Altered Tephra Research Articles

Protolith identification of bedding-parallel, smectite-bearing shear zones in argillaceous and siliceous marine sediments: Discriminating between tephra-derived shear zones and host-rock-derived fault gouges

Smectite-bearing shear zones are important in a variety of geoscientific and geoengineering fields owing to the unique physicochemical properties of smectite. Such shear zones occur globally in argillaceous and siliceous marine sediments, and are typically a product of either host-rock deformation, or exploitation of weak, altered tephra horizons within the sedimentary sequence. However, it is difficult to discriminate between a tephra-derived and host-rock-derived origin based on simple fault-rock observations. Here, a method is proposed that integrates two discriminators: the ratio of smectite to illite, sm/(ill + sm) (wt%/wt%), in the clay fractions of shear zone samples; and the ratio of conservative elements, Al2O3/TiO2 (wt%/wt%), in the bulk chemistry of samples. Although high sm/(ill + sm) ratios may indicate a tephra-derived origin, deformation processes can lower the ratio, making protolith identification difficult. Previous studies have suggested that different Al2O3/TiO2 ratios between shear zones and host rocks may also imply a tephra-derived origin. However, the Al2O3/TiO2 ratios within shear zones may be intrinsically comparable with those of the host rock, even if the shear zones were derived from tephra layers. Despite these ambiguities, application of these two indicators to the protolith identification of bedding-parallel, smectite-bearing shear zones in a folded siliceous mudstone (Hokkaido, Japan) demonstrates that combining sm/(ill + sm) and Al2O3/TiO2 ratios is a useful starting point when attempting to identify the protolith of smectite-bearing shear zones in argillaceous and siliceous marine mudstones.

Clay mineralogy of altered tephra beds and facies correlation between the Permian-Triassic boundary stratigraphic sets, Guizhou, south China

The Permian-Triassic boundary (PTB) successions in south China contain numerous altered volcanic ash (tephra) beds (K-bentonites), which presents the opportunity to correlate the PTB position in both marine and non-marine sections, especially when no feasible biostratigraphic markers are available in the profiles. Clay mineralogical and geochemical studies of two altered ash beds in the Zhongzai (ZZ) and Tucheng (TC) sections, in Guizhou Province, south China, deposited in littoral and interactive marine-terrestrial environments, respectively, permit an investigation of the alteration of ash and correlation of ash beds between disparate facies. The results show that the two ZZ altered ashes consist of mainly R1 I/Sm and minor R0 I/Sm. Sample ZZ-1 has slightly more R1 I/Sm but less R0 I/Sm relative to sample ZZ-2. The TC ash samples contain mainly kaolinite and mixed-layer kaolinite/smectite (K/Sm). The poorly-crystallized kaolinite is present in pseudo-hexagonal plates, and the well-crystallized kaolinite occurs in book-like aggregates in veins or cavities. K/Sm minerals are derived from the transformation of smectite to kaolinite. Obviously, the TC ashes experienced terrestrial weathering and resedimentation prior to final burial and preservation, and local microenvironmental conditions control the formation of clay minerals. The ZZ ash samples have markedly higher 87Sr/86Sr values than those of the TC samples. The notable difference in 87Sr/86Sr value of ash beds between the sections is attributed to variations in RbSr partitioning during the chemical weathering process in different environments. The ZZ ash samples have notably higher 143Nd/144Nd ratios than those of the TC samples, suggesting that the ZZ ashes are likely derived from eruptions involving continental crust and the TC ashes originate from eruptions involving new continental island arcs, in agreement with the REE distributions and the Ti vs. Zr, TiO2 vs. Al2O3, and Zr/TiO2 vs. Nb/Y discrimination plots. The occurrence of tephras from such markedly different volcanic source materials different in PTB stratigraphic sets previously believed to be synchronous, Guizhou, south China, suggests that correlation between disparate facies by an ash marker requires geochemical fingerprinting of the materials to confirm correlations or otherwise.

