Heidi E. Höfer

The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis

Abstract We have established a method to determine the oxidation state of Fe using the electron microprobe “the flank method.” This method has the possibility to analyze simultaneously the majorelement chemistry on the same spot. The method is based on FeL X-ray emission spectra that show a concomitant change of both the intensity and the wavelength of the soft FeLα and FeLβ emission lines. The method has been developed for garnets using well-characterized synthetic samples, and an empirical correction scheme has been established for self-absorption of the FeL X-ray emission. This enables the unambiguous correlation of measured intensity ratios of FeLα/FeLβ X-ray emission to the oxidation state in garnet. The flank method can be used with routine microprobe settings (e.g., 15 kV, 60 nA) and, for garnets, with minimum beam size. This way, Fe3+/ΣFe and major and trace elements can be determined within 10 minutes on the microscale. The resulting accuracy of Fe3+/ΣFe is then ±0.04 for garnets at 10 wt% total Fe. Application to natural garnet megacrysts with 8 wt% total Fe and Fe3+ known from Mössbauer spectroscopy yields excellent agreement between the two methods. For practical application, the calibration can be achieved by using three natural standards and fitting the data to Fe2+ and Fe3+ with linear equations. Through repeated measurements on homogeneous samples, the error in Fe3+/ΣFe becomes approximately ±0.02 (1σ). Optimization of analytical parameters such as beam current, measurement time, and repetition of analyses, will further lower the uncertainty.

Comparison of two electron probe microanalysis techniques to determine ferric iron in synthetic wüstite samples

Two techniques are compared for determining the ferric iron content in Fe x O using the electron microprobe: (i) a direct method termed the flank method (Hofer et al., 1994), which is based on the concomitant changes of both the intensity ratio and the wave length of the Fe Lα and Fe Lβ X-ray emission lines with Fe 2+ /Fe 3+ , and (ii) an indirect method by calculating Fe 2+ /Fe 3+ from elemental microprobe analyses which may include oxygen. The precision and accuracy of Fe 3+ /∑Fe determined with the flank method is ± 0.02 (1s). Similar values are achieved with the indirect method by determining the Fe and O contents, while measurement of iron alone and calculation of the oxygen content from the difference to 100% is less accurate. The results are valid only for this relatively simple two component system and cannot be transferred to multiple component systems without further calibration. At present, the flank method can be applied if the bulk chemical composition and crystal structures of the samples are known and calibration curves with similar materials have been established. The indirect method is restricted to samples with Fe as the only multivalent cation. The precision of the flank method is largely affected by the accuracy of the spectrometer position. Thus, a method is developed to optimize the spectrometer calibration using the Fe Kα 1 9th order emission line.

High-pressure synthesis of priderite and members of the lindsleyite-mathiasite and hawthorneite-yimengite series

Experiments investigating simplified members of the alkali-titanate mineral series priderite, lindsleyite-mathiasite (LIMA) and hawthorneite-yimengite (HAWYIM) confirm their stability at pressures between 3.5 and 5 GPa for priderite and LIMA, 4.3 and 5 GPa for HAWYIM at likely mantle solidus temperatures. Starting materials with K/Ba=1 resulted in solid-solutions for LIMA, whereas HAWYIM produced separate hawthorneite (K/Ba approx. 0.5) and nearly pure yimengite (K), indicating the existence of an asymmetric solvus for HAWYIM at high pressures. Priderite was examined with iron present as either Fe3+ or Fe2+; both are stable at 5 GPa, confirming the possible coexistence of priderite with diamond. X-ray powder diffraction data on end-members synthesized at high pressures and calculated reflection patterns improve the scope of diffraction data on these rare mantle minerals, with only small differences existing between the patterns for the synthetic minerals and those observed or expected for natural minerals.

