More Like this

1. Sulfide geochemistry of cumulates from the Lesser Antilles arc

   Pollock, T ; Cruz-Uribe, AM ; Marschall, HR ; Blundy, J ( August 2018 , Goldschmidt Abstracts)

   The sulfur isotope composition of volcanic rocks in arcs can be difficult to constrain because significant fractionation can occur during degassing. Mafic and ultramafic cumulates represent the least degassed part of the magmatic arc system, thereby offering an opportunity to investigate undegassed sulfur in arcs. Recent work on high pressure metamorphic rocks has suggested that subducted materials can retain their original isotopic composition to sub-arc depths. In particular, extreme negative δ34S values can be retained in subducted sediments. The purpose of this project is to investigate to what extent these deep subduction zone processes are reflected in the sulfur isotope signature of arc magmas. In the Lesser Antilles arc, there is a gradual decrease in terrigenous sediment being subducted from south to north. An estimated ~15% subducted sediment in the south and ~2% in the north is reflected in the chemical and isotopic composition of the Lesser Antilles arc magmas. Sulfides in these magma- derived cumulates record the earliest stages of magma evolution and are a more faithful monitor of the sulfur isotopic composition of the magma source region in the mantle than erupting lavas. We hypothesize that the decrease in terrigenous sediment being subducted from the south to north will be reflected in the S isotopes in cumulate samples. Samples of mafic and ultramafic cumulates have been collected from fourteen islands across the Lesser Antilles arc. Primary rock types are olivine gabbro, amphibole gabbro, plagioclase gabbro, and olivine gabbronorite. Sulfide minerals include pyrite, chalcopyrite, and pyrrhotite, and typically occur as spherical blebs. Sulfides are found primarily as inclusions in clinopyroxene, amphibole, olivine, and plagioclase. Sulfides occur less frequently as inclusions in magnetite and within the matrix. Analyses of sulfur isotopes in cumulate sulfides are currently underway. The decrease in the amount sediment being subducted from south to north in the Lesser Antilles arc should result in δ34S values that increase from south to north (more sediment subducted = more negative δ34S values). 

   more » « less
2. Temperature and co-crystallization effects on Zr isotopes

   https://doi.org/10.1016/j.gca.2023.05.004  

   Kirkpatrick, Heather M. ; Harrison, T. Mark ; Ibañez-Mejia, Mauricio ; Tissot, François L.H. ; MacLennan, Scott A. ; Bell, Elizabeth A. ( July 2023 , Geochimica et Cosmochimica Acta)

   We undertook Zr isotope measurements on zircon, titanite, biotite, amphibole, and whole rocks from the La Posta pluton (Peninsular Ranges, southern California) together with trace element analyses and U-Pb age measurements to understand the controls on Zr isotope fractionation in igneous rocks, including temperature, crystallization sequence, and kinetic effects. We find large (>0.6‰) Zr isotope fractionations (expressed as δ94/90Zr) between titanite and zircon forming at approximately the same temperature. Using equilibrium fractionation factors calculated from ionic and ab initio models, we infer the controls on Zr isotope evolution to include the relative order in which phases appear on the liquidus, with titanite fractionation resulting in isotopically lighter melt and zircon fractionation resulting in isotopically heavier melt. While these models of Zr fractionation can explain δ94/90Zr variations in zircon of up to ∼1.5‰, crystallization order, temperature and presence of co-crystallizing phases do not explain all aspects of the intracrystalline Zr isotopic distribution in zircons in the La Posta pluton or the large range of Zr isotopic values among zircons (>2‰). Without additional constraints, such as knowledge of co-crystallizing phases and a better understand of the true causes of Zr isotope fractionation, Zr isotopes in zircon remains an ambiguous proxy of magmatic evolution. 

   more » « less
3. The 3.1 Ma to 100 ka evolution of the Goat Rocks Volcanic Complex: Persistence and evolution of magmatism at a long-lived major andesite locus on the Cascades Arc

   Wall, Kellie T. ( July 2022 , Theses and dissertations Oregon State University)

