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1. Expedition 355 Preliminary Report: Arabian Sea Monsoon

   https://doi.org/10.14379/iodp.pr.355.2015  

   Pandey, D.K. ; Clift, P.D. ; Kulhanek, D.K. ; Ando, S. ; Bendle, J.A.P. ; Bratenkov, S. ; Griffith, E.M. ; Gurumurthy, G.P. ; Hahn, A. ; Iwai, M. ; et al ( July 2015 , Preliminary report)

   The Indian (southwest) summer monsoon is one of the most intense climatic phenomena on Earth. Its long-term development has been linked to the growth of high topography in South and Central Asia. The Indian continental margin, adjoining the Arabian Sea, offers a unique opportunity to investigate tectonic–climatic interactions and the net impact of these processes on weathering and erosion of the western Himalaya. During International Ocean Discovery Program Expedition 355, two sites (U1456 and U1457) were drilled in Laxmi Basin in the eastern Arabian Sea to document the coevolution of mountain building, weathering, erosion, and climate over a range ofmore »timescales. In addition, recovering basement from the eastern Arabian Sea provides constraints on the early rifting history of the western continental margin of India with special emphasis on continental breakup between India and the Seychelles and its relationship to the plume-related volcanism of the Deccan Plateau. Drilling and coring operations during Expedition 355 recovered sediment from Sites U1456 and U1457 in the Laxmi Basin, penetrating 1109.4 and 1108.6 m below seafloor (mbsf), respectively. Drilling reached sediment dated to 13.5–17.7 Ma (late early to early middle Miocene) at Site U1456, although with a large hiatus between the lowermost sediment and overlying deposits dated to <10.9 Ma. At Site U1457, a much longer hiatus occurs near the base of the cored section, spanning from 10.9 to ~62 Ma. At both sites, hiatuses span ~8.2–9.2 and ~3.6–5.6 Ma, with a possible condensed section spanning ~2.0–2.6 Ma, although the total duration for each hiatus is slightly different between the two sites. A major submarine fan draining the western Himalaya and Karakoram must have been supplying sediment to the eastern Arabian Sea since at least ~17 Ma. Sand mineral assemblages indicate that the Greater Himalayan Crystalline Sequence was fully exposed to the surface by this time. Most of the recovered sediment appears to be derived from the Indus River and includes minerals that are unique to the Indus Suture Zone, in particular glaucophane and hypersthene, most likely originating from the structural base of the Kohistan arc. Pliocene sandy intervals at Site U1456 were deposited in lower fan “sheet lobe” settings, with intervals of basin plain turbidites separated by hemipelagic muddy sections deposited during the Miocene. Site U1457 is more distal in facies, reflecting its more marginal setting. No major active lobe appears to have affected the Laxmi Basin since the Middle Pleistocene (~1.2 Ma). We succeeded in recovering sections spanning the 8 Ma climatic transition, when monsoon intensity is believed to have changed strongly, although the nature of this change awaits postcruise analysis. We also recovered sediment from a large mass transport deposit measuring ~330 and ~190 m thick at Sites U1456 and U1457, respectively. This section includes an upper sequence of slump-folded muddy and silty rocks, as well as underlying calcarenites and limestone breccias, together with smaller amounts of volcanic clasts, all of which are likely derived from the western Indian continental shelf. Identification of similar facies on the regional seismic lines in Laxmi Basin suggests that these deposits form parts of one of the world’s largest mass transport deposits. Coring of igneous basement was successful at Site U1457. Recovery of massive basalt and associated volcaniclastic sediment at this site should address the key questions related to rifting and volcanism associated with formation of Laxmi Basin. Geochemical analysis is required to understand the petrogenesis and thus the tectonic setting of volcanism that will reveal whether it is oceanic basalt or volcanic rock contaminated by underlying continental crust or continental flood basalt. However, the fact that the lavas are massive and have few vesicles implies water depths of eruption likely deeper than 2000 m. This precludes opening of the basin in the presence of a major mantle thermal anomaly, such as that associated with the Deccan Large Igneous Province. Other observations made at the two sites during Expedition 355 provide vital constraints on the rift history of this margin. Heat flow measurements at the two drill sites were calculated to be ~57 and ~60 mW/m2. Such heat flow values are compatible with those observed in average oceanic crust of 63–84 Ma age, as well as with the presence of highly extended continental crust. Postcruise analyses of the more than ~1722 m of core will provide further information about the nature of tectonic–climatic interactions in this global type area for such studies.« less
2. The effects of discontinuity topography in the mantle transition zone on global geodynamic observables and mantle heterogeneity

   https://doi.org/10.1093/gji/ggac074  

   Glišović, Petar ; Grand, Stephen P ; Lu, Chang ; Forte, Alessandro M ; Wei, S Shawn ( March 2022 , Geophysical Journal International)