A new attraction-detachment model for explaining flow sliding in clay-rich tephras

Abstract Altered pyroclastic (tephra) deposits are highly susceptible to landsliding, leading to fatalities and property damage every year. Halloysite, a low-activity clay mineral, is commonly associated with landslide-prone layers within altered tephra successions, especially in deposits with high sensitivity, which describes the post-failure strength loss. However, the precise role of halloysite in the development of sensitivity, and thus in sudden and unpredictable landsliding, is unknown. Here we show that an abundance of mushroom cap–shaped (MCS) spheroidal halloysite governs the development of sensitivity, and hence proneness to landsliding, in altered rhyolitic tephras, North Island, New Zealand. We found that a highly sensitive layer, which was involved in a flow slide, has a remarkably high content of aggregated MCS spheroids with substantial openings on one side. We suggest that short-range electrostatic and van der Waals interactions enabled the MCS spheroids to form interconnected aggregates by attraction between the edges of numerous paired silanol and aluminol sheets that are exposed in the openings and the convex silanol faces on the exterior surfaces of adjacent MCS spheroids. If these weak attractions are overcome during slope failure, multiple, weakly attracted MCS spheroids can be separated from one another, and the prevailing repulsion between exterior MCS surfaces results in a low remolded shear strength, a high sensitivity, and a high propensity for flow sliding. The evidence indicates that the attraction-detachment model explains the high sensitivity and contributes to an improved understanding of the mechanisms of flow sliding in sensitive, altered tephras rich in spheroidal halloysite.

Provenance of bentonite layers in the Palaeocene strata of the Central Basin, Svalbard: implications for magmatism and rifting events around the onset of the North Atlantic Igneous Province

A fold-and-thrust belt developed between Greenland and Svalbard during the Palaeogene, with an associated foreland basin forming in what is now Spitsbergen. This Central Basin is comprised of the Van Mijenfjorden Group, a ~2.3km thick sandstone-shale dominated succession that contains prominent and laterally continuous bentonite layers in the lower formations. These altered tephra layers can be used as stratigraphic markers that connect the basin development with regional explosive volcanism and changes to relative plate motions. We sampled and analysed bentonites from nine borehole cores across the Central Basin. Each layer shows evidence of alteration, with mobile elements such as alkali and alkali earth metals particularly disrupted. However, immobile elements including rare earth elements (REE) and preserved igneous minerals retain a magmatic signature, allowing for comparisons with potential volcanic sources to be made. The majority of bentonites are both evolved and strongly alkaline, with chemical signatures that are much closer to the continental rift events around Ellesmere Island and North Greenland than to the early activity of the North Atlantic Igneous Province (NAIP). There is a clear difference between tephra layers in the mid Palaeocene versus late Palaeocene strata. The early bentonites have a REE signature comparable to the volcanics of the Kap Washington Group exposed in North Greenland. The later bentonites have likely come from volcanic centres in the Nares Strait that are also the source of abundant volcaniclastic sediments in the Judge Daly Promontory, Ellesmere Island. These findings suggest that a mid to late Palaeocene change in locus of volcanic provenance may reflect changes in relative plate motions related to the formation of the West Spitsbergen fold-and-thrust belt and the emplacement of the NAIP. However, the lack of bentonites matching NAIP sources suggests that explosive volcanism was of insufficient magnitude to lead to deposition in the Central Basin at this time.

K-bentonites: A review

Pyroclastic material in the form of altered volcanic ash or tephra has been reported and described from one or more stratigraphic units from the Proterozoic to the Tertiary. This altered tephra, variously called bentonite or K-bentonite or tonstein depending on the degree of alteration and chemical composition, is often linked to large explosive volcanic eruptions that have occurred repeatedly in the past. K-bentonite and bentonite layers are the key components of a larger group of altered tephras that are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration under the influence of fluids with high-Mg content and that leach alkali elements. Smectite composition is partly controlled by parent rock chemistry. Studies have shown that K-bentonites often display variations in layer charge and mixed-layer clay ratios and that these correlate with physical properties and diagenetic history. The following is a review of known K-bentonite and related occurrences of altered tephra throughout the timescale from Precambrian to Cenozoic.

