Periodic Reporting for period 2 - OXYGEN (The redox evolution of arc magmas: from the oxygenation of the Earth’s atmosphere to the genesis of giant hydrothermal ore deposits)
Reporting period: 2022-07-01 to 2023-12-31
Arc magmatism at subduction zones is responsible for much of the mass transfer of chemical elements between the Earth’s lower and upper spheres. Arc magmas are significantly more oxidized and richer in volatile elements than other voluminous magma types on Earth. These characteristics promote the genesis of large magmatic-hydrothermal ore deposits, which provide much of the mineable base and precious metal resources for our society. The significance of such mineral deposits is ever increasing in the face of new global challenges. For example, copper is essential for our society’s transition to carbon-neutral clean energy sources, and about 75% of the present copper production is derived from magmatic-hydrothermal ore deposits.
The project OXYGEN aims to identify the processes that lead to the oxidation of arc magmas and the interplay between these processes and the magmatic abundance of metallic and volatile elements critical for ore genesis. This will help improve genetic models for magmatic-hydrothermal ore formation leading to better exploration strategies for such ore deposits. Another important objective is to assess the large-scale geochemical cycle of important heterovalent elements such as sulfur and iron at subduction zones to determine if the emergence of continents about 2.5 billion years ago and associated changes in magma genesis and ascent may have contributed to the oxygenation of the Earth’s atmosphere.
The project OXYGEN aims to identify the processes that lead to the oxidation of arc magmas and the interplay between these processes and the magmatic abundance of metallic and volatile elements critical for ore genesis. This will help improve genetic models for magmatic-hydrothermal ore formation leading to better exploration strategies for such ore deposits. Another important objective is to assess the large-scale geochemical cycle of important heterovalent elements such as sulfur and iron at subduction zones to determine if the emergence of continents about 2.5 billion years ago and associated changes in magma genesis and ascent may have contributed to the oxygenation of the Earth’s atmosphere.
The controls on the redox state of magmas at their mantle source have so far been addressed by experimental investigation of the oxygen fugacity (fO2) range of the sulfide to sulfate transition at pressure (P) – temperature (T) conditions of subducted slab dehydration and the mobility of sulfur in slab-derived fluids. The results show that the redox transition of sulfur occur at more oxidizing conditions than expected, which may facilitate slab derived sulfur to oxidize the mantle wedge to an extent higher than previously assumed. At the same time, we also studied monogenetic volcanoes in the Trans-Mexican Volcanic Belt to determine if slab-derived fluids can oxidize the mantle wedge, and to identify what type of fluids act as the most efficient oxidizing agents. The results here also clearly indicate that the oxidation of the mantle wedge by subducted slab derived fluids is the primary reasons of the oxidized nature of arc magmas.
The possible oxidation of arc magmas during ascent through the continental crust has so far been addressed by studying the Parinacota and Taapaca volcanoes in the Central Volcanic Zone of the Andes. Results at Parinacota also speak for magma oxidation and sulfur enrichment at mantle depths and only a minor role of magma differentiation in regulating magma redox. Additional field work has been conducted at the northern termination of the Southern Volcanic Zone of the Andes (San Jose – Espiritu Santo – Cerro Marmolejo complex and Tupungato – Tupungatito volcanoes) and the samples from these locations are currently being studied. All the above field-based studies benefit from the analysis of microscopic-sized droplets of silicate melt entrapped in volcanic phenocrysts to investigate the relationship between volatile element and ore metal abundances and magma redox evolution. To enhance this, the development of new oxybarometers relying on the partitioning of heterovalent elements between igneous minerals and silicate melt is also in progress via high-P-T experimentation. The same experiments help constrain the fO2 dependence of the liquid line of descent of arc magmas.
Furthermore, we are also studying the speciation of sulfur in magmatic fluids and melts in situ at high P-T and controlled redox conditions by using a novel Raman spectrometer configuration and a prototype experimental apparatus. The speciation of S in aqueous fluids within the T range of 750 – 1000 oC at P = 200 MPa has been determined as a function of oxygen fugacity (fO2). The results show that the main sulfur species in magmatic fluids are SO2, H2S and HS-, and the abundance of sulfur species such as radical sulfur ions are negligible, whereas sulfate stability is restricted to saline high-density liquid like fluids at oxidizing conditions. The fO2 range of the sulfur redox transition is strongly temperature dependent.
The possible oxidation of arc magmas during ascent through the continental crust has so far been addressed by studying the Parinacota and Taapaca volcanoes in the Central Volcanic Zone of the Andes. Results at Parinacota also speak for magma oxidation and sulfur enrichment at mantle depths and only a minor role of magma differentiation in regulating magma redox. Additional field work has been conducted at the northern termination of the Southern Volcanic Zone of the Andes (San Jose – Espiritu Santo – Cerro Marmolejo complex and Tupungato – Tupungatito volcanoes) and the samples from these locations are currently being studied. All the above field-based studies benefit from the analysis of microscopic-sized droplets of silicate melt entrapped in volcanic phenocrysts to investigate the relationship between volatile element and ore metal abundances and magma redox evolution. To enhance this, the development of new oxybarometers relying on the partitioning of heterovalent elements between igneous minerals and silicate melt is also in progress via high-P-T experimentation. The same experiments help constrain the fO2 dependence of the liquid line of descent of arc magmas.
