Olivine-rich mafic volcanic rocks (picrites) are a common and important part of ocean island and flood basalt volcanism. Despite their primitive bulk compositions (high MgO, FeO, Mg#, and low SiO2), olivine-rich magmas are typically interpreted as the result of the addition of olivine from cumulate zones into more evolved basaltic liquids (MgO ≤ ~8 wt%). There are commonly two texturally distinct olivine populations in picrites: type 1 grains with planar dislocation (kink) bands, subgrains, or undulose extinction; and type 2 grains that lack these optical textures. Type 1 olivine is similar in texture to olivine from tectonized ultramafic rocks, suggesting that these textures result from plastic deformation, likely within cumulate zones. However, recently it has been proposed that type 1 olivine could also result from growth phenomena or crystal-crystal collisions. In the Kilauea Iki picrite samples used in this study, type 1 grains make up only 10–20% of the modal olivine; however they make up 30–65% of the total olivine by volume due to their large size. Therefore, type 1 grains make a large contribution to the overall composition of Kilauea Iki picrites. A combination of textural (optical defects, crystal size distributions, and minor element zoning) and geochemical analyses (trace element concentrations and diffusion of minor elements) suggests that type 1 and type 2 olivine grains have experienced distinctly different petrological histories and that they are antecrysts and autocrysts, respectively. Differences between type 1 and type 2 olivine are evident in the abundances of slow diffusing trace elements (Al, P, Ti, V), which are likely inherited from their distinct parent magmas. Type 1 and type 2 grains also define different slopes in crystal size distributions, and constraints from diffusion of P and Cr suggest that type 1 grains have longer magmatic residence times than type 2 grains. Type 1 grains likely derive from deformed cumulates within the plumbing system of Kilauea volcano, and our work supports the hypothesis that picrites from Kilauea Iki are formed by the accumulation of antecrystic olivine in more evolved basaltic liquid. Our work further supports models that type 1 olivine textures are formed during plastic deformation within cumulate zones and are not the result of growth phenomena. Our methods can be applied to other olivine-rich volcanic rocks to test the cumulate model for the formation of type 1 olivine textures, which are relatively common in picritic and related rocks from other settings.
Abstract Quantitative models of petrologic processes require accurate partition coefficients. Our ability to obtain accurate partition coefficients is constrained by their dependence on pressure temperature and composition, and on the experimental and analytical techniques we apply. The source and magnitude of error in experimental studies of trace element partitioning may go unrecognized if one examines only the processed published data. The most important sources of error are relict crystals, and analyses of more than one phase in the analytical volume. Because we have typically published averaged data, identification of compromised data is difficult if not impossible. We addressed this problem by examining unprocessed data from plagioclase/melt partitioning experiments, by comparing models based on that data with existing partitioning models, and evaluated the degree to which the partitioning models are dependent on the calibration data. We found that partitioning models are dependent on the calibration data in ways that result in erroneous model values, and that the error will be systematic and dependent on the value of the partition coefficient. In effect, use of different calibration datasets will result in partitioning models whose results are systematically biased, and that one can arrive at different and conflicting conclusions depending on how a model is calibrated, defeating the purpose of applying the models. Ultimately this is an experimental data problem, which can be solved if we publish individual analyses (not averages) or use a projection method wherein we use an independent compositional constraint to identify and estimate the uncontaminated composition of each phase.
The well-studied Cascadia subduction zone has enriched our general understanding of global subduction zones. This Elements issue explores the interconnected set of processes that link geodynamics, tectonics, and magmatism at depth and the surface expressions of these processes, which shape the landscape and give rise to natural hazards in the Cascadia region. This issue also addresses the impact of subduction zone processes on human populations using cultural records, and reviews the state of knowledge of Cascadia while highlighting some key outstanding research questions.
Fluids or melts derived from a subducting plate are often cited as a mechanism for the oxidation of arc magmas. What remains unclear is the link between the fluid, oxygen fugacity, and other major and trace components, as well as the spatial distribution of the impact of those fluids. To test the potential effects of addition of a subduction-derived fluid or melt to the sub-arc mantle, olivine-hosted melt inclusions from primitive basaltic lavas sampled from across the central Oregon Cascades (43°–45°N) have been analyzed for major, trace and volatile elements and fO2. Oxygen fugacity was determined in melt inclusions from sulfur speciation determined by electron microprobe and from olivine–chromite oxygen geobarometry. The overall range in fO2 based on sulfur speciation measurements is from <–0·25 log units to + 1·9 log units (ΔFMQ, where FMQ is fayalite–magnetite–quartz buffer). Oxygen fugacity is positively correlated with fluid-mobile trace element and light rare earth element contents in basalts generated by relatively low-degree partial melting. Establishing a further correlation between fO2 and fluid-mobile trace element abundances with position along the arc requires the basalts to be subdivided into shoshonitic, calc-alkaline, low-K tholeiite and enriched intraplate basalt groups. Melt inclusions from enriched intraplate and shoshonitic lavas show increasing fO2 and trace element abundances closer to the trench, whereas calc-alkaline melt inclusions exhibit no significant across-arc variations. Low-K tholeiitic melt inclusions record an increase in incompatible trace elements closer to the trench; however, there is no correlated increase in fO2. The correlation observed in enriched intraplate and shoshonitic melt inclusions is interpreted to reflect a progressively greater proportion of a fluid-rich, oxidized subduction component in magmas generated nearer the subduction zone. Significantly, calc-alkaline melt inclusions with high ratios of large ion lithophile elements to high field strength elements, characteristic of 'typical' arc magmas, have oxidation states indistinguishable from low-K tholeiite and enriched intraplate basalt melt inclusions. The lack of across-arc geochemical variation in calc-alkaline melt inclusions may suggest that these basalts are not necessarily the most appropriate magmas for examining recent addition of a subduction component to the sub-arc mantle. Flux and batch melt model results produce a wide range of predicted amounts of melting and subduction component added to the mantle source; however, general trends characterized by increased melting and proportion of the subduction component from enriched intraplate, to low-K tholeiite, to calc-alkaline are robust. The model results do not require enriched intraplate, low-K tholeiite and calc-alkaline magmas to be produced from the same more fertile mantle source. However, enriched intraplate magmas, in contrast to calc-alkaline and low-K tholeiite magmas, cannot be generated from a depleted mantle source. Flux or batch melting of either the more fertile or depleted mantle sources used to generate the low-K tholeiite, calc-alkaline, and enriched intraplate magmas cannot reproduce shoshonitic compositions, which require a significantly depleted mantle source strongly metasomatized by a subduction component. The potential mantle source for shoshonitic basalts has a predicted fO2 (after oxidation) from + 0·3 to + 2·4 log units (ΔFMQ) whereas the mantle source for low-K tholeiite, calc-alkaline, and enriched intraplate magmas may range from –1·1 to + 0·7 log units (ΔFMQ).
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