How to create new subduction zones: A global perspective

The association of deep-sea trenches—steeply angled, planar zones where earthquakes occur deep into Earth’s interior—and chains, or arcs, of active, explosive volcanoes had been recognized for 90 years prior to the development of plate tectonic theory in the 1960s. Oceanic lithosphere is created at mid-ocean ridge spreading centers and recycled into the mantle at subduction zones, where down-going lithospheric plates dynamically sustain the deep-sea trenches. Study of subduction zone initiation is a challenge because evidence of the processes involved is typically destroyed or buried by later tectonic and crust-forming events. In 2014 and 2017, the International Ocean Discovery Program (IODP) specifically targeted these processes with three back-to-back expeditions to the archetypal Izu-Bonin-Mariana (IBM) intra-oceanic arcs and one expedition to the Tonga-Kermadec (TK) system. Both subduction systems were initiated ~52 million years ago, coincident with a proposed major change of Pacific plate motion. These expeditions explored the tectonism preceding and accompanying subduction initiation and the characteristics of the earliest crust-forming magmatism. Lack of compressive uplift in the overriding plate combined with voluminous basaltic seafloor magmatism in an extensional environment indicates a large component of spontaneous subduction initiation was involved for the IBM. Conversely, a complex range of far-field uplift and depression accompanied the birth of the TK system, indicative of a more distal forcing of subduction initiation. Future scientific ocean drilling is needed to target the three-dimensional aspects of these processes at new converging margins

Timescales of magmatic processes and eruption ages of the Nyiragongo volcanics from 238U-230Th-226Ra-210Pb disequilibria

Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 288 (2009): 149-1157, doi:10.1016/j.epsl.2009.09.017.The silica-undersaturated Nyiragongo volcanics, located in the East African rift, have globally unique chemical compositions and unusually low viscosities, only higher than carbonatite lavas, for terrestrial silicate magmas. We report 238U-230Th-226Ra-210Pb series disequilibria in 13 recent and prehistoric lava samples from Nyiragongo including those from the 2002 flank eruption and a 2003 lava lake sample. (230Th/238U) ranges from 0.90- 0.97 in the recent lavas and from 0.94-1.09 in the prehistoric lavas. To explain the variable 230Th and 238U excesses in these lavas, we hypothesize that different processes with opposite effects in terms of fractionating Th/U in the mantle source are involved. These processes include: 1) low degree partial melting of a phlogopite-bearing mantle source (consistent with low K/Rb) with residual garnet (consistent with high chondrite-normalized Dy/Yb), to produce the observed 230Th excesses; and, 2) carbonate metasomatism for the 238U enrichment, consistent with high Zr/Hf in the Nyiragongo lavas. The Nyiragongo volcanics have higher (230Th/232Th) values than observed in most mantle-derived rocks, especially ocean-island basalts, suggesting that their mantle-source was affected by carbonate metasomatism less than 300 ka ago. Several Nyiragongo samples display significant 226Ra excesses implying rapid magma transport (less than 8 ka) from the mantle-source to the surface. Modeling the observed (226Ra/230Th) versus Zr/Hf correlation in the lavas indicates that the 2002, 2003 and a few pre-historic lavas incorporated 50-60% of a carbonate-metasomatized mantle source while the other prehistoric lavas show 10-22% contribution of this source. This result indicates that the Nyiragongo lavas were derived from a heterogeneous, non-uniformly carbonated mantle source. The 2002 lava shows (210Pb/226Ra) equilibrium, whereas the 2003 lava lake sample shows initial (210Pb/226Ra) < 1. The latter observation suggests that Nyiragongo magmas degas as they rise to the surface over years or decades before eruption. (210Pb/226Ra) equilibrium in the 2002 lava suggests that the 2002 magma may have stagnated for more than a decade before eruption. The high CO2 content, high emission rates, extreme fluidity, along with the inferred short residence time and our inferences of rapid magma transport and high eruptive frequency suggest that the volcanic hazards of Nyiragongo, both from lava flows and gas emissions, are higher than previously estimated.Lava samples were collected as a part of a field work in Nyiragongo, supported by UN-OCHA grants. The field team included Paolo Papale, Alba Santo, Dario Tedesco and Orlando Vaselli with support from the staff of Goma Volcanological Observatory, D. R. Congo. The 2003 lava lake sample was collected by Jacques Durieux. Funding for U-series analyses was covered by NSF-EAR 063824101 and NSF-EAR 083887800 to KWWS. 210Po analyses were funded with EAR0738776 to MR. Sample preparation and dissolution was covered by NSF-EAR 0732679 to ARB. RC acknowledges guest student award from WHOI