Altered marine tephra deposits as potential slope failure planes?

Weathering of tephra results in increasing proportions of mechanically weak, authigenic clay minerals (smectite). This suggests that altered tephra represent inherent weak layers in slope sediment sequences, and these may facilitate slope failure in submarine and other aquatic environments. In drained direct shear experiments, tephra in different alteration stages were compared to common sand–clay mixtures for geotechnical reference. Attention is drawn to the influence of particle shape on shear strength. The results revealed volcanic ash to have (1) a high strength end-member at low alteration stages due to particle roughness and angularity and (2) a low strength end-member after complete diagenetic alteration, both under static conditions. This would suggest that strongly altered volcanic ash layers could potentially be responsible for slope failures. However, a review of ODP and IODP Expedition reports shows that advanced ash alteration mostly occurs at depths below (>800 mbsf) those commonly observed for slope failure initiation (<400 mbsf). This, in turn, suggests that volcanic ash alteration does not play an important role in the initiation of slope failure.

Stratigraphic correlations using trace elements in apatite from Late Ordovician (Sandbian-Katian) K-bentonites of eastern North America

The early Late Ordovician sedimentary rocks of eastern North America contain a relatively large number (>100) of widespread heavily altered tephra layers (K-bentonites). These beds represent an intense period of subaerial volcanism that occurred from ca. 455 to 449 Ma. The sedimentary rocks that contain these K-bentonites display complex regional lithostratigraphic relationships ranging from clastic foreland basin facies to cratonic carbonate platform facies. Accurate correlation of these ancient ash-fall beds is essential for testing chronostratigraphic hypotheses that attempt to connect these different tectono-sedimentary provinces. Despite the relatively thorough study of a few of these K-bentonites over the past several decades, the full stratigraphic potential of these beds has yet to be realized. To test the utility of the apatite trace-element K-bentonite correlation method on a larger scale, we studied over 200 K-bentonite samples from the Mohawkian Stage of eastern North America and statistically compared our results with previous studies on the same suites of K-bentonites. Electron microprobe (EPMA) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) results show that apatite trace-element data provide unique bed discriminators. Each of the K-bentonite layers exhibits unique and reproducible trends in Mg, Cl, Mn, Fe, Ce, Y, and other trace-element concentrations in apatite. Statistical evaluation of results from our apatite analyses suggests correlations for 12 K-bentonite beds, providing a significant improvement in stratigraphic resolution. The stratigraphic relations indicated by these new K-bentonite fingerprints provide a rigorous means by which to evaluate some previous interpretations of biostratigraphic, chemostratigraphic, and sequence stratigraphic studies in eastern North America.

Jarosite in a Pleistocene East African saline-alkaline paleolacustrine deposit: Implications for Mars aqueous geochemistry

[1] Jarosite occurs within altered tephra from the saline-alkaline paleolake deposits of Pliocene-Pleistocene Olduvai Gorge, Tanzania. Zeolites (mainly phillipsite), authigenic K-feldspar, and Mg/Fe-smectites dominate the mineral assemblage, indicating saline-alkaline diagenetic conditions (pH > 9). As jarosite is ordinarily an indicator of acidic conditions on Earth and Mars, its association with such undisputed high-pH indicators is unexpected. Of 55 altered tephra samples collected from the paleolake basin and margin deposits, eleven contained jarosite detectable by X-ray Diffraction (XRD) (>0.15%). Mössbauer spectroscopy, Fourier Transform Infrared Reflectance (FTIR), Electron Probe Microanalysis (EPMA), X-ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM) analyses confirm the presence and nature of the jarosite. This paper documents this occurrence and presents mechanisms that could produce this unusual and contradictory mineral assemblage. We favor a mechanism by which jarosite formed recently, perhaps as modern ground and meteoric water interacted with and oxidized paleolacustrine pyrite, providing local and temporary acidic conditions. However, local groundwater (at modern springs) has a pH > 9. In recent studies of Mars, the presence of jarosite or other Fe or Mg sulfates is often used to indicate dominantly acidic conditions. Regardless, the current study shows that jarosite can form in sediments dominated by alkaline minerals and solutions. Its coexistence with Mg/Fe smectites in particular makes it relevant to recent observations of Martian paleolakes.