Volatile-rich Metasomatism in the Cratonic Mantle beneath SW Greenland: Link to Kimberlites and Mid-lithospheric Discontinuities

The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from 100–170 km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150 Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene–spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopitebearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at 90–110 km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590–550 Ma extended to >210 km depth, and suggests loss of 40 km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval ( 90–110 km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 2311 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 12, 2311–2338 doi: 10.1093/petrology/egy009 Advance Access Publication Date: 14 February 2018

A Reconnaissance Study of Ti-minerals in Cratonic Granulite Xenoliths and their Potential as Recorders of Lower Crust Formation and Evolution

A comprehensive petrographic and in situ major and trace element study of rutile, ilmenite and Ti-magnetite was undertaken in six lower crustal xenoliths of metabasaltic (?underplate) and metasedimentary (subduction) origin from the Diavik kimberlites (central Slave Craton, Canada). The aims of the study were to improve our understanding of trace element incorporation into these Ti-minerals, and to use these systematics to obtain insights into lower continental crust formation and evolution. Abundant (oxy)exsolution of titanomagnetite lamellae, blocky rutile, as well as minor pleonaste and zircon in ilmenite from metabasaltic granulites are proposed to reflect cooling from magmatic or metamorphic temperatures and subsequent secular mantle cooling. This explains the large spread in Zr-in-rutile temperatures (>200 C) and may partly be responsible for the substantial heterogeneity of other trace element concentrations in rutile and ilmenite. Even after accounting for trace element heterogeneity and modal uncertainties, mass-balance calculations indicate that both Ti and Nb in lower crustal granulites are largely controlled by rutile and ilmenite. Rutile U–Pb data define discordia arrays that yield upper intercept ages broadly coincident with the 1 27 Ga giant Mackenzie dike swarm event, suggesting reheating of the lower crust above the rutile U–Pb closure temperature, whereas lower intercept ages roughly correspond to the age of Cretaceous to Eocene kimberlite magmatism. Subsequent cooling led to partial resetting and data spread along the concordia. Closer inspection reveals that inter-grain concentrations of elements that are compatible in rutile (Nb, Ta, W, U), but highly incompatible in the abundant silicate minerals (in equilibrium with melt), are heterogeneous and contrast with the more homogeneous concentrations of the transition metals (NiO, V). This may indicate that local reaction partners for diffusive homogenization of these element concentrations were absent. Nb/Ta is also highly variable at the sample scale. This may be explained by prograde growth from high-Nb/Ta mineral precursors (e.g. biotite) in the metasedimentary granulites and crystallization of the protoliths to the metabasaltic granulites from a mafic magma that had experienced fractionation of ilmenite with low Nb/Ta in a crustal magma chamber. Thus, (Fe)–Ti minerals represent high field strength element ‘islands’ in the granulite silicate matrix. The lack of homogenization and persistence of high-energy grain boundaries, such as exsolution lamellae, further indicate that the lower continental crust remained essentially dry and did not recrystallize, possibly since Neoarchaean metamorphism.

Electron microprobe technique for the determination of iron oxidation state in silicate glasses

Abstract We present a new calibration for the determination of the iron oxidation state in silicate glasses by electron probe microanalysis (EPMA) with the “flank method.” This method is based on the changes in both intensity and wavelength of the FeLα and FeLβ X-ray emission lines with iron oxidation state. The flank method utilizes the maximum difference for the FeLα and FeLβ spectra observed at the peak flanks between different standard materials, which quantitatively correlates with the Fe2+ content. Provided that this correlation is calibrated on reference materials, the Fe2+/ΣFe ratio can be determined for samples with known total Fe content. Two synthetic Fe-rich ferric and ferrous garnet end-members, i.e., andradite and almandine, were used to identify the FeLα and FeLβ flank method measuring positions that were then applied to the measurement of a variety of silicate glasses with known Fe2+/ΣFe ratio (ranging from 0.2 to 1.0). The measured intensity ratio of FeLβ over FeLα at these flank positions (Lβ/Lα) is a linear function of the Fe2+content (in wt%). A single linear trend can be established for both garnets and silicate glasses with 4–18 wt% FeOT (total iron expressed as FeO). In glasses with up to 18 wt% FeOT and 15 wt% TiO2, no systematic compositional (matrix) effects were observed. A possible influence of Ti on the Fe2+ determination has only been observed in one high-Ti glass with ~25 wt% TiO2, a content that is not typical for natural terrestrial silicate melts. The accuracy of the Fe2+/ΣFe determination, which depends on both the Fe2+ content determined with the flank method and on the total Fe content, is estimated to be within ±0.1 for silicate glasses with FeOT > 5 wt% and within ±0.3 for silicate glasses with low FeOT ≤ 5 wt%. The application of the flank method on silicate glasses requires minimization of the EPMA beam damage that can be successfully achieved by continuous movement of the sample stage under the electron beam during analysis, e.g., with a speed of 2 μm/s.

