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Iron oxides are ubiquitous minerals in the earth’s crust and earth scientist are utilizing their chemical complexity to characterize rock/ore forming conditions. Compared to magnetite, only few studies exist on hematite, although alpha-Fe2O3 is an abundant alteration/weathering/metamorphic product in primarily magnetite-bearing rocks and ores – under certain conditions hematite represents the main iron oxide mineral. Hematitization of magnetite (including pseudomorphism after magnetite, i.e., martitization) take place by coupled dissolution-reprecipitation process (CDR) under redox and non-redox fluid-rock reactions in static or dynamic structural state. These mechanisms have impact on the iron oxide trace elemental budget. In order to understand better the chemistry of metamorphic- and alteration-induced hematitization in various ore settings (iron oxide apatite IOA, Fe-skarns, iron formation and related hematite ore), we report in-situ LA-ICP-MS data from hematite and precursor magnetite. The aims are to test the use of hematite in magnetite discrimination diagrams, and to define element proxies to characterize certain oxide transformation conditions. Results show that hematitization significantly modify trace metal budgets in iron oxides. Metamorphic hematitization in various domains shows rather consistent depletion of low-valent and enrichment of high-valent elements. Alteration-induced martitization show contrasting mobility, allowing discrimination of hydrothermal from weathering-related condition, even within the same deposit. Elements commonly used in magnetite discrimination diagrams (particularly Al, Ti, Mn, Ni, Co, but also HFSE or REE) are variously modified by up to several orders of magnitudes, meaning that hematite analyses should be avoided in those diagrams. Further examination will aim to explain characteristic element mobility and “fingerprint” ore-forming/-alterating processes, including fluid-rock ratios.

Analysis of Au in geological materials has been proven useful for understanding geological processes related to the formation of various Au-rich ore deposits. However, the Au concentration in the source rocks is extremely low and the methodologies allowing analysis at sub-ng/g levels are time-consuming, costly and generally involve handling of dangerous chemicals. Here we present a new, faster, cheaper and safer method for low Au concentration analysis by Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS) of pressed-powder-pellets (PPP), which has been developed in the Laboratory for Environmental and Raw Materials Analysis (LERA) at Karlsruher Institut für Technologie (KIT). Fine instrumental tuning for key parameters such as ICP-MS sample gas flow, torch position, RF power, laser frequency, ablation style and laser gas flow allows us to increase the Au sensitivity while keeping oxide production low. Indeed, the latter is critical when analysing low concentration Au samples as the ICP-MS analysis is generally impaired by Ta181O16 and the Hf180O16H1 interferences. Reducing the oxides allows the PPP-LA-ICP-MS method for better quantification and correction of these interferences. The new method improves the detection limit down to 0.05-0.1 ng/g Au. Routine analysis of reference materials shows that Au analysis by PPP-LA-ICP-MS is both accurate and precise. This contribution highlights that the method is suitable for investigating Au distribution and behaviour in Au depleted rocks.

The Gardiner (E-Greenland) and Kovdor (Russia) alkaline complexes display a similar succession of rock types comprising dunites-pyroxenites, ijolite series rocks, melilitolites and carbonatites. Although similar melanephelinitic parental magmas are suggested for both complexes, they display enrichment in HFSE at strikingly different evolutionary stages: At Kovdor, melilitolites are barren but carbonatites are mineralized with HFSE. In contrast, melilitolites at Gardiner contain ore-grade accumulates of perovskite having wt.%-level contents of Nb, Ta and REE, while associated carbonatites are barren. Previous studies suggested that HFSE-poor carbonatites at Gardiner were formed by liquid immiscibility while Kovdor carbonatites result from fractional crystallization and retained high HFSE contents. These two evolutionary trends were explained by a different CO2-dependent stability of melilite vs. clinopyroxene+nepheline+calcite during the ijolite stage [1]. However, it is poorly investigated how the HFSE budget is affected by the crystallization of Ti-phases during different stages of the magmatic evolution, which are stabilized depending on magma composition (i.e. aTiO2, aSiO2) but also on intensive parameters such as P, T, and fO2 [2]. Preliminary results suggest that, in contrast to Kovdor, magmas at Gardiner had physiochemical conditions which favoured abundant crystallization of Ti-phases along with co-precipitation of HFSE earlier in the sequence. This is supported by (1) pyroxenites with abundant Ti-magnetite and ilmenite, (2) titanite-rich ijolites and (3) perovskite-rich melilitolites. Possibly, Ti-rich melts reflect a distinct mantle regime beneath E-Greenland, which also produced anomalously Ti-enriched flood basalts ~6-10 Ma before. [1] Veksler et al. (1998) J.Pet. 39, 2015-2031; [2] Marks et al. (2008) CG 257, 153-172