Element Distribution Between Coexisting Authigenic Mineral Phases in Argillic and Zeolitic Altered Tephra, Olduvai Gorge, Tanzania

AbstractThe current study demonstrates how co-existing zeolite and clay minerals formed by the alteration of tephra in a closed-basin lacustrine and lake-margin environment can retain the overall composition of the original bulk tephra for many elements, even when diagenetic conditions and resulting authigenic mineral assemblages change. Zeolite and clay minerals co-exist in the closed-basin, saline-alkaline lacustrine altered tephra of Pleistocene Olduvai Gorge, Tanzania, and their diagenetic histories can be reconstructed using variations in their textures and compositions. The authigenic minerals in the altered tephra of the Olduvai paleolake form a classic ‘bull’s-eye’ pattern, with clay-dominated tephra in the distal lake margin, chabazite and phillipsite in the proximal margin, and phillipsite ± K-feldspar in the intermittently dry lake and lake center. Fifteen representative samples of altered volcanic ash lapilli (designated Tuff IF) were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), electron probe microanalysis (EPMA), and scanning electron microscopy (SEM) to determine their authigenic mineral assemblages and bulk compositions, and to texturally and compositionally compare their clay mineral and zeolite components.Textural observations indicate that clay minerals formed first, followed by zeolites and finally feldspars. Clay minerals, however, persist even in the most altered samples. The overall composition of Tuff IF shows only limited change in Fe, Si, Al, and Na between fresh, clay-altered, and zeolite-dominated diagenetic environments, despite significant differences in authigenic assemblage. Where zeolites dominate the assemblage, the remaining clay minerals are rich in Mg, Fe, and Ti, elements that are not readily incorporated in zeolite structures. Where clay minerals dominate, they are more Al-rich. A ‘mixing model’ combining clay-mineral and zeolite compositions yields a close approximation of the original volcanic glass for most elements (exceptions including Mg, Ca, and K). This initial composition was preserved in part by the redistribution of elements between co-existing clay minerals and zeolites.

Visible, near‐infrared, and middle infrared spectroscopy of altered basaltic tephras: Spectral signatures of phyllosilicates, sulfates, and other aqueous alteration products with application to the mineralogy of the Columbia Hills of Gusev Crater, Mars

We studied visible to near‐infrared (VNIR, 0.35–2.5 μm) and middle infrared (MIR, 1800–250 cm−1, also called thermal infrared and vibrational) spectra of basaltic tephras from Mauna Kea volcano that were altered under ambient, hydrothermal (hydrolytic and acid sulfate), and dry heat conditions. Although models of MIR spectra of altered tephras generally produce fits whose quality is reduced compared to deconvolutions of primary lithologies, they successfully identify major alteration phases (cristobalite, oxide, phyllosilicate, and sulfate) except in palagonites. MIR spectra of the &lt;45 μm fraction of all altered tephra samples exhibit an H2O peak at ∼1640 cm−1, but it cannot be used as an indicator of H2O content. This feature is present with band strengths &gt;1% in spectra of the 500–1000 μm fraction only if phyllosilicates are present. Although Mauna Kea palagonitic tephra is considered a VNIR analog to Martian dust, comparison of MIR altered tephra spectra (&lt;45 μm fraction) to dust spectra retrieved from Mars Global Surveyor and Mars Exploration Rover instruments do not provide good spectral matches. The best MIR match is a tephra that has a strong plagioclase feldspar transparency feature and was altered under dry, high‐temperature, oxidizing conditions. This sample is not a VNIR analog and is not a process analog, but it emphasizes the mineralogical importance of plagioclase feldspar in Martian dust. No single tephra is a good spectral analog across the VNIR and MIR. We found no evidence for substantial sulfates or phyllosilicates in Mini‐Thermal Emission Spectrometer (Mini‐TES) spectra from Gusev Crater.

