International Ocean Discovery Program Scientific Prospectus
Portions of this work may have been published in whole or in part in other IODP documents or publ... more Portions of this work may have been published in whole or in part in other IODP documents or publications. This IODP Scientific Prospectus is based on precruise JOIDES Resolution Facility advisory panel discussions and scientific input from the designated Co-Chief Scientists on behalf of the drilling proponents. During the course of the cruise, actual site operations may indicate to the Co-Chief Scientists, the Expedition Project Manager/Staff Scientist, and the Operations Superintendent that it would be scientifically or operationally advantageous to amend the plan detailed in this prospectus. It should be understood that any substantial changes to the science deliverables outlined in the plan presented here are contingent upon the approval of the IODP JRSO Director and/or JOIDES Resolution Facility Board.
International Ocean Discovery Program (IODP) Expedition 399 collected new cores from the Atlantis... more International Ocean Discovery Program (IODP) Expedition 399 collected new cores from the Atlantis Massif (30°N; Mid-Atlantic Ridge), an oceanic core complex that hosts the Lost City hydrothermal field (LCHF). Studies of the Atlantis Massif and the LCHF have transformed our understanding of tectonic, magmatic, hydrothermal, and microbial processes at slow-spreading ridges. The Atlantis Massif was the site of four previous expeditions (Integrated Ocean Drilling Program Expeditions 304, 305, and 340T and IODP Expedition 357) and numerous dredging and submersible expeditions. The deepest IODP hole in young (<2 My) oceanic lithosphere, Hole U1309D, was drilled ~5 km north of the LCHF and reached 1415 meters below seafloor (mbsf ) through a series of primitive gabbroic rocks. A series of 17 shallow (<16.4 mbsf ) holes were also drilled at 9 sites across the south wall of the massif during Expedition 357, recovering heterogeneous rock types including hydrothermally altered peridotites, gabbroic, and basaltic rocks. The hydrologic regime differs between the two locations, with a low permeability conductive regime in Hole U1309D and a high (and possibly deep-reaching) permeability regime along the southern wall.
International Ocean Discovery Program expedition publications, May 2, 2016
Finally, although not an objective of the expedition, we were serendipitously able to generate a ... more Finally, although not an objective of the expedition, we were serendipitously able to generate a high-resolution (20 m per pixel) multibeam bathymetry map across the entire Atlantis Massif and the nearby fracture zone, Mid-Atlantic Ridge, and eastern conjugate, taking advantage of weather and operational downtime. This will assist science party members in evaluating and interpreting tectonic and mass-wasting processes at Atlantis Massif.
Supplemental data were provided by the authors and may not conform to IODP publication formats. E... more Supplemental data were provided by the authors and may not conform to IODP publication formats. ESO expedition photos are the property of IODP and are in the public domain. Some core photographs have been tonally enhanced to better illustrate particular features of interest. High-resolution images are available upon request. Cover photograph shows a unit of talc-amphibole-chlorite schist overlying a unit of serpentinized dunite with sheared zones of talc-tremolite metasomatism (Core 357-M0072B-6R-1). Photo credit: IODP ESO.
Overall, 73 sites were drilled with core recovery from 63 (Figures 7 and 8). A total of 320 seabe... more Overall, 73 sites were drilled with core recovery from 63 (Figures 7 and 8). A total of 320 seabed landings were made and seabed photographs taken. On a further 87 attempted landings the drill rig fell over and had to be picked up again. The drill operated successfully at its maximum design depth of 4500 m. The BGS BRIDGE drill performance was very successful over a sustained period and the system has definitely progressed from prototype to a working system. A07_CORE_VCDs Pdf files showing digital images of cores, lithological units, curation information and brief summary description. Locations, orientations, seafloor frame grabs etc. are also given. A legend for the VCDs is included.
