State of the Arc (SOTA) 2003 - An Overview

The third in a series of ~triennial SOTA workshops, held at Timberline Lodge on the slopes of Mt. Hood (16-21 August, 2003), brought together an international group of some 90 leading experts and students of subduction zones and their related magmatism. The workshop theme, 'Energy and Mass Fluxes in Volcanic Arcs', provided a broad focus for discussion and debate concerning a number of cutting edge issues and developments. The goal of this report is to summarize the proceedings and to convey the spirit and accomplishments of the meeting - that is, the 'state of the art', as currently viewed by the participants.

Structure
The conference was organized around a thematic set of keynote presentations by leading researchers on the topics of [1] thermal structures of subduction zones (SZs) from petrologic and geophysical perspectives, [2] sources, processes, and rates of mass fluxes originating below the Moho, [3] chronologies and rates, mass contributions, and impacts of crustal level processes that influence arc magmatism, [4] energetics and dynamics of magma feeder systems, and [5] origins, budgets, and influences of magmatic volatiles. Much of the meeting was devoted to moderated in-depth discussions of these topics. Parallel poster sessions provided opportunities for participants to display current research and to interact in detail with interested colleagues. A concluding workshop was organized along the lines of small group discussions focusing on set of broad questions and issues that arose during the meeting, with the goal of identifying profitable avenues and approaches for future research. An intra-conference field trip to visit representative exposures of volcanic deposits from Mt. Hood provided yet another forum for informed but informal interaction. Pre- and post-conference field trips to Mt. St. Helens and to the central Oregon Cascades and Crater Lake doubtless enhanced the attraction of this meeting to a broad and international group. This format, the secluded nature of the venue, and high fugacities of catalyzing fluids resulted in a stimulating level of discourse and collaboration.

Formal presentations and discussion topics
Current constraints on thermal structures of SZs were reviewed from petrologic and geophysical perspectives by Glenn Gaetani and Chris Kincaid, respectively. Whereas much progress has been made in recent years concerning conditions of melting of near-anhydrous mantle materials (e.g., mid-ocean ridge settings), the effects of H₂O and CO₂ on melting in subduction systems are comparatively less constrained. There is a basic need to reinvestigate the vapor-saturated solidi of common mantle materials using modern analytical tools, as well as the conditions of formation of volatile-rich magmas (including calcalkaline basalts and high-Mg# andesites). Numerical simulations of thermal conditions in subducting slabs and within the subarc mantle wedge have resulted in improved understanding of temperatures in these domains, but the results are sensitive to modeling approach and assumed boundary conditions. Until such models can be 'calibrated' by independent observations, there remain large uncertainties on predicted temperatures and geothermal gradients. Questions remain concerning dehydration behavior of slabs, conditions under which slab materials melt, and the relative contributions of flux melting versus decompression melting of convecting wedge mantle to arc magma production. First-order constraints on magma temperatures derived from geothermobarometry of arc lavas or exposed intrusive equivalents and from laboratory experiments suggest that the subarc mantle may be relatively warm compared to many numerical models. Scaled fluid dynamic experiments potentially provide higher resolution, and seemingly imply that two-dimensional numerical models (i.e., crossections of arcs) may underestimate actual thermal conditions; i.e., along-strike components of mantle convection may have a significant influence on thermal structure. It is expected that, with incorporation of T-dependent viscosity, SZ thermal conditions predicted by numerical and physical models may be reconciled with those based on petrologic constraints can be reconciled, but the details remain to be worked out. Still, all indirect modeling studies need to be tested against independent constraints or benchmarks before they can be applied confidently to understanding petrologic processes. Moreover, variance in the thermal conditions of SZs can result in a broad range in their behavior and this factor must be considered when comparing the outputs of magmatism and fluid fluxes from different arcs or arc segments.

