Abstract: ⁴⁰Ar/³⁹Ar results from The Pleiades volcanic center indicate episodic eruptive activity between 830 ka and the present. The Pleiades (72°40'S 165°30'E) is a 13 kilometer long sequence of volcanic cones and domes, lava flows and associated pyroclastic rocks situated on the crest of the Transantarctic Mountains (TAM) at the head of the Mariner Glacier. The Pleiades is composed of silica-undersaturated alkalic lavas that range from primitive basanites/tephrites to highly evolved peralkaline trachytes. Four previously published conventional K/Ar ages for lavas from The Pleiades were young (<50 ka) but too imprecise (±125-465%) to be useful.

Fifteen ⁴⁰Ar/³⁹Ar incremental heating ages suggest that at least four episodes of volcanism occurred at The Pleiades center between 830 ka and present. Three trachytic cones erupted at ~830 ka mark the earliest activity at The Pleiades. The oldest dated lava flow (832±56 ka) is from a small cone ~100 meters north of Alcyone Cone. Two separate cones of approximately the same age, 832±56 and 826±4 ka, erupted ~2 km northeast and northwest of Taygete Cone, respectively. At 627±10 ka, phonolitic lavas erupted from a cone ~4 km northwest of Taygete Cone, marking the only occurrence of phonolite at The Pleiades. A ~300 ky year hiatus was followed by the eruption of less evolved lavas. A phonotephritic cone erupted at the northern edge of The Pleiades at 337±37 ka. A tephriphonolitic dike and associated lava flows erupted two kilometers to the east of Taygete Cone at 312±6 ka. The most intense period of volcanic activity at The Pleiades center began ~100 ka. At 93±4 ka, a trachytic flow erupted ~3 km east of Taygete Cone at the eastern edge of The Pleiades. A significant phase of cone building occurred at Mt. Atlas, the largest volcanic cone at The Pleiades, at approximately 65 ka, as evidenced by two benmoreite lava flows (61±4 and 68±4 ka). At approximately 45 ka, lava flows were erupted on the western flank of Mt. Pleione (42±4 and 38±8 ka) and near the summit of Alcyone Cone (48±2 ka). On the west side of Mt. Pleiones, a benmoreitic lava from a series of interlayered benmoreitic and trachytic flows yields an age of 20±7 ka. The youngest activity (6±6 ka) at The Pleiades occurred at Taygete Cone, an endogenous dome of peralkaline trachyte. This near-zero age for Taygete Cone is consistent with evidence of recent volcanism, including fresh hydrothermal activity and compositionally similar pumice lapilli scattered over surfaces in the northern Pleiades.

Ash layers from explosive eruptions associated with dated Pleiades lavas are potentially useful as regional marker horizons. Preliminary geochemical data suggest that some englacial tephra from blue ice areas were erupted from The Pleiades. Although direct dating of these englacial layers has been problematic, further geochemical work may yield one-to-one correlations with ⁴⁰Ar/³⁹Ar dated Pleiades lavas, allowing them to be used as precise, reliable time-stratigraphic markers within the Antarctic Ice Sheet.

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Abstract: Historically erupted anorthoclase phenocrysts (ca. 1984) from Mt. Erebus, Antarctica yield K/Ar and ⁴⁰Ar/³⁹Ar apparent ages as old as 700 ka, indicating the presence of excess argon. ⁴⁰Ar/³⁹Ar furnace step-heating results from anorthoclase reveal a positive relationship between Cl/K ratio (determined from ³⁸Ar_Cl/³⁹Ar_K) and apparent age. Because chlorine is present in melt inclusions (up to 1700 ppm) and absent in the anorthoclase, the positive correlation suggests that the excess argon is associated with glass melt inclusions (MI) trapped within the anorthoclase during rapid crystal growth.

Confirmation of the source of excess argon comes from two sample aliquots separated from single phenocrysts. One aliquot, crushed to approximately 500 mm, contained up to 30% MI (by volume). The second aliquot was crushed to approximately 175 mm and treated with 15% hydrofluoric acid to preferentially remove any exposed glass. The treated anorthoclase aliquot typically contained less than 1% MI. The untreated aliquots (30% MI) yielded the highest Cl/K ratios and apparent ages; ages in excess of 200 ka result from heating steps below 1200^oC, whereas steps above 1200^oC yield ages of approximately 100 ka. The treated aliquots (<1% MI) consistently yield the lowest Cl/K ratios and apparent ages for steps below 1200^oC; plateaus are obtained for the low-temperature steps yielding ages as young as 8±2 ka. Above 1200^oC, ages are only slightly younger than those observed for the high-temperature steps of the aliquots containing about 30% MI.

The distribution of MI within anorthoclase grains controls the degassing behavior of the excess argon during furnace extraction. MI exposed on the exterior of anorthoclase grains principally degas during furnace extraction at temperatures less than 1200^oC. MI enclosed within anorthoclase grains degas at temperatures greater than 1200^oC. The latter degassing behavior is a result of the entrapment of MI gas by the anorthoclase lattice. It is only when the anorthoclase lattice begins to melt (~1200^oC) that pathways are created allowing the argon contained within the MI to escape.

The presence of excess argon in MI has a large impact on the dating of Mt. Erebus and, potentially, other young volcanoes. Whereas the removal of the MI leads to more accurate age spectra results, the use of single-crystal laser-fusion to date extremely young anorthoclase and sanidine may neglect problems of melt-inclusion hosted excess argon.

