Scientific objectives and overview of own research

One major aim of geochemistry is the evaluation of inputs and sinks that control ocean’s chemistry today and through time. Main sources and sinks for most of the elements relate to the riverine and hydrothermal fluxes into seawater, carbonate precipitation, and the alteration of the oceanic crust. Serving as a crucial part for the geochemical cycles for many elements it is thus important to understand and quantify hydrothermal circulation (Edmond et al., 1979; Von Damm, 1985; Palmer and Edmond, 1989). However, the impact of hydrothermal circulation on the composition of seawater is hard to estimate because of large uncertainties in the heat and water Figure 6: Tectonic map of the New Hebrides back-arc. The back-arc rifts are indicated in purple and spreading centers in blue. The active hydrothermal vent field at Nifonea volcano is indicated by the yellow star. Modified from Anderson et al.

(2016). DER = d’Entrecasteaux Ridge

| 15 fluxes. Vent systems transfer approximately 11 TW of heat to the ocean, which is about 25 to 30% of the total heat loss of the earth (Stein and Stein, 1994; Jaupart and Mareschal, 2003). The estimates on the global discharge of hydrothermal vent fluids are in the order of 3.5 * 10¹² kg/year for high temperature vent fluids (Elderfield and Schultz, 1996). To estimate the hydrothermal element fluxes into the ocean, the broad chemical range of hydrothermal vent fluids needs to be investigated and the processes and variables controlling the fluid chemistry need to be understood.

The fact that it is often speculated about the processes, which affect the vent fluid’s chemistry, manifests that the current knowledge on how to quantify and unravel the processes which occur during hydrothermal circulation and particularly their effect on single components in vent fluids and on the alteration of the oceanic crust is still rather incomplete. Individual factors have been studied intensively in experimental studies (e.g. Foustoukos and Seyfried, 2007) and natural systems (e.g. Edmond et al., 1982, James et al., 1995; Von Damm, 1995). Many processes during hydrothermal circulation have already been quantified. However, experimental setups cannot simulate the complexity of natural hydrothermal vent sites, which makes it challenging to apply experimental data on natural systems.

It has long been stated that vent fluids from MOR settings control the ocean chemistry and hence, most studies on natural submarine hydrothermal systems were conducted along the global MOR systems. However, vent fluids from arc and back-arc settings have a higher variability and some of their components (e.g. Al, B, and CO2) are considerably enriched relative to MOR fluids (Craddock et al., 2010, Mottl et al., 2011, de Ronde and Stucker, 2015). Still there is only limited data for vent fluids from the subduction-influenced settings. Equally important in this context is not only the limited dataset of vent fluids from arc and back-arc basins, but as well the question, which processes might lead to the high variability in these fluids. Case studies help to expand the datasets, which can be used to understand vent fluid compositions and to estimate hydrothermal fluxes in the ocean.

Vent fluids, which were chosen for the PhD project, are from two different back-arc basins in the Western Pacific (Manus Basin, New Hebrides back-arc). Vent fluids in these settings cover a wide compositional range (Craddock et al., 2010; Reeves et al., 2011, Seewald et al., 2015;

Schmidt et al., 2017). To understand and unravel the different processes (magmatic input, phase separation, water-rock interaction) and their influence on the vent fluid’s composition, the focus of this dissertation will be on radiogenic strontium (Sr) isotope ratios and “non-traditional” stable metal isotopes (lithium (Li), boron (B) and magnesium (Mg) isotope ratios). These isotopes were analysed in hydrothermal vent fluids and samples from fresh and altered volcanic crust.

The high variability of vent fluids from Manus Basin and the New Hebrides back-arc allow a detailed study on fluids, which are influenced by different sources (different host rock

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compositions, altered versus fresh oceanic crust, magmatic gases) and processes (water-rock interaction at different temperatures and different fluid acidities, phase separation and segregation). Due to their different properties (e.g. fluid mobility, volatility, ionic radii) (Chapter 1.2), the examined isotope systems are affected differently during hydrothermal circulation.