Furthermore, we are also studying the speciation of sulfur in magmatic fluids and melts in situ at high P-T and controlled redox conditions by using a novel Raman spectrometer configuration and a prototype experimental apparatus. The speciation of S in aqueous fluids within the T range of 750 – 1000 oC at P = 200 MPa has been determined as a function of oxygen fugacity (fO2). The results show that the main sulfur species in magmatic fluids are SO2, H2S and HS-, and the abundance of sulfur species such as radical sulfur ions are negligible, whereas sulfate stability is restricted to saline high-density liquid like fluids at oxidizing conditions. The fO2 range of the sulfur redox transition is strongly temperature dependent.
Within the first segment of the project focusing on mantle processes, we have so far experimentally identified that the sulfide to sulfate transition occurs at more oxidizing conditions than anticipated, which facilitates relatively high-degree of mantle oxidation by sulfate-bearing slab-derived fluids. This is consistent with the findings in natural samples from the Trans-Mexican Volcanic belt, which show positive correlation between magmatic sulfur concentrations and oxidation state. Indeed, some of these magmas are characterized by extremely high sulfur concentrations (the highest reported to date worldwide) and rather high oxidation state. Relatively primitive mafic magmas at Parinacota also show high oxidation state and relatively high magmatic sulfur abundances. At the same time, there is no significant variation in magma redox as a function of the site and duration of magma storage in the crust. Therefore, these above two field areas indicate that the oxidized nature of arc magmas is primarily obtained at their source in the mantle, answering one of the key questions of the project.
In addition, the study on sulfur speciation at upper crustal pressures found that the fO2 range of the redox transition of sulfur is temperature dependent with significant differences compared to the results of previous ex situ experimental studies and thermodynamic predictions. Furthermore, these experiments have conclusively shown that though radical sulfur ions are present in magmatic fluids, their concentration is below the limits of detection of non-resonant Raman spectrometry over the entire studied range of P-T-fO2 conditions. This is in sharp contrast to recent findings by also in situ studies, which used resonant Raman spectroscopy. These findings have significant implications with regards to ore metal transport in magmatic fluids. In addition, the stability of sulfate species in magmatic fluids was found to be more limited than previously proposed based on ex situ experiments. This has implications for the mechanisms of volcanic sulfate aerosol production, which in turn affect the Earth’s climate and redox balance.
Experiments in progress at mantle pressures quantify sulfur transport in various slab fluids as a function of P-T and fluid composition, whereas field-based investigations will put additional constraints on volatile element – ore metal – redox systematics during magma generation and ascent. The sulfur speciation dataset will be complemented by studying P-T-fO2-melt compositional effects on sulfur speciation in silicate melts. Ultimately, all above data will be used to support model calculations of the geochemical cycle of heterovalent elements in subduction zones and the net long-term effect of subduction zone magmatism on the redox state of the atmosphere – hydrosphere system. In addition, the improved understanding of the interplay between magma redox evolution, sulfur and ore metal budgets on the path from mantle to surface in volcanic arcs will be used to improve models of magmatic-hydrothermal ore genesis.
In addition, the study on sulfur speciation at upper crustal pressures found that the fO2 range of the redox transition of sulfur is temperature dependent with significant differences compared to the results of previous ex situ experimental studies and thermodynamic predictions. Furthermore, these experiments have conclusively shown that though radical sulfur ions are present in magmatic fluids, their concentration is below the limits of detection of non-resonant Raman spectrometry over the entire studied range of P-T-fO2 conditions. This is in sharp contrast to recent findings by also in situ studies, which used resonant Raman spectroscopy. These findings have significant implications with regards to ore metal transport in magmatic fluids. In addition, the stability of sulfate species in magmatic fluids was found to be more limited than previously proposed based on ex situ experiments. This has implications for the mechanisms of volcanic sulfate aerosol production, which in turn affect the Earth’s climate and redox balance.
Experiments in progress at mantle pressures quantify sulfur transport in various slab fluids as a function of P-T and fluid composition, whereas field-based investigations will put additional constraints on volatile element – ore metal – redox systematics during magma generation and ascent. The sulfur speciation dataset will be complemented by studying P-T-fO2-melt compositional effects on sulfur speciation in silicate melts. Ultimately, all above data will be used to support model calculations of the geochemical cycle of heterovalent elements in subduction zones and the net long-term effect of subduction zone magmatism on the redox state of the atmosphere – hydrosphere system. In addition, the improved understanding of the interplay between magma redox evolution, sulfur and ore metal budgets on the path from mantle to surface in volcanic arcs will be used to improve models of magmatic-hydrothermal ore genesis.