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1. 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. 

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2. Mush, Melts and Metasediments: a History of Rhyolites from the Okataina Volcanic Centre, New Zealand, as Captured in Plagioclase

   https://doi.org/10.1093/petrology/egab038  

   Sas, May ; Shane, Phil ; Kuritani, Takeshi ; Zellmer, Georg F ; Kent, Adam J ; Nakagawa, Mitsuhiro ( August 2021 , Journal of Petrology)

   null (Ed.)

   Abstract The Okataina Volcanic Centre (OVC), located in the Taupo Volcanic Zone, New Zealand, is a dominantly rhyolitic magmatic system in an arc setting, where eruptions are thought to be driven by mafic recharge. Here, Sr–Pb isotopes, and compositional and textural variations in plagioclase phenocrysts from 10 rhyolitic deposits (two caldera, one immediately post-caldera, four intra-caldera, and three extra-caldera) are used to investigate the OVC magmatic system and identify the sources and assimilants within this diverse mush zone. Plagioclase interiors exhibit normal and reverse zoning, and are commonly in disequilibrium with their accompanying glass, melt inclusions, and whole-rock compositions. This indicates that the crystals nucleated in melts that differed from their carrier magma. In contrast, the outermost rims of crystals exhibit normal zoning that is compositionally consistent with growth in cooling and fractionating melts just prior to eruption. At the intra-crystal scale, the total suite of 87Sr/86Sr ratios are highly variable (0·7042–0·7065 ± 0·0004 average 2SE); however, the majority (95 %) of the crystals are internally homogeneous within error. At whole-crystal scale (where better precision is obtained), 87Sr/86Sr ratios are much more homogeneous (0·70512–0·70543 ± 0·00001 average 2SE) and overlap with their host whole-rock Sr isotopic ratios. Whole-crystal Pb isotopic ratios also largely overlap with whole-rock Pb ratios. The plagioclase and whole-rock isotopic compositions indicate significant crustal assimilation (≥20 %) of Torlesse-like metasediments (local basement rock) by a depleted mid-ocean ridge mantle magma source, and Pb isotopes require variable fluid-dominant subduction flux. The new data support previous petrogenetic models for OVC magmas that require crystal growth in compositionally and thermally distinct magmas within a complex of disconnected melt-and-mush reservoirs. These reservoirs were rejuvenated by underplating basaltic magmas that serve as an eruption trigger. However, the outermost rims of the plagioclase imply that interaction between silicic melts and eruption-triggering mafic influx is largely limited to heat and volatile transfer, and results in rapid mobilization and syn-eruption mixing of rhyolitic melts. Finally, relatively uniform isotopic compositions of plagioclase indicate balanced contributions from the crust and mantle over the lifespan of the OVC magmatic system. 

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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. 

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4. The Atlantis Bank Gabbro Massif, Southwest Indian Ridge

   https://doi.org/org/10.1186/s40645-019-0307-9  

   Dick, Henry J. ; Robinson, Paul T. ; MacLeod, C.J. MacLeod ; Kinoshita, Hajimu ( November 2019 , Progress in earth and planetary science)

   This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation. Five crossings of the gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearly complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment. There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor. Inliers of the dike-gabbro transition show that the uppermost gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the gabbro massif also found few primitive gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex. The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north. 

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5. Expedition 391 summary

   Sager, W. ; Hoernle, K. ; Höfig, T.W. ; Avery, A.J. ; Bhutani, R. ; Buchs, D.M. ; Carvallo, C.A. ; Class, C. ; Dai, Y. ; Dalla Valle, G. ; et al ( October 2023 , Proceedings of the International Ocean Discovery Program Expedition reports)

