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Small, but significant fractionation of Fe isotopes occurs in high-temperature igneous rocks. Despite increasing study, the exact causes of this fractionation are still debated. Therefore, a systematic study of a complete igneous intrusive system is presented to evaluate the extent and causes of Fe isotope fractionation. The Eocene Skaergaard layered mafic intrusion, SE Greenland has a relatively simple magmatic history and localized superimposed alteration, which makes it a good setting to study Fe isotope fractionation. High-precision Fe isotope compositions ([delta]Fe56, relative to igneous rocks, in ‰) are presented for bulk rocks and bulk Fe-Ti oxides (magnetite-ilmenite) from a suite of 26 gabbros and ferrodiorites that encompasses the magmatic/alteration history of the Skaergaard intrusion. The [delta]Fe56 values of bulk rocks differ only by 0.074‰ (-0.043±0.013‰ to +0.031±0.027; avg. = -0.007±0.012‰, 2-SE; n = 11) and show a lack of or a slight variation with magmatic evolution as measured by plagioclase compositions and bulk-rock geochemistry. These bulk-rock [delta]Fe56 values remain within the 2-SE of the average mafic-intermediate composition of the Earth's crust (0.00±0.08‰ ). The lack of clear measurable trends in [delta]Fe56 values of Skaergaard bulk rocks as a function of magmatic evolution outside of analytical error is consistent with data from other mafic and ultramafic intrusions in that [delta]Fe56 values do not vary as much as in high-silica igneous rocks. In contrast, [delta]Fe56 values of Fe-Ti oxides throughout the evolution vary significantly, up to 0.69‰ (-0.179±0.010‰ to +0.508±0.012‰ , 2-SE; n = 26). A positive correlation between [delta]Fe56 values and Fe3+:Fe2+ ratios for bulk Fe-Ti oxides shows that their Fe isotope compositions are controlled by their Fe3+:Fe2+ ratio. Similar trends of [delta]Fe56 variations in Fe-Ti oxides and previously modeled fO2 values through the Layered Series seem to suggest that Fe isotope fractionation in Fe-Ti oxides may be driven by redox changes of the magma, but this is not seen in bulk rocks. The removal of iron via partial or complete magnetite dissolution by hydrothermal fluids may account for Fe isotope variations in bulk Fe-Ti oxides from highly altered rocks. Secondary minerals, not analyzed but replacing altered Fe-Ti oxides, likely have the light Fe isotopes removed from magnetite that results in high [delta]Fe56 values, and this difference can explain why altered and fresh bulk rocks do not differ in [delta]Fe56 values. Compared to the average Fe isotope composition of the Earth's mantle and chondritic meteorites, the average [delta]Fe56 value of Skaergaard bulk rocks is higher by 0.053‰ and 0.081‰, respectively. The higher [delta]Fe56 average of Skaergaard rocks is similar to that of terrestrial basalts, which record an average [delta]Fe56 value (+0.006±0.007‰ ) ~ 0.07‰ higher than mantle values. Further detailed Fe isotope studies of large mafic igneous intrusions will help better understand the mechanisms that produce the observed differences in [delta]Fe56 values between terrestrial mafic rocks and those of other planetary bodies.
