Geological setting of high-temperature hydrothermal activity and massive sulphide formation on fast- and slow-spreading ridges

Krasnov, S. G.; Poroshina, Irina; Cherkashev, G. A.

doi:10.1144/gsl.sp.1995.087.01.03

Cited by 18 publications

(14 citation statements)

References 26 publications

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“…Additionally, the magnetic field lows are long, continuous features, and at 17°28 0 S, over 100 m wide and over 2 km long. These dimensions are quite different from those of hydrothermal fields (see Haymon [1996], Embley et al [1998], andKrasnov et al [1995] for summaries).…”

Section: Dominant Contributions To the Axial Magnetic Field Lowmentioning

confidence: 99%

Episodic dike swarms inferred from near‐bottom magnetic anomaly maps at the southern East Pacific Rise

et al. 2003

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[1] Near-bottom, high-resolution magnetic field data gathered at the southern East Pacific Rise near 17°28 0 S, 18°14 0 S, and 18°37 0 S, using the autonomous underwater vehicle Autonomous Benthic Explorer (ABE) echo various geologic structures, including void space within lobate caverns, recent pillow mounds, and hydrothermal vent activity. This study is focused on a magnetic field low extending several kilometers along axis, coincident with a trough created by the draining of a lava lake during a highly effusive fissure eruption at 17°28 0 S. Similar lows are observed at three other drained lava lake troughs, including one which is at least 1800 years old, residing 400 m away from the present-day axis. We attribute these lows to the presence of shallow dike swarms. The degree to which other geologic features may contribute to the lows is constrained using geologic, geophysical, and geochemical observations and forward modeling. Compositional analyses of Alvin samples at 17°28 0 S do not support Fe or Ti variations as a primary source. Hypotheses requiring hydrothermal alteration and porosity variations are both inconsistent with geologic observations and near-bottom gravity data analysis from similar areas. Previous mappings between paleointensity variations and the observed magnetic field over distances of several kilometers from the axis suggest that such variations do not create the field low. The dominant source of the magnetization low is most likely the presence of a 100-200 m wide region of shallow dikes which are poorly magnetized relative to extrusives, or a region heated above magnetic blocking or Curie temperatures by intrusions during the most recent eruption (though the latter interpretation cannot explain the low at the fossil trough). In the first case, this extrusive thinning implies a change in eruptive behavior over the last 750-1500 years given the local spreading rate. For the latter case, thermal models suggest the anomaly had to have been created by a dike swarms totaling at least 45 m width during the most recent eruption(s), corresponding to $300 years of plate spreading. Models indicate that the source of the low is centered slightly east of the axial trough. This offset suggests that the axis has been progressively migrating westward over the past millennium, consistent with other studies covering greater length and timescales. Westward migration provides an explanation for the preferential emplacement of recent lavas flows west of the axis, evident in ABE bathymetry and submersible observations.

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Section: Dominant Contributions To the Axial Magnetic Field Lowmentioning

confidence: 99%

Episodic dike swarms inferred from near‐bottom magnetic anomaly maps at the southern East Pacific Rise

et al. 2003

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“…Early evidence for the importance of ultramafic rock‐hosted systems in the global hydrothermal budget was the discovery of widespread methane anomalies in plume surveys along the northern MAR [ Charlou and Donval , 1993; Charlou et al , 1998]. Recent discoveries of high‐temperature serpentinite‐hosted hydrothermal systems on the MAR at 14.45°N [ Krasnov et al , 1995; Sudarikov and Roumiantsev , 2000] and at 36°14'N [ German et al , 1996; Douville et al , 2002], as well as the moderate‐temperature (40°–75°C) Lost City vent site at 30°N [ Kelley et al , 2001] confirm that such systems encompass a broad range of hydrothermal environments. Preliminary studies at these vent sites indicate pronounced differences in fluid chemistry and mineralogy when compared to basalt‐hosted hydrothermal systems [ Douville et al , 2002; Krasnov et al , 1995].…”

Section: Introductionmentioning

confidence: 99%

Discovery of ancient and active hydrothermal systems along the ultra‐slow spreading Southwest Indian Ridge 10°–16°E

