Wasser-Gesteins-Wechselwirkungen in hydrothermalen Systemen bei Fluid-Gas- Gleichgewichtsbedingungen: eine experimentelle Studie bei Site 1309
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
Very high metal and rare-earth element (REE) concentrations with unusual (‘atypical’) normalized REE patterns are documented in fluids from active hydrothermal vent fields on the Mid-Atlantic Ridge 5°S (MAR 5°S) and the East Scotia Ridge. Those fluids show relative enrichment of middle heavy REEs and almost no Eu anomalies in chondrite-normalized patterns. Given the fact that many sections of slow-spreading ridges are at water depths greater than 3000m, the submarine hydrothermalism at 5°S on the Mid Atlantic Ridge with extreme fluxes of heat, metals and unique rare earth element chemistry is an example with general significance for oceanic element budgets. However, transport-reaction processes in these high-pressure (p) / high-temperature (T) hydrothermal systems are still poorly constrained. We proposed (1) experiments under p-T-conditions similar to those at MAR 5°S for the study of seawater-rock interaction and resulting element partitioning as well as (2) of time-resolved rock alteration and generation of secondary minerals and (3) subsequent modelling of time-integrated changes of fluid and rock chemistry. Thus, we ran series of experiments, basically in which natural bottom seawater or synthetic aqueous solutions (NaCl, NaCl-MgCl2 or NaCl-CaCl2) were reacted with gabbro and gabbro mineral assemblages from 300 to 475°C and 40 and 100MPa and some series with natural bottom seawater and unaltered basalt. The p-T conditions in all of the experiments were representative for water-rock interactions in hydrothermal root and discharge zones. In addition, fluid flux variability and kinetics were addressed in the experiments by varying the water-to-rock mass ratios (w/r) from 0.5 to 10 and using different run durations from 3 to 720h. Our experimental results showed that only seawater and synthetic MgCl2-bearing fluid mobilized significant amounts of REEs, Si, Ca, Fe, and Mn from gabbro, from clinopyroxene, and from plagioclase. At 425°C and 40MPa, fluids were initially acidic with pH (25°C) of 2 increasing to values between 4 and 7 upon progressing reactions. Rare earth element and Fe concentrations peaked within 3-6h after interaction with gabbroic mineral grains (125-500lm) at w/r of 5 (REEs) and 2-5 (Fe) but decreased with continuing reaction without strong REE fractionation. Most of the REEs that were leached from primary minerals and dissolved in the fluids became redeposited into solid reaction products after 720h. Contents of dissolved SiO2 were shown to be pressure-dependent, being about twofold higher at 100MPa than at 40MPa (425°C), and were below quartz saturation with gabbro and clinopyroxene as solid starting material but close to quartz saturation with plagioclase reactant. However, Si in fluids from the rock-dominated experiments at 100MPa with gabbro (w/r 0.5-1) dropped to very low concentrations. A concomitant decrease in chlorinity suggests that these changes may be due to the breakdown of olivine and the formation of serpentine and Fe-hydroxy chlorides. Regardless of the starting solid reactants, fluid REE patterns seem to be dominantly controlled by w/r. Atypical fluid REE patterns and very high fluid REE contents were obtained at high w/r (≥5). Whereas typical REE patterns known from many mid-ocean ridge vent fluids, showing relative enrichments of light REEs and a positive Eu anomaly, were obtained at low w/r of 0.5-1. Hence, our results clearly show that REE contents and patterns of vent fluids are sensitive to variations in the w/r. In summary, our experimentally derived water-rock partitioning behaviour of transition metals and REE under sub- and supercritical conditions as well as thermodynamic modelling added to our knowledge of seawater-rock interaction processes and improved our understanding of transport reactions and element fluxes during ascent in hydrothermal systems that run at extreme temperatures and great water depths.
Projektbezogene Publikationen (Auswahl)
- (2012) Hydrothermal alteration of basalt by seawater and resulting formation of secondary minerals – An electron microprobe study. In: Minerals as Advanced Materials II (ed. S.V. Krivovitchev), pp. 61-79, Springer publisher (Berlin, Heidelberg)
Kusebauch Ch, Holzheid A, Garbe-Schönberg C-D
(Siehe online unter https://doi.org/10.1007/978-3-642-20018-2_7) - (2012) Seawater-MORB interaction beyond the critical point of seawater: An experimental approach. 14th International Conference on Experimental Mineralogy, Petrology and Geochemistry - EMPG XIV, Kiel
Beermann O, Garbe-Schönberg D, Holzheid A, Kusebauch C
- (2012) The chemistry of seawater-like fluids reacted with gabbro at conditions below and above the critical point of seawater. 1st European Mineralogical Conference EMC² 2012, Frankfurt/Main
Beermann O, Garbe-Schönberg D, Holzheid A, Kusebauch C
- (2014) Generation of high reactive fluids by rapid clinopyroxene-seawater interaction: An experimental study at 425°C, 40 and 100MPa. EGU General Assembly 2014, Vienna, Austria, Geophysical Research Abstracts, Vol. 16, EGU 2014
Beermann O, Garbe-Schönberg D, Schächinger S, Arzi L, Holzheid A
- (2017) On the origins of very high Fe and REE concentrations in some black smoker fluids: Evidence from experiments with gabbro up to 475°C, 100MPa. – PACON Conference, Zhoushan, China
Garbe-Schönberg D, Beermann O, Bach W
- (2017) Time-resolved interaction of seawater with gabbro: An experimental study up to 475°C, 100MPa. Geochimica et Cosmochimica Acta 197, 167-192
Beermann O, Garbe-Schönberg D, Bach W, Holzheid A
(Siehe online unter https://doi.org/10.1016/j.gca.2016.10.016)
