Origin Of Iron Isotope Fractionation In High-silica Igneous Rocks

Iron is the most abundant polyvalent metal that partitions into minerals and fluids in two oxidation states?Fe2+ and Fe3+?during magmatic processes and plays a key role in a variety of magmatic and mineralizing processes on Earth.Furthermore,igneous rocks show remarkable fractionation in iron isotopes??0.6‰?,in which high-silica igneous rocks have Fe isotope compositions that significantly heavier than those of low-silica igneous rocks.Thus,iron isotopes can potentially serve as a powerful tool in tracking magmatic evolution and crustal differentiation.However,the mechanism controlling Fe isotope fractionation in high-silica igneous rocks remains a highly controversial topic.Two most important mechanisms,fluid exsolution and fractional crystallization,have been proposed to explain the origin of heavy Fe isotope composition in high-slica igneous rocks.In order to address the problem of iron isotope fractionation in high-silica igneous rocks,a series of representative rhyolitic and granitic samples were selected for whole rock Fe isotope analysis.Three suites of volcanic-sedimentary sequences from South China and northern Vietnam were selected for this study.The ?⁵⁶Fe values for the rhyolitic rocks show a large variation from 0.05±0.05‰ to 0.55±0.05‰,and they increase with increasing degree of magmatic differentiation.Geochemical indicators of fluid exsolution?e.g.,REE tetrad effect,low Zr/Hf and K/Rb?are all absent in the rhyolitic rocks,precluding significant fluid-melt/rock interaction for the studied samples.In addition,modelings of volcanic degassing further indicate that the maximum shift in ?⁵⁶Fe of the residual melt does not exceed 0.06‰,which is too small to explain the obsever ?⁵⁶Fe values of up to 0.5‰.In contrast to bulk rock,large dispersions in Fe isotopic compositions are observed between different mineral separates according to available data.Therefore,removal of isotopically light Fe-bearing minerals?e.g.biotite and Fe-Ti oxides?is proposed as the main casuse of Fe isotope variation in silicic melts during magmatic evolution.The ?⁵⁶Fe values for the investigated granites also show a large variation from 0.05±0.07 to 0.42±0.01‰.Nevertheless,compared with rhyolitic rocks,?⁵⁶Fe values of the granites show three different trends with increasing SiO2 contents:?1?the Paleoproterozoic Longwangzhuang granites from southern margin of the North China Craton is enriched in light iron isotopes;?2?the Late Jurassic granites from western Nanling Range,such as the Qianlishan and Qitialing granites,become enriched in heavy iron isotopes;?3?the Late Jurassic Ganfang-Guyangzhai granites from the huge Jiuling composite batholith have limited variation in Fe isotope composition.Exolution of fluids with isotopically light Fe isotopes is obviously insufficient to produce different trends mentioned obve.Even for the highly differentiated granites that experienced extensive fluid-melt/rock interaction by strong evidence of geology and geochemistry,their ?⁵⁶Fe values do not show clear correlations with indicators of fluid exsolution,indicating that fluid exsolution has limited effects on Fe isotope fractionation during magmatic-hydrothermal process.Thus,fractional crystallization of minerals plays a key role in controlling Fe isotope compositions in high-silica granites.The crystallizing mineral assemblages define the different ?⁵⁶Fe trends with increasing degree of magmatic differentiation:?1?removal of minerals with isotopically heavy Fe isotopes?e.g.magnetite?will drive the evolving melt towards to lower 856Fe values;?2?removal of low-?⁵⁶Fe minerals?e.g.hornblende,biotite and ilmenite?will drive the evolving melt towards to higher ?⁵⁶Fe values;and?3?the Fe isotope effect of high-?⁵⁶Fe mienrals will be compensated by low-856Fe minerals if they crystallize contemporaneously with Fe2+-bearing minerals.In one word,this study suggests that contributions of fluid exsolution to Fe isotope variation is very limited,and fractional crystallization is the dominant factor that controls iron isotope fractionation in high-silica igneous rocks.