Oxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe3+/Fe2+ between amphibole and glass synthesized in ...piston-cylinder and cold-seal apparatus experiments is developed as an oxybarometer, applicable to magmas ranging from basaltic to dacitic composition. The partitioning of Fe2+ is strongly dependent on melt polymerization; the relative compatibility of Fe2+ in amphibole decreases with increasing polymerization. The Fe2+/Mg distribution coefficient between amphibole and melt is a relatively constant value across all compositions and is, on average, 0.27. The amphibole oxybarometer is applied to amphibole in mafic enclaves, cumulates, and basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe3+/FeTotal. An average Fe3+/Fe2+ amphibole-glass distribution coefficient for basalt is used to convert the Fe3+/FeTotal of amphibole in samples from Shiveluch to magmatic oxygen fugacity relative to NNO. The fO2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently, higher fO2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. We identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before the eruption and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight into magmatic fO2 but also potentially details of shallow magmatic processes.
Laboratory spectral libraries of well-characterized natural samples are necessary for accurate interpretation of remote sensing spectral data. Tephra deposits, the result of explosive volcanic ...eruptions, are potentially found on all differentiated terrestrial bodies and are important chronologic and compositional marker beds on Earth. Here we present a visible/near-infrared (VNIR, 0.35–2.5 μm) and mid-infrared (MIR, 3.5–25 μm) spectral library composed of nineteen natural tephra samples from ten volcanic sources that span a range of compositions and components. The bulk, glass, and mineral phase compositions of each sample are measured and spectra from multiple size fractions of each sample were collected. The library can be found via the Terrestrial Analog portal (DOI: https://doi.org/10.5066/P9O54M4Q).
•Library of visible-near-infrared and mid-infrared spectra of tephra samples.•Tephra span wide-range of terrestrial compositions, including alkaline endmembers.•Tephra compositions and phase assemblages were characterized.
This paper presents a new X-ray absorption spectroscopy (XAS) method for making two-dimensional maps of Fe3+ in-situ in polished glass samples, which opens the door to study redox changes associated ...with magmatic processes such as crystallization, assimilation, ascent, and eruption. Multivariate analysis (MVA) allows selection of specific channels in a spectrum to inform predictions of spectral characteristics. Here, the sparse model of the least absolute shrinkage and selection operator (Lasso) is used to select key channels in XAS channels that can be used to predict accurate in-situ Fe3+ analyses of silicate glasses. By tuning the model to use only six channels, analytical time is decreased enough to allow mapping of Fe3+ variations in samples by making gridded point analyses at the scale of the XAS beam (1-2 µm). Maps of Fe3+ concentration can then be constructed using freely available, open source software (http://cars.uchicago.edu/xraylarch/). This result shows the enormous potential of using MVA to select indicative spectral regions for predicting variables of interest across a wide variety of spectroscopic applications. Redox gradients in lunar picritic glass beads first observed with point analyses are confirmed through this XAS mapping and suggest degassing processes during ascent and eruption are responsible for the range of Fe3+ values measured in these samples.
Experimental silicate glasses are often used as analog and calibration material for terrestrial and planetary materials. Measurements of Fe oxidation state using electron energy loss spectroscopy ...(EELS) in an aberration-corrected scanning transmission electron microscope (ac-STEM) show that a suite of experimental silicate (e.g., basaltic, andesitic, rhyolitic) glasses have spatially heterogeneous oxidation states at scales of tens of nanometers. Nano-crystals are observed in several of the glasses, indicating nucleation and incipient crystallization not seen at the scale of electron microprobe analysis (EMPA). Glasses prepared in air are uniformly oxidized while glasses prepared at the iron-wustite (IW) or quartz-fayalite-magnetite (QFM) buffers range from reduced to highly oxidized. EELS spectral shapes indicate that oxidized glasses have tetrahedral Fe3+ The nanoscale compositional and structural heterogeneities present in the experimental glasses mean that the suitability of such glasses as analogs for natural materials and calibration standards depends strongly on the scale of the measurements being done. The electron beam quickly damages silicate glass, but data showing changes in oxidation state among and within samples can be obtained with careful control of the beam current and dwell time. Determination of oxidation state in silicate glasses via STEM-EELS is very challenging, and accurate and reliable measurements of Fe3+/ΣFe require careful sample preparation and control of microscope conditions and benefit from comparison to complementary techniques.
