Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.
more »
« less
This content will become publicly available on December 1, 2024
Gas-mediated trace element incorporation into rhyolite-hosted topaz: A synchrotron microbeam XAS study
Magmatic gas exsolving during late-stage cooling of shallow magmas has been considered an important facilitator of low-pressure alteration and metal transport. However, the chemical properties of such gas, particularly its metal transport mechanisms and capacity, remain elusive. Trace elements in minerals produced by gas-mediated surface reaction or precipitation from gas capture details of gas composition and reaction pathways. However, interpretation of mineral trace element contents is dependent on understanding crystallographic controls on gas/mineral partitioning. This work investigates the structural accommodation of As, Mn, Ga, Ge, Fe, and Ti in vapor-deposited topaz of vesicular topaz rhyolite from the Thomas Range, Utah, through single-crystal synchrotron microbeam X-ray techniques on picogram quantities of those trace elements. X-ray absorption near edge structure (XANES) data indicates that these elements are incorporated into topaz as As5+, Fe3+, Mn3+, Ti4+, Ga3+, and Ge4+. Extended X-ray absorption fine structure (EXAFS) analysis for these trace elements, compared to EXAFS of structural Al and Si, reveals that As5+ and Ge4+ are incorporated directly into the tetrahedral site of the topaz structure, with the octahedral site accommodating Mn3+, Fe3+, Ga3+, and Ti4+. For As5+ and Fe3+, the structural impact of substitution extends to at least second neighbors (other elements were only resolvable to first neighbors). Further interpretation of the EXAFS results suggests that the substitution of Ti4+ results in increased distortion of the octahedral site, while the other trace elements induce more uniform expansion correlating in magnitude to their ionic radius. Comparison of quantified X-ray fluorescence (XRF) data for two topaz crystals from this rhyolite reveals variable trace element concentrations for As5+, Fe3+, Ga3+, and Ti4+, reflective of a source gas undersaturated in these trace elements changing in concentration over the period of topaz deposition. The identical Ge4+ content of the two topaz crystals suggests that Ge4+ in the gas was buffered by the growth of another Ge4+-bearing phase, such as quartz. The very low Mn3+ content in the topaz crystals does not reflect the abundance of Mn3+ in the gas (saturation of Mn is evidenced by coexisting bixbyite). Instead, it suggests a strong Jahn-Teller inhibitory effect to the substitution of Mn3+ for Al3+ in the distorted octahedral site of topaz. It is proposed that exsolution of an HF-enriched gas from cooling rhyolitic magma led to local scouring of Al, Si, and trace metals from the magma. Once topaz crystals nucleated, self-catalyzed reactions that recycle HF led to continued growth of topaz.
more » « less- Award ID(s):
- 2105876
- NSF-PAR ID:
- 10520494
- Publisher / Repository:
- MSA
- Date Published:
- Journal Name:
- American Mineralogist
- Volume:
- 108
- Issue:
- 12
- ISSN:
- 0003-004X
- Page Range / eLocation ID:
- 2153 to 2163
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)Symmetry-dependent properties such as ferroelectricity are suppressed at room temperature in Pb-free ABO 3 perovskites due to antiferrodistortive dynamics (octahedral rotations/tilts), resulting in the preferential stabilization of centrosymmetric crystals. For this reason, defect engineering (Ca doping, oxygen vacancy, etc. ) has been leveraged to break the symmetry of these crystals by inducing symmetry/structural transitions to modify the local A/B-site environment. This work demonstrates the use of in situ / ex situ photoluminescence spectroscopy to systematically detect symmetry/structural transformations in prototypical ferroelectric ABO 3 perovskites. These baseline optical responses are compared to recently synthesized Ca x Sr 1−x NbO 3 (CSNO) nanocrystals, which undergoes similar ferroelectric/structural phase transitions. Furthermore, the resultant PL response is corroborated with X-ray diffraction (XRD) and absorption spectroscopy (XAS) measurements to confirm the structural changes. This ability to directly monitor the local site symmetry within ABO 3 perovskites via photoluminescence spectroscopy can be used to screen for temperature- and defect-induced ferroelectric transitions.more » « less
-
