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Regularly alternating reduction-oxidation (redox) patterns attributed to variations in the Earth's orbit and axis (Milankovitch cycles) are widely recorded in marine sediment successions of the Phanerozoic and attest to a dynamic history of biospheric oxygen in response to astronomically forced climate change. To date, however, such astronomical redox control remains elusive for much older, Precambrian intervals of the geological record that were characterized by a globally anoxic and iron-rich ocean, i.e., prior to Earth's atmospheric oxygenation (ca. 2.4–2.2 billion years ago). Here we report a detailed cyclostratigraphic and geochemical investigation of marine-sedimentary redox cycles identified in the ca. 2.46 billion-year-old Joffre Member of the Brockman Iron Formation, NW Australia, suggesting the imprint of Earth's climatic precession cycle. Around the base and top of regularly intercalated mudrock layers, we identify sharp enrichments in redox sensitive elements (Fe, S, Ca, P) that appear to represent chemical reaction fronts formed during nonsteady state diagenesis. Using a reactive transport model, we find that the formation of characteristic double S peaks required periods of increased organic matter deposition, coupled to strongly declining Fe2+ concentrations in the overlying water column. This combination, in turn, implies a periodic deepening of the iron chemocline due to enhanced oxygenic photosynthesis in marine surface waters, and is interpreted as the result of precession-induced changes in monsoonal intensity that drove variations in runoff and associated nutrient delivery. Our study results point to a dynamic redox evolution of Precambrian oceanic margin environments in response to Milankovitch forcing, and offer a temporal framework to investigate linkages between biological activity and the early build-up of oxygen in Earth's ocean-atmosphere system.

A geochemical analysis was performed on the Kuruman banded iron formation (BIF) from the Griqualand West Basin in South Africa, in order to study the depositional environment and controls on BIF deposition. This study was supplemented with a preliminary spectral analysis to identify a possible Milankovitch cyclicity in the Kuruman formation. The Kuruman BIF (~2.46 Ga) was deposited pre-GOE (Great Oxygenation Event) and provides a window into Pre-Cambrian conditions on Earth. High-resolution sampling (1 meter-scale) was performed on the lower half of the UU BH-1 core, which comprises the lower half of the Kuruman formation. An optimised carbonate extraction scheme was applied on the samples in order to more closely study this fraction, as it has been found to most closely resemble a primary seawater signal. The Kuruman formation is characterised by an absence of Ce-anomalies, positive Eu-anomalies and Y/Ho values, and low abundances of continentally derived elements (i.e. Al, Zr and Ti). These observations indicate that the Kuruman formation was deposited in a highly reducing environment, with a strong hydrothermal component and limited input of continentally derived material. A gradual shift from an oxide dominated lower half of the core to a carbonate dominated upper half reflects the shallowing of the depositional setting of the Kuruman formation. Rare-earth elements + Yttrium (REE+Y) and Al concentrations were highly variable at the meter-scale and were found to show a periodicities of 2.8 and/or 3.7-3.9 meters, along with periods of 4-6 and ~9 meters. The periodicities observed in the elemental data, along with the dependency of BIF deposition on climatologically controlled factors such as nutrient availability and organic productivity, leads me to conclude that climate exerts a primary control on BIF deposition in the form of Milankovitch cycles.

Extensive Cenozoic intra-plate volcanism in West-Arabia reflects a geodynamic setting relating convergent, transform, and plume related divergent plate boundaries. I present a detailed analysis of mineral- and recrystallized melt inclusions from an extensive set of >1000 early crystallized olivine hosts derived from alkali basalts of the Harrat al-Shamah in Syria. Major and trace element geochemistry of olivine crystals and inclusions was determined by SEM, EMPA, and LA-ICPMS. Volume corrections for recrystallized melt inclusions were used in order to determine bulk major element compositions without re-homogenization. On the basis of a compositionally variable dataset, I discerned three distinct melt groups documenting a compositional progression. Covariance of major and trace elements in these primary melts indicates compositional control by polybaric melting and binary mixing of spinel- and garnet peridotite derived end-member melts. High Dy/Yb, Zr/Y, and low Zr/Nb ratios in incompatible element enriched magmas indicate a magma source of Red-Sea MORB-type mantle which was locally re-fertilized by OIB-type material. Olivine-melt thermometry indicates the majority of melt-generation to be located at the lithosphere-asthenosphere boundary, consistent with thermal erosion by hot mantle. I argue for West-Arabian intra-plate volcanism to be driven by migrating plume material which locally exploited tectonically generated zones of lithospheric weakness. My data shows no evidence for melting of metasomatic veins and/or a separate mantle plume underneath West-Arabia.

