Deciphering petrogenic processes using Pb isotope ratios from time-series samples at Bezymianny and Klyuchevskoy volcanoes, Central Kamchatka Depression

The Klyuchevskoy group of volcanoes in the Kamchatka arc erupts compositionally diverse magmas (high-Mg basalts to dacites) over small spatial scales. New high-precision Pb isotope data from modern juvenile (1956–present) erupted products and hosted enclaves and xenoliths from Bezymianny volcano reveal that Bezymianny and Klyuchevskoy volcanoes, separated by only 9 km, undergo varying degrees of crustal processing through independent crustal columns. Lead isotope compositions of Klyuchevskoy basalts–basaltic andesites are more radiogenic than Bezymianny andesites ([superscript 208]Pb/[superscript 204]Pb = 37.850–37.903, [superscript 207]Pb/[superscript 204]Pb = 15.468–15.480, and [superscript 206]Pb/[superscript 204]Pb = 18.249–18.278 at Bezymianny; [superscript 208]Pb/[superscript 204]Pb = 37.907–37.949, [superscript 207]Pb/[superscript 204]Pb = 15.478–15.487, and [superscript 206]Pb/[superscript 204]Pb = 18.289–18.305 at Klyuchevskoy). A mid-crustal xenolith with a crystallization pressure of 5.2 ± 0.6 kbars inferred from two-pyroxene geobarometry and basaltic andesite enclaves from Bezymianny record less radiogenic Pb isotope compositions than their host magmas. Hence, assimilation of such lithologies in the middle or lower crust can explain the Pb isotope data in Bezymianny andesites, although a component of magma mixing with less radiogenic mafic recharge magmas and possible mantle heterogeneity cannot be excluded. Lead isotope compositions for the Klyuchevskoy Group are less radiogenic than other arc segments (Karymsky—Eastern Volcanic Zone; Shiveluch—Northern Central Kamchatka Depression), which indicate increased lower-crustal assimilation beneath the Klyuchevskoy Group. Decadal timescale Pb isotope variations at Klyuchevskoy demonstrate rapid changes in the magnitude of assimilation at a volcanic center. Lead isotope data coupled with trace element data reflect the influence of crustal processes on magma compositions even in thin mafic volcanic arcs.University of Washington. Department of Earth and Space Science

The Complexity of Arc Magmatism: A Geochemical Investigation of Crustal Processes Beneath a Highly Productive Volcanic Center, Kamchatka, Russia

Understanding crustal modification of magmas in volcanic arcs is a fundamental problem of igneous petrology and geochemistry. This dissertation examines the role of crustal processes including crustal assimilation, magma mixing, magma differentiation, and volatile transport in generating erupted magmas from four volcanic systems in the Kamchatka Volcanic arc: Bezymianny, Klyuchevskoy, Shiveluch, and Karymsky. High-precision Pb isotope data from the Central Kamchatka Depression (CKD) record lower crustal assimilation and magma mixing of less radiogenic crust and melt into the magmas of Bezymianny volcano. Unradiogenic Pb isotope compositions in CKD magmas suggest that lower crustal assimilation occurs to a larger extent beneath the CKD than in other segments of the Kamchatka arc. Klyuchevskoy compositional variations support this argument and also show that the degree of assimilation beneath an individual volcanic center may vary on a decadal timescale. Documented lower crustal assimilation by magmas beneath the CKD modifies uranium-series isotope disequilibrium compositions of CKD magmas. U-series isotopes are the only radioactive isotope system with half-lives useful for understanding the timing of volcanic processes. Constraining the affect of crustal processes that alter U-series compositions is essential to interpret isotope signatures. Chapter 2 of this dissertation shows that lower crustal assimilation and fractional crystallization of high-pressure clinopyroxene modifies U-excess to form Thexcess compositions. Global documentation of Th-excess reveals a correlation of Th-excess disequilibria with hot lower crustal geologic settings. Th-excess may therefore be an important chemical tracer of lower crustal assimilation. Shallow magma storage and degassing is investigated using (210Pb)/(226Ra) compositions at periodically erupting volcanoes with well-constrained degassing and eruptive histories. Current models that predict (210Pb)/(226Ra) as a function of gas loss do not accurately predict the observed compositions of erupted products in Kamchatka. This paradox has important implications for the timescales of magma storage and degassing in the crust. Chapter 4 of this work is a detailed isotopic study of environmental uranium contamination from an abandoned open-pit mine. Uranium isotope data fingerprint alteration of the mine source and transport of uranium from the mine to sediment in Red Rock Creek on National Forest Service recreational land