John Eiler and Simon Turner reviewed geochemical and petrologic constraints on the mantle origins of arc magmas with particular emphases on source components, processes, and rates of melt formation and ascent. A broad diversity of mafic magma types is recognized ranging from common calcalkaline variants to those with affinities to OIB and MORB. A key and longstanding question concerns the relative roles of subducted sediments, oceanic crust, and variably hydrated slab mantle versus potentially heterogeneous wedge mantle lithologies in producing the observed spectrum. An important advance will be discerning the effects of compositional heterogeneity in the mantle and slab from the effects of melting regimes and how they can be reconciled with physical models. Considerable discussion of transport properties and elemental partitioning among hydrous fluids, silicate melts, supercritical fluids and mantle or slab minerals ensued bearing on the important issue of whether or not, from a geochemical perspective, there is compelling evidence for slab melting. Evidence from short-lived U-Th series radioisotopes added additional fuel to the discussion - a particularly acute unsolved problem in this regard is the partitioning behavior of Th between melts and fluids in the sub-arc environment. Young mafic lavas commonly exhibit large ²²⁶Ra and ²³⁰Th excesses that often correlate with anomalies in other robust slab tracers (e.g., ¹⁰Be). Such observed isotopic disequilibrium in many arc lavas provides strong evidence for short (e.g., <10⁴ years) time scales for their formation and transport from near slab depths. Because the time scales appear to differ from one parent-daughter system to another, what these data imply concerning the mechanisms of elemental transport from slab to wedge and melting processes remains an open question. Moreover, the inferred short transport times carry significant implications concerning mechanisms of transport that remain to be fully explored. Turning to shallower levels within arc systems, Georg Zellmer reviewed available U-Th-Ra isotopic studies of the full spectrum of arc lavas. These data, coupled with petrography and information on crystal size distributions and element diffusion profiles, provide important constraints on time scales of magma differentiation, storage, and transport. A variety of studies document the entrainment of older crystal populations, magma mingling, and other open-system processes that attest to significant complexity in the formation of many evolved arc lavas, and raise questions concerning interpretation of the disequilibrium data. The differences in time-scales implied by Th-Ra and U-Th disequilibrium in evolved magmatic systems reveal that further investigation is needed into the extent to which such magmas inherit their isotopic disequilibria from parental basalts (some of which seemingly carry signals of slab-derived elemental fractionations) as opposed to higher level processes (e.g., melt reactions with hydrothermally altered rocks or other materials at crustal levels, or magma mixing). Combined studies using other isotopic and elemental tracers offer potential to resolve this question. Jon Davidson provided an overview of the problems associated with unraveling crustal contributions, and Mike Dungan provided a detailed case study showing how extensive yet geochemically subtle crustal modification may be. On a mineralogical scale, microanalytical studies of phenocrysts commonly reveal complex isotopic and elemental variations further attesting to common operation of open system processes in arc lavas. At the scale of a long-lived volcanic complex, petrographic, mineralogical, and geochemical studies commonly reveal intricate relations best interpreted in terms of magma mingling, and digestion (or disaggregation and entrainment) of cogenetic cumulates and perhaps other rocks in the shallow crust. The message that comes through clearly is that many, and perhaps most, arc lavas experience one or more form of open system modification that may obscure details of subcrustal magmatic petrogenesis. Although this caveat may be most important for intermediate and evolved lavas, even apparently primitive basaltic lavas may be affected to some degree. The extent to which this may be evident will depend, element by element, on the leverage exerted by the 'crustal filter'. Some features, such as the U-Th series disequilibria and ¹⁰Be signals are hard to explain except as slab-derived signals, but may be 'diluted' as a consequence of crustal influence.

The origins of intermediate and felsic arc magmas were further considered from other viewpoints. George Bergantz considered the role of crustal formation in terms of available experimental data for melting of suitable crustal protoliths, potential heat sources, and geological investigations of the exposed roots of older subduction complexes. A review of experimentally produced melts suggests that magmas most closely resembling estimated average crustal composition (i.e., granodiorite/andesite) generally require water-undersaturated melting of mafic to intermediate amphibolitic protoliths. However formed, such protoliths may melt in response to underplating or intrusion of hot basaltic liquids, but numerical models considering thermal diffusion and time scales place restrictive geometric bounds on the details of this process. Unfortunately, the rare basement exposures that have been studied to date do not corroborate such a model. Kelly Russell discussed attempts to combine simultaneously thermodynamic, kinetic, and energetic constraints to simulate compositional variations in magmas undergoing cooling, fractionation, and open system interaction with wall rocks or other assimilant material. Such models, bound by energetic constraints (often not accounted for by petrologists), provide more realistic and self-consistent tests to evaluate geochemical details of arc magma evolution. A central point is that rising magmas are strongly out of equilibrium with their surroundings and have the potential to react on very short time scales. Finally, Jon Blundy described experimental and numerical modeling attempts to simulate the development of dacitic magmas similar to those erupted at Mt. St. Helens. This model also invoked periodic inputs of basaltic magma into the deep crust, and remelting of differentiated products of those magmas to produce broadly dacitic liquids. Experimental studies coupled with petrographic investigations also provide new insights concerning temperatures and processes of magma differentiation, crystallization behavior, and volatile budgets. These presentations provoked considerable discussion and comments concerning magma conduits, storage, petrographic implications, etc. Clearly, the evolution of specific arc magma suites can differ from one volcano to another - even on small distance scales - such that general petrogenetic models require some customization in detail. Nevertheless, it appears that in most cases the fundamental energy driving arc volcanism derives from inputs of basaltic magma from mantle sources. The crustal 'filter' (perhaps better conceptualized for Earth scientists as a 'distillery') in turn imparts unique attributes to the outputs seen at the surface. These latter effects may be difficult to distinguish from subcrustal inputs; they are a nuisance to those interested in mantle reservoirs but are central issues to volcanologists.

