› › Examining The Iron Isotope Geochemistry Of Olivine In The Skaergaard Layered Mafic Intrusion East Greenland

EXAMINING THE IRON ISOTOPE GEOCHEMISTRY OF OLIVINE IN THE SKAERGAARD LAYERED MAFIC INTRUSION, EAST GREENLAND.

Author : Allison R Murrie

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Page : 76 pages

File Size : 29,31 MB

Release : 2018

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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.



A Re-examination of Hydrothermal Alteration and Fe-Ti Oxide Relationships in the Skaergaard Layered Mafic Intrusion, East Greenland

Author : Tiffany Lynn Cummings

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Page : 209 pages

File Size : 22,58 MB

Release : 2018

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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.



The Iron Isotope Composition of Pyroxene from the Skaergaard Layered Mafic Intrusion, East Greenland

Author : Brett Anthony Pertunen

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Page : 98 pages

File Size : 50,71 MB

Release : 2018

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Iron isotopes show small fractionations in igneous rocks emplaced at high temperature, but the processes by which these fractionations originate are still debated. The Skaergaard layered mafic intrusion of East Greenland is a good system to study how iron stable isotopes may fractionate in igneous systems because it formed from a single pulse of tholeiitic magma. High-precision (0.023 [per mille], 2-SE) iron isotope ratios were obtained for pyroxene from fresh and altered gabbros and ferrodiorites representative of the Skaergaard intrusion. The [delta]56Fe values of pyroxene vary from -0.102 [per mille] 0.025 [per mille] to +0.114 [per mille] 0.018 [per mille] (2-SE, relative to igneous rocks), with an overall range of 0.216 [per mille] (n = 13). Poikilitic pyroxene from gabbros of the Lower Zone a and the Upper Border Series g have the lowest [delta]56Fe values, whereas mosaic pyroxene in an altered ferrodiorite of the Upper Border Series [gamma] has the highest [delta]56Fe value. The possible effect of hydrothermal alteration on the iron isotope compositions was investigated for this pyroxene, but it was determined that the pyroxene contains abundant inclusions of magnetite, which increase its [delta]56Fe value, and that hydrothermal alteration did not fractionate iron isotopes in pyroxenes from Skaergaard. The iron isotope composition of pyroxene was investigated in relation to the fractional crystallization of coexisting Fe-Ti oxides magnetite and ilmenite. In the Lower Zone, the [delta]56Fe values of pyroxene systematically increase with magma evolution from the Lower Zone a (-0.102 [plus or equal to] 0.025 [per mille]) to the Lower Zone c (-0.005 [plus or equal to] 0.012 [per mille]). Ferric iron is increasingly incorporated into pyroxene with magmatic evolution in the Lower Zone, allowing for incorporation of more heavy isotopes that result in the increase in [delta]56Fe values. When magnetite saturation occurs at the top of the Lower Zone c, Fe3+ becomes preferentially partitioned into magnetite, and pyroxene [delta]56Fe values then start to generally decline slightly with evolution in the Middle Zone. Fractionation factors between Fe-Ti oxides and pyroxene were calculated and compared with theoretical Fe isotope fractionation factors to determine equilibration temperatures and their significance. Calculated fractionation factors between Fe-Ti oxides and pyroxene range from -0.08 [per mille] to 0.58 [per mille]. The ratio of magnetite:ilmenite was found to strongly control the calculations of the temperature of iron-isotope equilibration between Fe-Ti oxides and pyroxene. When magnetite:ilmenite ratios representative of each rock and various pyroxene varieties were used to calculate weighted theoretical b factors for the oxides, some Fe-Ti oxide – pyroxene pairs yield low temperatures (~200 °C - ~900 °C), whereas others yield temperatures near the temperature of coexisting plagioclase crystallization (~1030 °C - ~1140 °C). Others yield calculated temperatures of equilibrium iron isotope fractionation >1300 °C. Anomalously low or high temperatures of fractionation calculated between Fe-Ti oxides and pyroxene pairs were found to be most likely the result of a lower magnetite:ilmenite ratio than estimated, and changing that ratio to include more ilmenite was shown to lower the calculated temperature of equilibrium iron isotope fractionation. Iron isotope fractionation factors between Fe-Ti oxides and coexisting pyroxene from Skaergaard are small, are affected by magnetite:ilmenite ratios and the composition of pyroxene, and reflect high-temperature magmatic fractionation.



IRON ISOTOPE GEOCHEMISTRY OF BULK ROCKS AND Fe-Ti OXIDES FROM THE SKAERGAARD LAYERED MAFIC INTRUSION, SE GREENLAND.

Author : Alex Joseph Hammerstrom

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Page : 162 pages

File Size : 44,22 MB

Release : 2019

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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.



Layered Intrusions

Author : R.G. Cawthorn

Publisher : Elsevier

Page : 543 pages

File Size : 41,88 MB

Release : 1996-10-18

Category : Science

ISBN : 0080535402

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Book Description

Written by leading experts in the field, this work summarises the important aspects relating to layered intrusions, with almost universal coverage of the subject. Each chapter is a complete review of a theme or specific geological intrusion. The first part of the book covers the general principles and processes which apply to all intrusions. Those chapters on individual intrusions concentrate on factual information. A detailed full colour geological map of the Skaergaard intrusion is also included. This book will appeal to a wide audience - university libraries, research students in igneous petrology, state organisations such as exploration companies - as well as being an ideal textbook for university courses on igneous petrology, and geochemistry and petrography.





Meddelelser Om Grønland

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Page : 80 pages

File Size : 37,4 MB

Release : 1986

Category : Earth sciences

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Invites papers that contribute significantly to studies in Greenland within any of the fields of geoscience ...



Layered Intrusions

Author : Bernard Charlier

Publisher : Springer

Page : 749 pages

File Size : 17,98 MB

Release : 2015-05-18

Category : Science

ISBN : 9401796521

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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.



Oxide Minerals

Author : Donald H. Lindsley

Publisher : Walter de Gruyter GmbH & Co KG

Page : 524 pages

File Size : 50,50 MB

Release : 2018-12-17

Category : Science

ISBN : 1501508687

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Book Description

Volume 25 of Reviews in Mineralogy was published to be used as the textbook for the Short Course on Fe-Ti Oxides: Their Petrologic and Magnetic Significance, held May 24-27, 1991, organized by B.R. Frost, D.H. Lindsley, and SK Banerjee and jointly sponsored by the Mineralogical Society of America and the American Geophysical Union. It has been fourteen and a half years since the last MSA Short Course on Oxide Minerals and the appearance of Volume 3 of Reviews in Mineralogy. Much progress has been made in the interim. This is particularly evident in the coverage of the thermodynamic properties of oxide minerals: nothing in Volume 3, while in contrast, Volume 25 has three chapters (6, 7, and 8) presenting various aspects of the thermodynamics of oxide minerals; and other chapters (9, 11, 12) build extensively on thermodynamic models. The coverage of magnetic properties has also been considerably expanded (Chapters 4, 8, and 14). Finally, the interaction of oxides and silicates is emphasized in Chapters 9, 11, 12, 13, and 14. Because Volume 3 is out of print and will not be readily available to newcomers to our science, as much as possible we have tried to make Volume 25 a replacement for, rather than a supplement to, the earlier volume. Chapters on crystal chemistry, phase equilibria, and oxide minerals in both igneous and metamorphic rocks have been rewritten or extensively revised.