts were found in Shield
The volatiles' budget: from primary magmas to mantle source
The Primitive Mantle-normalized trace element patterns of basanitic and basaltic MI are well comparable, suggesting that they could have been generated by different partial melting degree of a similar source (Fig. 4). As above described the highest measured and estimated primary H2O and CO2 contents in basanitic MI are 2.64 wt% and 8800 ppm, respectively. Their corresponding F and Cl contents are 888 and 810 ppm, although some other basanitic MI can get up to 1377 and 1336 ppm of F and Cl,
Conclusions
The study of olivine-hosted MIs from Northern Victoria Land (Antarctica) allowed us to speculate about the nature and origin of volatile content of lithospheric and sub-lithospheric mantle responsible for the onset of Cenozoic magmatism as response of the formation of the Western Antarctic Rift System. The occurrence of mantle xenoliths embedded in alkaline lavas in few neighboring localities, provided an important link between lithospheric mantle and olivine-hosted MIs, permitting to quantify
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was funded by PRIN2017 (Piano di Ricerca di Interesse Nazionale, Italian Ministry of Education, University and Research) “Micro to macro - How to unravel the nature of the Large Magmatic Events” funded to Prof. Massimo Coltorti. Field work was made possible thanks to PNRA (Piano Nazionale Ricerca Antartide) funding and the logistic support at Mario Zucchelli Station at Terra Nova Bay. We are thankful with Raul Carampin and Leonardo Tauro (IGG-CNR, Padua, Italy) for the technical help
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