We conducted a petrologic study of apatite within one LL chondrite, six R chondrites, and six CK chondrites. These data were combined with previously published apatite data from a broader range of ...chondrite meteorites to determine that chondrites host either chlorapatite or hydroxylapatite with ≤33 mol% F in the apatite X-site (unless affected by partial melting by impacts, which can cause F-enrichment of residual apatite). These data indicate that either fluorapatite was not a primary condensate from the solar nebula or that it did not survive lower temperature nebular processes and/or parent body processes. Bulk-rock Cl and F data from chondrites were used to determine that the solar system has a Cl/F ratio of 10.5 ± 1.0 (3σ). The Cl/F ratios of apatite from chondrites are broadly reflective of the solar system Cl/F value, indicating that apatite in chondrites is fluorine poor because the solar system has about an order of magnitude more Cl than F. The Cl/F ratio of the solar system was combined with known apatite-melt partitioning relationships for F and Cl to predict the range of apatite compositions that would form from a melt with a chondritic Cl/F ratio. This range of apatite compositions allowed for the development of a crude model to use apatite X-site compositions from achondrites (and chondrite melt rocks) to determine whether they derive from a volatile-depleted and/or differentiated source, albeit with important caveats that are detailed in the manuscript. This study further highlights the utility of apatite as a mineralogical tool to understand the origin of volatiles (including H2O) and the diversity of their associated geological processes throughout the history of our solar system, including at its nascent stage.
Igneous phenocrysts commonly exhibit zoning in major and trace element composition, reflecting (and potentially constraining) the differentiation and/or mixing histories of their parent melts. To ...date, little work has been done characterizing zonation of oxygen isotopes in minerals from mafic and ultramafic rocks. We present 259 ion probe (CAMECA ims-1280) measurements of δ18O in 34 natural magmatic and mantle olivines and pyroxenes from five hand samples from diverse igneous environments. We compare δ18O variations with zonation in other elements especially P; analyzed by electron microprobe analysis (EMPA) and nano-secondary ionization mass spectrometry (nanoSIMS). There is generally a close (average within ∼0·1-0·2 ‰) agreement between average δ18O values of olivines measured by SIMS (standardized against San Carlos olivine) and independently known values for bulk separates from the same samples measured by laser fluorination. These data demonstrate that current ion microprobe techniques are not only precise but also accurate enough for study of sub-per-mil oxygen isotope variations in silicates (within ∼0·2 ‰), provided samples are prepared and analyzed following strict guidelines. All but one of the 34 studied grains are homogeneous in δ18O within a small multiple of analytical precision estimated ±0·2‰, 1σ for most data; poorer for a subset of measurements made on small (∼5 µm) spots. This population of isotopically homogeneous grains includes some with oscillatory micrometer-scale P banding. The lack of δ18O variations suggests that whatever factors lead to this common mode of trace element zonation have no detectable effect on melt-crystal partitioning of oxygen isotopes. Large (2‰) oxygen isotope variations are observed in one olivine glomerocryst from Mauna Kea, Hawaii. This glomerocryst contains P-rich domains that are either equant or skeletal or feathery in outline, and these P-rich domains are systematically low in δ18O compared with adjacent, later-grown, P-poor olivine. This unusual oxygen isotope zonation pattern might reflect a kinetic fractionation during nucleation and growth of the cores of some olivine phenocrysts. We tested this hypothesis through measurements of δ18O distributions in synthetic olivines grown at a range of rates and exhibiting diverse patterns of P zoning. These synthetic olivines are homogeneous in δ18O, within the limits of our analyses (± 0·3-0·4‰ in this case) and show no connection between P zonation and oxygen isotope heterogeneity. We therefore think it more plausible that unusual O isotope zonation in the Mauna Kea glomerocryst reflects addition of a low-δ18O component to some Hawaiian magmas just before nucleation of olivine. More generally, this study demonstrates the utility of modern SIMS techniques for in situ study of the subtle (∼1‰ range) oxygen isotope variations characteristic of common mafic and ultramafic rocks.
Pre-edge features in the K absorption edge of X-ray absorption spectra are commonly used to predict Fe3+ valence state in silicate glasses. However, this study shows that using the entire spectral ...region from the pre-edge into the extended X-ray absorption fine-structure region provides more accurate results when combined with multivariate analysis techniques. The least absolute shrinkage and selection operator (lasso) regression technique yields %Fe3+ values that are accurate to ±3.6% absolute when the full spectral region is employed. This method can be used across a broad range of glass compositions, is easily automated, and is demonstrated to yield accurate results from different synchrotrons. It will enable future studies involving X-ray mapping of redox gradients on standard thin sections at 1 × 1 µm pixel sizes.