Melt inclusions in zircon (MIZ) directly reflect the physicochemical state of the magma during zircon growth. However, their potential as geothermometers and petrogenetic indicators is still poorly explored. Therefore, we investigated MIZ from well-characterized mafic and felsic rocks of the Bushveld Complex (South Africa) and acquired a novel dataset of major, trace and volatile element contents. Re-homogenized MIZ of all rock types display rhyolitic compositions (65-78 wt% SiO2) and similar H2O contents (1.6-4.0 wt%). Liquidus temperatures of MIZ obtained from normative Qz-Ab-Or and rhyolite-MELTS modelling indicate melt entrapment at 930–850°C (at 200 MPa), tailing down to 700°C in some samples. For rutile-bearing mafic cumulates of the lower BC (Marginal and Critical Zone), these temperatures overlap with TiO2 saturation temperatures of MIZ as well as with Ti-in-zircon of host crystals using aTiO2=aSiO2=1.0 [1], in accordance with textural associations of zircon+rutile+quartz. In contrast, MIZ in all rutile-free, magnetite-ilmenite-titanite- and quartz-bearing rocks of the upper BC (Upper Zone ferrogabbros, granitic rocks), display strikingly lower Ti contents, but also higher ƩREE and lower Th/U. Cross-calibration of TiO2 saturation (MIZ) and Ti-in-zircon thermometers with MIZ liquidus temperatures suggests that zircon crystallized at highly reduced aTiO2~0.3, significantly below aTiO2~0.6 previously estimated for rutile-free rocks in the literature, usage of which would underestimate zircon crystallization temperatures by 50-100 °C. In summary, MIZ may inherit chemical signatures of host rocks, are powerful zircon geothermometers and provide constraints for aTiO2 in Rt-free rocks. [1] Ferry & Watson (2007) CMP 154, 429–437; [2] Hayden & Watson (2007) EPSL 258, 561-568

Zircons of magmatic rocks can show enormous variations in Th/U ratios (0.2 to 100) and extreme Th/U zoning. We present data from felsic and mafic rocks of the Bushveld Complex in South Africa. Zircon grains in mafic cumulate rocks reveal Th/U ratios up to 70, those in felsic rocks barely exceed 1.0. In mafic rocks zircon mostly occur together with Rt-Bt-Kfs-Qtz in intercumulus domains, and crystallized during final magma cooling between 900 and 700°C, after >75% of fractional crystallization. The resulting zircons reveal very distinct Th/U zoning trends. Group (1) zircons show systematic increase in Th/U from core to rim (from 0.5 to 20), accompanied by a systematic decrease in U content (from >170 to 10 ppm), group (2) zircons the opposite trend, and group (3) zircons nearly no zoning. Modelling result reveal that all three zoning trends can be explained by minor differences in Bt-Rt-Zrc crystallization history. Trend (1) results from Rayleigh-like fractionation due to zircon growth (±Rt), having different partition coefficients for U ≫ Th. Trend (2) results from zircon growth after onset of biotite-in reaction, causing breakdown of previously formed rutile, thereby releasing U≫Th. Trend (3) results from mass balance constrains, causing mutual compensation of fractionation effects. The absence of pronounced Th/U zoning of zircons in felsic Bushveld rocks also results from compensation of zircon fractionation due to coeval crystallization of abundant rock-forming minerals (Opx-Cpx-Hbl-Pl-Kfs-Qtz) at an early stage of fractional crystallization (10-20%), all being highly incompatible for Th and U [1]. [1] Gudelius et al. (2020). Chemical Geology 546, 119647.