Magnetitite deposits like El Laco (Chile) are rare and have controversial origins. An unusual magnetitite lava flow overlying a rhyolite unit occurs in the north-central Alaska Range and originally covered ~ 750 km2 of the Miocene Nenana basin. Dating of the rhyolite and relationships between the magnetitite and sedimentary rocks indicate that both are of Late Miocene age. The magnetitite flow is mainly magnetite with some post-eruptive alteration to hematite. Both the rhyolite flow and the magnetitite flow are vesicular, but the magnetitite flow also has small, millimetre-scale columnar jointing. The vesicular zones in the magnetitite flow grade into massive rock on the scale of a thin section, suggesting a degassing lava origin. Samples of the magnetitite flow contain between 12 and 26 wt.% SiO2 and between 45 and 75 wt.% FeO. Rare earth elements (REE) and trace elements from the magnetitite and rhyolite have similar patterns but with lesser abundance in the magnetitite. Both the rhyolite and the magnetitite have light-REE-enriched REE profiles with negative Eu anomalies. Electron microscopic analysis shows that most of the silica and trace element content of the magnetitite flow comes from very finely disseminated silicate minerals and glass in the magnetite. This suggests that the magnetitite was derived from a magma that had undergone unmixing into a silica-rich phase and an iron-rich phase prior to its eruption. Fractures and vesicles within the magnetitite flow contain minor rhyolitic glass and minerals suggesting that the rhyolite magma invaded columnar joints in the solidified magnetitite flow, and is a subvolcanic sill-like body at the studied locality. The magnetitite flow erupted prior to the emplacement of the rhyolite, which may be extrusive on a regional scale. The features of the Nenana magnetitite, and its geological relationships, are consistent with genetic models that invoke unmixing of magma into immiscible Fe-rich and Si-rich liquids during ascent.more » « less
-
Pellegrini, M ; Saccani, C ; Guzzini, A (Ed.)more » « less
Twenty novel Mn, Fe, and Cu complexes of ethylene cross-bridged tetraazamacrocycles with potentially copolymerizable allyl and benzyl pendant arms were synthesized and characterized. Multiple X-ray crystal structures demonstrate the cis-folded pseudo-octahedral geometry forced by the rigidifying ethylene cross-bridge and show that two cis coordination cites are available for interaction with substrate and oxidant. The Cu complexes were used to determine kinetic stability under harsh acidic and high-temperature conditions, which revealed that the cyclam-based ligands provide superior stabilization with half-lives of many minutes or even hours in 5 M HCl at 50–90 °C. Cyclic voltammetry studies of the Fe and Mn complexes reveal reversible redox processes indicating stabilization of Fe2+/Fe3+ and Mn2+/Mn3+/Mn4+ oxidation states, indicating the likelihood of catalytic oxidation for these complexes. Finally, dye-bleaching experiments with methylene blue, methyl orange, and rhodamine B demonstrate efficient catalytic decolorization and allow selection of the most successful monomeric catalysts for copolymerization to produce future heterogeneous water purification materials.
-
A set of 11 clinoamphibole standards with known Fe3+/ΣFe were analyzed for X-ray absorption spectra at the Advanced Photon Source at Argonne National Laboratory. Fe3+/ΣFe of each sample is known from Mössbauer data, and span from 0% to 100% Fe3+/ΣFe. Due to the extreme absorption anisotropy of crystals, we determined the orientation of clinoamphiboles crystals using single-crystal X-ray diffraction and spindle stage techniques. Clinoamphiboles crystal were then analyzed with the beam propagation and polarization axes along known direction relative to the crystallographic basis. Absorption anisotropy was then interpolated to predict the range of absorption magnitudes a clinoamphibole may exhibit at each energy. Partial least square fits for Fe3+/ΣFe indicate predictions are closest to the known Fe3+/ΣFe when comparing the same orientation of each sample in the dataset. Predictions including all orientations, while less accurate, still yield strong results with an R-squared of 0.9 and RMSE of 4.7. The results from this dataset demonstrate the efficacy of XAS as a microanalytical technique for crystalline materials, and functions as a reference dataset to research in oxybarometry.more » « less
