Diapirs as the source of the sediment signature in arc lavas

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 4 (2011): 641-646, doi:10.1038/ngeo1214Many arc lavas show evidence for the involvement of subducted sediment in the melting process. There is debate whether this “sediment melt” signature forms at relatively low temperature near the fluid-saturated solidus or at higher temperature beyond the breakdown of trace-element-rich accessory minerals. We present new geochemical data from high- to ultrahigh-pressure rocks that underwent subduction and show no significant depletion of key trace elements in the sediment melt component until peak metamorphic temperatures exceeded ~1050ºC from 2.7 to 5 GPa. These temperatures are higher than for the top of the subducting plate at similar pressures based on thermal models. To address this discrepancy, we use instability calculations for a non-Newtonian buoyant layer in a viscous half-space to show that, in typical subduction zones, solid-state sediment diapirs initiate at temperatures between 500–850ºC. Based on these calculations, we propose that the sediment melt component in arc magmas is produced by high degrees of dehydration melting in buoyant diapirs of metasediment that detach from the slab and rise into the hot mantle wedge. Efficient recycling of sediments into the wedge by this mechanism will alter volatile fluxes into the deep mantle compared to estimates based solely on devolatilization of the slab.Funding for this work was provided by NSF and WHOI’s Deep Ocean Exploration Institute

Reconstruction of the Talkeetna intraoceanic arc of Alaska through thermobarometry

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): B03204, doi:10.1029/2007JB005208.The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper mantle through the volcanic carapace is well exposed. We reconstruct the vertical section of the Talkeetna arc by determining the (re)crystallization pressures at various structural levels. The thermobarometry shows that the tonalites and quartz diorites intruded at ∼5–9 km into a volcanic section estimated from stratigraphy to be 7 km thick. The shallowest, Tazlina and Barnette, gabbros crystallized at ∼17–24 km; the Klanelneechena Klippe crystallized at ∼24–26 km; and the base of the arc crystallized at ∼35 km depth. The arc had a volcanic:plutonic ratio of ∼1:3–1:4. However, many or most of the felsic plutonic rocks may represent crystallized liquids rather than cumulates so that the liquid:cumulate ratio might be 1:2 or larger. The current 5- to 7-km structural thickness of the plutonic section of the arc is ∼15–30% of the original 23- to 28-km thickness. The bulk composition of the original Talkeetna arc section was ∼51–58 wt % SiO2.Funded by NSF EAR-9910899

Role of melt supply in oceanic detachment faulting and formation of megamullions

Author Posting. © Geological Society of America, 2008. This article is posted here by permission of Geological Society of America for personal use, not for redistribution. The definitive version was published in Geology 36 (2008): 455-458, doi:10.1130/G24639A.1.Normal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identified on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when ~30%–50% of total extension is accommodated by magmatic accretion and that there is significant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges.This research was supported by the National Science Foundation and by the Henry Bryant Bigelow Chair in Oceanography to Tucholke at Woods Hole Oceanographic Institution

Timescales for the growth of sediment diapirs in subduction zones

Author Posting. © The Author(s), 2012. This article is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 190 (2012): 1361–1377, doi:10.1111/j.1365-246X.2012.05565.x.In this study, we calculate timescales for the growth of gravitational instabilities forming in the sediment layer on the downgoing slab at subduction zones. Subducted metasediments are buoyant with respect to the overlying mantle and may form diapirs that detach from the slab and rise upwards into the mantle wedge. We use a particle-in-cell, finite-difference method to calculate growth rates for instabilities forming within a buoyant, wet-quartz metasediment layer underlying a dense mantle half-space composed of wet olivine. These growth rates are used to determine where sediment diapirs initiate and detach from the slab over a range of subduction zone thermal structures. We find that, given a sufficient layer thickness (200–800 m, depending on slab-surface and mantle-wedge temperatures), sediment diapirs begin to grow rapidly at depths of ∼80 km and detach from the slab within 1–3 Myr at temperatures ≤900 °C and at depths roughly corresponding to the location of the slab beneath the arc. Diapir growth is most sensitive to absolute slab temperature, however it is also affected by the viscosity ratio between the sediment layer and the mantle wedge and the length-scale over which viscosity decays above the slab. These secondary affects are most pronounced in colder subduction systems with old slabs and faster subduction rates. For a broad range of subduction zone thermal conditions, we find that diapirs can efficiently transport sediments into the mantle wedge, where they would melt and be incorporated into arc magmas. Thus, we conclude that sediment diapirism is a common feature of many subduction zones, providing a potential explanation for the ‘sediment signature’ in the chemistry of arc magmas.This work was supported by NSF Grant EAR-0652707 and a WHOI Deep Ocean Exploration Institute Fellowship to MB

Analysis of reported error in Monte Carlo rendered images

Evaluating image quality in Monte Carlo rendered images is an important aspect of the rendering process as we often need to determine the relative quality between images computed using different algorithms and with varying amounts of computation. The use of a gold-standard, reference image, or ground truth (GT) is a common method to provide a baseline with which to compare experimental results. We show that if not chosen carefully the reference image can skew results leading to significant misreporting of error. We present an analysis of error in Monte Carlo rendered images and discuss practices to avoid or be aware of when designing an experiment