Phenocryst composition as a tool for correlating fresh and altered tephra, Bed I, Olduvai Gorge, Tanzania

Olduvai Gorge, Tanzania, often cited as a textbook example of continental stratigraphic study (as depicted by Hay 1976 and others) is well known for its abundant Plio-Pleistocene fauna, hominin fossils, stone artifacts, and well-dated tephra layers used widely in the calibration of human evolution. However, precise stratigraphic correlation between various sites within Olduvai has been hampered by the rarity of unaltered volcanic glass suitable for geochemical characterization. Geochemical characterization of tephra for stratigraphic correlation purposes is difficult when fresh glass is absent or secondarily altered. A multi-component approach using the major and minor element compositions of phenocrysts and glass (where present) provides successful results at Olduvai and is proposed here as a viable methodology for use elsewhere. Six widespread Bed I (~2.1-1.79 myr) tephra layers of the Olduvai area are geochemically fingerprinted and used here to show the viability of themulti-component method. This method for geochemically fingerprinting tephra successfully provides a means for distinguishing similar looking tephra layers and for correlating fresh and altered tephra layers amongst a variety of depositional and diagenetic environments (freshwater wetlands, saline-alkaline lake, and fluvial plain). Application of this technique at Olduvai is providing high-resolution stratigraphic correlations and should prove applicable to other volcaniclastic sequences where tephra is poorly preserved.

Detection of soluble and fixed NH 4+ in clay minerals by DTA and IR reflectance spectroscopy: a potential tool for planetary surface exploration

Nitrogen is an essential element for life. It is the only element among the six major biogenic elements, C, N, S, O, P, H, whose presence in the Martian soil has not been positively and directly established. We describe here a study assessing the ability to detect NH 4 + in soils using two methods: differential thermal analysis (DTA) and infrared (IR) reflectance spectroscopy. Four standard clay minerals (kaolinite, montmorillonite, illite and attapulgite) and an altered tephra sample from Mauna Kea were treated with NH 4 + in this study, then leached in order to remove the non-chemically bound ammonium species. Aliquots of these NH 4 +-treated and then leached samples were analyzed by DTA and IR reflectance spectroscopy to quantify the detectability of soluble and sorbed/fixed NH 4 +. An exotherm at 270–280°C was clearly detected in the DTA curves of NH 4 +-treated (non-leached) samples. This feature is assigned to the thermal decomposition reaction of NH 4 +. Spectral bands observed at 1.56, 2.05, 2.12, 3.06, 3.25, 3.55, 4.2, 5.7 and 7.0 μm in the reflectance spectra of NH 4 +-treated and leached samples are assigned to the sorbed/fixed ammonium in the clays. The montmorillonite spectra have shown the most intense absorption features due to fixed ammonium among the leached samples in this study, as a result of its high cation sorption capacity. It is concluded that the presence of fixed NH 4 + in clays may be detected by IR reflectance or emission spectroscopy. Distinction between soluble and fixed NH 4 + may be achieved through the presence or absence of several spectral features assigned to the fixed NH 4 + moiety and, specifically, by use of the 4.2 μm feature assigned to solution NH 4 +. Thermal analyses furnish supporting evidence of ammonium in the clays/soil through detection of N released at temperatures of 270–330°C. Based on the results of this study, it is estimated that IR spectra measured from a rover should be able to detect ammonium if present above a few mg NH 4 + per g sample in the surface layers. Orbital IR spectra and thermal evolved-gas analyses measured on a rover may be able to detect ammonium in soils as well, but only if present at higher abundances. The spectral features at 3.06 and 7.0 μm due to bound NH 4 + in clays and altered volcanic tephra appear to be the most promising for detection by orbital spectrometers. If N species are present in a solid phase on the Mars surface, sedimentary deposits may be the best regions to look for them.