Peridotites in the upper Austroalpine Ulten zone (Eastern Italy) sample the subduction-modified Variscan mantle wedge. Metasomatism of peridotites during four stages of mantle wedge evolution includes: (1) Intrusion of alkaline melts from an inner, subduction-modified wedge and cryptic enrichment of spinel lherzolites (SL); (2) Reaction with siliceous crustal melts after pressure increase, generating coarse-grained garnet amphibole peridotites (GAP); (3) Crystallisation of abundant amphibole (± apatite and dolomite) from residual hydrous fluids during and/or after peak metamorphism recorded by fine-grained GAP; [4] Subsequent influx of crustal fluids, causing retrograde formation of spinel chlorite amphibole peridotites (SAP) [1-5]. SL and coarse GAP are apparently more fertile, whereas fine GAP and SAP retain the most depleted major-element characteristics. Overall, samples fall on partial melting trends consistent with extraction of low degrees of melt (F≤0.15) at 2-1 GPa. SL and coarse GAP have ±flat PGE patterns normalised to Primitive Upper Mantle (PUM), or show small decreases or increases from compatible to incompatible PGE. This suggests retention of primary sulphide liquid at low degrees of melting, during which PGE concentrations are little fractionated [6]. Indeed, broad positive correlations between the PGE suggest a common host, likely sulphide, observed in the samples as assemblages of pn ± po and cpy. Most fine-GAP share these patterns, indicating robustness against massive hydrous fluid influx, while Os/Ir > PUM argue against strong Os scavenging by highly oxidising hydrous fluids. Nevertheless, elevated Ru/IrPUM in a subgroup of samples may indicate a role for spinel addition under oxidising conditions. Most samples have 187Os/188Os >PUM, despite sub-PUM Re/Os, which requires addition of, or isotopic equilibration with, 187Os-rich crustal components, most likely via the precipitation of metasomatic sulphide. [1] Nimis and Morten (2000) J Geodyn 30: 93-115; [2] Rampone and Morten (2001) J Petrol 42: 207-219; [3] Tumiati et al. (2003) Earth Planet Sci Lett 210: 509-526; [4] Sapienza et al. (2009) Contrib Mineral Petrol 158: 401-420; [5] Scambelluri et al. (2006) Contrib Mineral Petrol 151: 372-394; [6] Mungall and Brenan (2014) Geochim Cosmochim Acta 125: 265-289.

Metasomatized mantle wedge peridotites exhumed within high-pressure terranes of continental collision zones provide unique insights into crust-mantle interaction and attendant mass transfer, which are critical to our understanding of terrestrial element cycles. Such peridotites occur in high-grade gneisses of the Ulten Zone in the European Alps and record metasomatism by crustal fluids at 330 Ma and high-pressure conditions (2.0 GPa, 850 °C) that caused a transition from coarse-grained, garnet-bearing to fine-grained, amphibole-rich rocks. We explored the effects of crustal fluids on canonically robust Lu-Hf peridotite isotope signatures in comparison with fluid-sensitive trace elements and Nd-Li isotopes. Notably, we found that a Lu-Hf pseudo-isochron is created by a decrease in bulk-rock 176Lu/177Hf from coarse- to fine-grained peridotite that is demonstrably caused by heavy rare earth element (HREE) loss during fluid-assisted, garnet-consuming, amphibole-forming reactions accompanied by enrichment in fluid-mobile elements and the addition of unradiogenic Nd. Despite close spatial relationships, some peridotite lenses record more intense fluid activity that causes complete garnet breakdown and high field strength element (HFSE) addition along with the addition of crust-derived unradiogenic Hf, as well as distinct chromatographic light REE (LREE) fractionation. We suggest that the observed geochemical and isotopic provinciality between peridotite lenses reflects different positions relative to the crustal fluid source at depth. This interpretation is supported by Li isotopes: inferred proximal peridotites show light δ7Li due to strong kinetic Li isotope fractionation (−4.7–2.0‰) that accompanies Li enrichment, whereas distal peridotites show Li contents and δ7Li similar to those of the depleted mantle (1.0–7.2‰). Thus, Earth's mantle can acquire significant Hf-Nd-Li-isotopic heterogeneity during locally variable ingress of crustal fluids in continental subduction zones.

The orogenic peridotites and pyroxenites of the Ulten Zone (north Italy) record multistage metasomatism by crust-derived melts and fluids within a Late-Variscan mantle wedge. We acquired new major and trace element data as well as garnet and whole-rock iron speciation for a representative suite of samples, with the aim to further constrain element cycling and the redox state attending the development of the major mineralogical and textural rock types that occur within the Ulten Zone. Initially, spinel peridotites were refertilized by mafic melts in the hot and shallow mantle wedge, followed by garnet formation as the peridotites were carried towards a cool, subducting slab of continental crust by corner flow. Upon exhumation, ingress of aqueous, crust-derived fluids provoked amphibole-forming reactions, which caused gradual consumption of garnet and clinopyroxene and transformation from coarse- to fine-grained assemblages. Since Si, Al, Na, Ti, Ca and HREE formerly stored in reactants were not fully accommodated in newly-formed phases, these elements were partially removed from the bulk-rock, generating more depleted compositions resembling residues after partial melting. Unexpectedly, the remaining garnet retains low Fe3+/Sigma Fe (