Ralf Halama

A geochemical and Sr-Nd-O isotopic study of the Proterozoic Eriksfjord Basalts, Gardar Province, South Greenland: Reconstruction of an OIB signature in crustally contaminated rift-related basalts

Abstract Basalts from the volcano-sedimentaiy Eriksfjord Formation (Gardar Province, South Greenland) were erupted at around 1.2 Ga into rift-related graben structures. The basalts have compositions transitional between tholeiite and alkaline basalt with MgO contents <7 wt.% and they display LREE-enrichment relative to a chondritic source. Most of the trace element and REE characteristics are similar to those of basalts derived from OIB-Iike mantle sources. Initial 87Sr/86Sr ratios of clinopyroxene separates range from 0.70278 to 0.70383 and initial εNd values vary from -3.2 to +2.1. The most unradiogenic samples overlap with the field defined by carbonatites of similar age and can be explained by mixing of isotopically depleted and enriched mantle components. Using AFC modelling equations, the Sr-Nd isotope data of the more radiogenic basalts can successfully be modelled by addition of <5% lower crustal granulite-facies gneisses as contaminants. δ18Ov-smow values of separated clinopyroxene range from +5.2 to +6.0%o and fall within the range of typical mantle-derived rocks. However, up to 10% mixing with an average lower crustal component are permitted by the data.

Lithium and its isotopes in tourmaline as indicators of the crystallization process in the San Diego County pegmatites, California, USA

In the lithium-cesium-tantalum-type pegmatite dikes of San Diego County, California, USA, tourmaline is the main reservoir for Li, except in the cores and the pockets of the dikes where other Li-bearing minerals also occur. Tourmaline from three subhorizontal dikes was analyzed for bulk Li concentrations and Li isotope ratios. The bottom portion of each dike includes rhythmically layered aplite called line-rock. Above the aplite is the lower pegmatite zone that crystallized upward whereas the hanging pegmatite zone crystallized downward. The lower and hanging pegmatite zones are joined at the core zone. Pockets that were once fluid-filled occur in the core zone. Tourmaline in the line-rocks and the upper border zones has 22–70 ppm Li and in the pegmatite zones 53–450 ppm Li. Large tourmaline blades in the cores have 174–663 ppm Li. Elbaite rims on prismatic tourmaline in the pockets have up to 5075 ppm Li. The progressive enrichment in Li from the wall-zones to the pockets is attributed to inward fractional crystallization of the dikes. The line-rock in each dike appears to have crystallized until the melt reached fluid saturation, at which point the melt and the fluid began to unmix to form the pegmatite zones and the pockets. The estimated initial Li concentration in the magma that produced the dikes is ~ 630 ppm. At this low concentration, Li has had much smaller effect on crystallization of the dikes than H 2 O. δ 7 Li in tourmaline in the line-rocks, the cores, and the pockets ranges from +11.2 to +16.1 ‰ with no systematic difference between these textural zones. However, in radial tourmalines δ 7 Li is > 19 ‰. The very elevated δ 7 Li may reflect Li isotope fractionation between the melt and the exsolving fluid at the time of crystallization of these tourmalines, with 7 Li preferring the more strongly-bonded occupancy in the silicate melt over a hydrated ion occupancy in the fluid. Alternatively, the elevated δ 7 Li may also have been caused by preferential accumulation of the slower-diffusing 7 Li ahead of the rapidly-growing radial tourmalines. The overall elevated δ 7 Li values of the dikes may have been acquired by Li isotope exchange with wall-rocks during passage of the pegmatite melts from their sources.

Introduction to the special issue on SFB 574 “Volatiles and fluids in subduction zones: climate feedback and trigger mechanisms for natural disasters”

combine field work on land with marine cruises probing the seafloor. During the course of the SFB 574, many different aspects of subduction zone processes were investigated. Geophysical investigations identified and quantified the input of water through hydration of the bend-faulted subducting plate as far as mantle depths. This process and water release from the subducting slab deeper in the subduction zone was investigated by numerical modeling. These studies highlighted the important role of hydration and dehydration of mantle rocks for the global water cycle. Subducted oceanic fracture zones are another conduit for water transported deep into the subduction zone. Compactional and thermal dehydration of the subducted sediment layer affects the strength of interplate coupling and the depth and lateral extension of the seismogenic zone where disastrous earthquakes are generated. The fluids, largely generated by clay mineral transformation, are expelled through the forearc by splay faults. At the seafloor, the sites of cold seeps, often associated with mud volcanoes, are populated by biota that control the carbon transfer to the ocean. The most prominent manifestations of cold seeps are authigenic carbonates that form from anaerobic oxidation of methane and which serve as archives of forearc processes. The largest cold seep emissions occur at faults generated by the subduction of volcanic seamounts. Petrological analyses of ancient, exhumed subduction zone metamorphic rocks revealed the important role of pervasive, typically channelized fluids in high-pressure metamorphic reactions of dewatering subducted igneous crust and mantle. Slab-derived fluids hydrate the mantle wedge along the slab–wedge interface and form a subduction channel in which mixing of different fluids and fluid–rock interaction causes metasomatic overprinting. The element redistributions associated with the liberation of fluids from Sonderforschungsbereiche (SFBs) are a successful funding model in use by the German Science Foundation (DFG) for over 30 years to strengthen basic research first locally at universities and later also supra-regionally by including academic institutions at different cities and states. Literally translated, SFB means “special research area” that comprises research that complements but does not duplicate research at participating institutions and departments. The English terminology used by the DFG is “Collaborative Research Centre,” which better describes the expected approach by emphasizing collaboration and interdisciplinary efforts in such a way that the overall result is better than the sum of individual results. The SFB 574 had united more than 70 scientists with expertise in structural geology, geophysics, sedimentology, geochemistry, empirical and experimental petrology, volcanology, and biology for 11 years (2001–2012). The overarching theme addressed the role of volatiles in subduction zone tectonic, hydrological, metamorphic and magmatic processes, and resulting hazards. The main areas of research were the subduction zones of Central America and southern Chile. Both extend across shorelines from deepsea trenches to arc-volcano summits and thus required to

