Investigation of iron isotope variability in the bimodal Aztec Wash Pluton, Eldorado Mountains, Nevada

iron;geochemistry;granite;pluton;magma;Geology;differentiation;non traditional stable isotopes;thermal migration;thermal diffusion

Kehoe, Kelsey ; Lundstrom ; Craig C.

关键词: iron;    geochemistry;    granite;    pluton;    magma;    Geology;    differentiation;    non traditional stable isotopes;    thermal migration;    thermal diffusion;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/73025/Kelsey_Kehoe.pdf?sequence=1&isAllowed=y

美国|英语

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

【 摘 要 】

Over the course of the past decade, our understanding of silicic plutons has undergone a fundamental shift, from envisioning pluton emplacement as one large magmatic intrusion to the concept of incremental emplacement, in which plutons form from multiple smaller injections of magma over hundreds of thousands to million-year time scales (Coleman et al., 2004; Glazner et al., 2004). While this concept helps to assuage previously held concerns about pluton emplacement, it also raises questions about the formation of features shared by many felsic intrusions, such as compositional zonation and the presence of significant volumes of relatively homogeneous granite. Water-rich temperature gradient experiments (Huang et al., 2009) have been able to produce compositional zonation similar to that found in zoned plutons. The process that alters andesitic starting material to granitic and mafic end members within a temperature gradient is called thermal migration zone refining (Lundstrom, 2009). In addition to the compositional changes that occur within the temperature gradient, an isotopic signature was also observed: heavy isotopes of Fe, Mg, O, H and Li became enriched in the cold, felsic end of the gradient (Bindeman, et al., 2013; Huang et al., 2009). This isotopic trend is strikingly similar to one found in igneous systems, in which non-traditional stable isotopes such as Fe and Si become increasingly heavy as the silica content increases (Poitrasson and Freydier, 2005; Schoenberg and von Blanckenburg, 2006; Heimann et al., 2008; Savage, et al., 2011). Could the isotopic trend found in magmatic systems be related to temperature gradients formed as a result of multiple intrusions? There are only a few investigations of the role that temperature gradients have in driving isotopic fractionation within magmatic systems (Zambardi et al., 2014; Gajos, 2014). In order to test this hypothesis, I have analyzed samples collected from transects oriented paleo-vertically and paleohorizontally through the Miocene-aged Aztec Wash Pluton (AW). AW is a bimodal, reversely zoned pluton, consisting of an outer “rim” of granite (the granite zone) underlain by the heterogeneous zone, which contains co-existing mafic, felsic and intermediate rocks. Rotation of the intrusion during Basin and Range extension exposed a sub-vertical slice of AW from the roof downwards, allowing for the paleovertical and paleohorizontal transects. Major element compositions and iron isotope ratios were measured for samples from four transects: PV, a paleovertical transect through the top of the pluton, ELT and 11-22, two paleohorizontal transects, and 11-20, a short transect through monzonite and monzodiorite layers. Fe isotope ratio results do not match reasonable Rayleigh fractionation models for fractional crystallization or fluid exsolution. The scattering of a range of iron isotope compositions of samples throughout the transects may be indicative of thermal diffusion signatures. However, the complicated open system history of AW has likely obscured the presence of simple diffusional gradients like those seen in the experiments (Huang et al. 2009; Bindeman et al. 2013).