Unlike in an oxygen-rich setting, pyrite is a persistent mineral in anoxic environments and is therefore capable of recording environmental changes that occurred during its crystal growth phases. Pyrites from the Barberton Greenstone Belt (BGB) in the Kaapval craton in South Africa have an age of 3.55-3.22 Ga, and can therefore give a unique insight in the environmental changes that happened in Paleoarchean era. In this thesis study, the chemistries of pyrite crystals from the BGB, which were sampled from the Mapepe formation at depths of 309.58 meters and 440.30 meters, were studied with the Electron Microprobe Analyser (EMPA). Furthermore, the Raman spectrometer was used to (1) test the biogenicity of the framboidal pyrite cores and (2) to determine the maximum metamorphic temperature, using the carbonaceous material (CM) as a geothermometer. The EMPA provided chemical maps of the pyrites that showed that one grain could contain up to 8 different pyrite generations. The pyrite overgrowths were identified by changes in enrichments in Co, Ni, As or Cu. The pyrites in sample 309.58m contained detrital pyrite cores enriched in Cu, whereas the grains in sample 440.30m were nearly completely depleted from Cu and occasionally contained an As enriched core of detrital origin. The detrital cores from both drill core depths contained overgrowths enriched in both Co and As, and in both, As and Ni. Enrichments in Co + As, Co or Ni are attributed to either hydrothermal (Co + As) or metamorphic fluid (Ni) interactions and the As + Ni enriched pyrites and the depleted pyrites are associated with diagenetic crystal growth. Cobalt, Nickel and Copper are proposed to be sourced from chemically weathered mafic rocks that were subsequently transported and incorporated within the pyrites. The As enrichment is associated with marine sediments from As-rich ancient oceans. Many pyrite overgrowths were attributed to fluid-crystal interactions, implying that hydrothermal and metamorphic flu

The 1.6-2.0Ga Gamsberg deposit is a >200 Mt zinc reserve located in the Aggeneys-Gamsberg mining district in the Northern Cape Province of South Africa, which is host to a cluster of world-class base metal deposits. These four proximal SEDEX-type deposits were formed in an extensional basin, in which Gamsberg occupies the eastern side of the district. The Gamsberg deposit is characterized by an anomalous enrichment in manganese, which greatly complicates ore re?finement. Previous work has shown a high abundance and variability of Mn in sphalerite minerals throughout the stratigraphy at the West orebody of Gamsberg. This study goes beyond previous work by determining for the fi?rst time sulfur isotope ratio variability in individual sphalerite and iron sul?fide minerals with stratigraphic position through the deposit. LA-ICP-MS analysis of the Fe-sulfi?de- and sphalerite minerals in the samples from the West orebody revealed initially low d34S values around 0‰ that increased abruptly to high d34S values of up to +40‰ relative to Vienna Canyon Diablo Troilite (VCDT). This was interpreted as the source of reduced sul- fur for initial mineralization being decoupled from the source of reduced sulfur after initial mineralization, thermochemcial reduction of seawater derived sulfate. The strongly positive d34S values were interpreted as the result of Gamsberg developing later than the other Aggeneys deposits in the closed depositional basin. Manganese variability is interpreted as a fi?nite sediment enrichment as a distal expression of the other Aggeneys SEDEX deposits which was remobilized and progressively depleted during the formation of the Gamsberg deposit. Metamorphism and associated metasomatism during the Namaquan orogeny (1.06-1.03 Ga) were interpreted as a mechanism for introducing secondary sulfi?de minerals to ore units with an oxidized primary mineral assemblage and mobilizing manganese for incorporation in metamorphic gangue minerals.

