T44C-01 INVITED
Geochemical studies of the Izu-Bonin-Mariana subduction system: highlights, progress and future directions
Significant progress has been made over the last few years in understanding the source of subduction-related melts and the efficiency of recycling processes at both the Central American and Izu-Bonin-Mariana (IBM) margins. These arc systems have distinct physical and geochemical characteristics, which influences how slab- derived components are fluxed through their respective subduction factories. Here we report results from our recent and ongoing melt inclusion studies in the IBM system. However, our presentation will also highlight geochemical studies presented at this year's joint NSF-MARGINS and IFREE IBM workshop. We present volatile, major and trace element data for olivine-hosted melt inclusions from several subaerial and submarine volcanoes within the Izu-Bonin-Mariana subduction zone system. The aim of this work is to quantitatively assess how volatiles are cycled through the IBM subduction system and evaluate their relationship to physical subduction parameters and geochemical slab tracers. Our results show significantly higher CO2 contents in the cross chain samples (i.e., from volcanoes located behind the volcanic front and trending across the arc) as compared to the arc front samples, suggesting either enhanced decarbonation at depth, or that the cross chain samples have experienced less degassing. We find large variations in water contents along the arc system from 0.2 wt% at Maug volcano up to close to 6 wt% at NW Rota, Agrigan and Nijima volcanoes. Ongoing studies examining the dehydration process across the arc show that high water contents in the cross chain samples show similar values to the arc-front samples, suggesting that volatile release is a continuous process across the arc. This finding is consistent with experimental results 1 and melt inclusion studies across the Central American arc 2. Intriguingly, our data set shows a strong decoupling of water and slab fluid tracers such as Ba/La and B contents. The highest water contents are not found in samples with the highest Ba/La or B contents. The IBM system is therefore distinctly different from the Central American system where these simple relationships seem to hold 3; this implies that either the source of IBM fluids is different or there are key differences in the dehydration/melting regime in IBM as compared to Central America. 1Schmidt and Poli (1998) EPSL 163, 361–379 2Walker et al. (2003) Contrib. Mineral. Petrol. 146, 62-77 3Wade et al., in prep.
T44C-02
Geomorphologic study of the serpentine seamounts in the Mariana forearc region
In order to understand the origin of serpentine seamounts in the Mariana forearc region, we obtained new bathymetric map, which cover approximately 770 km with an area of 52000 km2 along the arc, during cruises YK03-07 in 2003 and KR06-15 in 2006. We synthesized with the precise bathymetric map and previously reported geological papers, and classified the geomorphologic characteristics of the serpentine seamounts into three types: the conical type, the dome (including coin shape) type, and the plateau type. The plateau type seamount is considered to be a horst relatively uplifted oceanic crust with serpentine layers. On the otherhand, the other two types indicate a circular or orbital boundary in the bathymetric maps. These shapes imply that effusive materials are supplied from a central vent. Conical-type serpentine seamounts have concave or straight slopes in profile around the summit.Acoustically high reflective areas, which thier outlines are similar to a basaltic lava flow shape, around the summit. The edifices, which may be a mud flow deposit, are not thick enough for their morphological outlines to be detectable. At the same time, wrinkled slopes are observed on their flanks. Dome-type serpentine seamounts have convex upward slopes in profile around the summit. Some small domes, which are less than 10 km in diameter and 500 m in height, are located on the summit. Some of the domes have ramp structures on their boundaries. Remarkable wrinkled slopes are present and often indicate a curved ridge, approximately 100 m high, on their surface, known as an ogive ridge. This slope morphology is very similar to a viscous silicic coulee. The thickness is assumed to be more than 200m. Some of the dome-type seamounts have remarkable ramp structures, 100-150 m high, on their boundaries. These geomorphologic characteristics imply that the serpentine seamounts are produced by highly viscous materials expelled episodically from the central vent. If serpentine seamounts are a result of the accumulation of thin, less than 10 m, serpentine mud flows as proposed previously, the shape of serpentine seamounts should be similar to that of the shield volcano. The above geomorphologic characteristics indicate the presence of 22 serpentine seamounts including nine of which are newly identified. These 22 serpentine seamounts are not evenly distributed along the arc, but are localized in three areas: a northern area, N18° to N20°, a central area, N15° to N15°10', and a southern area, N14° to N14°40'. In the northern and central areas, the effusive centers of the serpentine seamounts are located roughly on the edge of the polygonal graben delineated by faults. The geomorphologic characteristics of the serpentine seamounts and their geometric arrangement are analogous to those of silicic lava domes developed on a caldera rim. These common characteristics suggests that the distribution pattern of the serpentine seamounts is more closely related to the polygonal depressions than to the tectonic setting.
