The GyPSM-S (Geodynamic and Petrological Synthesis Model for Subduction) scheme couples a petrological model with a 2-D thermal and variable viscosity flow model to describe and compare fundamental processes occurring within the subduction mantle wedge, including the development of a low-viscosity channel (LVC) (Hebert et al., 2009, Earth and Planetary Science Letters, v. 278, p. 243–256). Here we supplement the basic coupled model result with more sophisticated treatments of trace element partitioning in the fluid phase and melt transport regimes. We investigate the influences of slab fluid source lithology and fluid transport mechanisms on melt geochemistry, the implications of mantle source depletion related to fluid fluxing, and potential melt migration processes. This study describes two model cases that can be compared to geochemical datasets for the Izu–Bonin intra-oceanic subduction system and the Central Costa Rican part of the Central American arc. We find that there is a progression of geochemical characteristics described in studies of cross-arc and along-arc lavas that can be approximated assuming (i) limited fluid–rock interaction within the mantle wedge and (ii) that melt migration preserves the spatial distinction among melts initiated in different areas of the wedge. Specifically, volcanic front lavas have significant contributions from shallower slab fluid sources, and rear-arc lavas have significant contributions from deeper slab fluid sources. Evidence for limited fluid–rock interaction could imply either a rapid fluid transport mechanism or a fluid-dominated trace element budget within the LVC. Although we do not include a back-arc in these models, interpretations of the results lead to several potential mechanisms to explain hydrous inputs to back-arc source regions.
Naturally occurring greenish quartz found within the context of amethyst-bearing deposits is not simply the result of the exposure of amethyst to thermal bleaching or exposure to the sun. Rather, it can represent a set of distinct color-varieties resulting from the changing chemical and thermal nature of the precipitating solution. Greenish quartz occurs at the Thunder Bay Amethyst Mine Panorama (TBAMP), Thunder Bay, Ontario, Canada, in several distinct varieties. Yellowish green quartz and dark green quartz with purple hues occur as loose detritus, and pale greenish gray quartz occurs as part of a color-gradational sequence of mineralization involving macrocrystalline quartz of other colors and chalcedony. The TBAMP system contains a number of color varieties of quartz including greenish, amethyst, colorless, and smoky. Spectroscopic, irradiation and controlled heating studies show that changes in salinity and temperature of the hydrothermal system that produced the TBAMP deposit are reflected in the changing coloration of the quartz. The greenish quartz, especially the greenish gray variety, has increased turbidity and fluid inclusions in comparison with the adjacent amethyst. Analysis of different colors on major (r = {101 1}) and minor (z = {011 1}) rhombohedral sectors within the quartz indicates that changes in the growth rate also have influenced color development. As the system evolved, two factors contributed to the color changes. A minor ferric component appears to change position from interstitial to substitutional within specific growth-sectors, and the trace-element composition of the quartz evolved. The samples from the TBAMP deposit are compared to isolated samples of greenish quartz collected from three other amethyst-bearing localities: Farm Kos and Farm Rooisand (Namibia), Kalomo-Mapatiqya (Zambia), and southern Bahia (Brazil). All included similar greenish hues with the exception of the yellowish green variety. Colors within the quartz are consistently correlated with the speciation of hydrous components. Darker green samples incorporate larger amounts of molecular H_2O than either pale greenish gray samples, colorless samples, or amethyst. The appearance of strong hydroxyl peaks in the infrared spectra is limited to amethyst and colorless varieties.
Melt focusing at mid‐ocean ridges is necessary to explain the narrowness of the zone of crustal accretion and the formation of large but localized on‐axis seamounts at slow and ultraslow spreading centers. It has been proposed that melt focusing is facilitated by the presence of a barrier to upward melt migration at the base of the thermal boundary layer (TBL). We assess the development of a melt impermeable boundary by modeling the geochemical evolution and crystallization history of melts as they rise into the TBL of mid‐ocean ridges with different spreading rates. A permeability barrier, associated with a crystallization front controlled by the conductive thermal regime, exists for melt trajectories at slow to fast spreading ridges (≥10 mm/yr half rate). The effective lateral scope of the barrier, where the slope of the barrier exceeds a critical value that allows buoyant melt transport to the axis, generally increases with spreading rate. At all distances from the axis at ultraslow ridges and off‐axis at slow spreading ridges, the weak crystallization front may prohibit formation of an efficient barrier and lead to the possibility that some fraction of melt may be incorporated into the lithospheric mantle, allowing refertilization. The protracted crystallization history and potential absence of an effective permeability barrier may explain the dearth of volcanism at ultraslow ridges and calls for a revision of lateral melt focusing scenarios at ultraslow spreading rates.
