Periodic Reporting for period 1 - ANTELOPE (Non-trAditioNal sTable mEtaL isotOPE (Fe, Mg, Zn) fractionation: a case study from the Bushveld Complex, South Africa)
Période du rapport: 2018-04-23 au 2020-04-22
Mafic layered intrusions, formed by the slow cooling and differentiation of basaltic magmas within the crust often contain enormous metallic ore resources, of vital importance for industrial and societal applications. These precious metals, including chromium (Cr) and Platinum-Group Elements (PGE), are often hosted within continuous, nearly monomineralic dense oxide layers. Despite many decades of scientific research, the processes that resulted in the formation of these spectacular layering features and the accumulation of precious metals to such high concentrations are heavily debated. Part of the problem arises from the fact that compositional data suggests the influence of post-magmatic processes and fluid circulation operating in the magma chamber. Such processes are likely to have modified the primary cumulate pile and influenced precious metal distribution, but the timing and extent to which these rocks have been subjected to subsolidus modification is not known. The ca. 2.0 Ga Bushveld Complex in South Africa and the ca. 60 Ma Rum layered intrusion in Scotland both contain PGE-rich mineralized layers, and have been cornerstones for understanding open-system magmatic processes for decades.
In this work I developed geochemical tools to quantify the influence of post-magmatic processes, using high-precision iron (Fe) isotope measurements on ferromagnesian sililcates (olivine and pyroxene) and oxides (Cr-spinel).
In particular, I address the following questions:
(i) what is the extent of inter-mineral Fe isotope variations?
(ii) are minerals in isotopic equilibrium or disequilibrium with each other?
(iii) what are the potential mechanisms that control Fe isotope fractionation?
(iv) what is the potential of Fe isotopes as tracers for magmatic processes involved in the formation of mafic layered intrusions?
The results show that most mineral pairs in our samples are in isotopic equilibrium. The magnitude of inter-mineral Fe isotope fractionation indicate that primary mineral compositions have been extensively modified by iron-magnesium exchange occurring down to subsolidus temperatures (~ 800ºC). However despite evidence for isotopic equilibrium, the sign of Fe isotope partitioning between Cr-spinel and silicates (olivine and pyroxenes) does not agree with that predicted by consideration of charge and coordination environment. In particular, This can be attributed to strong compositional effects on equilibrium partitioning of Fe in Cr-rich spinel specifically, as this is the case in particular in highly magnesian lavas and layered intrusions. This work thus highlights the importance of mineral compositional effects in controlling Fe isotope behaviour between minerals in mantle-derived rocks, an aspect that is not well understood and often not taken into consideration. Because Fe isotopes are routinely used as a fingerprinting tool to investigate magmatic processes, the conclusions of this work have far-reaching implications for the application of high-temperature stable isotope geochemistry across the fields of igneous and metamorphic petrology as well as ore geology and cosmochemistry.
In this work I developed geochemical tools to quantify the influence of post-magmatic processes, using high-precision iron (Fe) isotope measurements on ferromagnesian sililcates (olivine and pyroxene) and oxides (Cr-spinel).
In particular, I address the following questions:
(i) what is the extent of inter-mineral Fe isotope variations?
(ii) are minerals in isotopic equilibrium or disequilibrium with each other?
(iii) what are the potential mechanisms that control Fe isotope fractionation?
(iv) what is the potential of Fe isotopes as tracers for magmatic processes involved in the formation of mafic layered intrusions?
The results show that most mineral pairs in our samples are in isotopic equilibrium. The magnitude of inter-mineral Fe isotope fractionation indicate that primary mineral compositions have been extensively modified by iron-magnesium exchange occurring down to subsolidus temperatures (~ 800ºC). However despite evidence for isotopic equilibrium, the sign of Fe isotope partitioning between Cr-spinel and silicates (olivine and pyroxenes) does not agree with that predicted by consideration of charge and coordination environment. In particular, This can be attributed to strong compositional effects on equilibrium partitioning of Fe in Cr-rich spinel specifically, as this is the case in particular in highly magnesian lavas and layered intrusions. This work thus highlights the importance of mineral compositional effects in controlling Fe isotope behaviour between minerals in mantle-derived rocks, an aspect that is not well understood and often not taken into consideration. Because Fe isotopes are routinely used as a fingerprinting tool to investigate magmatic processes, the conclusions of this work have far-reaching implications for the application of high-temperature stable isotope geochemistry across the fields of igneous and metamorphic petrology as well as ore geology and cosmochemistry.