T44C-03
Petrology, Pressures, and Temperatures Along the Mariana Sub-Forearc Slab-Mantle Interface
The protrusion of sedimentary serpentinite deposits from the Mariana forearc on the seafloor has constructed large km-scale seamounts, probably upwelling through fault-bounded conduits in the suprasubduction-zone mantle before reaching the seafloor. Clasts of basic metamorphic rocks and minerals, interpreted to come from the subducting Pacific plate, have been sampled in these serpentinites and are typically of the moderate- to high- pressure and low-temperature blueschist facies conditions expected for sub-forearc subduction zone environments. The majority of minerals have no indication of retrograde metamorphism or metasomatism, indicating that most of the clasts ascended relatively rapidly through the suprasubduction-zone mantle wedge and forearc crust without any significant chemical interaction with the surrounding environment. This material is from the top of the slab and therefore reflects the conditions at the slab-mantle interface. The material is likely to have interacted mechanically and perhaps chemically with the serpentinites and peridotites from the base of the forearc mantle wedge. Furthermore, the temperatures encountered will be hotter than those of the cool interior of the slab. Many terrestrial blueschist deposits originally from slab interiors would therefore not be representative of these boundary conditions. The Mariana blueschists allow these conditions to be estimated in an active modern subduction zone. Bulk rock compositions have been calculated for these clasts using a combination of element mapping and modal analysis. Bulk compositions are generally basaltic, but with unusually high Mg and low Al, resembling altered and possibly alkalic basalt (possibly ocean island basalt), potentially remnants of underplated seamounts that have been smeared off the slab. Inferred conditions of metamorphism are approximately 0.45 to 0.6 GPa (~20 km below seafloor) and approximately 250 to 300 C. These temperatures are among the most robust sub-forearc slab temperature estimates to date and imply 12-20% shear-heating along the slab-mantle interface using the Peacock (1992) thermal model.
T44C-04
Enormous Power Output and Fluid Fluxes Driven Through a few Mid-plate Outcrops
Hydrothermal circulation on ridge flanks, areas far from the magmatic influence of plate creation, is responsible for roughly 30% of the heat loss from oceanic lithosphere and a fluid mass flux on the order of 1015 kg/yr. The thermal state of subducting oceanic lithosphere influences melting conditions in the overlying mantle wedge, the extent of sediment dewatering associated with mineralogical and rheological transitions, and the nature of tectonic and seismic processes at depth. Understanding the distribution and processes associated with ridge flank hydrothermal circulation has been limited by a lack of detailed co-located bathymetric, seismic reflection, and thermal data. We have collected and analyzed such a composite data set on a portion the Cocos Plate approaching the subduction zone along the Middle America Trench, offshore of the Nicoya Peninsula, Costa Rica. This survey covers a seafloor area of 14,500 km2 where crustal ages vary between 18 and 24 Ma. Low- permeability sediments in the study area are thick (400-500 m) and continuous, except where seamounts and other highly-permeable basalt outcrops penetrate the seafloor. Observational data show that enormous quantities of heat and fluid flow from the crust into the ocean through small areas of basement exposure. The seafloor conductive heat flux in most of this area is 10-30% of predicted lithospheric values, requiring an integrated advective power output of 800-1,400 MW. Upper basement temperatures in this area are 5–40 oC, requiring a fluid mass flux of 0.1–3 x 1012 kg/yr to sustain the advective power output. The ten basement outcrops mapped in this region comprise <2% of the seafloor area, and only a few of these features show evidence for fluid outflow. Modeling studies suggest that discharge is favored from smaller outcrops, consistent with our observations. Even if the advective power output were distributed evenly across all ten basement outcrops in this area, each outcrop would discharge as much heat as a high-temperature vent field. Observations of such large fluid fluxes emanating from ridge-flank outcrops indicate the importance of these features to lithospheric fluid and heat budgets, elucidate crustal physical properties and forces required to sustain enormous fluxes, and provide constraints on the thermal state of the upper oceanic lithosphere prior to subduction.
