ALBERT

Description: (19.03.17‐26.03.17)

Description: In 2013, high-temperature vent fluids were sampled in the Nifonea vent field. This field is located within the caldera of a large shield-type volcano of the Vate Trough, a young extensional rift in the New Hebrides back-arc. Hydrothermal venting occurs as clear and black smoker fluids with temperatures up to 368 °C, the hottest temperatures measured so far in the western Pacific. The physico-chemical conditions place the fluids within the two-phase field of NaCl–H2O, and venting is dominated by vapour phase fluids with Cl concentrations as low as 25 mM. The fluid composition, which differs between the individual vent sites, is interpreted to reflect the specific geochemical fluid signature of a hydrothermal system in its initial, post-eruptive stage. The strong Cl depletion is accompanied by low alkali/Cl ratios compared to more evolved hydrothermal systems, and very high Fe/Cl ratios. The concentrations of REY (180 nM) and As (21 μM) in the most Cl-depleted fluid are among the highest reported so far for submarine hydrothermal fluids, whereas the inter-element REY fractionation is only minor. The fluid signature, which has been described here for the first time in a back-arc setting, is controlled by fast fluid passage through basaltic volcanic rocks, with extremely high water-rock ratios and only limited water-rock exchange, phase separation and segregation, and (at least) two-component fluid mixing. Metals and metalloids are unexpectedly mobile in the vapour phase fluids, and the strong enrichments of Fe, REY, and As highlight the metal transport capacity of low-salinity, low-density vapours at the specific physico-chemical conditions at Nifonea. One possible scenario is that the fluids boiled before the separated vapour phase continued to react with fresh glassy lavas. The mobilization of metals is likely to occur by leaching from fresh glass and grain boundaries and is supported by the high water/rock ratios. The enrichment of B and As is further controlled by their high volatility, whereas the strong enrichment of REY is also a consequence of the elevated concentrations in the host rocks. However, a direct contribution of metals such as As from magmatic degassing cannot be ruled out. The different fluid end-member composition of individual vent sites could be explained by mixing of vapour phase fluids with another fluid phase of different water/rock interaction history.

Description: The potential of mining seafloor massive sulfide deposits for metals such as Cu, Zn, and Au is currently debated. One key challenge is to predict where the largest deposits worth mining might form, which in turn requires understanding the pattern of subseafloor hydrothermal mass and energy transport. Numerical models of heat and fluid flow are applied to illustrate the important role of fault zone properties (permeability and width) in controlling mass accumulation at hydrothermal vents at slow spreading ridges. We combine modeled mass-flow rates, vent temperatures, and vent field dimensions with the known fluid chemistry at the fault-controlled Logatchev 1 hydrothermal field of the Mid-Atlantic Ridge. We predict that the 135 kilotons of SMS at this site (estimated by other studies) can have accumulated with a minimum depositional efficiency of 5% in the known duration of hydrothermal venting (58,200 year age of the deposit). In general, the most productive faults must provide an efficient fluid pathway while at the same time limit cooling due to mixing with entrained cold seawater. This balance is best met by faults that are just wide and permeable enough to control a hydrothermal plume rising through the oceanic crust. Model runs with increased basal heat input, mimicking a heat flow contribution from along-axis, lead to higher mass fluxes and vent temperatures, capable of significantly higher SMS accumulation rates. Nonsteady state conditions, such as the influence of a cooling magmatic intrusion beneath the fault zone, also can temporarily increase the mass flux while sustaining high vent temperatures.

Description: (06.03.17‐12.03.17)

Description: Highlights • The genetic model for Algoma-type BIF is modified taking into account S-MIF results. • Metal and sulfur sources are decoupled and reflect diverse microbial metabolisms. • Sulfur deposited with oxide-facies BIF is mostly atmospheric in origin. • Little juvenile sulfur is found, despite the proximity to volcanic sources. Abstract Neoarchean Algoma-type banded iron formations (BIFs) are widely viewed as direct chemical precipitates from proximal volcanic–hydrothermal vents. However, a systematic multiple sulfur isotope study of oxide-facies BIF from a type locality in the ca. 2.74 Ga Temagami greenstone belt reveals mainly bacterial turnover of atmospheric elemental sulfur in the host basin rather than deposition of hydrothermally cycled seawater sulfate or sulfur from direct volcanic input. Trace amounts of chromium reducible sulfur that were extracted for quadruple sulfur isotope (32S–33S–34S–36S) analysis record the previously known mass-independent fractionation of volcanic SO2 in the Archean atmosphere (S-MIF) and biological sulfur cycling but only minor contributions from juvenile sulfur, despite the proximity of volcanic sources. We show that the dominant bacterial metabolisms were iron reduction and sulfur disproportionation, and not sulfate reduction, consistent with limited availability of organic matter and the abundant ferric iron deposited as Fe(OH)3. That sulfur contained in the BIF was not a direct volcanic–hydrothermal input, as expected, changes the view of an important archive of the Neoarchean sulfur cycle in which the available sulfur pools were strongly decoupled and only species produced photochemically under anoxic atmospheric conditions were deposited in the BIF-forming environment.

Description: Manganese nodules, Co-rich crusts, and Seafloor massive Sulfides (SMS) are commonly seen as possible future resources that could potentially add to the global raw materials supply. At present, a proper global assessment of these resources is not possible due to a severe lack of information regarding their size, global distribution, and composition. The sizes of the most prospective areas that need to be explored for a global resource assessment are vast. Future deep-sea minerals exploration has to provide higher-resolution data and at the same time needs to cover large areas of the seafloor in a fast and cost-efficient manner. While nodules and crusts are 2-dimensional occurrences and an assessment of their distribution at the seafloor itself seems sufficient, seafloor massive sulfides are 3-dimensional sites and a proper resource assessment will always require drilling. Here the development of methods to image the subseafloor and to recognize economically interesting sites prior to drilling is of importance.

Description: Mining the deep seabed is fraught with challenges. Untapped mineral potential under the shallow, more accessible continental shelf could add a new dimension to offshore mining and help meet future mineral demand.

Description: Seafloor massive sulfide (SMS) deposits form on and just below the seafloor along submarine tectonic plate boundaries. The deposits form from seawater that circulates through the underlying crust, is heated, leaches metals and sulfur from the surrounding rock, and then ascends and vents at the seafloor, forming sulfide mineral accumulations rich in Cu, Zn, Pb, Au, and Ag. Hydrothermal circulation through the crust is driven by shallow magmatic heat sources along the plate boundaries. Although high temperature “black smoker” chimneys and the unique ecosystems that they support are the most recognizable features of these vent sites, the mineral deposits can take on a variety of forms, from individual chimneys of less than a meter tall to large mounds with diameters of several hundred meters. The description of the deposits as “massive” refers to the high proportion (typically over 60%) of sulfide minerals that make up the deposits. Other minerals that commonly occur in SMS deposits are sulfates (barite and anhydrite), amorphous silica, and clay minerals. At the time of writing, more than 500 sites of high temperature seafloor hydrothermal systems and related mineral deposits have been found of the seafloor.