Bach

Banerjee

Dick

et al. 2002

Geochem Geophys Geosyst

113

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[1] We report the discovery of active and fossil hydrothermal systems during R/V Knorr Cruise 162, Legs VII and IX along a 400 km long segment of the ultra-slow Southwest Indian Ridge (SWIR) between 10 and 16°E, where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date (8.4 mm yr À1 full rate). Eight of forty-one optical/temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others. Fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias, four deposits ofsepiolite and silica, as well as Mn-oxide and nontronite cemented breccias. The massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep-seated normal fault bounding the southern rift valley wall. Silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low-to moderate-temperature ultramafic-hosted hydrothermal activity. The abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates, and hence the magmatic heat input, along this section of the SWIR are lower than on any other explored segment of the global mid-ocean ridge system. This observation suggests that high mantle upwelling and magma supply rates are not required to drive mid-ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra-slow end of the ridge spreading spectrum. The frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation, with hydrothermal vent sites being preferentially associated with long-lived faults that provide fluid pathways. Our results suggest the contribution of magma-starved ridge segments to the global ocean-lithosphere geochemical budget is potentially much larger than previously thought.

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“…Alternatively evidence has been found for along-axis magma penetration from the central part of a rift segment to the tip, resulting in a lateral shift of hydrothermal activity with time [e.g. [42]]. …”

Section: Implications For Hydrothermal Activity Within the Damage Zonementioning

confidence: 99%

Numerical modelling of non-transform discontinuity geometry: Implications for ridge structure, volcano-tectonic fabric development and hydrothermal activity at segment ends

Tyler

Bull

Parson

et al. 2007

Earth and Planetary Science Letters

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Ocean ridge discontinuities partition and offset spreading centres at a range of scales. Large scale discontinuities (10's-100's km) are synonymous with first-order transform faults, which have well defined linear fault zone valleys. In contrast, NonTransform Discontinuities (NTDs) are diffuse, smaller scale offsets (0 to b 20 km), characterised by central basins or topographic highs. The geometry of NTD offsets can be categorised by the sense of offset, either right-stepping or left-stepping, and by the relative positions of the segment tips. The segment tip configurations include under-lapping, over-lapping or simple across-axis jumps or stepping in the ridge axis. In this study finite difference software is used to model segment geometry at a slow-spreading ridge under a normal tensile-stress within a homogeneous and isotropic medium. Along-and across-axis segment separations were varied incrementally for left-and right-stepping senses. The results show that the ratio of along-axis to across-axis segment tip separation is a dominant control of stress field rotation within an NTD. Features which most clearly show rotation within an NTD include basins and tectonically controlled constructional ridges. The obliquity of these features along with measurements of the surrounding fault fabrics are used as a way of observing and determining stress rotations within NTDs along the Central Indian Ridge (CIR). These rotations were used to obtain segment geometries from models where the central tensor showed an equivalent rotation. The results show that geometry has a profound effect on stress field rotation under which large-and small-scale volcanotectonic fabrics form. In addition, a shortfall of the predicted model tip relative to interpreted positions, along with morphology and observation of the ridge fabrics at the terminations to some segments, suggests the existence of a zone, broadly analogous to the process zone observed in fracture mechanics, which we call a damage zone. Given the criteria for the promotion of hydrothermal circulation, this damage zone would have a greater potential for hosting hydrothermal activity.

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Geological setting of high-temperature hydrothermal activity and massive sulphide formation on fast- and slow-spreading ridges

Cited by 18 publications

References 26 publications

Episodic dike swarms inferred from near‐bottom magnetic anomaly maps at the southern East Pacific Rise

Episodic dike swarms inferred from near‐bottom magnetic anomaly maps at the southern East Pacific Rise

Discovery of ancient and active hydrothermal systems along the ultra‐slow spreading Southwest Indian Ridge 10°–16°E

Numerical modelling of non-transform discontinuity geometry: Implications for ridge structure, volcano-tectonic fabric development and hydrothermal activity at segment ends

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