Changes in groundwater chemistry have been proposed as earthquake precursors. These include changes in radon count rates 1-2 , concentrations of dissolved elements 3-5 and stable isotope ratios 4-5. Other proposed precursors include changes in seismic wave velocities 6 , water levels in boreholes 7 , micro-seismicity 8 and shear wave splitting 9. These phenomena have often been attributed to rock

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Hydrogeochemical monitoring of a basalt-hosted aquifer, which contains Ice Age meteoric water and is situated at 1220 m below sea level in the Tjö rnes Fracture Zone, northern Iceland, has been ongoing since July 2002. Based on hydrogeochemical changes following an earthquake of magnitude (M w ) 5.8 on 16 September 2002, we constrained the timescales of post-seismic fault sealing and water-rock interaction. We interpret that the earthquake ruptured a hydrological barrier, permitting a rapid influx of chemically and isotopically distinct Ice Age meteoric water from a second aquifer. During the two subsequent years, we monitored a chemical and isotopic recovery towards pre-earthquake aquifer compositions, which we interpret to have been mainly facilitated by fault-sealing processes. This recovery was interrupted in November 2004 by a second rupturing event, which was probably induced by two minor earthquakes and which reopened the pathway to the second aquifer. We conclude that the timescale of fault sealing was approximately 2 years and that the approach to isotopic equilibrium (from global meteoric water line) was approximately 18% after >10 4 years.

Iron is the most important metal for modern industry and Sweden is by far the largest iron-producer in Europe, yet the genesis of Sweden's main iron-source, the ‘Kiruna-type’ apatite-iron-oxide ores, remains enigmatic. We show that magnetites from the largest central Swedish ‘Kiruna-type’ deposit at Grängesberg have δ18O values between −0.4 and +3.7‰, while the 1.90−1.88 Ga meta-volcanic host rocks have δ18O values between +4.9 and +9‰. Over 90% of the magnetite data are consistent with direct precipitation from intermediate to felsic magmas or magmatic fluids at high-temperature (δ18Omgt > +0.9‰, i.e. ortho-magmatic). A smaller group of magnetites (δ18Omgt ≤ +0.9‰), in turn, equilibrated with high-δ18O, likely meteoric, hydrothermal fluids at low temperatures. The central Swedish ‘Kiruna-type’ ores thus formed dominantly through magmatic iron-oxide precipitation within a larger volcanic superstructure, while local hydrothermal activity resulted from low-temperature fluid circulation in the shallower parts of this system.

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Eclogites, blueschists and greenschists are found in close proximity to one another along a 1-km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2-1.9 GPa and 410-530°C followed, at 43-38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5-2.1 GPa and 520-580°C. We explain co-existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10-100-m and veins on a scale of 1-10-cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11-16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high-P nappe stack consisting of belts in which high-P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension. K E Y W O R D Sbulk composition, Cyclades Blueschist Unit, HP-LT metamorphism, metamorphic fluids, Syros

[1] Drilling results from the Integrated Ocean Drilling Program's Arctic Coring Expedition (ACEX) to the Lomonosov Ridge (LR) document a 26 million year hiatus that separates freshwater-influenced biosilica-rich deposits of the middle Eocene from fossil-poor glaciomarine silty clays of the early Miocene. Detailed micropaleontological and sedimentological data from sediments surrounding this mid-Cenozoic hiatus describe a shallow water setting for the LR, a finding that conflicts with predrilling seismic predictions and an initial postcruise assessment of its subsidence history that assumed smooth thermally controlled subsidence following rifting. A review of Cenozoic tectonic processes affecting the geodynamic evolution of the central Arctic Ocean highlights a prolonged phase of basin-wide compression that ended in the early Miocene. The coincidence in timing between the end of compression and the start of rapid early Miocene subsidence provides a compelling link between these observations and similarly accounts for the shallow water setting that persisted more than 30 million years after rifting ended. However, for much of the late Paleogene and early Neogene, tectonic reconstructions of the Arctic Ocean describe a landlocked basin, adding additional uncertainty to reconstructions of paleodepth estimates as the magnitude of regional sea level variations remains unknown.

Geothermobarometric and geochronological work indicates a complete Eocene/ early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure-temperature (P-T) method of THERMOCALC coupled with detailed textural work, we separate an early blueschist facies event at 576 AE 16 to 619 AE 32°C and 15.5 AE 0.5 to 16.3 AE 0.9 kbar from a subsequent greenschist facies overprint at 384 AE 30°C and 3.8 AE 1.1 kbar. Multi-mineral Rb-Sr isochron dating yields crystallization ages for near peak-pressure blueschist facies assemblages between 40.5 AE 1.0 and 38.3 AE 0.5 Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high-P rocks from south Naxos were exhumed to upper mid-crustal levels in the late Eocene and early Oligocene at rates of 7.4 AE 4.6 km/Ma, completing a full blueschist-/greenschist facies metamorphic cycle soon after subduction within c. 8 Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid-controlled metamorphic re-equilibration, and is unrelated to the strong high-T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.