Constraining Subduction Zone Processes Through Local, Regional, and Global Chemical Systematics

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Constraining Subduction Zone Processes Through Local, Regional, and Global Chemical Systematics

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Title: Constraining Subduction Zone Processes Through Local, Regional, and Global Chemical Systematics
Author: Turner, Stephen
Citation: Turner, Stephen. 2015. Constraining Subduction Zone Processes Through Local, Regional, and Global Chemical Systematics. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Abstract: Subduction zones recycle material from Earth’s surface into the mantle, and are an important means of continent building. The subduction system serves as a stamp, imprinting the distinct chemical characteristics of our planet’s geological reservoirs, and distinguishing it within the solar system. As such, the elemental exchanges mediated by this system are a long-standing focus of geochemical and geophysical research. Advances in geochemical techniques and improved geophysical models of subduction have illuminated the processes which give rise to arc volcanism. Great strides have been made in answering the question of what goes down, and what comes back up, though much remains unknown. Arc volcanoes provide a valuable window into the complex subduction environment, and so a comprehensive understanding of arc magma petrogenesis provides a means to resolve significant outstanding questions.
The processes that regulate the compositions of erupted arc magmas are complicated, however. In order to use arc magmas as a tool for constraining elemental fluxes across large-scale geochemical reservoirs, we must trace the path of lavas sampled on Earth’s surface back down through the lithosphere. Once we reach the asthenosphere, we require constraints on the conditions from which the magma was generated – a mantle source, fluxed by some hydrous material originating from the subducting plate. We can determine which elements have been added to the mantle at this point only if we know the composition of the mantle before the hydrous addition. None of these processes are likely to be identical from one volcano to the next (or, indeed, at a single volcano over time). Instead, these processes will vary dependent on the physical conditions present. Thus, this work requires an understanding of the physical conditions across the earth, as well as their effects of physical processes on elemental transfer within the system.
Chapter 1 of this dissertation addresses the question of how magma ascent through the crust can vary on short timescales (~50 years) at a single location, specifically at Bezymianny Volcano, in Kamchatka, Russia. This project was conducted following two field seasons at Bezymianny. Field experience provided an on-the-ground understanding of this volcano’s unique magma system, and fostered multi-disciplinary interactions with geophysicists and seismologists that informed the interpretation of its geochemistry. Bezymianny often erupts multiple times per year. The sample set used in this study was collected by several different volcanologists over five decades, and provides unprecedented temporal resolution of sampling for this time period. The compositions of Bezymianny magmas varied regularly throughout the eruptive cycle. Using whole rock trace element compositions and thermobarometry from amphiboles, it was possible to characterize magma mixing at Bezymianny that varied in proportions from three separate crustal reservoirs -- each positioned at a different depth within the crust. A comparison is then drawn between the magma plumbing systems of Bezymianny and other volcanoes with similar surface features. This analysis demonstrates that the form of a volcano on the surface does not necessarily reflect the structure of its roots.
Chapters 2 and 3 were motivated by Chapter 4, rather than the other way around. Chapter 4 is a regional investigation of chemical variability along the Chilean Southern Volcanic Zone (SVZ). The SVZ is a classic study area for igneous chemistry, in which the compositions of erupted magmas vary along and across the strike of the volcanic arc. Along with magma chemistry, multiple physical parameters that may influence the subduction system (or simply, “subduction parameters”), also vary along-strike. From south to north within the SVZ the crust becomes thicker, while the depth of the slab increases significantly. In addition, there is a decrease in the angle of the slab beneath the arc front. To resolve whether the magma variability in the SVZ is due to variation of subduction parameters within the overriding plate or the subducting slab (or both) is non-trivial. In part, this is because variations in the flux from the slab, the melting processes, and in intra-crustal processing, can have similar chemical consequences.
This ambiguity motivated a re-examination of the relationships between subduction parameters and global magma chemistry. In many ways this project builds upon the study of Plank and Langmuir (1988), but also utilizes the extensive literature database that has been developed in the interim. The new data enable assessment of not only major elemental variation, but also trace elements and isotopes. Chapter 2 presents the systematics of a global dataset, which includes several new observations of global correlations between trace elements and trace element ratios. The global correlations with magma chemistry also extend to correlations with crustal thickness. There are strong correlations among incompatible elements that are typically separated into groups. The ratio Dy/Yb also correlates with incompatible elements, suggesting involvement of garnet.
In Chapter 2, this dataset is used to investigate whether the global trends might arise from intra-crustal processes. This possibility is supported by the correlations between chemical parameters and the thickness of the crust. The main crustal processes considered are high-pressure crystal fractionation and mixing between primary magmas and an enriched crustal component. High-pressure fractionation trends are not found to be more abundant at arcs with thick crust, however, and the composition of the hypothetical global contaminant is unlikely to exist in nature. The global magma variation is therefore most plausibly primary in nature, arising from processes in the slab or mantle, rather than the crust.
Chapter 3 investigates whether variable slab fluxes or melting processes are responsible for the global correlations in magma chemistry. The correlations with crustal thickness, if not produced by processes within the crust itself, are suggestive of a melting process. The chemical parameters also correlate, however, with the slab “thermal parameter,” implicating processes within the downgoing plate. In addition to the arc front chemical systematics, it is shown that rear-arc volcanic compositions, after filtration to minimize the effects of slab input, have strong correlations between Sr and Nd isotopes. Rear-arc Nd isotopes also correlate well with the Nd isotope values of the arc front. To constrain the potential effects of slab and mantle processes, quantitative models are developed for two different scenarios: In one scenario, global chemical diversity is produced by a variable slab flux, while the mantle thermal structure is held constant. In a second scenario, global chemical diversity is produced by a variable mantle thermal structure, while slab flux is relatively constant. Both models are able to reproduce the global trends, though the observed correlations between filtered rear-arc and arc front Nd isotopes are difficult to reconcile with potentially large fluxes of slab material to the mantle wedge. Both models have implications as well for the flux of H2O, both from the slab to the mantle, and from the mantle back to the exosphere.
Finally, in Chapter 4, we apply this global modeling framework back to the problem of the SVZ. It is demonstrated that the systematics of the SVZ mimic those of the global system in a remarkable way. The correlations between elements within the global dataset are also present in the SVZ, and these trends overlap. An extensive dataset of rear arc SVZ samples is used to demonstrate control of Nd isotopes and other compositional features by variable mantle heterogeneity, rather than variable slab flux. The along-strike chemical trends of the SVZ are consistent with the scenario of variable mantle thermal structure, but not with the scenario of variable slab flux. This conclusion is quantitatively tested using a model in which both mantle heterogeneity and mantle thermal structure are varied along the arc. The model is successful in reproducing the observed chemical variations. Because the SVZ and global systematics are so similar, it is likely that conclusions drawn from this region can be extrapolated back to the global framework.
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