Ferropericlase inclusions in ultradeep diamonds from Sao Luiz (Brazil): high Li abundances and diverse Li-isotope and trace element compositions suggest an origin from a subduction mélange

The most remarkable feature of the inclusion suite in ultradeep alluvial and kimberlitic diamonds from Sao Luiz (Juina area in Brazil) is the enormous range in Mg# [100xMg/(Mg + Fe)] of the ferropericlases (fper). The Mg-richer ferropericlases are from the boundary to the lower mantle or from the lower mantle itself when they coexist with ringwoodite or Mg- perovskite (bridgmanite). This, however, is not an explanation for the more Fe-rich members and a lowermost mantle or a “D” layer origin has been proposed for them. Such a suggested ultra-deep origin separates the Fe-rich fper-bearing diamonds from the rest of the Sao Luiz ultradeep diamond inclusion suite, which also contains Ca-rich phases. These are now thought to have an origin in the uppermost lower mantle and in the transition zone and to belong either to a peridotitic or mafic (subducted oceanic crust) protolith lithology. We analysed a new set of more Fe-rich ferropericlase inclusions from 10 Sao Luiz ultradeep alluvial diamonds for their Li isotope composition by solution MC-ICP-MS (multi collector inductively coupled plasma mass spectrometry), their major and minor elements by EPMA (electron probe micro-analyser) and their Li-contents by SIMS (secondary ion mass spectrometry), with the aim to understand the origin of the ferropericlase protoliths. Our new data confirm the wide range of ferropericlase Mg# that were reported before and augment the known lack of correlation between major and minor elements. Four pooled ferropericlase inclusions from four diamonds provided sufficient material to determine for the first time their Li isotope composition, which ranges from δ7Li + 9.6 ‰ to −3.9 ‰. This wide Li isotopic range encompasses that of serpentinized ocean floor peridotites including rodingites and ophicarbonates, fresh and altered MORB (mid ocean ridge basalt), seafloor sediments and of eclogites. This large range in Li isotopic composition, up to 5 times higher than ‘primitive upper mantle’ Li-abundances, and an extremely large and incoherent range in Mg# and Cr, Ni, Mn, Na contents in the ferropericlase inclusions suggests that their protoliths were members of the above lithologies. This mélange of altered rocks originally contained a variety of carbonates (calcite, magnesite, dolomite, siderite) and brucite as the secondary products in veins and as patches and Ca-rich members like rodingites and ophicarbonates. Dehydration and redox reactions during or after deep subduction into the transition zone and the upper parts of the lower mantle led to the formation of diamond and ferropericlase inclusions with variable compositions and a predominance of the Ca-rich, high-pressure silicate inclusions. We suggest that the latter originated from peridotites, mafic rocks and sedimentary rocks as redox products between calcite and SiO2.

Foreword: European Mineralogical Conference 2012

The first European Mineralogical Conference was held in early September 2012 at the Johann Wolfgang Goethe Universitat in Frankfurt, Germany. The idea for a joint meeting of European Mineralogical Societies was borne from the success of previous joint meetings of the Mineralogical Society of Great Britain and Ireland (MinSoc) with the Mineralogical Societies of America (MSA), Canada (MAC), France (SFMC) and Germany (DMG) in Cambridge in 2007 and in Edinburgh in 2009. Enthusiasm for a combined conference of European Mineralogical Societies was growing, and a central location in Europe was sought as venue. Frankfurt is an easy place to reach from virtually anywhere, and, when asked, we happily agreed to organize such a meeting in conjunction with the potential participating societies. An open invitation was extended to which seven societies initially responded. A first meeting of the presidents of …