2. Wochenbericht MSM75

02.07.2018 - 08.07.201

1. Wochenbericht MSM75

29.06.2018 – 01.07.201

2. Wochenbericht SO237

22.12.14 - 28.12.1

Geochemistry of volcanic rocks from the Woodlark Basin

EGU2011-12864 The Woodlark Basin east of Papua New Guinea represents one of the few places on Earth where a spreading axis propagates into continental crust. This special tectonic setting allows insights into the evolution of magma composition as continental extension and break-up changes to the formation of ocean crust. We report here geochemical results on samples collected in 2009 from the four segments closest to the continental breakup, from segment 1 which abuts the detachment fault responsible for continental extension on Moresby Seamount in the West, to segment 4, representing mature oceanic crust in the East. A total of 208 glass samples have been analyzed for their major (EMPA) and trace element (LA-ICPMS) compositions. The data show strong E-W variations. Samples ranging from tholeiitic basalt and basaltic andesite to andesite and rhyolite are found on Segment 1. They have generally high alkali values and a wide range of trace element contents and ratios. Segments 2 to 4 magmas in contrast only comprise tholeiitic basalt with lower alkali contents and a more restricted range of trace element chemistry. The geochemical differences between the segments cannot be attributed to differentiation processes alone, and different sources are required. High Ba/La, (La/Sm)N, Rb/Sr, and Th/La on Segment 1 suggest a derivation from an enriched mantle source, while low Nd/Pb and Nb/U suggest that some of the enrichment may also reflect the influence of continental crust during magma genesis. Whether this continental signature is present in the form of recycled material in the mantle or as rafted continental blocks in the axial region is at present unclear. In contrast to rocks from segment 1, trace element compositions of volcanic glasses from segments 2 to 4 show a stronger MORB signature, presumably reflecting more mature spreading in this part of the basin. The influence of continental material appears to be minimal, suggesting that uncontaminated asthenosphere quickly flows into the rift and/or that continental blocks are not retained in the axial region for long time periods following the rifting-spreading transition

Computer modelling studies of mackinawite, greigite and cubic FeS

In this thesis we develop rigorous theoretical models for the simulation of the iron sulfides mackinawite, greigite and cubic FeS using both ab initio and interatomic potential methods. The mineral mackinawite (tetragonal FeS) takes a layered PbO-type structure, with Fe atoms coordinated tetrahedrally to S ligands. We have used GGA+U calculations to show that the inter-layer interaction is very difficult to accurately describe using this form of DFT, and instead a single-layer formulism is developed which allows the modelling of the electronic and magnetic properties of a single layer of mackinawite. These results are used to derive an interatomic potential to investigate the surfaces of this phase, and we use the calculated surface energies to successfully reproduce the observed crystal morphology of mackinawite. The effect of impurity atoms in the interlayer sites is investigated, and it is found that these contribute considerably to the stabilisation of the mackinawite structure. Greigite (Fe3S4) is the iron sulfide analogue of the famous iron oxide magnetite. We use spin-polarised GGA+U calculations to model the magnetic and electronic structure of greigite, and this phase is found to be most accurately described using an applied Ueff value of 1 eV. Further calculations show that a Verwey-type low temperature transition in greigite is energetically unfavourable. Cubic FeS takes the cubic sphalerite structure at room temperature. A low temperature transition to an antiferromagnetic orthorhombic structure has been observed experimentally. GGA+U calculations demonstrate that applying a value for the Hubbard Ueff parameter of 2 eV provides an excellent description of both the lowand high-temperature structures. It is found that the previously derived potential for mackinawite predicts the cubic FeS structure as well as non-spin-polarised GGA. The work described in this thesis has provided a greater understanding of the electronic, magnetic and structural properties of these iron sulfides

1. Wochenbericht SO237

14.12.14 - 21.12.1

3. Wochenbericht MSM75

09.07.2018 - 15.07.201