Banded iron formations (BIF), deposited prior to and concurrent with the Great Oxidation Event (GOE) at ~2.4 Ga, record changes in the oceanic and atmospheric chemistry during this critical time interval. Three previously unstudied drill-cores from the western Transvaal Basin, South Africa, capturing the rhythmically mesobanded Kuruman BIF and the overlying granular Griquatown BIF, were sampled every ~20 m. along core depth. These samples were analysed for mineralogy, geochemistry and bulk Fe and C-isotopes. Bulk Fe-isotopic values of 50 samples show an apparent relationship with mineralogy. The lower δ56Fe values (< -1.3) correlate with carbonate-rich samples, whereas higher δ56Fe values (>0.0) correspond to samples rich in bulk modal magnetite. To further investigate this relationship, a 3-step sequential extraction protocol was developed to separate the three main Fe-hosting fractions (Fe-carbonates, Fe-oxides and Fe-silicates). Rare Earth Element (REE) patterns were resolved for the individual fractions and using the leachate destruction protocol of Henkel et al. [1] we were able to measure for the first time species specific Fe-isotopes of bulk-BIF samples. Species specific Fe-isotopes are probably a better proxy for the Palaeoproterozoic ocean than bulk-rock values, since the latter are strongly influenced by the modal mineralogy of each sample. We used bulk-rock C-isotope data combined with the species specific REE and δ56Fe to argue that the Fe-carbonates (and possibly Fe-silicates) in the Transvaal BIFs record primary chemical signatures. It follows that chemical signatures can be preserved, through changes of the textural appearance of minerals in BIF during diagenesis and low-grade metamorphism [2]. Preliminary data indicate that the Fe-oxides (dominated by magnetite) are probably formed by recycling and mixing of precursor Fe-(oxy)hydroxides and ferrous sea- or pore-waters, since their positive δ56Fe values deviate strongly and consistently from the nega

Banded iron formations (BIF) deposited during the Great Oxidation Event (GOE) at ~2.4Ga, potentially capture chemical changes in the Paleoproterozoic atmosphere and ocean. The Kuruman and Griquatown BIFs in the Griqualand West Basin, South Africa were deposited concurrent with the GOE. Continuous drill-cores thereof provide important insights into the Paleoproterozoic environment. Bulk-rock samples of these cores show an apparent relationship between modal mineralogy and Fe-isotopes: low δ56Fe-values (<-1.3‰) correlate with samples rich in Fe-carbonate, whereas positive δ56Fe-values correspond to magnetite-rich samples. To interrogate this further, an existing sequential extraction scheme was modified to accommodate the BIF mineralogy [1]. Since ferric oxides are insignificant in the rocks we studied, the hydroxylamine and dithionite leaches could be omitted. The remaining extractions were optimized for the dissolution of three main Fe-hosting mineral fractions (Fe-carbonates, magnetite and Fe-silicates). The final silicate residue was dissolved in HF-HClO4-HNO3. The tests established a 3-step sequential extraction procedure to quantitatively separate the Fe-mineral fractions. To measure the species specific Fe-isotopes a processing-protocol was applied to break down the organic leachates [2]. Combining the species specific data (δ56Fe and REE) with bulk-rock data (C-isotopes and mineralogy) it becomes apparent that the BIFs have recorded primary signals from Paleoproterozoic seawater. References: [1] Poulton & Canfield (2005) Chem. Geo., 214, 209-221. [2] Henkel et al., (2016) Chemical Geology, 421, 93-102