Vatiation of crustal thicknes, i.e. Moho depth, can be calculated by examining variations of grav... more Vatiation of crustal thicknes, i.e. Moho depth, can be calculated by examining variations of gravity in different reference levels. In this model, we assume that the sediment layer is very thin, hence its gravity attraction is negligible. [4a] Free Air Anomaly (g): gravity anomaly from the observation point, i.e. water level. FAA [4b] Mantle Bouguer Anomaly (g): gravity anomalies obtained by subtracting from the g MBA FAA the attraction of the topography (removing effects from upward continued bathymetry) and of an assumed homogenous 6 km thick crust (as a first approach of Moho depth). The anomalies here no longer mimics the sea floor, but negative anomalies depicting the cooling of the oceanic lithosphere are still quite dominant. [4c] Residual Mantle Bouguer Anomaly (g): gravity anomalies obtained by subtracting RMBA from the g the attraction from the cooling of the lithosphere (g), obtained from Section 3. MBA T The anomalies are no longer depicting the ridge, hence the only attraction that is left would serve as an approach of to observe the crustal thickness variation. [4d] Residual Crustal Thickness (RCT): the vertical difference between the assumed homogenous 6 km thick crust to the 'real' depth of the Moho. This is obtained by adding the inverted g signals to the assumed crustal depth of 6 km. The depth of the Moho itself will act RMBA as on of the most important horizon in the model. Based on this result, we can see that the detachment fault zone tend to have thinner crust than the magmatic zone. Alteration from one mode to another can still be seen, especially north of Atlantis FZ. Conclusions Ÿ Thinner crust is identifiable in detachment mode of spreading. In this area, it is ~1 km thinner than in magmatic mode. This finding is consistent with previous published works on the differentiation between one spreading mode to another. Ÿ The dips of the magnetic anomalies in detachment mode of spreading tend to be more anisotropic than in magmatic mode. Blobby textures with unclear magnetic reversals are expected, in contrast with consistent and symmetrical textures observed in magmatic mode. Ÿ Gravity response of the model coincides well with slightly higher amplitude on peaks. These are depicted as the model is still being built 2-dimensionally, thus the lateral effect are still there. This amplitude difference is not to be expected in the 3-dimensional models. Ÿ Lateral changes, both/either in magnetic remanence and/or susceptibility need to be included in the physical model in order to have the expected magnetic response. Further Works Ÿ Test if the findings are still consistent in other area(s) which has similar data quality, followed by testing in area(s) which only consist of global/satellite-derived data. Ÿ Test models in area(s) in which seismic survey(s) has/have been carried out. Ÿ Construct 3-dimensional models. Based on our observation in [5a-5c], we found that magnetic signatures do not always behave similarly where OCCs are in place. We hypothize that positive Magnetic Anomaly (MA) is found around active OCC(s) while negative MA is found around inactive one(s). We also characterize the different types of spreading by examining the dip anisotropy, i.e. the directional components. In detachment fault mode [5d-5e], the magnetic anomaly (stripes) is scattered, blobby, with reversals that are not clearly seen. The polar histograms also depict the undulation in the directions other than the original WNW-ESE dip. While in magmatic mode [5f-5g], the stripes have more consistent texture with clearly seen reversals. Both polar histograms depict mostly the WNW-ESE dip with minor undulations in other directions.
The Lost City hydrothermal field (LCHF) is hosted in serpentinite at the crest of the Atlantis Ma... more The Lost City hydrothermal field (LCHF) is hosted in serpentinite at the crest of the Atlantis Massif, an oceanic core complex close to the mid-Atlantic Ridge. It is remarkable for its longevity and for venting low-temperature (40-91°C) alkaline fluids rich in hydrogen and methane. IODP Hole U1309D, 5 km north of the LCHF, penetrated 1415 m of gabbroic rocks and contains a near-conductive thermal gradient close to 100°C km À1. This is remarkable so close to an active hydrothermal field. We present hydrothermal modelling using a topographic profile through the vent field and IODP site U1309. Long-lived circulation with vent temperatures similar to the LCHF can be sustained at moderate permeabilities of 10 À14 to 10 À15 m 2 with a basal heatflow of 0.22 W m À2. Seafloor topography is an important control, with vents tending to form and remain in higher topography. Models with a uniform permeability throughout the Massif cannot simultaneously maintain circulation at the LCHF and the near-conductive gradient in the borehole, where permeabilities <10 À16 m 2 are required. A steeply dipping permeability discontinuity between the LCHF and the drill hole is required to stabilize venting at the summit of the massif by creating a lateral conductive boundary layer. The discontinuity needs to be close to the vent site, supporting previous inferences that high permeability is most likely produced by faulting related to the transform fault. Rapid increases in modelled fluid temperatures with depth beneath the vent agree with previous estimates of reaction temperature based on geochemical modelling.
I. ABSTRACT In recent years there has been a profound rise in appreciation of the importance that... more I. ABSTRACT In recent years there has been a profound rise in appreciation of the importance that exposure of mantle material, and resultant reactions with seawater to produce serpentinite play as fundamental, and central Earth processes. Serpentinization profoundly affects the rheology, gravity, and seismic structure of ocean crust formed by slow-spreading. Because magnetite is a product of serpentinization, hydration of olivine-rich rocks has a major effect on the magnetic signature of the ocean floor. The geochemical consequences of serpentinization are also important to sea floor studies. Serpentinite-hosted hydrothermal vents are common along slow and ultraslow spreading ridges. These vents host unique microbiological ecosystems supported by hydrogen and methane released during peridotite- seawater interactions. Serpentinization is also common away from mid-ocean ridges, in fore-arc regions and at rifted continental margins. Serpentine formation and dehydration plays a dominant...