   Hotspot tracks (chains of seamounts, ridges, and other volcanic structures) provide important records of plate motions, as well as mantle geodynamics, magma flux, and mantle source compositions. The Tristan-Gough-Walvis Ridge (TGW) hotspot track, extending from the active volcanic islands of Tristan da Cunha and Gough through a province of guyots and then along Walvis Ridge to the Etendeka flood basalt province, forms one of the most prominent and complex global hotspot tracks. The TGW hotspot track displays a tight linear age progression in which ages increase from the islands to the flood basalts (covering ~135 My). Unlike Pacific tracks, which are often simple, nearly linear chains of seamounts, the TGW track is alternately a steep-sided narrow ridge, an oceanic plateau, subparallel linear ridges and chains of seamounts (most are flat-topped guyots). The track displays isotopic zonation over the last ~70 My. The zonation appears near the middle of the track just before it splits into two to three chains of ridge- and guyot-type seamounts. Walvis Ridge, forming the older part of the track, is also overprinted with age-progressive late-stage volcanism, which was emplaced ~30–40 My after the initial eruptions and has a distinct isotopic composition. The plan for Expedition 391 was to drill at six sites, three along Walvis Ridge and three in the seamounts of the Guyot Province, to collect igneous rocks to better understand the formation of volcanic edifices, the temporal and geochemical evolution of the hotspot, and the variation in paleolatitudes at which the volcanic edifices formed. After a delay of 18 days to address a shipboard Coronavirus (COVID-19) outbreak, Expedition 391 proceeded to drill at four of the proposed sites: three sites on Walvis Ridge around Valdivia Bank, an ocean plateau within the ridge, and one site on the lower flank of a guyot in the Center track of the Guyot Province, a ridge located between the Tristan subtrack (which extends from the end of Walvis Ridge to the islands of Tristan da Cunha) and the Gough subtrack (which extends from Walvis Ridge to Gough Island). The first hole was drilled at Site U1575, located on a low portion of the northeastern Walvis Ridge just north of Valdivia Bank. At this location, 209.9 m of sediments and 122.4 m of igneous basement were cored. The sediments ranged in age from Late Pleistocene (~0.43–1.24 Ma) to Late Cretaceous (Campanian; 72–78 Ma). The igneous basement comprised 10 submarine lava units consisting of pillow, lobate, sheet, and massive lava flows, the thickest of which was ~21 m. Most lavas are tholeiitic, but some alkalic basalts were recovered. A portion of the igneous succession consists of low-Ti basalts, which are unusual because they appear in the Etendeka flood basalts but have not been previously found on Walvis Ridge. Two holes were drilled at Site U1576 on the west flank of Valdivia Bank. The first of these holes was terminated because a bit jammed shortly after entering the igneous basement. Hole U1576A recovered a remarkable ~380 m thick sedimentary section consisting mostly of chalk covering a nearly complete sequence from Late Pleistocene (~0.43–1.24 Ma) to Late Cretaceous (Campanian; ~79–81.38 Ma). These sediments display short and long cyclic color changes that imply astronomically forced and longer term paleoenvironmental changes. The igneous basement recovered in Hole U1576B yielded 11 submarine lava units (total thickness = ~65 m). The flows range from pillows to massive flows with compositions varying from tholeiitic basalt to basaltic andesite, only the second occurrence of the latter composition recovered from the TGW track thus far. These units are separated by seven sedimentary chalk units that range 0.1–11.6 m in thickness, implying a long-term interplay of sedimentation and lava eruptions. These intercalated sediments revealed Upper Cretaceous (Campanian) ages of ~77–79 Ma for the upper two interbeds and ~79–81.38 Ma for the lower beds. Coring at Site U1577, on the extreme eastern flank of Valdivia Bank, penetrated a 154.8 m thick sedimentary section ranging from the Paleocene (Thanetian; ~58.8 Ma) to Upper Cretaceous (Campanian; ~81.43–83.20 Ma). Igneous basement coring progressed only 39.1 m below the sediment/basalt contact, recovering three massive submarine tholeiitic basalt lava flows that are 4.1, 15.5, and >19.1 m thick, respectively. Paleomagnetic data from Sites U1577 and U1576 indicate that the former volcanic basement formed just before the end of the Cretaceous Normal Superchron and the latter during Chron 33r, shortly afterward. Biostratigraphic and paleomagnetic data suggest that Valdivia Bank becomes younger from east to west. Site U1578, located on a Center track guyot, provided a long and varied igneous section. After coring through 184.3 m of pelagic carbonate sediments mainly consisting of Eocene and Paleocene chalk (~55.64–63.5 Ma), Hole U1578A cored 302.1 m of igneous basement. Basement lavas are largely pillows but are interspersed with sheet and massive flows. Lava compositions are mostly alkalic basalts with some hawaiite. Several intervals contain abundant olivine (some fresh), and some of the pillow stacks consist of basalt with remarkably high Ti content. The igneous sequence is interrupted by 10 sedimentary interbeds consisting of chalk and volcaniclastics and ranging 0.46–10.19 m in thickness. Investigations of toothpick samples from the intercalated sediments were examined, each revealing the same age range of ~63.5–64.81 Ma (lower Paleocene; Danian). Paleomagnetic data display a change in basement magnetic polarity ~100 m above the base of the hole. Combining magnetic stratigraphy with biostratigraphic data, the igneous section is inferred to span >1 My. Nearly 7 months after Expedition 391, JOIDES Resolution transited from Cape Town to the north Atlantic. During this transit (Expedition 397T), 7.9 days of ship time were used to drill two holes (U1584A and U1585A) at sites on the Gough and Tristan tracks that had been omitted because of COVID-19–related time loss on the earlier cruise. For both, coring was begun only a short distance above the igneous basement to save time. The 75.2 m thick section drilled in Hole U1584A contains two sedimentary units: clay-rich carbonate sediments overlie a pumice-dominated volcaniclastic deposit containing basalt fragments. Because the goal was to core basalt and the base of the volcaniclastic deposit was not imaged in the seismic profile, the hole was terminated early to save operation time for the next site. In Hole U1585A, coring penetrated a 273.5 m thick sediment section overlying an 81.2 m thick pile of massive basalt flows. The sediment section is divided into four units: The uppermost unit consists of nannofossil chalk; The two intermediate units contain alternating chalk and volcaniclastic sediments containing several breccia units; and The lowermost unit consists of volcanic breccia containing juvenile blocks, bombs, and accretionary lapilli. This thick sedimentary section documents a transition from shallow-water volcanism to open-ocean sedimentation as the seamount subsided. The thick underlying basalt section is made up of four sparsely to highly phyric massive flows, the thickest of which is >43 m thick. Samples of these units are mostly basalt with a few trachybasalts and one trachyandesite. Although the igneous penetration was less than planned, coring during Expeditions 391 and 397T obtained samples that clearly will lead to an improved understanding of the evolution of the TGW hotspot and its track. Reasonable recovery of fresh basalt in some holes provides ample samples for geochemical, geochronologic, and paleomagnetic studies. Good recovery of Late Cretaceous and early Cenozoic chalk successions provides samples for paleoenvironmental study. 

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