Analysis Of Glass Shards Research Articles

Geochemical characteristics of the infilling of ground wedges at Puerto Deseado (Santa Cruz, Argentina): palaeoenvironmental and chronological implications

Ground wedge structures of cryogenic origin are common in the Quaternary sediments along the coast of the Patagonia, and their formation is related to climatic cold events experienced by this area in the Late Quaternary. The infilling sediments of two wedges generations were analyzed in the area of Puerto Deseado. Bulk chemistry (major elements), X-ray diffraction (XRD), morphoscopic observations with Scanning Electronic Microscope (SEM) and chemical analyses of volcanic glass shards were undertaken to provide indications about infilling sediment provenience, along with chronological constraint for wedge formation. Bulk chemistry and XRD patterns indicate a significant SiO2-enriched composition of the sediment infilling compared to the most of the loess deposits of the North Argentina and the present day dust originated in Patagonia. This was interpreted as due to the nature of the bedrock present over the Deseado Massif. SEM morphoscopic characteristics of glass shards evidence typical aeolian reworking features, with impact structures and indented edges of the volcanic fragments. Chemical analyses of the glass shards indicate that they were probably generated by the H0 eruption (17,300-17,400 cal yr BP) of the Hudson volcano. Volcanological data indicate that H0 eruption dispersed toward NE, but volcanic glasses were available for reworking due to a WNW component in the western wind direction. Over the Deseado Massif structural high the glass shards mixed with sediments enriched in SiO2, and were eventually deflated further to SE reaching the present coastal area and infilling the frost cracks. The age of the glass shards (17,300-17,400 cal yr BP) and that of the sandy layer affected by cryogenic structures (14,670±750 yr BP) well constrain to the Late Glacial both wedge generations.

Systematic tapping of independent magma chambers during the 1 Ma Kidnappers supereruption

The 1.0 Ma Kidnappers supereruption (~ 1200 km 3 DRE) from Mangakino volcanic centre, Taupo Volcanic Zone, New Zealand, produced a large phreatomagmatic fall deposit followed by an exceptionally widespread ignimbrite. Detailed sampling and analysis of glass shards and mineral phases have been undertaken through a proximal 4.0 m section of the fall deposit, representing the first two-thirds of erupted extra-caldera material. Major and trace element chemistries of glass shards define three distinct populations (types A, B and C), which systematically change in proportion through the fall deposit and are inferred to represent three magma types. Type B glass and biotite first appear at the same level (~ 0.95 m above base) in the fall deposit suggesting later tapping of a biotite-bearing magma. Plagioclase and Fe–Ti oxide compositions show bimodal distributions, which are linked to types A and B glass compositions. Temperature and pressure (T–P) estimates from hornblende and Fe–Ti oxide equilibria from each magma type are similar and therefore the three magma bodies were adjacent, not vertically stacked, in the crust. Most hornblende model T–P estimates range from 770 to 840 °C and 90 to 170 MPa corresponding to storage depths of ~ 4.0–6.5 km. Hornblende model T–P estimates coupled with in situ trace element fingerprinting imply that the magma bodies were individually well mixed, and not stratified. Compositional gaps between the three glass compositional types imply that no mixing between these magmas occurred. We interpret these data, coupled with the systematic changes in shard compositional proportions through the fall deposit, to reflect that three independent melt-dominant bodies of magma contributed large (A, ~ 270 km 3), medium (B, ~ 90 km 3) and small (C, ~ 40 km 3) volumes (as reflected in the fall deposits) and were systematically tapped during the eruption. We propose that the systematic evacuation of the three independent magma bodies implies that there was tectonic triggering and linkage of eruptions. Our results show that supereruptions can be generated by near simultaneous multiple eruptions from independent magma chambers rather than the evacuation of a large single unitary magma chamber.

Distal occurrence of mid-Holocene Whakatane Tephra on the Chatham Islands, New Zealand, and potential for cryptotephra studies

The Whakatane Tephra, a rhyolitic tephra erupted ca. 5500 cal. BP from Okataina Volcanic Centre, central North Island, has been identified on the Chatham Islands which lie ∼900 km east of Christchurch, New Zealand. The visible tephra layer, ∼5 mm in thickness and preserved within peat on Pitt Island, was identified using both radiocarbon dating and analysis of glass shards by electron microprobe. Whakatane Tephra is the first Holocene tephra to be identified on the Chatham Islands, and it is the most-distal Holocene tephra yet recorded in the New Zealand region, being ∼850 km from source. The Pitt Island occurrence extends the tephra’s dispersal area markedly, by an order of magnitude, possibly to ∼300,000 km 2. An estimated dispersal index ( D) of approximately 10 5 km 2 indicates that the eruption generated a very high plinian column, possibly exceeding ∼30 km in height, with strong winds blowing the ash plume southeastwards. This new discovery of distal Whakatane Tephra as a thin but visible layer strongly implies that cryptotephras are likely to be preserved on the Chatham Islands and within adjacent ocean floor sediments. Therefore the potential exists to develop enhanced cryptotephrostratigraphic records from these distal areas, which in turn would help facilitate precise correlation via tephrochronology of palaeoenvironmental records (such as NZ-INTIMATE) from mainland New Zealand, the southwest Pacific Ocean, and the Chatham Islands.