Alteration of hydrovolcanic basaltic ash: Observations with visible and near‐infrared spectrometry

Ash of basaltic composition that is produced by hydrovolcanic activity occurs frequently on Earth and may be much more widespread on Mars. Altered basaltic tephras from tuff rings and tuff cones across the Basin and Range were examined using visible and near infrared (Vis/IR) reflectance spectrometry and a variety of other techniques. It was found that Vis/IR Spectrometry is sensitive to subtle changes in the hydration and oxidation state of these tephras. Hydration produces an increase in overall reflectance with gradual development of water and hydroxyl absorptions at 1.4 μm and 1.9 μm. The oxidation of basaltic tephras can be observed spectrally through changes in the position and slope of the Fe3+ ‐ O2− charge transfer edge, inflections on that edge due to ferric oxide minerals and changes in the position and depth of the 1 μm iron crystal‐field band. Significantly in the tuffs examined, ferrous and ferric iron minerals produce one composite absorption feature rather than two resolvable bands. Different styles of alteration were noted between thinly bedded tuff ring deposits emplaced by high energy, largely dry, pyroclastic surge and massively bedded tuff cone deposits emplaced by wetter, lower energy pyroclastic flow. The former class of tephras become hydrated and only moderately oxidized with some limited development of dioctahedral smectite clay minerals and minimal palagonitization. The latter class of tephras are highly palagonitized which implies a high degree of hydration and oxidation. In fact, in the most highly altered tephras, all the Fe2+ that is initially in the volcanic glass is converted to Fe3+ within nanophase and bulk ferric oxide phases in the palagonite. There is also more extensive development in the tuff cone beds of zeolite and phyllosilicate minerals. The differences in reflectance spectra of altered basaltic tephras can be traced back to initial differences in the water/magma ratio extant at the volcanic vent; thus, Vis/IR spectrometry holds the potential to be used as a tool for investigations into the volcanic history and paleoclimate of volcanic terrains.

Trace element mobility during early diagenesis of volcanic ash: Applications to stratigraphic correlation

We have analyzed the relative mobility of trace elements in five rhyolitic and two basaltic tephra layers preserved in both altered and unaltered states, as a means of developing a rapid, whole-rock method for correlating altered tephra layers. The tephra layers were altered to zeolites or clay minerals in either saline-alkaline pore fluids, characteristic of small ephemeral lakes, or slightly more dilute pore fluids associated with perennial lakes and brackish coastal groundwaters. Comparison of whole rock trace elements analyses from the unaltered and altered portions of each tephra layer indicates that immobile elements, such as Ti, Zr, Hf, and Ta, retain their initial concentrations. Relatively more reactive alkaline and alkaline-earth elements, such as Rb, Sr, and Cs, are generally mobile. The lanthanide elements display variable mobility. The Light Rare Earth Elements (LREE) generally retain their initial concentrations, whereas the Heavy Rare Earth Elements (HREE) are generally depleted in alkaline solutions but immobile in more dilute pore fluids. We suggest that differences between LREE and HREE mobility result from the combined effects of LREE partitioning into stable accessory minerals and HREE transport as solution complexes, although our data set was not designed for quantitative analysis of these proposed mechanisms. Where both the diagenetic environment, and the accessory mineral compositions are well-constrained, the trace element signatures of the unaltered and altered tephra layers can be used to correlate an altered tephra layer to its unaltered equivalent, and to distinguish between different altered tephra layers. Four of the seven altered tephra layers can̈be distinguished on the basis of ratios of immobile elements, e.g. Zr Ti vs. Hf Ti . The remaining three, all W-type rhyolitic ashes, can be largely differentiated using cluster analysis on a combination of immobile elements and elements with similar mobilities.