   PhD Dissertation Abstract: The imposing andesite stratovolcano is the characteristic expression of subduction zone magmatism, posing hazards to coastal populations and bearing insight into deep Earth processes. On a map of a typical volcanic arc, one can easily distinguish the approximately linear alignment and regular spacing of these major edifices that stand out from a diffuse distribution of mafic volcanoes (e.g. the Quaternary Cascades; Hildreth, 2007). The andesitic composite volcanoes have a reputation for being complex, open systems: crystal zoning “stratigraphies,” diverse crystal cargoes including antecrysts or xenocrysts, quenched magmatic inclusions, and variations in isotopic signatures are among the many lines of evidence that these systems involve a variety of igneous processes and melt sources. To investigate the development and evolution of such transcrustal magma factories, I have conducted a detailed temporal, spatial, and geochemical characterization of a long-lived arc volcanic center in the southern Washington Cascades, the Goat Rocks volcanic complex. Results from ⁴⁰Ar/³⁹Ar and U/Pb geochronology constrain the lifespan of the Goat Rocks volcanic complex from ~3.1 Ma to ~100 ka. During this time, four major composite volcanoes were built (as well as several smaller volcanoes). From oldest to youngest, these are Tieton Peak, Bear Creek Mountain, Lake Creek volcano, and Old Snowy Mountain. Four volcanic stages are defined based on the lifespans of these centers and distinct compositional changes that occur from one to the next: Tieton Peak stage (3.1-2.6 Ma), Bear Creek Mountain stage (1.6-1.1 Ma), Lake Creek stage (1.1 Ma to 456 ka), and Old Snowy Mountain stage (440 ka to 115 ka). Two lava flow remnants also have ages in the interim between Tieton Peak stage and Bear Creek Mountain stage (2.3 Ma and 2.1 Ma), and their sources are not yet identified. The ages of the Bear Creek Mountain and Lake Creek stages in fact overlap, and the gap between Lake Creek stage and Old Snowy Mountain stage is only on the order of 10⁴ years. Based on supporting compositional evidence, the Bear Creek Mountain, Lake Creek, and Old Snowy Mountain stage volcanoes are considered to be the migrating surface expressions of a continuous magmatic system that was active over at least ~1.5 million years. It remains uncertain whether the gaps between the Tieton Peak stage, scattered early Pleistocene andesites, and Bear Creek Mountain stage are due to incomplete exposure/sampling or real quiescent periods earlier in the development of the Goat Rocks volcanic complex. Throughout the construction of the andesitic complex, mafic volcanoes were active on its periphery. These include the Miriam Creek volcano (~3.6-3.1 Ma), Devils Washbasin volcano (3.0-2.7 Ma), Hogback Mountain (1.1 Ma – 891 ka), Lakeview Mountain (194 ka), and Walupt Lake volcano (65 ka). Two basalt and basaltic andesite units (Qob₁ and Qob₂, 1.4 and 1.3 Ma; Hammond, 2017) also erupted from the Goat Rocks area, likely an older incarnation of Hogback Mountain. The suite of mafic magmas erupted in this region are all calcalkaline basalt (or basaltic andesite; CAB), but two compositional groups emerge from the trace element and isotopic data. Group 1 is LILE and LREE-enriched, with higher ⁸⁷Sr/⁸⁶Sr isotopes, and includes compositions from Devils Washbasin, Lower Hogback Mountain, and Lakeview Mountain. Group 2 is less enriched in LILE and LREE and lower in ⁸⁷Sr/⁸⁶Sr, and includes the compositions of Miriam Creek, Qob1, Upper Hogback Mountain, Walupt Lake, and Coleman Weedpatch. The two groups are recurrent through time and with no geographic distinction; in fact, both types were tapped by the Hogback Mountain volcano. Together both of these groups, alongside CABs from Mount Adams and various basalts from Mount St. Helens, form a compositional array between the basalts of the High Cascades and the intraplate-type basalts (IPB) of Mount Adams and Simcoe volcanic field. These results lead to three conclusions. 1) Variably subduction-modified mantle is distributed across the region, perhaps either as stratified layers or a web-like network of fluid pathways amongst less metasomatized mantle. 2) Transitional compositions between the IPBs and typical “High Cascades” CAB/HAOT signature suggest a broader influence of the mantle domain that feeds IPBs—if asthenospheric mantle through a slab window, as suggested by Mullen et al. (2017), then perhaps it bleeds in smaller quantities over a broader area. This compositional trend solidifies the interpretation of the southern Washington Cascades as a unique and coherent “segment” of the arc (the Washington segment of Pitcher and Kent, 2019). 3) The recurrence of variable mafic magma types through time, and with no geographic boundaries, indicates that the compositional evolution of the Goat Rocks volcanic complex was not likely driven by a change in mafic input. Indeed, the Sr, Nd, Hf, and Pb isotope ratios of the intermediate to felsic suite are closely aligned with the local basalts and suggest a limited role of crustal assimilation. Importantly, several mineral thermometers (zircon, ilmenite-magnetite pairs, and amphibole) align in recording higher crystallization temperatures in Bear Creek Mountain to early Lake Creek time, a cooling trend through the Lake Creek stage, and a more diverse range of temperatures in the transition to Old Snowy Mountain stage. Thus, it is suggested that the compositional evolution at Goat Rocks represents a thermal cycle of waxing and waning magmatic flux: where the period of Bear Creek Mountain to early Lake Creek volcanism was the climactic phase of a vertically extensive magma homogenization factory, then the system waned and cooled, ultimately losing its ability to filter, homogenize, and enrich magmas. 