Multisite Investigation of Outcomes With Implementation of CYP2C19 Genotype-Guided Antiplatelet Therapy After Percutaneous Coronary Intervention

OBJECTIVES: This multicenter pragmatic investigation assessed outcomes following clinical implementation of CYP2C19 genotype-guided antiplatelet therapy after percutaneous coronary intervention (PCI). BACKGROUND: CYP2C19 loss-of-function alleles impair clopidogrel effectiveness after PCI. METHODS: After clinical genotyping, each institution recommended alternative antiplatelet therapy (prasugrel, ticagrelor) in PCI patients with a loss-of-function allele. Major adverse cardiovascular events (defined as myocardial infarction, stroke, or death) within 12 months of PCI were compared between patients with a loss-of-function allele prescribed clopidogrel versus alternative therapy. Risk was also compared between patients without a loss-of-function allele and loss-of-function allele carriers prescribed alternative therapy. Cox regression was performed, adjusting for group differences with inverse probability of treatment weights. RESULTS: Among 1,815 patients, 572 (31.5%) had a loss-of-function allele. The risk for major adverse cardiovascular events was significantly higher in patients with a loss-of-function allele prescribed clopidogrel versus alternative therapy (23.4 vs. 8.7 per 100 patient-years; adjusted hazard ratio: 2.26; 95% confidence interval: 1.18 to 4.32; p = 0.013). Similar results were observed among 1,210 patients with acute coronary syndromes at the time of PCI (adjusted hazard ratio: 2.87; 95% confidence interval: 1.35 to 6.09; p = 0.013). There was no difference in major adverse cardiovascular events between patients without a loss-of-function allele and loss-of-function allele carriers prescribed alternative therapy (adjusted hazard ratio: 1.14; 95% confidence interval: 0.69 to 1.88; p = 0.60). CONCLUSIONS: These data from real-world observations demonstrate a higher risk for cardiovascular events in patients with a CYP2C19 loss-of-function allele if clopidogrel versus alternative therapy is prescribed. A future randomized study of genotype-guided antiplatelet therapy may be of value

Grain-size distribution in the mantle wedge of subduction zones

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): B10203, doi:10.1029/2011JB008294.Mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones, including mantle flow and fluid migration. To investigate the grain-size distribution in the mantle wedge, we coupled a two-dimensional (2-D) steady state finite element thermal and mantle-flow model with a laboratory-derived grain-size evolution model. In our coupled model, the mantle wedge has a composite olivine rheology that incorporates grain-size-dependent diffusion creep and grain-size-independent dislocation creep. Our results show that all subduction settings lead to a characteristic grain-size distribution, in which grain size increases from 10 to 100 μm at the most trenchward part of the creeping region to a few centimeters in the subarc mantle. Despite the large variation in grain size, its effect on the mantle rheology and flow is very small, as >90% of the deformation in the flowing part of the creeping region is accommodated by grain-size-independent dislocation creep. The predicted grain-size distribution leads to a downdip increase in permeability by ∼5 orders of magnitude. This increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches ∼100 km depth compared with shallower depths, potentially providing an explanation for the relatively uniform subarc slab depth. Seismic attenuation derived from the predicted grain-size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones, without requiring a significant contribution by the presence of melt.Funding for this research was provided by the National Science Foundation through a MARGINS Postdoctoral Fellowship (NSF OCE‐0840800) and NSF grant EAR‐0854673

Galapagos-OIB signature in southern Central America: mantle refertilization by arc-hot spot interaction

[1] Although most Central American magmas have a typical arc geochemical signature, magmas in southern Central America (central Costa Rica and Panama) have isotopic and trace element compositions with an ocean island basalt (OIB) affinity, similar to the Galapagos-OIB lavas (e.g., Ba/La 10, 206Pb/204Pb > 18.8). Our new data for Costa Rica suggest that this signature, unusual for a convergent margin, has a relatively recent origin (Late Miocene ∼6 Ma). We also show that there was a transition from typical arc magmas (analogous to the modern Nicaraguan volcanic front) to OIB-like magmas similar to the Galapagos hot spot. The geographic distribution of the Galapagos signature in recent lavas from southern Central America is present landward from the subduction of the Galapagos hot spot tracks (the Seamount Province and the Cocos/Coiba Ridge) at the Middle American Trench. The higher Pb isotopic ratios, relatively lower Sr and Nd isotopic ratios, and enriched incompatible-element signature of central Costa Rican magmas can be explained by arc–hot spot interaction. The isotopic ratios of central Costa Rican lavas require the subducting Seamount Province (Northern Galapagos Domain) component, whereas the isotopic ratios of the adakites and alkaline basalts from southern Costa Rica and Panama are in the geochemical range of the subducting Cocos/Coiba Ridge (Central Galapagos Domain). Geological and geochemical evidence collectively indicate that the relatively recent Galapagos-OIB signature in southern Central America represents a geochemical signal from subducting Galapagos hot spot tracks, which started to collide with the margin ∼8 Ma ago. The Galapagos hot spot contribution decreases systematically along the volcanic front from central Costa Rica to NW Nicaragua