International Ocean Discovery Program Scientific Prospectus
Portions of this work may have been published in whole or in part in other IODP documents or publ... more Portions of this work may have been published in whole or in part in other IODP documents or publications. This IODP Scientific Prospectus is based on precruise JOIDES Resolution Facility advisory panel discussions and scientific input from the designated Co-Chief Scientists on behalf of the drilling proponents. During the course of the cruise, actual site operations may indicate to the Co-Chief Scientists, the Expedition Project Manager/Staff Scientist, and the Operations Superintendent that it would be scientifically or operationally advantageous to amend the plan detailed in this prospectus. It should be understood that any substantial changes to the science deliverables outlined in the plan presented here are contingent upon the approval of the IODP JRSO Director and/or JOIDES Resolution Facility Board.
International Ocean Discovery Program (IODP) Expedition 399 collected new cores from the Atlantis... more International Ocean Discovery Program (IODP) Expedition 399 collected new cores from the Atlantis Massif (30°N; Mid-Atlantic Ridge), an oceanic core complex that hosts the Lost City hydrothermal field (LCHF). Studies of the Atlantis Massif and the LCHF have transformed our understanding of tectonic, magmatic, hydrothermal, and microbial processes at slow-spreading ridges. The Atlantis Massif was the site of four previous expeditions (Integrated Ocean Drilling Program Expeditions 304, 305, and 340T and IODP Expedition 357) and numerous dredging and submersible expeditions. The deepest IODP hole in young (<2 My) oceanic lithosphere, Hole U1309D, was drilled ~5 km north of the LCHF and reached 1415 meters below seafloor (mbsf ) through a series of primitive gabbroic rocks. A series of 17 shallow (<16.4 mbsf ) holes were also drilled at 9 sites across the south wall of the massif during Expedition 357, recovering heterogeneous rock types including hydrothermally altered peridotites, gabbroic, and basaltic rocks. The hydrologic regime differs between the two locations, with a low permeability conductive regime in Hole U1309D and a high (and possibly deep-reaching) permeability regime along the southern wall.
International Ocean Discovery Program expedition publications, May 2, 2016
Finally, although not an objective of the expedition, we were serendipitously able to generate a ... more Finally, although not an objective of the expedition, we were serendipitously able to generate a high-resolution (20 m per pixel) multibeam bathymetry map across the entire Atlantis Massif and the nearby fracture zone, Mid-Atlantic Ridge, and eastern conjugate, taking advantage of weather and operational downtime. This will assist science party members in evaluating and interpreting tectonic and mass-wasting processes at Atlantis Massif.
Supplemental data were provided by the authors and may not conform to IODP publication formats. E... more Supplemental data were provided by the authors and may not conform to IODP publication formats. ESO expedition photos are the property of IODP and are in the public domain. Some core photographs have been tonally enhanced to better illustrate particular features of interest. High-resolution images are available upon request. Cover photograph shows a unit of talc-amphibole-chlorite schist overlying a unit of serpentinized dunite with sheared zones of talc-tremolite metasomatism (Core 357-M0072B-6R-1). Photo credit: IODP ESO.
Overall, 73 sites were drilled with core recovery from 63 (Figures 7 and 8). A total of 320 seabe... more Overall, 73 sites were drilled with core recovery from 63 (Figures 7 and 8). A total of 320 seabed landings were made and seabed photographs taken. On a further 87 attempted landings the drill rig fell over and had to be picked up again. The drill operated successfully at its maximum design depth of 4500 m. The BGS BRIDGE drill performance was very successful over a sustained period and the system has definitely progressed from prototype to a working system. A07_CORE_VCDs Pdf files showing digital images of cores, lithological units, curation information and brief summary description. Locations, orientations, seafloor frame grabs etc. are also given. A legend for the VCDs is included.