T44C-05 INVITED
An Overview of Volatile and Isotopic Variations Along the Central American Volcanic Front
An overview of the volatile (H2O, CO2, S, halogens) and isotopic (Sr-Nd-Pb-Hf) variations along the Central American volcanic front and preferred explanations of their origin will be presented. Volatile data from 1200 melt inclusions in olivine, pyroxene and feldspar phenocrysts from Central American mafic through felsic rocks provide an unprecedented picture of volatile cycling through subduction zones. In primitive olivine-hosted melt inclusions from Central America, H2O and CO2 show maxima and S, Cl and F minima in Nicaraguan samples. In evolved melt inclusions in pyroxene and feldspar, F shows a maxima in Nicaragua and Cl shows a systemic decrease from Costa Rica to Guatemala. As suggested by previous studies, the high water contents in Nicaraguan magmas are likely to reflect the greatest dehydration of serpentinite where the downgoing slab is deepest beneath the volcanic front. Seismic data indicate that serpentinite is indeed present in the incoming plate outboard of Nicaragua and that the mantle beneath Nicaragua may contain more water-rich melts than beneath Costa Rica. High CO2 content in primitive melt inclusions from Nicaragua and the rear arc (Walker et al., 2003, CMP, 146:62-77) may reflect greater decarbonation with increasing depth of the subducting slab and/or greater degassing of volcanic front melts from Costa Rica and Guatemala. Volatile fluxes estimated for mafic Central American melts are similar to global arc averages (Wallace, 2005, JVGR 140:217- 240). Systematic variations in isotope geochemistry occur along the volcanic front in Central America from Costa Rica to Guatemala: Sr, Nd and Hf isotope ratios display maxima and Pb isotopes a minima in Nicaragua. Highest Sr isotope ratios in Nicaragua may be related to the dehydration of serpentinites in the subducting plate beneath Nicaragua. The systematic variation in isotope ratios from central Costa Rica to NW Nicaragua and the lead isotope geochemistry of Costa Rica volcanic rocks can be explained through addition of fluids/melts from the subducting seamount province of the Galapagos hotspot track to the mantle wedge beneath central Costa Rica and flow of this mantle beneath NW Nicaragua. Seismic velocity anisotropy provides additional support for an arc- parallel component of flow in the mantle wedge beneath the Costa Rican and Nicaraguan volcanic fronts. A decrease in Nd and Hf isotopic composition in the volcanic front from NW Nicaragua to NW Guatemala cannot be explained by addition of sediment melts to the source or crustal assimilation and therefore most likely reflect the increasing contribution of an enriched component located in the mantle wedge (possibly lithospheric mantle) to the melts.
T44C-06
Volcanic Segmentation, Nb Depletion, Reactive Flow Path Length and Volcano Size are Linked in Central America
Central America is unusual among the global set of arcs because the volcanic front is segmented by abrupt right steps of up to 40 km. Nb depletion (Zr/Nb) jumps at these steps (Bolge, 2005). Nb depletion is greater (Zr/Nb higher) at the volcano closer to the trench. Between steps, Nb depletion decreases to the SE, creating a saw- tooth pattern. Given the smooth Benioff zone revealed by seismicity and the slight anticlockwise rotation of the linear volcanic segments relative to the isobaths to the seismic zone, the variation in Nb depletion correlates with depth to the seismic zone. Thus, short paths have maximum Nb depletion and long paths have minimum depletion. This suggests a melt/fluid generation with a Nb retaining phase in the residue to create the initial depletion. A reactive flow through the mantle wedge then progressively reduces the initial depletion. The full set of incompatible elements is consistent with simple AFC models of this process. Furthermore, the uneven distribution of volcanic output along the volcanic front appears related to the variation in path length. A short path generates maximum Nb depletion and a small volcano. An intermediate length path generates a large volcano and moderate Nb depletion. A long path generates minimum Nb depletion and, again, a small volcano. This suggests that the magma generating region has an optimal depth or distance from the trench for magma productivity. Finally, although the origin of the volcanic lines is not explained, it must be a facet of upper plate tectonics, rather than a segmented slab.
