Potassium isotope systematics in subducting materials and arc volcanics

Potassium is an active element in geological systems. This thesis includes studies of the behavior of K in relation to low-temperature processes at the Earth’s surface, and in relation to high- temperature processes in subduction zones. The thread that connects these studies is that low- temperature processes influence the K content and K-isotope composition of mafic oceanic crust and marine sediments—and these materials are important inputs to subduction zones. The principal chemical analytical tool in this thesis is the measurement of stable K-isotope ratios (41K/39K) in natural samples. These data are obtained by utilizing a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS). Potassium-isotope measurements have become much more precise in recent years as a result of innovations in the construction and application of MC-ICPMS instruments. One aspect of this thesis is an examination of the effects of hydrothermal alteration on mafic oceanic crust. This includes a study of altered mafic materials from the ~485 Ma Bay of Islands ophiolite of Newfoundland, Canada. It also includes later work on altered materials from International Ocean Drilling Program (IODP) Sites 801 and C0012. In the Bay of Islands ophiolite, we observe large variations in 41K/39K ratios and a correlation with 87Sr/86Sr ratios. We interpret these variations as reflecting addition of heavy (presumed seawater-derived) K to the ophiolitic materials during seafloor hydrothermal alteration. By contrast, the materials at IODP Sites 801 iii and C0012 have K-isotope compositions that are variable but, on the whole, much lighter than seawater. At these sites, it appears that the added K was highly fractionated relative to seawater, possibly resulting from kinetic or equilibrium isotope effects in which alteration clays preferentially incorporate light K. Another aspect of this thesis is an analysis of the likely controls on the K-isotope compositions of deep-sea sediments at IODP Site 1149 in the western Pacific (near the Izu trench) and Site C0011 in the Shikoku Basin (near the Nankai Trough). Sediments at these sites comprise a mixture of terrigenous and volcanogenic materials. At both sites, K-isotope ratios fall mostly within a narrow range and are on average slightly lighter—by about 0.05 to 0.10‰—than a recently published estimate for the upper continental crust (UCC). Deviations from the UCC composition in the direction of lighter K-isotope compositions likely reflect two processes: (1) variable extents of chemical weathering of terrigenous silicates, and (2) variable proportions of an isotopically light altered ash component in the sediment mixtures. A final aspect of this thesis is a study of K-isotope variations among lavas of the Izu arc. We find that the Izu lavas generally have heavy K-isotope compositions, relative to estimates of the upper mantle (based on published data for mid-ocean-ridge basalts), and relative to estimates for subducting altered oceanic crust (IODP Site 801) and sediments (IODP Site 1149). Moreover, we observe an across-arc shift in K-isotope ratios: lavas of the frontal arc are heavier than the mantle by about 0.22‰, whereas those of the rear arc are heavier by only about 0.08‰ (median values). This leads us to conclude that, in addition to reflecting source characteristics, the K-isotope compositions of arc lavas may reflect isotopic fractionation in association with slab dehydration or melting—with heavy K partitioning preferentially into fluids. A model is proposed iv in which loss of heavy K from the slab under the frontal arc results in a residual slab that is isotopically lighter. The residual slab, in turn, releases fluids with relatively light K under the rear arc. We consider it likely that the behavior of K is coupled to a significant degree to that of Pb and Sr, and less so to that of Nd and Hf, consistent with an interpretation of across-arc changes in radiogenic-isotope ratios. According to our model, across-arc variations in K-isotope ratios in the Izu arc reflect the progressive depletion of the descending slab in K and similarly mobile elements.

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