Anomalous fractionation of the minor isotopes of sulfur (Δ33S, Δ36S) in Archean pyrite is thought to reflect photochemical reactions in an anoxic atmosphere, with most samples falling along a reference array with Δ36S/Δ33S ≈ −1. Small deviations from this array record microbial sulfate reduction or changes in atmospheric source reactions. Here, we argue that reworking of atmospheric sulfur with distinct minor sulfur isotope ratios (Δ36S/Δ33S ≠ −1) produced additional variability in sulfide Δ33S and Δ36S-values in a 3.52 Ga hydrothermal barite deposit at Londozi, Barberton Greenstone Belt, Swaziland. In situ measurement of the four stable sulfur isotopes in pyrite revealed Δ36S–Δ33S relationships and a Δ36S/Δ33S trend (−3.2 ± 0.4), which is significantly different from the co-variation between Δ36S and Δ33S in the co-existing barite that reflects ambient Paleoarchean seawater sulfate. This argues against biological or thermochemical sulfate reduction at the time of barite deposition, and requires incorporation of sulfide generated in a chemically distinct atmosphere before 3.52 Ga. We propose a model that combines reworking of this sulfur by hydrothermal leaching, deep mixing with juvenile sulfur and surface mixing with biogenic sulfide to explain the observed variation in δ34S, Δ33S and Δ36S. These interactions between abiotic and biological processes in the Londozi hydrothermal system complicate the interpretation of biosignatures based on deviations in Δ33S and Δ36S from the Archean reference array.

Many active volcanoes host a crater lake that receives input from magmatic-hydrothermal fluids. The chemical properties of such a lake are to a large extent determined by the injection of these fluids. The chemistry of the water can therefore provide insight in the transport of chemical elements from the sources to the surface. Monitoring the changing conditions can be an important tool in assessing the dynamic changes in subsurface parts of the system. Interpretations of lake compositions are not straightforward and rely on detailed insights into the processes involved in controlling the presence and concentration of the various chemical species involved. This study focuses on a group of trace elements that are known to be highly mobile in volcanic environments, but have rarely been investigated as yet. Based on available analytical data, it uses the active crater lake of Poás volcano in Costa Rica as an example to explore the behavior of B, Zn, As, Mo, Sn, Sb, Tl and Pb, and to identify possible sources and sinks in volcanic lake systems. Each of these elements show enrichment relative to magnesium, arguing against congruent rock dissolution, but consistent with input through subaqueous fumarolic gas. Time-series trends indicate increased concentrations when the lake system was most active. There is a general correlation between concentration levels and lake temperature, and an inverse correlation with pH. This points to an enhanced input of the elements during intervals of increased volcanic activity. Additionally, during periods of lower volcanic activity, when conduits below the lake were presumably blocked, lower concentration levels were found. Still, this group of very mobile elements remains relatively more enriched compared to elements that predominantly partition into brine water, suggesting that in these low-activity intervals gas/vapor transport was less affected by reduced permeability than the input of hydrous fluid. Though the time-series trends for

We performed a series of experiments at high pressures and temperatures to determine the partitioning of a wide range of trace elements between ilmenite (Ilm), armalcolite (Arm) and anhydrous lunar silicate melt, to constrain geochemical models of the formation of titanium-rich melts in the Moon. Experiments were performed in graphite-lined platinum capsules at pressures and temperatures ranging from 1.1 to 2.3GPa and 1300-1400°C using a synthetic Ti-enriched Apollo 'black glass' composition in the CaO-FeO-MgO-Al

The ~533 Ma Venetia Diamond Mine is located between the Kaapvaal and Zimbabwe Cratons and the study of selected xenoliths provides the opportunity to investigate the temporal evolution of the sub-continental lithospheric mantle (SCLM) underneath southern

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Over the last several hundred years, Stromboli has been characterized by steady-state Strombolian activity. The volcanic products are dominated by degassed and highly porphyritic (HP-magma) black scoria bombs, lapilli and lava flows of basaltic shoshonitic composition. Periodically (about one to three events per year), more energetic explosive eruptions also eject light coloured volatile-rich pumices with low phenocryst content (LP-magma) that have more mafic compositions than the HP-magma. An in situ major and trace element and Sr isotope microanalysis study is presented on four samples chosen to characterize the different modes of activity at Stromboli: a lava flow (1985-1986 effusive event), a scoria bomb from the 'normal' present-day activity of Stromboli (April 1984), and a scoria and coeval pumice sample from a recent more explosive eruption (September 1996). Plagioclase (An

This paper reports the results from a second characterisation of the 91500 zircon, including data from electron probe microanalysis, laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS) and laser