Vesuvianite in high-pressure-metamorphosed oceanic lithosphere (Raspas Complex, Ecuador) and its role for transport of water and trace elements in subduction zones

Metamorphosed, vesuvianite-bearing dykes occur in serpentinised peridotites of the Raspas Complex (Ecuador), which represents a piece of oceanic lithosphere that has experienced high-pressure, subduction-related metamorphism. The serpentinite mantle protoliths are geochemically indistinguishable from modern oceanic lithosphere entering subduction zones. Positive Eu anomalies (Eu/Eu* = 1.3-7.2) and relative LREE enrichments (LaN/SmN = 1.2-5.5) point to hydrothermal alteration of the peridotite precursor rocks at or near the seafloor. Major mineral phases in the metamorphosed dykes include chlorite, diopside, amphibole and vesuvianite. In each dyke, only two of these phases − either amphibole + vesuvianite, diopside + chlorite, or amphibole + chlorite dominate the modal mineralogy with >∼90 vol.%, suggesting metasomatic replacement at elevated P-T conditions during subduction, controlled by an external fluid. This fluid caused the decrease in coexisting mineral phases and overprinting of initial Sr isotope ratios (0.7025-0.7031). Preserved geochemical signatures from the dyke protoliths, including positive Eu anomalies (Eu/Eu* = 1.2-2.0) and Na enrichment due to spilitisation, reveal that the dykes originated as oceanic olivine gabbros and troctolites.Vesuvianite in the Raspas Complex formed by hydration and silica removal from gabbroic mineral assemblages during subduction. It has a wide stability in P-T space for hydrated and silica deficient bulk compositions so that it potentially represents a significant repository for the cycling of elements during subduction. In addition to Ca, Mg and Al, incorporation of significant amounts of Ti, Fe and Na (up to 2.4, 1.7 and 1.6 atoms per formula unit, respectively) in vesuvianite bears evidence for the potential of vesuvianite as petrogenetic indicator, although lack of relevant thermodynamic and experimental data precludes the extraction of quantitative information. For cold subduction zones in particular, vesuvianite appears to be able to carry significant amounts of water to mantle depths. Preferential incorporation of HREE (up to 2.2 ppm Yb), Sr (up to ∼ 300 ppm) and Pb (up to 4.5 ppm) in vesuvianite underlines its potentially important role for the storage, transport and release of these key elements in radiogenic isotope geochemistry during subduction zone cycling

Fluid-induced breakdown of white mica controls nitrogen transfer during fluid–rock interaction in subduction zones

ABSTRACT In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ15N = + 2.1 to +3.6‰). Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.

Mineral dissolution and reprecipitation mediated by an amorphous phase

Fluid-mediated mineral dissolution and reprecipitation processes are the most common mineral reaction mechanism in the solid Earth and are fundamental for the Earth’s internal dynamics. Element exchange during such mineral reactions is commonly thought to occur via aqueous solutions with the mineral solubility in the coexisting fluid being a rate limiting factor. Here we show in high-pressure/low temperature rocks that element transfer during mineral dissolution and reprecipitation can occur in an alkali-Al–Si-rich amorphous material that forms directly by depolymerization of the crystal lattice and is thermodynamically decoupled from aqueous solutions. Depolymerization starts along grain boundaries and crystal lattice defects that serve as element exchange pathways and are sites of porosity formation. The resulting amorphous material occupies large volumes in an interconnected porosity network. Precipitation of product minerals occurs directly by repolymerization of the amorphous material at the product surface. This mechanism allows for significantly higher element transport and mineral reaction rates than aqueous solutions with major implications for the role of mineral reactions in the dynamic Earth.Fluid-mediated mineral dissolution is a key mechanism for mineral reactions in the Earth. Here, the authors show that element transport during mineral dissolution and reprecipitation reactions can be mediated by an amorphous phase, which can contain significant amounts of metals.

Rb-Sr and in situ 40Ar/39Ar dating of exhumation-related shearing and fluid-induced recrystallization in the Sesia zone (Western Alps, Italy)

Ralf Halama1,2, Johannes Glodny3, Matthias Konrad-Schmolke2,4, and Masafumi Sudo2 1School of Geography, Geology and the Environment, Keele University, Keele ST5 5BG, UK 2Institute of Earth and Environmental Science, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany 3GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany 4Department of Earth Sciences, University of Gothenburg, Guldhedsgatan 5a, 40530 Gothenburg, Sweden GEOSPHERE