The essential part of the work performed during this project involves: (i) major and trace elements analyses of rocks and minerals (olivine, spinel, pyroxenes) from three oxide layers (one in the Bushveld and two in the Rum layered intrusion) and surrounding gabbroic cumulates, and (ii) dissolution, Fe purification and isotope analyses of the characterized samples by Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS).
Diffusion modelling was performed in order to assess the important point of whether the mineral assemblages are in isotopic equilibrium. Length and timescales of equilibration were determined using published Fe-Mg interdiffusion coefficients for the minerals of interest, and compared with estimates of the cooling rates of the Bushveld magmas. The results are used to predict mineral pairs to be in Fe isotopic equilibrium.
The precise Fe isotopes measurements in bulk rocks and minerals have been used in conjunction with petrological information to gain insights into the post-magmatic evolution of the Bushveld magmas, showing that primary mineral compositions have been extensively modified by Fe-Mg exchange occurring down to subsolidus temperatures (~ 800ºC). Moreover, these results point to a strong compositional effect on the equilibrium partitioning of Fe in spinel, a mechanism that has not been previously demonstrated. This has critical implication for the application for Fe isotope systematics in mantle rocks.
The results obtained on the Bushveld Complex were presented at several international conferences and local seminars over the course of the action. They are presented in a manuscript which will be submitted promptly to Geochimica et Cosmochimica Acta.
The work on the Rum layered intrusion has suffered a delay due to the global pandemic situation which resulted in the closure of the laboratory facilities. Fe isotope results for the Rum samples are expected to be produced during the summer, and will result in an additional two manuscripts which will pertain to the interpretation of the isotopic data, and address the issues of post-magmatic processes and timescales of cooling in both layered intrusions.
Diffusion modelling was performed in order to assess the important point of whether the mineral assemblages are in isotopic equilibrium. Length and timescales of equilibration were determined using published Fe-Mg interdiffusion coefficients for the minerals of interest, and compared with estimates of the cooling rates of the Bushveld magmas. The results are used to predict mineral pairs to be in Fe isotopic equilibrium.
The precise Fe isotopes measurements in bulk rocks and minerals have been used in conjunction with petrological information to gain insights into the post-magmatic evolution of the Bushveld magmas, showing that primary mineral compositions have been extensively modified by Fe-Mg exchange occurring down to subsolidus temperatures (~ 800ºC). Moreover, these results point to a strong compositional effect on the equilibrium partitioning of Fe in spinel, a mechanism that has not been previously demonstrated. This has critical implication for the application for Fe isotope systematics in mantle rocks.
The results obtained on the Bushveld Complex were presented at several international conferences and local seminars over the course of the action. They are presented in a manuscript which will be submitted promptly to Geochimica et Cosmochimica Acta.
The work on the Rum layered intrusion has suffered a delay due to the global pandemic situation which resulted in the closure of the laboratory facilities. Fe isotope results for the Rum samples are expected to be produced during the summer, and will result in an additional two manuscripts which will pertain to the interpretation of the isotopic data, and address the issues of post-magmatic processes and timescales of cooling in both layered intrusions.
Fe isotopes are widely used as a tool to answer fundamental questions pertaining to the geochemical cycling between the Earth’s surface and deep interiors, planetary differentiation, and the generation of mineral deposits. In all these applications, the interpretation of natural isotopic variations critically relies on a complete understanding of the mechanisms that govern the partitioning of Fe isotopes between rock-forming minerals.
The most innovative part of the project is related to the finding that the observed partitioning of Fe isotopes between the different minerals in the Bushveld unexpectedly contradicts the established theoretical predictions on stable Fe isotope partitioning. These results highlight the importance of previously overlooked compositional effects on the behaviour of Fe isotopes in spinel. Spinel is a ubiquitous mineral in the mantle and in mantle-derived magmatic and volcanic rocks. The results of this project thus have far-reaching implications for researchers in the fields of high-temperature geochemistry as well as cosmochemistry and ore geology.
The most innovative part of the project is related to the finding that the observed partitioning of Fe isotopes between the different minerals in the Bushveld unexpectedly contradicts the established theoretical predictions on stable Fe isotope partitioning. These results highlight the importance of previously overlooked compositional effects on the behaviour of Fe isotopes in spinel. Spinel is a ubiquitous mineral in the mantle and in mantle-derived magmatic and volcanic rocks. The results of this project thus have far-reaching implications for researchers in the fields of high-temperature geochemistry as well as cosmochemistry and ore geology.