Despite reports of significant fractionation of iron isotopes occurring under magmatic and hydrothermal conditions, the mechanisms that cause and control fractionation in magmatic systems remain poorly understood. Previous studies suggest that iron isotopes fractionate during fractional crystallization and hydrothermal alteration, but this has yet to be determined in a simple, closed igneous system. The Eocene-aged Skaergaard layered mafic intrusion, East Greenland underwent closed-system evolution, as it cooled and crystallized inward from the floor, roof, and walls, and later underwent hydrothermal alteration, making it a natural laboratory for studying the processes that control iron isotope fractionation under magmatic and hydrothermal conditions. This study focuses on the iron isotope composition of olivine, coexisting phases, and the host rocks of the Skaergaard intrusion, in addition to determining the extent and possible causes of fractionation. Presented here are new iron isotope compositions, relative to igneous rocks ([delta]56FeIgR), for bulk-mineral separates of olivine (n=9), coexisting Fe-Ti oxides (n=9), coexisting pyroxene (n=9), and bulk-rocks (n=6), obtained via multi-collector – inductively coupled plasma – mass spectrometry, for a suite of gabbros from the Layered Series of the Skaergaard intrusion. Samples were selected to represent the range of compositions and the varying extent of alteration within the Layered Series. The [delta]56Fe values range from -0.12 0.05[per mille] to +0.01 ± 0.01[per mille] (2-SE) for olivine, -0.09 ± 0.01[per mille] to +0.51 ± 0.01[per mille] for coexisting Fe-Ti oxides, -0.10 ± 0.03 to -0.01 ± 0.01[per mille] for coexisting pyroxene, and -0.02 ± 0.01[per mille] to +0.03 ± 0.03[per mille] for bulk-rocks. These values mostly fall within, but also vary from the average of mafic- to intermediate-composition crustal igneous rocks (0.00 ± 0.08[per mille]; 2-SD). Oxygen isotope compositions ([delta]18OVSMOW), obtained to investigate potential iron isotope fractionation due to hydrothermal alteration, range from 4.48 ± 0.13[per mille] to 4.77 ± 0.13[per mille] (2-SD) for olivine and 4.12 ± 0.08[per mille] to 5.49 ± 0.14[per mille] for bulk-rock, falling within and below the range for unaltered mafic rocks worldwide (5-6[per mille]). The [delta]56Fe values of unaltered olivine exhibit slight systematic variations in parts of the intrusion, and these changes coincide with slight variations in [delta]56Fe values of coexisting unaltered Fe-Ti oxides and pyroxene, and are interpreted as a reflection of fractionation due to fractional crystallization. Additionally, [delta]56Fe values of altered olivine appear slightly higher than expected, potentially indicating slight fractionation due to hydrothermal alteration. However, the iron isotope composition of the altered olivine fall within the range of fresh olivine, which suggests that no fractionation took place during alteration. In contrast, the [delta]56Fe values of bulk-rocks analyzed here vary by only 0.05[per mille] across the Layered Series with no systematic trends, and do not reflect fractionation due to fractional crystallization or hydrothermal alteration. Inter-mineral iron isotope fractionation factors between olivine and Fe-Ti oxides ([delta]56FeOx-Ol) from the Layered Series range from -0.02[per mille] to +0.54[per mille]. Using theoretical and experimental fractionation factors as a function of temperature, these fractionations are shown to yield, in some cases, temperatures close to those of crystallization, but in other cases produce an inaccurate wide range of temperatures. However, when oxide compositional variations are taken into account in the calculations, more realistic fractionation temperatures are obtained. In general, fractionation factors increase with evolution in the Middle Zone. Iron isotope fractionation factors between olivine and pyroxene ([delta]56FePx-Ol) have a narrower range from -0.08[per mille] to +0.06[per mille], and calculations yield unrealistically high fractionation temperatures. Overall, the unrealistic temperatures obtained are due to mineral compositional variations outside those used in the theoretical and experimental studies, extrapolations done to higher temperatures than those at which experiments were conducted, and in some cases, the effect of inclusions.
Fractionation between Fe isotopes in igneous systems and hydrothermally-altered rocks is small but significant, and detailed petrographic understanding of rocks is needed to discern the origin of such fractionations. One hypothesis proposed to explain the causes of Fe isotope fractionation in igneous rocks involves hydrothermal alteration. Movement of hydrothermal fluids through the system could result in removal of light Fe isotopes, leaving a heavier Fe isotope composition in the rocks. Previous oxygen isotope ([delta]18O) analyses of bulk-rocks, pyroxene, and plagioclase indicate that the Skaergaard intrusion hosted its own hydrothermal system. To provide petrographic context to interpret bulk-rock, bulk-mineral, and in-situ Fe isotope compositions (of Fe-Ti oxides), this study characterizes