Abstract: Improved Regional Ties to Global Geochronology using Pb-Isotope Signatures of Volcanic Glass Shards from Deep Water Gulf of Mexico Ash Beds

ABSTRACT Sedimentary beds rich in volcanic ash (ash beds) have been confirmed in several recent offshore Gulf of Mexico (GOM) exploratory wells. A petrographic/scanning electron microscope survey for volcanic glass shards on well cuttings and sidewall cores donated by the petroleum industry is currently ongoing at University of New Orleans (UNO). Fifteen separate ashes have been documented in our preliminary survey. We have also determined the paleontological ages of these GOM ashes with industry micro-paleo reports. The paleo-ages of these ash beds appear to be correlative with the ages of maximum flooding surfaces on industry sea-level curves. Due to their distinctive log and seismic response, thick ash beds are ideal candidates for use as precise time markers in the GOM subsurface if they can be tied to known eruptive events. Based on proximity in time, we believe that the most likely sources for these ashes are the Huckleberry, Lava Creek, Mesa Falls, Bishop, and other eruptive events that occurred in the Yellowstone Plateau from 0.6 to 12 mya. These eruptions had enormous ash volumes that covered much of the ancestral Mississippi drainage area. Isotopic analyses of glass shards from GOM ash beds indicate a lead isotope signature that matches volcanic tuffs in the Yellowstone Plateau and precludes other ash sources for the GOM ash beds.

Evidence for a Mazama-like tephra deposited ca. 10 000 BP at Copper Lake, Banff National Park, Alberta

Mazama tephra is a widespread mid-Holocene stratigraphic marker, dating to ca. 6845 BP, and is considered useful for the correlation of Holocene events in western North America. We present evidence from Copper Lake, Alberta, for a tephra layer that underlies Mazama tephra and appears to be a discrete tephra layer but that is indistinguishable from the conventional Mazama tephra by microprobe analysis of glass shards. Three alternative hypotheses are considered to explain this tephra underlying Mazama: in situ position of an earlier Mazama-like tephra, recycling of tephras, and settling of tephra slabs through the gyttja. The size distributions of birch pollen grains indicate that the Mazama-like tephra is in primary stratigraphic position in the early Holocene gyttja, and that sediment recycling has not destroyed the integrity of Copper Lake sediment stratigraphy. It is concluded that most probably a Mazama-like tephra was deposited between 9700 and 10 500 BP. If this in situ hypothesis is correct, the identification of a tephra as Mazama by major-element chemistry does not necessarily fix the age of the enclosing strata. Evidence to confirm the existence and plot the distribution of a Late Pleistocene – early Holocene, Mazama-like tephra is needed from other sites in northwestern America. A collateral result of this study is that ablation of valley-bottom ice in the vicinity of Castle Mountain took place prior to 10 500 BP.

Sedimentological Study in the Northwestern Gulf of Mexico Using Tephrochronology and Oxygen Isotope Stratigraphy: ABSTRACT

Microprobe analyses of tephra have been used in conjunction with oxygen isotope ({delta}O{sup 18}) and seismic reflection data in a sedimentary study of the 1,000-ft-deep deep-sea core E67-13 from the northwestern Gulf of Mexico (latitude 27{degree}28'N, longitude 95{degree}54'N). No megascopic tephra layers are found in this core, but electron microprobe analysis of glass shards from five intervals of dispersed rhyolitic tephra indicate that the tephra is both heterogeneous and altered. Extensive hydration and mobility of iron and other ions into carbonate material (pyritization) hinders accurate geochemical fingerprinting. The intervals of dispersed tephra coincide with increases in terrigenous material suggesting that the tephra is reworked from land deposits and redeposited via fluvial systems in the gulf. Volcanic tephra widespread in the region are absent from core E67-113. The oxygen isotope stratigraphy in the core indicates two hiatuses at 0-0.93 k.y. and 0.12-1.2 m.y. A discrete tephra layer found in another core (E67-126A) in the same region is geochemically and biostratigraphically correlated to the Lava Creek rhyolitic eruption from Yellowstone (0.6{plus minus}0.01-0.02 m.y.). This tephra, as well as many other region-wide tephra, fall into the missing time spans in E67-113. The absence of discrete tephra layers in core E67-113 is due tomore » deep-sea erosional processes that removed sediment from the area. The seismic reflection data show that core E67-113 is drilled in a channel adjacent to a large salt diapir. This channel cuts into onlapping sediments. As the diapir ascended through the overlying sediments, bottom currents in the gulf were diverted around the diapir eroding significant amounts of sediment and creating a channel or moat.« less