How to Create New Subduction Zones: A Global Perspective

The association of deep-sea trenches—steeply angled, planar zones where earthquakes occur deep into Earth’s interior—and chains, or arcs, of active, explosive volcanoes had been recognized for 90 years prior to the development of plate tectonic theory in the 1960s. Oceanic lithosphere is created at mid-ocean ridge spreading centers and recycled into the mantle at subduction zones, where down-going lithospheric plates dynamically sustain the deep-sea trenches. Study of subduction zone initiation is a challenge because evidence of the processes involved is typically destroyed or buried by later tectonic and crust-forming events. In 2014 and 2017, the International Ocean Discovery Program (IODP) specifically targeted these processes with three back-to-back expeditions to the archetypal Izu-Bonin-Mariana (IBM) intra-oceanic arcs and one expedition to the Tonga-Kermadec (TK) system. Both subduction systems were initiated ~52 million years ago, coincident with a proposed major change of Pacific plate motion. These expeditions explored the tectonism preceding and accompanying subduction initiation and the characteristics of the earliest crust-forming magmatism. Lack of compressive uplift in the overriding plate combined with voluminous basaltic seafloor magmatism in an extensional environment indicates a large component of spontaneous subduction initiation was involved for the IBM. Conversely, a complex range of far-field uplift and depression accompanied the birth of the TK system, indicative of a more distal forcing of subduction initiation. Future scientific ocean drilling is needed to target the three-dimensional aspects of these processes at new converging margins

Extinction-to-Backscatter Ratios of Lofted Aerosol Layers Observed During the First Three Months of CALIPSO Measurements

Case studies from the first three months of the Cloud and Aerosol Lidar and Infrared Pathfinder Spaceborne Observations (CALIPSO) measurements of lofted aerosol layers are analyzed using transmittance [Young, 1995] and two-wavelength algorithms [Vaughan et al., 2004] to determine the aerosol extinction-to-backscatter ratios at 532 and 1064 nm. The transmittance method requires clear air below the layer so that the transmittance through the layer can be determined. Suitable scenes are selected from the browse images and clear air below features is identified by low 532 nm backscatter signal and confirmed by low depolarization and color ratios. The transmittance and two-wavelength techniques are applied to a number of lofted layers and the extinction-to-backscatter ratios are compared with values obtained from the CALIPSO aerosol models [Omar et al., 2004]. The results obtained from these studies are used to adjust the aerosol models and develop observations based extinction-to-backscatter ratio look-up tables and phase functions. Values obtained by these techniques are compared to Sa determinations using other independent methods with a goal of developing probability distribution functions of aerosol type-specific extinction to backscatter ratios. In particular, the results are compared to values determined directly by the High Spectral Resolution Lidar (HSRL) during the CALIPSO CloudSat Validation Experiments (CCVEX) and Sa determined by the application of the two-wavelength lidar Constrained Ratio Aerosol Model-fit (CRAM) retrieval approach [Cattrall et al., 2005; Reagan et al., 2004] to the HSRL data. The results are also compared to values derived using the empirical relationship between the multiple-scattering fraction and the linear depolarization ratio by using Monte Carlo simulations of water clouds [Hu et al., 2006]

The Grizzly, December 6, 2018

Snapchat Story Leads to School Enforcement of Discriminatory Acts Policy • What\u27s in Store for Martin Luther King, Jr. Week • Ursinus Celebrated the Fifth Annual #Giving2UCday on Campus • Student Employee Profiles: Facilities • The Curtain Club and how Theatre Evolved at Ursinus • Opinions: It\u27s Time to Retire the War on Christmas ; Let Students Spend Dining Dollars Off-Campus • Athlete Spotlight: Junior Quarterback Tom Garlick • Eric Williams Jr. Knocks Down 1,000th Career Point for UC Men\u27s Basketballhttps://digitalcommons.ursinus.edu/grizzlynews/1610/thumbnail.jp