The last formal session concerned volatile species in volcanic arcs. Bernard Marty reviewed isotopic compositions of many volatile components in magmas and how these may be used to estimate their sources and fluxes in volcanic arcs. Most arcs display ³He/⁴He ratios that are close to the MORB ratio (8±1 R_a), implying that He predominantly originates from the upper mantle. Samples with lower values record contributions of crustal He, although at some localities lower values may derive from subducted slab materials. Both sediment/slab and mantle sources appear to contribute to the fluxes of N and C species, based on elevated CO₂/³He and N₂/³He ratios compared to the upper mantle. However, degassing from subducting slabs and from ascending arc magmas and, for some species, complex chemistry in the crust combine to obscure volatile budgets. Issues needing further work include forearc degassing fluxes (tied to metamorphic reaction paths in subducting slabs), reconciliation of large ('cryptic') fluxes estimated for C with lower estimates based on studies of quenched lavas and/or melt inclusions in phenocrysts, and effects of devolatilization on isotopic compositions of gas species (used in certain flux calculations). Improvements in remote sensing measurements are needed to better quantify local emissions, and global arc flux estimates can be improved by expanding existing studies to more arcs (particularly submarine volcanic plumes) and improving mass flux estimates for arc magmatism (i.e., arc growth rates). Sampling methods are also quite varied (fumaroles, hot springs, ground/airborne gas monitoring, and melt inclusion studies), sometimes with different results; these need to be fully reconciled or explained. Paul Wallace focussed on the measurement techniques and inventories for major volatile components (H₂O, CO₂, SO₂, and Cl) in arc magmas, and discussed the 'S problem'. The latter refers to an imbalance between monitored S emissions in volcanic plumes compared to lower petrologic estimates of S available in associated magma bodies; the excess S needed for a mass balance has important, but unresolved, implications for gas behavior in magmatic systems, volumes of magma involved, and possibly recycling of volatiles from shallow magmatic-hydrothermal systems. Estimated subduction inputs and magmatic outputs for H₂O, CO₂, and Cl are qualitatively similar, suggesting efficient recycling of these volatiles back to the surface; however, forearc fluxes and plutonic sinks currently are ignored for lack of adequate data. The effects of volatiles (mainly H₂O) in driving magma ascent, vesiculation, and explosive eruptions need to be quantified. In particular, success in predicting eruptive styles hinges on better understanding of volatile inventories and magmatic degassing.

Future directions and research objectives
During the course of the conference we developed a set of questions (below), the answers to which were considered important in developing a comprehensive understanding of arc processes, and that also seemed amenable to direct investigation. Participants split into groups to discuss potential approaches to resolve or constrain each. The deliberations of each group were presented and discussed in a concluding plenary session; brief summaries of these reports follow each question.

1. How can we better quantify thermal structures of SZs?

Minimal constraints can be derived from magma temperatures (geothermometry) and further refined via experimental studies (see #4 below). Experiments on subducting assemblages and analog studies of ultra-high pressure (UHP) terranes can shed light on thermal conditions attained within subducting slabs. Seismology (e.g., earthquake mechanisms and tomography) may constrain thermal structure of anomalous regions and/or unique P/T loci in SZs and mantle wedge domains. Reconciliation of reduced heat flow values and petrologic constraints (e.g., mantle and crustal melting estimates) with thermal models can help eliminate inappropriate models and provide insights to improve modeling efforts. Experiments on appropriate mantle materials with and without volatiles are needed to improve estimates of wedge rheology.

2. What are the mass and energy fluxes in arcs, and how do they influence arc structure?

Mass balances at arcs are, well, in a state of flux. Large uncertainties remain regarding temporal productivity, plutonic versus volcanic rock ratios, extent of recycling of older arc material to make younger magmas, and compositional inventories. For heat flux estimates, considerably more quality heat flow data, complementary studies of magma-derived species, and quantification of groundwater effects on heat flow are needed at various scales. Volatile fluxes are needed both along strike and across arcs. Existing data are mainly limited to subaerial volcanoes, but surveys of submarine volcanoes may provide a means to more rapidly expand the database. Magma flux estimates can be improved via detailed seismic studies of young arcs situated on oceanic lithosphere where igneous rock and volcanogenic sediment volumes and effects of loading and lithosphere flexure can be constrained more accurately.