   SUMMARY Despite progress in tomographic imaging of Earth's interior, a number of critical questions regarding the large-scale structure and dynamics of the mantle remain outstanding. One of those questions is the impact of phase-boundary undulations on global imaging of mantle heterogeneity and on geodynamic (i.e. convection-related) observables. To address this issue, we developed a joint seismic-geodynamic-mineral physical tomographic inversion procedure that incorporates lateral variations in the depths of the 410- and 660-km discontinuities. This inversion includes S-wave traveltimes, SS precursors that are sensitive to transition-zone topography, geodynamic observables/data (free-air gravity, dynamic surface topography, horizontal divergence of tectonic plates and excessmore »core-mantle boundary ellipticity) and mineral physical constraints on thermal heterogeneity. Compared to joint tomography models that do not include data sensitivity to phase-boundary undulations in the transition zone, the inclusion of 410- and 660-km topography strongly influences the inference of volumetric anomalies in a depth interval that encompasses the transition zone and mid-mantle. It is notable that joint tomography inversions, which include constraints on transition-zone discontinuity topography by seismic and geodynamic data, yield more pronounced density anomalies associated with subduction zones and hotspots. We also find that the inclusion of 410- and 660-km topography may improve the fit to the geodynamic observables, depending on the weights applied to seismic and geodynamic data in the inversions. As a consequence, we find that the amplitude of non-thermal density anomalies required to explain the geodynamic data decreases in most of the mantle. These findings underline the sensitivity of the joint inversions to the inclusion of transition-zone complexity (e.g. phase-boundary topography) and the implications for the inferred non-thermal density anomalies in these depth regions. Finally, we underline that our inferences of 410- and 660-km topography avoid a commonly employed approximation that represents the contribution of volumetric heterogeneity to SS-wave precursor data. Our results suggest that this previously employed correction, based on a priori estimates of upper-mantle heterogeneity, might be a significant source of error in estimating the 410- and 660-km topography.« 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 manymore »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.« less
4. Anomalous elastic behavior of phase egg, AlSiO3(OH), at high pressures

   Mookherjee, Mainak ; Panero, Wendy R. ; Wunder, Bernd ; Jahn, Sandro ( January 2019 , The American mineralogist)

   Phase egg, [AlSiO3(OH)], is an aluminosilicate hydrous mineral that is thermodynamically stable in lithological compositions represented by Al2O3-SiO2-H2O (ASH) ternary, i.e., a simplified ternary for the mineralogy of subducted sediments and continental crustal rocks. High-pressure and high-temperature experiments on lithological compositions resembling hydrated sedimentary layers in subducting slabs show that phase egg is stable up to pressures of 20–30 GPa, which translates to the transition zone to lower mantle depths. Thus, phase egg is a potential candidate for transporting water into the Earth’s mantle transition zone. In this study, we use first-principles simulations based on density functional theory to exploremore »the pressure dependence of crystal structure and how it influences energetics and elasticity. Our results indicate that phase egg exhibits anomalous behavior of the pressure dependence of the elasticity at mantle transition zone depths (~15 GPa). Such anomalous behavior in the elasticity is related to changes in the hydrogen bonding O-H···O configurations, which we delineate as a transition from a low-pressure to a high-pressure structure of phase egg. Full elastic constant tensors indicate that phase egg is very anisotropic resulting in a maximum anisotropy of compressional wave velocity, AvP ≈ 30% and of shear wave velocity, AvS ≈ 17% at zero pressures. Our results also indicate that the phase egg has one of the fastest bulk sound velocities (vP and vS) compared to other hydrous aluminous phases in the ASH ternary, which include topaz-OH, phase Pi, and d-AlOOH. However, the bulk sound velocity of phase egg is slower than that of stishovite. At depths corresponding to the base of mantle transition zone, phase egg decomposes to a mixture of d-AlOOH and stishovite. The changes in compressional DvP and shear DvS velocity associated with the decomposition is ~0.42% and –1.23%, respectively. Although phase egg may be limited to subducted sediments, it could hold several weight percentages of water along a normal mantle geotherm.« less
5. Evidence for the stability of ultrahydrous stishovite in Earth’s lower mantle

   https://doi.org/10.1073/pnas.1914295117  

   Lin, Yanhao ; Hu, Qingyang ; Meng, Yue ; Walter, Michael ; Mao, Ho-Kwang ( December 2019 , Proceedings of the National Academy of Sciences)

   The distribution and transportation of water in Earth’s interior depends on the stability of water-bearing phases. The transition zone in Earth’s mantle is generally accepted as an important potential water reservoir because its main constituents, wadsleyite and ringwoodite, can incorporate weight percent levels of H₂O in their structures at mantle temperatures. The extent to which water can be transported beyond the transition zone deeper into the mantle depends on the water carrying capacity of minerals stable in subducted lithosphere. Stishovite is one of the major mineral components in subducting oceanic crust, yet the capacity of stishovite to incorporate water beyondmore »at lower mantle conditions remains speculative. In this study, we combine in situ laser heating with synchrotron X-ray diffraction to show that the unit cell volume of stishovite synthesized under hydrous conditions is ∼2.3 to 5.0% greater than that of anhydrous stishovite at pressures of ∼27 to 58 GPa and temperatures of 1,240 to 1,835 K. Our results indicate that stishovite, even at temperatures along a mantle geotherm, can potentially incorporate weight percent levels of H₂O in its crystal structure and has the potential to be a key phase for transporting and storing water in the lower mantle.

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