Volcanically hosted venting with indications of ultramafic influence at Aurora hydrothermal field on Gakkel Ridge

Abstract

The Aurora hydrothermal system, Arctic Ocean, hosts active submarine venting within an extensive field of relict mineral deposits. Here we show the site is associated with a neovolcanic mound located within the Gakkel Ridge rift-valley floor, but deep-tow camera and sidescan surveys reveal the site to be ≥100 m across-unusually large for a volcanically hosted vent on a slow-spreading ridge and more comparable to tectonically hosted systems that require large time-integrated heat-fluxes to form. The hydrothermal plume emanating from Aurora exhibits much higher dissolved CH₄/Mn values than typical basalt-hosted hydrothermal systems and, instead, closely resembles those of high-temperature ultramafic-influenced vents at slow-spreading ridges. We hypothesize that deep-penetrating fluid circulation may have sustained the prolonged venting evident at the Aurora hydrothermal field with a hydrothermal convection cell that can access ultramafic lithologies underlying anomalously thin ocean crust at this ultraslow spreading ridge setting. Our findings have implications for ultra-slow ridge cooling, global marine mineral distributions, and the diversity of geologic settings that can host abiotic organic synthesis - pertinent to the search for life beyond Earth.

Fig. 1. Location map for the Aurora seamount, Gakkel Ridge.

a Location of the Aurora hydrothermal field (black star), together with eight additional hydrothermal plume sources (white stars) identified along the Gakkel Ridge during the 2001 Arctic Mid-Ocean Ridge Exploration (AMORE) expedition. Figure made with GeoMapApp (www.geomapapp.org) using data from the International Bathymetric Chart of the Arctic Ocean. b Multibeam bathymetric map showing tracklines for three high resolution deep-tow surveys (OFOBS-04, −07 and −08) that passed directly over the Aurora vent site (black star). Colored lines denote tracklines for each deployment, chevrons indicate direction of travel. The multibeam bathymetry displayed here was collected from the RV Kronprins Håkon in 2019 and gridded at 15 m. Black rectangle delineates the area of sidescan sonar survey shown in Fig. 2.

Fig. 2. Results from high resolution deep-tow (OFOBS) surveys of the seafloor at Aurora hydrothermal field.

a High-frequency sidescan sonar image of the seafloor crossing the Aurora vent-field, Gakkel Ridge from the OFOBS-07 survey. b Corresponding geological map based on sidescan interpretation coupled with deep-tow camera ground-truthing (dive tracks are in the same colors as Fig. 1, arrows show direction of travel). Observations of extinct sulfides are located as white triangles; observations of active fluid flow are located as red triangles. The ridge along which the vents are distributed is shown as a solid line with tick marks to indicate the downhill direction in (a) and (b) and the rectangle bracketing the confirmed hydrothermal field in both panels measures 50 m x 100 m. Note, however, that the southern limit to the Aurora field, along-ridge, remains unconstrained by camera ground-truthing. c Relict sulfides could be identified in photographs in multiple forms, all present in this image (from top left to lower right): upright, extinct chimneys; fallen sub-cylindrical chimney structures; and massive sulfide, often with distinctive red-orange oxidation weathering. d Active vent-sites could occasionally be identified from oblique photographs, as in this example, or more commonly from buoyant plumes of black billowing smoke that engulfed the cameras when the instrument passed more directly over a vent-source. e Schlieren effects from shimmering water together with pale, smooth-textured patches allowed us to identify lower-temperature hydrothermal flow and associated (putative) microbial mats. Scale bars in panels c, d, e: 1 m.

Fig. 3. Plots of Aurora hydrothermal plume composition.

a Location map for the trajectories of 11 CTD deployments over the Aurora mound, completed in 2014 projected over multibeam data from 2019 that is gridded at 20 m. Each CTD deployment was selected based on interpretation and prediction of ice-floe trajectories that dictated the direction of travel. Cumulative depth profiles are shown for b δ³He anomalies, c dissolved CH₄ concentrations and d dissolved Mn concentrations, as measured in water samples collected during the CTD casts. Shading illustrates the depth range (2800–3700 m) of the non-buoyant hydrothermal plume.

Fig. 4. Plot of dissolved CH4 concentrations vs dissolved Mn concentrations for the Aurora hydrothermal plume (solid circles) compared with the like-for-like trend (red line) for the Rainbow hydrothermal plume.

Ranges of CH₄:Mn ratios for high temperature (≥300 °C) vent-fluids at sites hosted in mafic rocks on slow and ultra-slow spreading ridges (Snakepit, TAG, Broken Spur and Piccard) are shown in pale orange (data from^,,). Corresponding ranges for high temperature (≥300 °C) vents influenced by ultramafic rocks (Rainbow, Logatchev I, Logatchev II, Ashadze I, Ashadze II) are shown in pale blue (data from^,,). Also shown, in purple, are the much higher CH₄:Mn values for systems influenced by terrigenous continental sediments of the kind reported at Aurora (ref. and reference. therein). The dashed orange line (Lucky Strike Vents) and dashed blue line (Kairei Vents) indicate two vent-systems that depart from the general case, each of which is discussed in detail in the text.

Fig. 5. Changes in basalt CaO/Al2O3 compositions along the Western Volcanic Zone (WVZ) of the Gakkel Ridge between the Lena Trough and the Sparsely Magmatic Zone (SMZ).

Lower ratios towards either end of the WVZ compared to its center can be attributed to (i) lower degrees of partial melting of the mantle causing less clinopyroxene to contribute CaO to the melt and resulting in reduced melt production, and/or (ii) high-pressure crystallization of clinopyroxene, leading to CaO depletion in the melt, within a thick lithosphere. In either case, the data support a restricted thickness of volcanic crust overlying the lithospheric mantle at the Aurora site in the western WVZ (location denoted by vertical arrow). Yellow stars: this study. Gray circles: glass analyses downloaded from the PetDB Petrology DataBase (earthchem.org/petdb).