3. How do we differentiate steady state processes from transient events in SZs, and how are these related to tectonic forcing functions?

The concept of 'steady state' is a function of the time scale considered. Over time, events such as arc migration, variations in slab geometry, changes in the character of subducting slabs (e.g., loci of fracture zones, aseismic ridges), etc. can perturb the thermal structure, the character of reservoirs involved in arc magmatism, and the loci and mechanisms of melting. Understanding such effects requires comparative studies of modern arcs (where tectonic factors are best constrained). Temporal variations in magmatic activity at specific arcs also need to be characterized (production rates, compositions, etc.) and inverted to define realistic physical models. There is also debate concerning the significance of high temperatures inferred on petrologic grounds in the uppermost mantle beneath some arcs; whether this reflects transient spikes due to injection of mantle derived magma or regionally high mantle temperatures has important implications for dynamic processes in the mantle wedge.

4. What is the composition of the mantle wedge, how does it melt, and what does it produce?

Comprehensive geochemical approaches can be organized to evaluate melt production (degree of melting) in a geographic context within one or more well-studied (intraoceanic?) arcs where slab additions and wedge depletion can be reasonably constrained. In principal, combined with a comprehensive phase diagram for peridotite, the above data can be inverted to a geographic map of P-T-H₂O-melt fraction. Development of such a phase diagram would provide a baseline from which departures or anomalies in melt chemistry could be interpreted and used subsequently to define further relevant experiments (e.g., for other mantle lithologies). Also, theoretical studies are needed to address melting processes under variable volatile contents, and for melt transport in a deforming mantle wedge.

5. What are the effects of the 'crustal filter' in modifying mantle magmatic inputs and/or in producing the observed compositional spectrum of arc magmas?

Fundamental parameters to determine include (1) the most primitive 'baseline' magma composition in an arc - and, specifically:, whether or not high-Mg basaltic andesites are an important primary magma; (2) composition, thickness, and thermal structure of the local crust; (3) geologic history and duration of magmatic activity; and (4) stratigraphy and volcanologic context. Processes to evaluate in specific magmatic systems include partial melting, crystallization, recharge/mixing, and assimilation. Comprehensive geologic, petrologic, geochemical, geophysical and experimental approaches are needed to fully constrain these processes and to identify the lithologic reservoirs involved. Moreover, understanding magma evolution in the crust will place constraints on the energetics required, on subcrustal heat inputs needed, and on related physical mechanisms.

6. How does the slab impart its signal (chemical/physical) to arc systems?

Major efforts are needed on several fronts. The depth, extent, and distribution of alteration (e.g., serpentinite and other hydrous minerals) in oceanic lithosphere are major factors controlling key chemical inventories in subducting slabs as well as their physical properties and behavior. Drilling, seismic studies, modeling studies and examination of exhumed UHP and arc terrains will provide further constraints. Experiments pertaining to slab dehydration and melting are needed for realistic compositions and realistic P-T paths. More comprehensive element partitioning studies (fluids, melts, near supercritical fluids) are needed at appropriate P-T conditions. Relations between spatial variations in arc outputs and subduction forcing functions (especially those linked to geophysical measurements) need examination. Particular focus on forearc domains can provide important constraints on early devolatilization history of subducting slabs.

7. How can we reconcile the disparate time scales across the compositional spectrum from U-Th series radioisotopes?

Several new studies suggest that shallow hydrous processes (e.g., dehydration of amphibole, phologopite, or other hydrous phases including water) and/or diffusion effects among source minerals attending dehydration melting may influence U-Th series daughters and contribute to observed anomalies. The importance of such effects may be evaluated using trace element signatures for minerals capable of imparting such a signal (e.g., amphibole or phlogopite), through studies of xenoliths, or by direct experimentation. Also, experiments are needed to establish closure temperatures for different U-Th series systems. Possible complications due to magma mixing or interaction between magmas and hydrothermal fluids or alteration products may be tested using trace element and stable isotope geochemistry. Direct slab sources for U-Th series disequilibria (especially in evolved arc lavas) may be tested by the presence of robust slab tracers (e.g., ¹⁰Be or other slab fluid indicators). Comparative differences among U-Th, Th-Ra, etc. and Sr diffusion ages in crystals are needed to determine if these systems record valid chronologies for specific processes.

