Spatially controlled Fe isotope variations at Torres Del Paine

Torres del Paine pluton;Fe isotopes;magmatic differentiation

Gajos, Norbert ; Lundstrom ; Craig C.

关键词: Torres del Paine pluton;    Fe isotopes;    magmatic differentiation;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/49527/Norbert_Gajos.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Recent advances in mass-spectrometry have led to identification of systematic trends of changing non-traditional stable isotope ratios in igneous rocks with differentiation index. We present new Fe isotope data for the Torres del Paine igneous complex in southern Chile. The multi-composition pluton consists of a 1.5 km vertical exposure of homogenous granite overlying a contemporaneous and possibly cogenetic 0.5 km mafic gabbro suite. This first-of-its-kind spatially dependent Fe isotope investigation of a convergent margin related pluton aims to understand the nature of granite and silicic igneous rock formation.Samples were collected along four well defined spatial transects, focusing on major plutonic contacts between the country rock, granite and mafic units. Results collected by bracketed double spike MC-ICP-MS (2s precision of ±0.04) show a trend of increasing δ56Fe with increasing silica content as well as an systematic increase in δ56Fe away from the mafic base of the pluton. Importantly, the marginal Torres del Paine granites are isotopically heavier (δ56Fe= +0.25 ± 0.02 2se.) compared to granites found in the interior pluton (δ56Fe= +0.17 ± 0.02 2se.). Cerro Toro country rock values are isotopically light (δ56Fe= +0.04 ± 0.04 2σ).The process responsible for Fe isotope variations remains debated but has been suggested to reflect four mechanisms: (1) crustal assimilation, (2) fractional crystallization, (3) late stage fluid exsolution and (4) thermal migration. Assimilation of isotopically light country rock would not produce the isotopically heavy Torres del Paine granites. Likewise, experimentally determined equilibrium fractionation factors argue against fractional crystallization producing the isotopically heavy granites. Loss of a magnetite equilibrated Fe-bearing fluid would enrich the high silica granites in isotopically heavy Fe; however, the need for unrealistically high amounts of fluid related Fe loss argues against a late stage fluid exsolution fractionation mechanism. Finally, temperature gradient driven isotope fractionation fits well with the top-down pluton emplacement sequence found by Michel et al. (2008) and Leuthold et al. (2012) and explains the spatial distribution of Fe isotope values found with depth in the pluton. We conclude that temperature gradient driven differentiation is the most likely process producing Fe isotope ratio variations in the Torres del Paine pluton. Findings from Torres del Paine have large implications for pluton emplacement, magma differentiation and the formation of granite in particular and continental crust in general.