Stefan Krumm

Metamorphism and deformation mechanisms in the Sierras Australes fold and thrust belt (Buenos Aires Province, Argentina)

Abstract The Sierras Australes fold and thrust belt is affected by an easternmost “anchizonal” metamorphism which grades into a western area of greenschist facies metamorphism. Both areas, and their inferred boundary, are distinguished on the basis of quartz recrystallization and illite crystallinity data. During the first deformation pressure solution and grain-boundary diffusion accompanied rigid-body rotation and grain-boundary gliding processes. These mechanisms together with intracrystalline quartz deformation and recrystallization contributed to increasing rock ductility in the area of greenschist facies metamorphism. Two deformational events, expressed by folding, cleavage formation, and thrusting, were followed by static annealing of quartz, which affects the southwestern part of the mountain range. Evidence for fluid and mass transfer also suggests convective fluid-driven heat transport as an important mechanism for the final metamorphic overprint under static conditions.

Oxygen isotopes in the Azores islands: Crustal assimilation recorded in olivine

Oxygen isotope ratios of olivine have become a widely used tool for the study of magmatic systems, especially in the interpretation of source heterogeneities in mantle plume–derived ocean island basalts. The underlying assumption is that fresh minerals provide a better guide to magma δ 18 O than bulk rock analyses and that olivine is also likely to be a major phenocryst phase in primitive magmas. However, distinctions between source compositions and the effects of subsequent magma evolution have not always been thoroughly scrutinized. For the Azores samples investigated here, we can demonstrate that the δ 18 O variation (+4.84‰ to +5.25‰ Vienna standard mean ocean water) observed in the olivine phenocryst population is closely linked to evolution in the host magmas during ascent to the surface. We observe a linear, positive correlation between forsterite (Fo) content and δ 18 O in all of the individual island lava suites. This forces us to conclude that the low oxygen isotope ratios result from combined assimilation and fractional crystallization processes, the assimilant being hydrothermally (temperature > 250 °C) altered, lower oceanic crust. Linear regression of the measured δ 18 O olivine values to Fo 89 suggests a homogeneous mantle source with δ 18 O =

Magmatic Evolution and Source Variations at the Nifonea Ridge (New Hebrides Island Arc)

The Nifonea submarine volcano rises 1000 m above the seafloor of the Vate Trough back-arc basin behind the New Hebrides island arc. This large volcanic edifice has a caldera of ∼8 km diameter and is connected to two ∼20 km long volcanic rift zones in the back-arc basin. We present new chemical and isotope data for volcanic glasses and whole-rocks from both the volcano and its rift zones. Lavas from Nifonea volcano show an evolution towards more incompatible element enrichment, with the most enriched lavas being the youngest eruption products on the caldera floor. These are products of significant fractional crystallization, show minor contamination by hydrothermal fluids (<0·3%) and reflect mixing of melts derived from depleted upper mantle and melts from an enriched source similar to those occurring in the North Fiji Basin. The enrichment in Nb of these lavas is comparable with that of some lavas from the New Hebrides island arc (e.g. Mota Lava island), where these coexist with typical island arc basalts. The lavas erupted along the rift zones in the Vate Trough back-arc basin are relatively depleted in incompatible elements, indicating melting of depleted upper mantle with a minor addition of a sediment-derived fluid. Our observations suggest that the mantle beneath Vate Trough is heterogeneous on a small scale (<20 km) and that the occurrence of these enriched and fertile mantle portions has a stronger control on melting processes than the influx from the subducting slab, as all samples were recovered at a similar distance from the trench.

Primitive andesites from the Taupo Volcanic Zone formed by magma mixing

Andesites with Mg# >45 erupted at subduction zones form either by partial melting of metasomatized mantle or by mixing and assimilation processes during melt ascent. Primitive whole rock basaltic andesites from the Pukeonake vent in the Tongariro Volcanic Centre in New Zealand’s Taupo Volcanic Zone contain olivine, clino- and orthopyroxene, and plagioclase xeno- and antecrysts in a partly glassy matrix. Glass pools interstitial between minerals and glass inclusions in clinopyroxene, orthopyroxene and plagioclase as well as matrix glasses are rhyolitic to dacitic indicating that the melts were more evolved than their andesitic bulk host rock analyses indicate. Olivine xenocrysts have high Fo contents up to 94%, δ18O(SMOW) of +5.1‰, and contain Cr-spinel inclusions, all of which imply an origin in equilibrium with primitive mantle-derived melts. Mineral zoning in olivine, clinopyroxene and plagioclase suggest that fractional crystallization occurred. Elevated O isotope ratios in clinopyroxene and glass indicate that the lavas assimilated sedimentary rocks during stagnation in the crust. Thus, the Pukeonake andesites formed by a combination of fractional crystallization, assimilation of crustal rocks, and mixing of dacite liquid with mantle-derived minerals in a complex crustal magma system. The disequilibrium textures and O isotope compositions of the minerals indicate mixing processes on timescales of less than a year prior to eruption. Similar processes may occur in other subduction zones and require careful study of the lavas to determine the origin of andesite magmas in arc volcanoes situated on continental crust.

Formation of andesite melts and Ca‐rich plagioclase in the submarine Monowai volcanic system, Kermadec arc

Andesites are typical rocks of island arcs and may either form by fractional crystallization processes or by mixing between a mafic and a felsic magma. Here we present new petrographic and geochemical data from lavas of the submarine Monowai volcanic system in the northern Kermadec island arc that display a continuous range in composition from basalt to andesite. Using petrology, major, trace, and volatile element data, we show that basaltic magmas mostly evolve to andesitic magmas by fractional crystallization. Our thermobarometric calculations indicate that the formation of the large caldera is related to eruption of basaltic-andesitic to andesitic magmas from a magma reservoir in the deeper crust. Small variations in trace element ratios between the caldera and the large active cone imply a homogeneous mantle source. Contrastingly, resurgent dome melts of the caldera stagnated at shallower depths are more depleted and show a stronger subduction input than the other edifices. The Monowai basaltic glasses contain less than 1 wt % H2O and follow typical tholeiitic fractionation trends. High-An plagioclase crystals observed in the Monowai lavas likely reflect mixing of H2O-saturated melt batches with hot and dry tholeiitic, decompression melt batches. The result is a relatively H2O-poor mafic magma at Monowai implying that partial melting of the mantle wedge is only partly due to the volatile flux and that adiabatic melting may play a significant role in the formation of the parental melts of the Monowai volcanic system and possibly other arc volcanoes.

Systematic variations in the trace element and sulphur isotope composition of pyrite with stratigraphic depth in the Skouriotissa volcanic-hosted massive sulphide deposit, Troodos ophiolite, Cyprus

Jenkin, G. R. T., Al-Bassam, A. Z.M., Harris, R. C., Abbott, A. P., Smith, D. J., Holwell, D. A., Chapman, R. J. and Stanley, C. J. 2015. The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals, Minerals Engineering, 87, 18–24. doi:10.1016/j.mineng.2015.09.026. Abbott, A. P., Harris, R. C., Holyoak, F., Frisch, G., Hartley, J. and Jenkin, G. R. T. 2015. Electrocatalytic Recovery of Elements from Complex Mixtures using Deep Eutectic Solvents, Green Chemistry, 17, 2172–2179. doi:10.1039/C4GC02246G.