Understanding the frequency, magnitude, and nature of explosive volcanic eruptions is essential for hazard planning and risk mitigation. Terrestrial stratigraphic tephra records can be patchy and ...incomplete due to subsequent erosion and burial processes. In contrast, the marine sedimentary record commonly preserves a more complete historical record of volcanic activity as individual events are archived within continually accumulating background sediments. While larger tephra layers are often identifiable by changes in sediment color and/or texture, smaller fallout layers may also be present that are not visible to the naked eye. These cryptotephra are commonly more difficult to identify and often require time‐consuming and destructive point counting, petrography, and microscopy work. Here we present several rapid, nondestructive, and quantitative core scanning methodologies (magnetic susceptibility, visible to shortwave infrared spectroscopy, and XRF core scanning) which, when combined, can be used to identify the presence of increased volcaniclastic components (interpreted to be cryptotephra) in the sedimentary record. We develop a new spectral parameter (BDI1000VIS) that exploits the absorption of the 1 µm near‐infrared band in tephra. Using predetermined mixtures, BDI1000VIS can accurately identify tephra layers in concentrations >15–20%. When applied to the upper ∼270 kyr record of IODP core U1396C from the Caribbean Sea, and verified by traditional point counting, 29 potential cryptotephra layers were identified as originating from eruptions of the Lesser Antilles Volcanic Arc. Application of these methods in future coring endeavors can be used to minimize the need for physical disaggregation of valuable drill core material and allow for near‐real‐time recognition of tephra units, both visible and cryptotephra.
Key Points
Tephra, both visible and invisible, can be identified in a rapid, nondestructive manner
VSWIR spectral parameters can be used to identify tephra and cryptotephra
The tephrochronologic record of the northern Antilles has been expanded
A suite of Hawaiian basalts that were variably altered in the presence of SO2-rich gases during the current summit eruptive episode at Halemaumau crater, Kilauea, were studied to determine their ...alteration phase assemblage and reactive pathways using electron microscopy, Mossbauer spectroscopy, and X-ray diffraction. The alteration conditions represent an acid fog environment. Alteration rinds on the basalts vary in thickness from tens of micrometers to the entirety of the rock and are composed of amorphous silica rims (85-95 wt% SiO2) overlain by sulfates. Sulfate mineralogy consisted of gypsum, anhydrite, and natroalunite-jarosite. No phyllosilicates were observed in any alteration assemblages. Phenocrysts and glass were both observed to be extensively reacted during alteration. The Halemaumau samples may provide good analogs for basalt alteration on other rocky planetary bodies, i.e., Mars, Venus, and Mercury, where S is ubiquitous and low fluid/rock ratios are common.
The surface of Venus is in contact with a hot (~470 °C), high pressure (92 bars), and caustic (CO
with S, but little H
O) atmosphere, which should cause progressive alteration of the crust in the ...form of sulfate and iron-oxide coatings; however, the exact rate of alteration and mineral species are not well constrained. Different experimental approaches, each with its own limitations, are currently being used to constrain mineralogy and alteration rates. One note is that no experimental approach has been able to fully replicate the necessary conditions and sustain them for a significant length of time. Furthermore, geochemical modeling studies can also constrain surface alteration mineralogy, again with different assumptions and limitations. Here, we review recent geochemical modeling and experimental studies to constrain the state of the art for alteration mineralogy, rate of alteration, open questions about the surface mineralogy of Venus, and what can be constrained before the fleet of missions arrives later this decade.
Combining the new results confirms that basalt on the surface of Venus should react quickly and form coatings of sulfates and iron-oxides; however, the mineralogy and rate of alteration are dependent on physical properties of the protolith (including bulk composition, mineralogy, and crystallinity), as well as atmospheric composition, and surface temperature. Importantly, the geochemical modeling results show that the mineralogy is largely controlled by atmospheric oxygen fugacity, which is not well constrained for the near-surface environment on Venus. Therefore, alteration experiments run over a range of oxygen and sulfur fugacities are needed across a wide range of Venus analog materials with varying mineralogy and crystallinity.
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