Karson Jeffrey A.
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PreprintMagnetic exploration of a low-temperature ultramafic-hosted hydrothermal site (Lost City, 30°N, MAR)( 2016-12) Szitkar, Florent ; Tivey, Maurice A. ; Kelley, Deborah S. ; Karson, Jeffrey A. ; Fruh-Green, Gretchen L. ; Denny, Alden R.A 2003 high-resolution magnetic survey conducted by the Autonomous Underwater Vehicle ABE over the low-temperature, ultramafic-hosted hydrothermal field Lost City reveals a weak positive magnetic anomaly. This observation is in direct contrast to recent observations of strong positive magnetic anomalies documented over the high-temperature ultramafic-hosted hydrothermal vents fields Rainbow and Ashadze, which indicates that temperature may control the production of magnetization at these sites. The Lost City survey provides a unique opportunity to study a field that is, to date, one of a kind, and is an end member of ultramafic-hosted hydrothermal systems. Our results highlight the key contribution of temperature on magnetite production resulting from serpentinization reactions. Whereas high temperature promotes significant production and partitioning of iron into magnetite, low temperature favors iron partitioning into various alteration phases, resulting in a magnetite-poor rock. Moreover, the distribution of magnetic anomalies confirms results of a previous geological survey indicating the progressive migration of hydrothermal activity upslope. These discoveries contribute to the results of 25 years of magnetic exploration of a wide range of hydrothermal sites, from low- to high-temperature and from basalt- to ultramafic-hosted, and thereby validate using high-resolution magnetics as a crucial parameter for locating and characterizing hydrothermal sites hosting unique chemosynthetic-based ecosystems and potentially mineral-rich deposits.
ArticleDetachment shear zone of the Atlantis Massif core complex, Mid-Atlantic Ridge, 30°N(American Geophysical Union, 2006-06-21) Karson, Jeffrey A. ; Fruh-Green, Gretchen L. ; Kelley, Deborah S. ; Williams, E. A. ; Yoerger, Dana R. ; Jakuba, Michael V.Near-bottom investigations of the cross section of the Atlantis Massif exposed in a major tectonic escarpment provide an unprecedented view of the internal structure of the footwall domain of this oceanic core complex. Integrated direct observations, sampling, photogeology, and imaging define a mylonitic, low-angle detachment shear zone (DSZ) along the crest of the massif. The shear zone may project beneath the nearby, corrugated upper surface of the massif. The DSZ and related structures are inferred to be responsible for the unroofing of upper mantle peridotites and lower crustal gabbroic rocks by extreme, localized tectonic extension during seafloor spreading over the past 2 m.y. The DSZ is characterized by strongly foliated to mylonitic serpentinites and talc-amphibole schists. It is about 100 m thick and can be traced continuously for at least 3 km in the tectonic transport direction. The DSZ foliation arches over the top of the massif in a convex-upward trajectory mimicking the morphology of the top of the massif. Kinematic indicators show consistent top-to-east (toward the MAR axis) tectonic transport directions. Foliated DSZ rocks grade structurally downward into more massive basement rocks that lack a pervasive outcrop-scale foliation. The DSZ and underlying basement rocks are cut by discrete, anastomosing, normal-slip, shear zones. Widely spaced, steeply dipping, normal faults cut all the older structures and localize serpentinization-driven hydrothermal outflow at the Lost City Hydrothermal Field. A thin (few meters) sequence of sedimentary breccias grading upward into pelagic limestones directly overlies the DSZ and may record a history of progressive rotation of the shear zone from a moderately dipping attitude into its present, gently dipping orientation during lateral spreading and uplift.
ArticleTemporal and spatial variability in the composition of lavas exposed along the Western Blanco Transform Fault(American Geophysical Union, 2005-11-15) Pollock, Meagen A. ; Klein, Emily M. ; Karson, Jeffrey A. ; Tivey, Maurice A.The northern scarp of the western Blanco Transform (BT) fault zone provides a "tectonic window" into crust generated at an intermediate-rate spreading center, exposing a ~2000 m vertical section of lavas and dikes. The lava unit was sampled by submersible during the Blancovin dive program in 1995, recovering a total of 61 samples over vertical distances of ~1000 m and a lateral extent of ~13 km. Major elements analyses of 40 whole rock samples exhibit typical tholeiitic fractionation trends of increasing FeO*, Na2O, and TiO2 and decreasing Al2O3 and CaO with decreasing MgO. The lava suite shows a considerable range in extent of crystallization, including primitive samples (Mg# 64) and evolved FeTi basalts (FeO>12%;TiO2>2%). Based on rare earth element and trace element data, all of the lavas are incompatible-element depleted normal mid-ocean ridge basalts (N-MORB;La/SmN<1). The geochemical systematics suggest that the lavas were derived from a slightly heterogeneous mantle source, and crystallization occurred in a magmatic regime of relatively low magma flux and/or high cooling rate, consistent with magmatic processes occurring along the present-day southern Cleft Segment. The BT scarp reveals the oceanic crust in two-dimensional space, allowing us to explore temporal and spatial relationships in the horizontal and vertical directions. As a whole, the data do not appear to form regular spatial trends; rather, primitive lavas tend to cluster shallower and toward the center of the study area, while more evolved lavas are present deeper and toward the west and east. Considered within a model for construction of the upper crust, these findings suggest that the upper lavas along the BT scarp may have been emplaced off-axis, either by extensive off-axis flow or off-axis eruption, while the lower lavas represent axial flows that have subsided with time. A calculation based on an isochron model for construction of the upper crust suggests that the Cleft Segment requires at least ~50 ka to build the lower extrusive section, consistent to first order with independent estimates for the construction of intermediate-spreading rate crust.
ArticleExploration of the Northern Guaymas Basin(The Oceanography Society, 2018-03) Soule, Samuel A. ; Seewald, Jeffrey S. ; Wankel, Scott D. ; Michel, Anna P. M. ; Beinart, Roxanne A. ; Escobar Briones, Elva ; Morales Dominguez, Esmerelda ; Girguis, Peter R. ; Coleman, Dwight ; Raineault, Nicole A. ; Wagner, Jamie K.S. ; Foulk, Aubrey ; Bagla, Anshika ; Karson, Jeffrey A.