T44C-07
Temperature of mantle melts beneath Central America: Integrating petrologic and seismic observations
Mantle melting beneath volcanic arcs is likely polybaric and polythermal, driven by a combination of decompression and hydration. Here we integrate petrologic and seismic observations to constrain the temperature and water content of the bulk melt and the hot zone within the mantle wedge. Results from the TUCAN seismic experiment in Central America show a vertical region, from 150-50 km, of high Vp/Vs beneath the volcanic front of Nicaragua, which may correspond to a melting column. Seismic attenuation tomography shows a wedge-shaped region of high attenuation (1/Q) centered at 75 km depth, which may correspond to the region of maximum temperature in the mantle wedge. The intersection of these regions predicts a zone of maximum melt generation at around 75 km, and melts from this region might dominate the bulk magma that erupts from the volcanoes. Maximum values of 1/Q are higher for the Nicaragua wedge than the Costa Rica wedge, and correspond to higher peak temperatures, assuming dry olivine with uniform grain size. To test this view, we calculate the olivine-liquid temperatures for the most primitive basaltic compositions (Mg# > 50) erupted from Nicaragua and Costa Rica volcanoes, using the thermometer of Sugawara (JGR 2000). Equilibrium olivine is added until whole rock compositions are in equilibrium with mantle olivine (Fo90), assuming 15% Fe3+/FeT. Temperatures calculated from anhydrous melt compositions at 2.5 GPa (75 km) are 1419 +/- 14 C (1 sd) and 1392 +/- 13 C for Nicaragua and Costa Rica magmas, respectively. The small regional difference in mean temperature derives from significant differences in the Fe90 of Nicaragua and Costa Rican volcanics. The sense of the temperature difference is in agreement with the predictions of the seismic attenuation, but the magnitude is small. Central American magmas, however, contain substantial dissolved water as measured in olivine-hosted melt inclusions (c.f., Benjamin, JVGR 2007), with Nicaragua magmas (4-6 wt% H2O) being wetter than Costa Rican magmas (2-4 wt%). Such high water contents in melts and equilibrium mantle olivines will affect both the olivine-liquid temperatures and attenuation-based temperatures. Assuming 4 and 2.5 wt% H2O in primary Nicaragua and Costa Rica magmas, respectively, the olivine-liquid temperatures drop to 1314 +/- 14 C (Nicaragua) and 1321 +/- 13C (Costa Rica). Thus, the temperature of the mantle wedge might actually be similar regionally, with the higher attenuation observed beneath Nicaragua due to higher water contents in the mantle. Peak wedge temperatures of 1315-1320 C beneath Central America are well below the dry mantle solidus at 2.5 GPa (1420 C; Hirschmann G3 2003), and would require water-fluxed melting throughout the mantle wedge.
T44C-08
Oxygen Isotopes of Silicic Volcanic Deposits of NW Costa Rica: A Key to Understanding Crustal Evolution in Central America?
NW Costa Rica has a long history of silicic volcanism, from > 6 my to 0.66 my recorded in silicic ignimbrite deposits. We have determined a preliminary stratigraphic sequence containing at least 23 eruptive units, which allows us to evaluate chemical variations with stratigraphic position. This stratigraphy is based on recently determined 40Ar/39Ar dating, supplemented by new stratigraphic field constraints. The regional chemical variations (including oxygen isotopes) of the silicic deposits in NW Costa Rica were discussed in a recent report (Vogel et al., 2006) on the silicic ignimbrites in Nicaragua and Costa Rica. In Nicaragua and Costa Rica the overall chemical trends of selected trace elements and oxygen isotopes in these silicic deposits are similar to the trends in the basaltic lavas from the active volcanic front. These observations led us to conclude that the silicic deposits were genetically related to the lavas. Oxygen isotopes of phenocrysts (clinopyroxene, orthopyroxene and magnetite) from Nicaraguan and Costa Rican pyroclastic deposits show an increase in δ18O from northwest to southeast of 1.5‰, except for a low δ18O excursion in NW Costa Rica, which is the subject of this report. We showed that the oxygen isotopes of the coexisting clinopyroxene, orthopyroxene and magnetite in these silicic deposits (glassy pumice samples) were in equilibrium and therefore we could use the ubiquitous magnetite as a proxy for the overall variation of oxygen isotopes of whole rock (magma). The δ18O (Mt) variation is independent of silica content of the samples and thus is not due to crystal fractionation. A better understanding of the stratigraphy in NW Costa Rica (Guanacaste province) allows us to evaluate the variation of δ18O of magnetite with time. Values of δ18O (Mt) decrease with time (the low δ18O excursion reported previously). The δ18O (Mt) of the oldest deposit are 3.49 ‰ and it decreases to 1.29 ‰ in the youngest deposits. The youngest ignimbrites were erupted from a nested caldera complex. However the source of the older deposits is not known because the younger deposits covered earlier calderas and continuous outcrops are absent. It is possible that all of these ignimbrites erupted from the same caldera complex. Interpretation of this remarkable oxygen isotopic trend is still in progress. One possibility is that the low δ18O magmas represent melting of previously emplaced calc-alkaline plutons that have interacted with hydrothermal systems (possibly meteoric). With continued evolution of the crust, each successive melting event produced silicic magmas from a source that was more hydrothermally altered. Thus the young silicic deposits represent a source that was more altered than the source for the older silicic deposits. The decreasing δ18O (Mt) values may represent a source area that is continually being reprocessed and hydrothermally altered leading to the development of a "continental- like" crust.