The Grizzly, November 8, 2018

UC\u27s First African-American Graduate Passes Away: W. Robert Crigler Graduated from Ursinus in 1956 • This is Their Home and They Belong Here : Ursinus Released a Statement Affirming the Rights of Trans Students in the Wake of Threats from President Trump • What\u27s Happening in Honduras? • Ursinus Introduces Vice President of Enrollment, Shannon Zottola • The Reader\u27s Recovery • The Cubed Curve • Opinions: Latin America Deserves Better Response from the U.S.; Ritter Should be the Next Update to Campus • Athlete Spotlight: Gabriela Howell • Men\u27s Basketball Participates in The Programhttps://digitalcommons.ursinus.edu/grizzlynews/1607/thumbnail.jp

The Social Sciences Interdisciplinarity for Astronomy and Astrophysics -- Lessons from the History of NASA and Related Fields

In this paper we showcase the importance of understanding and measuring interdisciplinarity and other -disciplinarity concepts for all scientists, the role social sciences have historically played in NASA research and missions, the sparsity of social science interdisciplinarity in space and planetary sciences, including astronomy and astrophysics, while there is an imperative necessity for it, and the example of interdisciplinarity between social sciences and astrobiology. Ultimately we give voice to the scientists across all fields with respect to their needs, aspirations and experiences in their interdisciplinary work with social sciences through an ad-hoc survey we conducted within the Astro2020 Decadal Survey scientific community

Oxygen isotope constraints on the sources of Central American arc lavas

Oxygen-isotope ratios of olivine and plagioclase phenocrysts in basalts and basaltic andesites from the Central American arc vary systematically with location, from a minimum δ18Oolivine value of 4.6 (below the range typical of terrestrial basalts) in Nicaragua near the center of the arc to a maximum δ18Oolivine value of 5.7 (above the typical range) in Guatemala near the northwest end of the arc. These oxygen-isotope variations correlate with major and trace element abundances and with Sr and Nd isotope compositions of host lavas, defining trends that suggest variations in δ18O reflect slab contributions to the mantle sources of these lavas. These trends can be explained by a model in which both a low-δ18O, water-rich component and a high-δ18O, water-poor component are extracted from the subducting Cocos slab and flux melting in the overlying mantle wedge. The first of these components dominates slab fluxes beneath the center of the arc and is the principal control on the extent of melting of the mantle wedge (which is highest in the center of the arc); the second component dominates slab fluxes beneath the northwestern margin of the arc. Fluxes of both components are small or negligible beneath the southeastern margin of the arc. We suggest that the low-δ18O component is a solute-rich aqueous fluid produced by dehydration of hydrothermally altered rocks deep within the Cocos slab (perhaps serpentinites produced in deep normal faults offshore of Nicaragua) and that the high-δ18O component is a partial melt of subducted sediment on top of the plate

Geodynamic Evolution of a Forearc Rift in the Southernmost Mariana Arc

The southernmost Mariana forearc stretched to accommodate opening of the Mariana Trough backarc basin in late Neogene time, erupting basalts now exposed in the SE Mariana Forearc Rift (SEMFR) 3.7 – 2.7 Ma ago. Today, SEMFR is a broad zone of extension that formed on hydrated, forearc lithosphere and overlies the shallow subducting slab (slab depth ≤ 30 – 50 km). It comprises NW-SE trending subparallel deeps, 3 - 16 km wide, that can be traced ≥ ~ 30 km from the trench almost to the backarc spreading center, the Malaguana-Gadao Ridge (MGR). While forearcs are usually underlain by serpentinized harzburgites too cold to melt, SEMFR crust is mostly composed of Pliocene, low-K basaltic to basaltic andesite lavas that are compositionally similar to arc lavas and backarc basin (BAB) lavas, and thus defines a forearc region that recently witnessed abundant igneous activity in the form of seafloor spreading. SEMFR igneous rocks have low Na8, Ti8, and Fe8, consistent with extensive melting, at ~ 23 ± 6.6 km depth and 1239 ± 40oC, by adiabatic decompression of depleted asthenospheric mantle metasomatized by slab-derived fluids. Stretching of pre-existing forearc lithosphere allowed BAB-like mantle to flow along SEMFR and melt, forming new oceanic crust. Melts interacted with preexisting forearc lithosphere during ascent. SEMFR is no longer magmatically active and post-magmatic tectonic activity dominates the rift