   more » « less
4. The formation of high-Sr/Y plutons in cordilleran-arc crust by crystal accumulation and melt loss

   https://doi.org/10.1130/GES02400.1  

   Brackman, Adam J. ; Schwartz, Joshua J. ( February 2022 , Geosphere)

   Bulk-rock data are commonly used in geochemical studies as a proxy for melt compositions in order to understand the evolution of crustal melts. However, processes of crystal accumulation and melt migration out of deep-crustal, crystal-rich mush zones to shallower storage regions raise questions about how faithfully bulk-rock compositions in plutons approximate melt compositions. This problem is particularly acute in the lower crust of arcs, where melt reservoirs are subject to periodic melt extraction that leaves behind a cumulate residue. Here, we examine bulk-rock data from the perspective of high-Sr/Y plutonic rocks in the lower crust of a well-exposed Early Cretaceous cordilleran-arc system in Fiordland, New Zealand. We test the validity of using high-Sr/Y bulk-rock compositions as proxies for melts by comparing bulk-rock compositions to melts modeled from >100 major- and trace-element analyses of 23 magmatic clinopyroxene grains from the same samples. The sampling locations of the igneous clinopyroxenes and encompassing bulk rocks are distributed across ~550 km2 of exhumed lower crust and are representative of Mesozoic lower-crustal arc rocks in the Median batholith. We confirm that bulk-rock data have characteristics of high-Sr/Y plutons (Sr/Y >50, Na2O >3.5 wt%, Sr >1000 ppm, and Y <20 ppm), features that have been previously interpreted to indicate the presence of garnet as a residual or fractionating phase. In contrast to bulk rocks, igneous clinopyroxenes have low Sr (<100 ppm), high Y (25–100 ppm), and low molar Mg# [100 × Mg/(Mg + Fe)] values (60–70), which are consistent with derivation from fractionated, low-Sr/Y melts. Chondrite-normalized rare-earth-element patterns and Sm/Yb values in clinopyroxenes also show little to no evidence for involvement of garnet in the source or in differentiation processes. Fe-Mg partitioning relationships indicate that clinopyroxenes are not in equilibrium with their encompassing bulk rocks but could have been in equilibrium with melt compositions determined from chemometry of coexisting igneous hornblendes. Moho-depth calculations based on bulk-rock Sr/Y values also yield Moho depths (average = 69 km) that are inconsistent with Moho depths based on bulk-rock Ce/Y, contact aureole studies, Al-in-horn- blende crystallization pressures, and our modeled clinopyroxene crystallization pressures. These data indicate that most Mesozoic high-Sr/Y bulk rocks in the lower crust of Fiordland are cumulates formed by plagioclase + amphibole + clinopyroxene accumulation and interstitial melt loss from crystal-rich mush zones. Our data do not support widespread fractionation of igneous garnet nor partial melting of a garnet-bearing source in the petrogenesis of these melts. We speculate that melt extraction and the production of voluminous cumulates in the lower crust were aided by unusually high heat flow and high magma addition rates associated with an Early Cretaceous arc flareup. We conclude that bulk-rock compositions are poor proxies for melt compositions in the lower crust of the Median batholith, and geochemical modeling of these high-Sr/Y bulk rocks would overemphasize the role of garnet in their petrogenesis. 

   more » « less
5. Flare Up of Hot-Dry-Reduced Ignimbrites Related to Extension in the Cascades Arc: The Deschutes Formation, Central Oregon

   https://doi.org/10.1093/petrology/egad058  

   Pitcher, Bradley W ; Grunder, Anita L ; Kent, Adam J ( August 2023 , Journal of Petrology)

   Abstract Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25–5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (<10%) dacite to rhyolites (mostly 65–77 wt.% SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO + 1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50–250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modeling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt.%) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and the production of hot-dry-reduced rhyolite melts. Depletion of incompatible elements in successive rhyolites implies progressive depletion in fertility of the protolith. Extension also allowed for the establishment of a robust hydrothermal system, and assimilation of hydrothermally-altered rocks by magmas residing in a shallow, complex storage network lead to low δ18O melts. Our findings suggest the integral role that extensional tectonics played in producing an unusual ignimbrite flare-up of hot-dry-reduced rhyolite magmas that are atypical of the Cascades arc and may be an important contributor to flare-ups at arcs worldwide. 

   more » « less