Temporal patterns of tephra alterations in various geologic settings: The Jemez Volcanic Field and the adjacent Morrison Formation in New Mexico and Colorado

Authigenic mineral assemblages and temporal constraints of tephra layers altered by hydrothermal activities, subaerial weathering, and burial diagenesis are described. Clay-size assemblages (< 2 mm) from hydrothermally altered, Miocene to Quaternary volcanic rocks of the Jemez volcanic field are characterized by argillic minerals (kaolinite, quartz, smectite, alunite, pyrite, etc.) from acid-sulfate reactions and propylitic minerals (illite, chlorite, quartz, calcite, pyrite, etc.) that formed from neutral-chloride fluids. Unlike the hydrothermally altered rocks, the diagenetic minerals from the altered Jurassic tephra layers (bentonites) of the Morrison Formation are zonally distributed from a marginal smectite zone that grades laterally to zeolites, feldspars, and illite/smectite clays toward the center of the basin. In the Jenez volcanic field, the hydrothermal illite/smectites in core hole VC-2A range between 0.66 and 0.83 Ma for subsurface Plio-Pleistocene tephra deposits (1.14–1.78 Ma) and interbedded volcaniclastic units, whereas similar clays from altered Miocene rocks yield ages between 5.6 and 8.1 Ma. These clays formed during hydrothermal alterations related to the waning phases of the various volcanic episodes of the area. Clinoptilotites from the zeolitized tephra units from the Late Jurassic Morrison Formation gave discordant dates ranging from 154 to 83 Ma, whereas the illite/smectites from the bentonite beds of the same formation yielded two age groups (29.69–38.2 and 53.7–56 Ma). All the K Ar results were interpreted using the K Ar isochron method and in most cases the isochron ages are similar to the conventional K Ar results. The altered tephra layers provide diagnostic regional marker horizons in addition to recording concisely the time of diagenetic processes in various geologic settings.

Spectral analysis and mapping of palagonite tuffs of Pavant Butte, Millard County, Utah

The Pavant Butte tuff cone in Millard County, Utah was erupted into the waters of glacial Lake Bonneville. The basaltic tephras that compose Pavant Butte are in various states of alteration; from nearly pristine to fully palagonitized. Their level of alteration is detectable by visible and near infrared reflectance spectrometry. Unaltered sideromelane has a very low reflectance and is characterized by Fe2+ crystal‐field bands near 1 and 2 μm. With hydration the reflectance of the tephra increases; water and hydroxyl absorption bands at 1.4 and 1.9 μm also become prominent. The most highly altered tephras are palagonite tuffs which are characterized by a relatively high reflectance, deep H2O and OH absorptions, and a steep Fe3+ absorption edge in the visible. Such altered basaltic tephras may also be good models for altered soils and dust on Mars, for which remote sensing is the primary source of data. The geologic origin of these near ubiquitous martian surface materials is still undetermined; hydrovolcanism is one possible source. The palagonite tuff deposits at Pavant Butte were remotely observed by two airborne imaging spectrometers, and the data used for compositional mapping. The palagonite tuff was used as an endmember in a linear spectral mixing model analysis. This method was also used to calibrate the remotely sensed data to reflectance. Reflectance spectra from the airborne systems validated the laboratory sample spectra and indicated the presence of both di‐ and trioctahedral clay minerals within the palagonite tuff.

A catalog of North-American ashes found in altered and unaltered states for use as a data base in studying tephra diagenesis

Information on tephra layers that can be found in both altered and unaltered states is cataloged in order to provide a comprehensive data base for studying diagenetic processes in tephra. Data summarized in the catalog include age and presumed source of each tephra layer, mineralogy of unaltered and altered tephra, and differences in the major-element chemistry between unaltered and altered tephra.