8. How can we better constrain the systematics and effects of degassing in the crust?

For specific well-studied and monitored volcanoes, volatile fluxes need to be constrained for deep-seated magmas via petrologic methods, and their shallower emanations quantified using combined surface monitoring and geophysical methods. Theoretical models for volatile behavior are needed for a range of pressures, and these reconciled with observed fluxes. To achieve satisfactory models, further experimental work is needed on partitioning of volatile species into melts of diverse composition - including the effects that volatiles may have on each other during partitioning. Rates of volatile production need to be quantified using radiogenic tracers as appropriate. Valuable insights can also be gained from parallel studies of plutonic rocks.

9. What drives crystallization and degassing in magmas?

Important types of investigations include the following. Couple direct investigations of magmatic volatiles to seismic or other signals of fluid migration in the deep crust and upper mantle. Experimentally determine partitioning of S, Cl, H₂O and other volatiles into CO₂-rich fluids at high pressures. Study the abundance and isotopic composition of volatile species in related but compositionally diverse suites of magmas. Combine studies of natural samples with experiments to determine styles of phenocryst zoning induced by degassing versus cooling. Develop robust criteria to identify phenocrysts in equilibrium with host melts. Develop new geobarometers and geothermometers for common phase assemblages (e.g., olivine + spinel) occurring in hydrous calcalkalic magmas. Devise experiments to distinguish between decompression versus cooling controls on magma degassing. Evaluate effects of crystallization (i.e., crystal content) on the composition of melt inclusions in terms of post-entrapment modifications within samples and temporal evolution of related suites of samples.

Closure and conclusion
The final function of the conference included overviews by several 'designated sages' of what was accomplished.

It was noted that SOTA 2003 brought to light an amusing lexicon of terms that may or may not become permanently installed in the arc literature. The concept of 'crystal cargo' refers to inherited material (or 'antecrysts', as coined by Wes Hildreth in the 2001 Penrose conference on silicic magmas) entrained in many arc lavas - and provides patent evidence for the open-system nature of many arc petrogenetic processes. The term 'illegitimate unwanted daughters' (with the contraceptive acronym, IUDs) was proffered by Jon Blundy to those studying excesses of offspring in short-lived radioisotope systematics. And in a self-analytical vein, many participants were torn between roles as 'splitters' vs. 'lumpers' depending on the scale of their perspective on different issues - the former being perhaps the derivative of the latter (or the latter being the integral of the former, as suggested by Adam Kent).

The success of SOTA 2003 can be attributed to the presence of a diverse group of active researchers, including a contingent of emerging young scientists who brought with them novel ideas and expertise with state-of-the-art approaches and technologies. The size of the group promoted active discussion and interchange, and this was enhanced in the smaller group breakouts. Finally, all appreciated the setting, which was inspirational, and the secluded and comfortable venue that promoted undistracted interactions - often extending into the wee hours of the night. In the end, the success will be measured at least partly by the number of new collaborations formed and new ideas developed.

We conclude with a perhaps unsurprising observation. The complex variations within and between volcanic arcs are products of the inherent variability in composition and history of the slab, wedge, and crustal reservoirs involved. Depending on our experience and perspective, as students of volcanic arcs, we each see different parts of the anecdotal elephant. Yet, through conferences like SOTA there is increasing communication and integration of expertise among the disparate specialists that ultimately may result in development of a 'unifying theory' to bring all these parts into common focus and eventually serve as a useful predictive tool.

Further information
Details of the conference, a list of participants, submitted abstracts, and PowerPoint files from most presenters are available for inspection on the SOTA 2003 WebPage ( www.ruf.rice.edu/~leeman/SOTA2003/info.html). In the next year a compilation of thematic papers submitted by conference participants will be published as a special volume of the Journal of Volcanology and Geothermal Research. There was unanimous support for continuing this series of conferences

Acknowledgements
SOTA 2003 received major funding from the U.S. National Science Foundation, as well as support from Rice University, Portland State University, the U.S. Geological Survey, the International Association for Volcanology and Chemistry of the Earth's Interior (IAVCEI), and the Volcanology and Magmatic Studies Group (U.K.). Many individuals assisted in many ways, but we particularly want to thank the field trip leaders (Charlie Bacon , Mike Clynne, Rick Conrey, Cynthia Gardner, Dan Miller, John Pallister, and Willy Scott) for sharing their expertise, the graduate students from Portland State University and Oregon State University for their help with logistics, and the staff at Timberline Lodge for taking such good care of us.