Abstract Using two‐dimensional dynamic models of the Northern Izu–Bonin (NIB) subduction zone, we show that a particular localized low‐viscosity (η LV = 3.3 × 10 19 − 4.0 × 10 20 Pa s), low‐density (Δρ ∼ −10 kg/m 3 relative to ambient mantle) geometry within the wedge is required to match surface observations of topography, gravity, and geoid anomalies. The hydration structure resulting in this low‐viscosity, low‐density geometry develops due to fluid release into the wedge within a depth interval from 150 to 350 km and is consistent with results from coupled geochemical and geodynamic modeling of the NIB subduction system and from previous uncoupled models of the wedge beneath the Japan arcs. The source of the fluids can be either subducting lithospheric serpentinite or stable hydrous phases in the wedge such as serpentine or chlorite. On the basis of this modeling, predictions can be made as to the specific low‐viscosity geometries associated with geophysical surface observables for other subduction zones based on regional subduction parameters such as subducting slab age.
Abstract Subduction‐related transport of water into the mantle has significant dynamical and geochemical implications. Dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle, potentially hydrating nominally anhydrous minerals (NAM) and impacting the viscosity and density structure of the mantle over a wide area. We present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab, focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle. A sharp decrease in water storage capacity across the lower boundary of the transition zone may produce “secondary dehydration” of hydrated NAM, leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone. Rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways. Slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle. Water stabilized in NAM and as a free fluid can initiate upwelling within and above the transition zone. A less abrupt change in water storage capacity across the base of the transition zone leads to high NAM water contents in a channel adjacent to the slab where viscosity is reduced. However, seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity.
[1] Throughout the global mid-ocean ridge system, transform faults offset spreading centers. Conductive cooling may be more efficient beneath transform faults, producing a thickened lithosphere that directs melt away from the transform. However, recent observations of thickened crust along transform faults at fast ridges suggest melt redistribution toward transforms, intra-crustal melt production, or efficient extraction of melt. We apply a 3-D model of melt migration and extraction along an oceanic transform domain bounded by ridge segments. Melt is assumed to travel vertically before collecting and migrating beneath a low-permeability boundary inclined towards the ridge axis. A melt extraction zone, which may be geologically interpreted as the presence of faults and/or dikes leading to rapid lateral and vertical melt migration toward plate boundaries, affects the pattern of crustal accretion at segmented ridges. First, we examine a generic ridge-transform-ridge geometry and then a model that represents the Siqueiros transform on the East Pacific Rise. On the basis of crustal thickness variations within the intra-transform spreading centers along the fast-slipping Siqueiros fault, we constrain the presence of a melt extraction zone within 10 km of the transform zone.
Crustal thickness variations at the ultraslow spreading 10-16°E region of the Southwest Indian Ridge are used to constrain melt migration processes.In the study area, ridge morphology correlates with the obliquity of the ridge axis with respect to the spreading direction.A long oblique "supersegment", nearly devoid of magmatism, is flanked at either end by robust magmatic centers (Joseph Mayes Seamount and Narrowgate segment) of much lesser obliquity.Plate-driven mantle flow and temperature structure are calculated in 3-D based on the observed ridge segmentation.Melt extraction is assumed to occur in three steps: (1) vertical migration out of the melting region, (2) focusing along an inclined permeability barrier, and (3) extraction when the melt enters a region shallower than ∼35 km within 5 km of the ridge axis.No crust is predicted in our model along the oblique supersegment.The formation of Joseph Mayes Seamount is consistent with an on-axis melt anomaly induced by the local orthogonal spreading.The crustal thickness anomaly at Narrowgate results from melt extracted at a tectonic damage zone as it travels along the axis toward regions of lesser obliquity.Orthogonal spreading enhances the Narrowgate crustal thickness anomaly but is not necessary for it.The lack of a residual mantle Bouguer gravity high along the oblique supersegment can be explained by deep serpentization of the upper mantle permissible by the thermal structure of this ridge segment.Buoyancy-driven upwelling and/or mantle heterogeneities are not required to explain the extreme focusing of melt in the study area.
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