Examining Canonical Theories of U-series Disequilibria in Volcanic Arcs in Light of a More Comprehensive, Global Database

Disequilibrium in the short-lived U-series isotopic system occurs during partial melting, differentiation, and volatile transport; therefore, the U-series decay chain is a unique tool to examine the magnitude and timing of magmatic processes. However, our understanding of U-series fractionation in subduction zones is incomplete. We use published data from volcanoes around the world and new data from volcanic systems in the Kamchatka Arc (Bezymianny, Klyuchevskoy, and Karymsky) to examine two theories regarding the behavior of U-series nuclides: 1) that Th-excess in arc magmas, (230Th)/(238U) >1, is a function of arc-thickness/garnet in the melting region, or magnetite fractionation and 2) that 210Pb deficits, (210Pb)/(226Ra) <1, are the result of continuous magma degassing. Our results show that neither of these theories explains the complete dataset produced by the U-series community. Th-excess is generally observed in MORB and attributed to decompression melting; however, global data also record Th-excess in fluid-fluxed subduction zones. Limited experimental data suggest preferential U transport over Th in subduction zone fluids and, therefore, U-excess rather than Th-excess should exist in arcs. The common explanation for arc Th-excess is melt interaction with thick continental crust where phases such as garnet retain high U/Th in crystalline residues. We record Th-excess at Bezymianny, (230Th)/(238U) from 1.04-1.06 and Klyuchevskoy, (230Th)/(238U) from 1.01 and 1.08, volcanoes, which are located on relatively thin (~35 km), primitive crust. These magmas have low Sr/Y (15.5-19.9) that preclude a significant influence of garnet. In addition, LA-ICP-MS measurements of in-situ U and Th mineral-melt partitioning on erupted mineral phases (plagioclase, pyroxene, Fe-Ti oxides, apatite) suggest that U-series disequilibria are transparent to shallow crustal processing. Th-excess at Klyuchevskoy inversely correlates with Ba/Th, Sr/Th, Dy/Yb, and Ce/Pb. We suggest that Th-excess is a function of decompression mantle melting beneath arcs and/or phase fractionation by lower crustal mineral assemblages. 210Pb deficits are interpreted as the result of 222Rn gas loss in magmatic systems. With 222Rn loss, the daughter product, 210Pb, cannot grow into magma. Two published models (Gauthier and Condomines 1999 and Condomines et al. 2010) simulate this process where the magnitude of 210Pb deficit increases with the duration of degassing. We record 210Pb deficits at Bezymianny and Karymsky volcanoes (0.846-0.966). However, for known eruption intervals, these 210Pb deficits are larger than current models predict. If physical constraints for gas behavior in magma are added to these models, the problem is magnified. Our results highlight a need for studies quantifying the fractionation of U-series nuclides among all phases (crystals, melts, and volatiles) relevant to subduction zones. Primitive, thin volcanic arcs that have a well-characterized volcanic history through time, similar to the Kamchatka Arc, are ideal places to begin

The application of radiochronometry during the 4th collaborative materials exercise of the nuclear forensics international technical working group (ITWG)

In a recent international exercise, 10 international nuclear forensics laboratories successfully performed radiochronometry on three low enriched uranium oxide samples, providing 12 analytical results using three different parent-daughter pairs serving as independent chronometers. The vast majority of the results were consistent with one another and consistent with the known processing history of the materials. In general, for these particular samples, mass spectrometry gave more accurate and more precise analytical results than decay counting measurements. In addition, the concordance of the U-235-Pa-231 and U-234-Th-230 chronometers confirmed the validity of the age dating assumptions, increasing confidence in the resulting conclusions

The application of radiochronometry during the 4th collaborative materials exercise of the nuclear forensics international technical working group (ITWG)

In a recent international exercise, 10 international nuclear forensics laboratories successfully performed radiochronometry on three low enriched uranium oxide samples, providing 12 analytical results using three different parent-daughter pairs serving as independent chronometers. The vast majority of the results were consistent with one another and consistent with the known processing history of the materials. In general, for these particular samples, mass spectrometry gave more accurate and more precise analytical results than decay counting measurements. In addition, the concordance of the 235U–231Pa and 234U–230Th chronometers confirmed the validity of the age dating assumptions, increasing confidence in the resulting conclusions.JRC.G.II.6-Nuclear Safeguards and Forensic