Consequently, the main objective of this thesis is to identify and understand the individual influences on Sr, Li, B and Mg isotopes during hydrothermal circulation in back-arc basins. This may help to broaden the knowledge on the individual processes during hydrothermal circulation in general and in arc and back-arc settings in specific.

1.4.2 Overview of own research

The research goals of this PhD project, which were outlined in chapter 1.4.1, were addressed within three individual manuscripts (Chapter 3-5). All manuscripts and figures within were written and designed by myself. Furthermore, all Li, B and Mg isotope ratios and most of the Sr isotope ratios in the hydrothermal fluids and volcanic rocks, which are presented in these manuscripts, were prepared and analysed by myself. Eoghan Reeves and Wolfgang Bach provided additional datasets (main and trace elements, Sr isotope ratios) for rocks and fluids from the Manus Basin. All co-authors contributed to the manuscripts through productive discussions, suggestions and comments on the interpretation of the data. The following section briefly summarizes the outcomes of each manuscript with respect to the before mentioned research goals.

Chapter 3: The influence of magmatic fluids and phase separation on B systematics in submarine hydrothermal vent fluids – case studies from the Manus Basin and Nifonea volcano

F. Wilckens, E. P. Reeves, W. Bach, A. Meixner, J. S. Seewald, A. Koschinsky, S. A. Kasemann This manuscript aims to unravel the influences of B concentrations and isotope ratios in a broad range of vent fluids. The study reveals that B in most fluids is influenced by water-rock interaction with fresh and altered oceanic crust and hence might be a good proxy for accessing water-rock interaction during hydrothermal circulation and the basement alteration in the hydrothermal circulation cell. However, the vapour-rich fluids from the Manus Basin and Nifonea volcano deviate from this trend. We found that phase separation at PT-condition well above the two-phase curve may lead to an enrichment of B in the vapour-rich fluids. Because the low B isotope ratios in these fluids apparently do not relate to phase separation, we suggest that they rather reveal evidence for a preferential mobilisation of B from the oceanic crust during interaction with vapour-rich fluids with long residence times in the subsurface. In contrast, fluids with shorter residence times in the subsurface define a trend towards lower B concentrations and might reflect a mixing between hydrothermal fluids and a magmatic fluid rising from the subsurface.

| 17 Chapter 4: Lithium isotope ratios in submarine hydrothermal vent fluids from Manus Basin and Nifonea volcano reveal evidence for negligible Li isotope fractionation during water-rock interaction

F. K. Wilckens, W. Bach, A. Mexiner, E.P. Reeves, J. S. Seewald, A. Koschinsky, S. A.

Kasemann

This manuscript reports on Li isotope systematics in the vent fluids and discusses Li isotope fractionation during water-rock interaction at hydrothermal conditions. The data shows that Li isotope ratios in vent fluids from back-arc basins differ from vent fluids at MOR settings. δ⁷Li values are on average about 4‰ lower at back-arc basins, although the composition of the oceanic crust is similar. The manuscript discusses potential reasons for the isotopic offset. Further, the data shows that Li in fluids, which are affected by extreme boiling, is depleted in the vapour-rich phase. In general, the data opens the questions whether leaching of Li from the oceanic crust differs from MOR settings and whether the estimated Li isotopic composition of the global hydrothermal Li flux is valid.

Chapter 5: Assessing water-rock interaction and basement alteration from B, Mg, Li and Sr isotopes in acid-sulfate fluids

F. Wilckens, W. Bach, A. Meixner, J. S. Seewald, S.A. Kasemann

The last manuscript presents data of Li, B, Mg and Sr isotope ratios in acid-sulfate fluids. Mg isotope ratios show that Mg in these fluids has not an isotopic signature of the oceanic crust, but is mostly seawater derived. Nevertheless, the combination of Sr, Li and B isotopes show that acid-sulfate fluids are influenced to a significant amount by water-rock interaction. Especially Li and B deviate in their leaching behaviour during argillic alteration. Li appears to be depleted fast in the oceanic crust, whereas B appears to be leached more slowly. The combination of Li and B isotope ratios might thus help to assess the alteration of the oceanic crust in these extreme acidic environments in order to estimate the alteration period during which the fluids have reacted with the host rocks.