Vatiation of crustal thicknes, i.e. Moho depth, can be calculated by examining variations of grav... more Vatiation of crustal thicknes, i.e. Moho depth, can be calculated by examining variations of gravity in different reference levels. In this model, we assume that the sediment layer is very thin, hence its gravity attraction is negligible. [4a] Free Air Anomaly (g): gravity anomaly from the observation point, i.e. water level. FAA [4b] Mantle Bouguer Anomaly (g): gravity anomalies obtained by subtracting from the g MBA FAA the attraction of the topography (removing effects from upward continued bathymetry) and of an assumed homogenous 6 km thick crust (as a first approach of Moho depth). The anomalies here no longer mimics the sea floor, but negative anomalies depicting the cooling of the oceanic lithosphere are still quite dominant. [4c] Residual Mantle Bouguer Anomaly (g): gravity anomalies obtained by subtracting RMBA from the g the attraction from the cooling of the lithosphere (g), obtained from Section 3. MBA T The anomalies are no longer depicting the ridge, hence the only attraction that is left would serve as an approach of to observe the crustal thickness variation. [4d] Residual Crustal Thickness (RCT): the vertical difference between the assumed homogenous 6 km thick crust to the 'real' depth of the Moho. This is obtained by adding the inverted g signals to the assumed crustal depth of 6 km. The depth of the Moho itself will act RMBA as on of the most important horizon in the model. Based on this result, we can see that the detachment fault zone tend to have thinner crust than the magmatic zone. Alteration from one mode to another can still be seen, especially north of Atlantis FZ. Conclusions Ÿ Thinner crust is identifiable in detachment mode of spreading. In this area, it is ~1 km thinner than in magmatic mode. This finding is consistent with previous published works on the differentiation between one spreading mode to another. Ÿ The dips of the magnetic anomalies in detachment mode of spreading tend to be more anisotropic than in magmatic mode. Blobby textures with unclear magnetic reversals are expected, in contrast with consistent and symmetrical textures observed in magmatic mode. Ÿ Gravity response of the model coincides well with slightly higher amplitude on peaks. These are depicted as the model is still being built 2-dimensionally, thus the lateral effect are still there. This amplitude difference is not to be expected in the 3-dimensional models. Ÿ Lateral changes, both/either in magnetic remanence and/or susceptibility need to be included in the physical model in order to have the expected magnetic response. Further Works Ÿ Test if the findings are still consistent in other area(s) which has similar data quality, followed by testing in area(s) which only consist of global/satellite-derived data. Ÿ Test models in area(s) in which seismic survey(s) has/have been carried out. Ÿ Construct 3-dimensional models. Based on our observation in [5a-5c], we found that magnetic signatures do not always behave similarly where OCCs are in place. We hypothize that positive Magnetic Anomaly (MA) is found around active OCC(s) while negative MA is found around inactive one(s). We also characterize the different types of spreading by examining the dip anisotropy, i.e. the directional components. In detachment fault mode [5d-5e], the magnetic anomaly (stripes) is scattered, blobby, with reversals that are not clearly seen. The polar histograms also depict the undulation in the directions other than the original WNW-ESE dip. While in magmatic mode [5f-5g], the stripes have more consistent texture with clearly seen reversals. Both polar histograms depict mostly the WNW-ESE dip with minor undulations in other directions.
The Lost City hydrothermal field (LCHF) is hosted in serpentinite at the crest of the Atlantis Ma... more The Lost City hydrothermal field (LCHF) is hosted in serpentinite at the crest of the Atlantis Massif, an oceanic core complex close to the mid-Atlantic Ridge. It is remarkable for its longevity and for venting low-temperature (40-91°C) alkaline fluids rich in hydrogen and methane. IODP Hole U1309D, 5 km north of the LCHF, penetrated 1415 m of gabbroic rocks and contains a near-conductive thermal gradient close to 100°C km À1. This is remarkable so close to an active hydrothermal field. We present hydrothermal modelling using a topographic profile through the vent field and IODP site U1309. Long-lived circulation with vent temperatures similar to the LCHF can be sustained at moderate permeabilities of 10 À14 to 10 À15 m 2 with a basal heatflow of 0.22 W m À2. Seafloor topography is an important control, with vents tending to form and remain in higher topography. Models with a uniform permeability throughout the Massif cannot simultaneously maintain circulation at the LCHF and the near-conductive gradient in the borehole, where permeabilities <10 À16 m 2 are required. A steeply dipping permeability discontinuity between the LCHF and the drill hole is required to stabilize venting at the summit of the massif by creating a lateral conductive boundary layer. The discontinuity needs to be close to the vent site, supporting previous inferences that high permeability is most likely produced by faulting related to the transform fault. Rapid increases in modelled fluid temperatures with depth beneath the vent agree with previous estimates of reaction temperature based on geochemical modelling.
I. ABSTRACT In recent years there has been a profound rise in appreciation of the importance that... more I. ABSTRACT In recent years there has been a profound rise in appreciation of the importance that exposure of mantle material, and resultant reactions with seawater to produce serpentinite play as fundamental, and central Earth processes. Serpentinization profoundly affects the rheology, gravity, and seismic structure of ocean crust formed by slow-spreading. Because magnetite is a product of serpentinization, hydration of olivine-rich rocks has a major effect on the magnetic signature of the ocean floor. The geochemical consequences of serpentinization are also important to sea floor studies. Serpentinite-hosted hydrothermal vents are common along slow and ultraslow spreading ridges. These vents host unique microbiological ecosystems supported by hydrogen and methane released during peridotite- seawater interactions. Serpentinization is also common away from mid-ocean ridges, in fore-arc regions and at rifted continental margins. Serpentine formation and dehydration plays a dominant...
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