hydrothermal alteration via oxygen isotope geochemistry of rocks and minerals and the nature of fine-scale exsolution relationships among Fe-Ti oxides in the Skaergaard intrusion. Petrographic, SEM-EDS, TEM, and XRD analyses show exsolutions of ilmenite, hematite, and spinel of different sizes and shapes within Ti-rich magnetite and ilmenite. Characterization of the textural relationships show the following associations: 1) discrete magnetite and ilmenite crystals (0.2–1 mm) in mutual contact, in which ilmenite exsolved from original titanomagnetite or crystalized separately; 2) very thin (1 [micro]m) exsolutions of hematite within ilmenite; 3) coarse (~50 [micro]m) triangular exsolutions of ilmenite within magnetite; 4) very-fine scale (0.1[micro]m), box-like exsolutions of Fe-Ti oxide within titanomagnetite visible only via SEM at 24,000x magnification; 5) very fine-grained (0.5–5 [micro]m) exsolutions of a transparent mineral phase. TEM analysis indicates that the very fine, box-type exsolutions within magnetite are secondary ilmenite containing small amounts of Mg, Al, and Mn, whereas thicker, triangular lamellae are early ilmenite close to pure Ti-Fe oxide in composition. Very fine-grained transparent phases correspond to Fe-Mg-Zn spinel ([Fe,Mg,Zn]Al2O4). Complex, fine-scale exsolutions complicate in-situ analysis of Fe-Ti oxides and mask variations in isotope and major- and trace-element contents. Mass-balance calculations and simple binary-mixing modeling of iron isotope compositions ([delta]56Fe) of magnetite and ilmenite and various mixtures of these oxides indicate that previously reported Fe isotope compositions measured in-situ in magnetite, as well as in bulk-magnetite separates, are unlikely to represent pure magnetite, and instead reflect a mixture of magnetite and ilmenite in varying proportions. New oxygen isotope compositions obtained by traditional laser-fluorination mass spectrometry are presented for bulk-rock gabbro and ferrodiorite (n=35) and bulk-mineral separates for the same rocks for olivine (n=10), pyroxene (n=23), Fe-Ti oxides (n=29), and amphibole (n=5) from the least to most hydrothermally altered layers of the Skaergaard intrusion. Bulk-rock [delta]18OVSMOW values range from -0.03±0.08[per mile] to 5.5±0.14[per mile] (1SD). Mineral separates have a range of [delta]18O from -0.61±0.26[per mile] to +5.29±0.16[per mile] for pyroxene, from +4.47±0.26[per mile] to +4.77±0.13[per mile] for olivine, from -3.13±0.16[per mile] to +3.66±0.16[per mile] for Fe-Ti oxides (magnetite-ilmenite-spinel), and from -0.12±0.13[per mile] to +1.91±0.13[per mile] (2SD) for amphibole. Inter-mineral oxygen isotope fractionation factors between olivine and pyroxene range from -0.09 to 0.28[per mile] and yield temperatures of 900-1200 [degrees] C. Fractionation factors for olivine-oxide and pyroxene-oxide pairs range from 1.19 to 1.82[per mile] and 0.45 to 2.13[per mile], respectively, and reflect hydrothermal alteration at temperatures of 700–800°C, and 800–1000°C respectively.
This edited work contains the most recent advances related to the study of layered intrusions and cumulate rocks formation. The first part of this book presents reviews and new views of processes producing the textural, mineralogical and geochemical characteristics of layered igneous rocks. The second part summarizes progress in the study of selected layered intrusions and their ore deposits from different parts of the world including Canada, Southwest China, Greenland and South Africa. Thirty experts have contributed to this update on recent research on Layered Intrusions. This highly informative book will provide insight for researchers with an interest in geology, igneous petrology, geochemistry and mineral resources.
Acknowledgements xix pioneering workers on igneous layering in Greenland xx Wbrkshop participants xxii Henning Sfl!rensen, University of Copenhagen, Dermark. Latte Melchior Larsen, Geological SUrvey of Greenland, Copenhagen, Dermark. Abstract 1 1 • Introduction 1 1. 1 The agpaitic rocks of the Ilimaussaq intrusion 3 2. Igneous layering in the Ilimaussaq intrusion 4 3. Mineralogy of the layered kakortokite series 15 4. Chemistry of the layered kakortokite series 19 5. Origin of the kakortokite layering 20 5. 1 Discussion 22 6. Conclusion 25 References 26 2. I. AYERn«;r CCMPl\CTIOO NID PCBJ. "--MN}tATIC ~ IN '!HE KLOKKEN INTRUSIOO 29 Ian Parsons and SUsanne M. Becker, University of Aberdeen, U. K. Abstract 29 1. Introduction 30 2. Age of the intrusion 31 3. General structure and mineral variation 31 vi TABLE OF CONTENTS 3. 1 Nomenclature of rock types 31 3. 2 Bulk chemical and modal variation 36 4. The contacts and wall-rocks 37 4. 1 Guter contact 37 4. 2 The gabbro sheath 37 4. 3 The unlaminated syenite sheath 39 4. 4 The gabbro-syenite transition 41 5. The layered series 43 5. 1 General relationships 43 5. 2 Granular syenites 43 5. 2. 1 Structure and cryptic variation 43 5. 2. 2 Origin of granular layers 46 5. 2. 3 Trace elements and chamber dlinensions 47 5. 3 Laminated syenites 48 5. 3. 1 General features 48 5. 3. 2 Mineral layering 51 5. 3.
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