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Abstract: Results of ⁴⁰Ar/³⁹Ar dating indicate that anorthoclase phonolite activity characteristic of Mt. Erebus is predominantly less than 120 ka, an order of magnitude younger than previously believed. Mt. Erebus, a 3794 meter high active polygenetic stratovolcano, is composed of voluminous anorthoclase phonolite (a.k.a. kenyte) overlying unknown volumes of poorly exposed, less differentiated lavas. The pre-anorthoclase phonolite lavas, (basanite to ne-benmoreite) crop out on Fang Ridge, an eroded remnant of a proto-Erebus volcano and other sporadic locations on the flanks of the Mt. Erebus edifice. Anorthoclase phonolite lava flows are exposed around the flanks and fill the major summit caldera. Anorthoclase feldspars found within the bombs and lavas are large (3/4 10 cm), abundant (~30-40%) and contain many melt (glass) inclusions trapped during crystal growth. This study employs the ⁴⁰Ar/³⁹Ar method to date these young, widespread anorthoclase phonolite flows as well as the older, less evolved lavas from Mt. Erebus.

The ⁴⁰Ar/³⁹Ar method reveals the complexities and limitations in dating historically erupted anorthoclase feldspar from Mt. Erebus. Armstrong (1978), using conventional K/Ar analyses on historically erupted anorthoclase, produced an age of 440 ka, suggesting that excess argon was a problem. In this study, we found a strong correlation between apparent age and degree of glass contamination for duplicate analyses of historically erupted summit phenocrysts (anorthoclase). In order to approach geologically reasonable ages it was necessary to remove excess-argon-containing melt inclusions during sample preparation. For very young samples (e.g. historically erupted anorthoclase), the amount of excess argon (⁴⁰Ar_E) generally overwhelms the radiogenic argon component (⁴⁰Ar*) even with the majority of melt inclusions removed (e.g. 8±2 ka for a 1984 erupted anorthoclase phenocryst).

Dates on anorthoclase phonolite flows at the caldera rim constrain the timing of collapse. The truncated caldera wall, formed from caldera collapse, contains flows as young as 93±6 ka. Anorthoclase phonolite flows filling the caldera are dated at 24±4 ka (Lower Hut Flow), 47±9 ka (Nausea Knob) and 75±5 ka (Northeast Flow). The latter date is from a flow that locally spills over the caldera wall. The dates for the caldera rim and Northeast Flow suggest caldera collapse occurred sometime between 93 and 75 ka.

Most anorthoclase phonolite flows on the flanks of Mt. Erebus have apparent ages from 26 to 120 ka. Three Sisters Cones (26±2 ka), Hooper's Shoulder (32±5 ka), Cape Evans (32±6 ka), William's Cliff (59±4 ka), Cape Royds (73±5 ka) and Cape Barne (89±3 ka) all yield relatively flat age spectra when compared to the discordant age spectra of historically erupted anorthoclase. A date for an unnamed flow northwest of Hooper's Shoulder (121±7 ka) provides an upper age limit for the current phonolitic activity on Mt. Erebus. Geochemical and age relationships of spatially separated flows prove insightful for the more recent geologic evolution of Mt. Erebus. For example, the present summit caldera may have collapsed due to the draining of a summit magma chamber which formed the voluminous flows at Cape Barne.

Older eruptive activity reveals the petrologic evolution of Mt. Erebus. A short pulse of trachytic volcanism occurred approximately 160-175 ky ago as evident by flows occurring at Bomb Peak (159±2 ka) and Aurora Cliffs (175±4). Older anorthoclase phonolite flows crop out principally in the southwest sector of the volcano occurring at and around Turks Head (243±5 ka). These flows at Turks Head currently represent the oldest dated stage of anorthoclase phonolite activity on Mt. Erebus although older rocks of phonolitic composition occur at Inaccessible Island (542±3 ka). Plagioclase from Tryggve Point and Turks Head ne-hawaiite yield ages of 370±9 ka and 377±5 while plagioclase from Abbott's Peak yields an age of 550±8 ka. Dating of Fang Ridge reveals an approximate eruptive interval for the proto-Erebus volcano. Plagioclase from an upper stratigraphic flow at Fang Ridge yields an age of 778±11 ka while whole rock from a lower stratigraphic flow yields an age of 1060±80 ka. The oldest dated rock on Mt. Erebus is a basanite dike (1310±7 ka) exposed on the shores of Cape Barne, next to and locally overlain by younger anorthoclase phonolite flows.

Implications of the new ⁴⁰Ar/³⁹Ar data from Mt. Erebus are far reaching. For example, dating of anorthoclase phonolite erratics in Ross Sea drift along the west side of McMurdo Sound may confirm their derivation from glacially scoured terrains at Capes Barne, Royds and Evans (89-32 ka) thereby constraining the timing of Ross Sea glaciation.

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Abstract: Blue ice at Allan Hills, Antarctica, contains abundant tephra layers as well as terrestrial windblown (?) dust layers, and mixed tephra and dust layers. The layers range from thin, faint laminae to distinct bands as thick as 50 cm. They dip from near-horizontal to near-vertical, depending on the geometry of local ice flow. Detailed GPS mapping reveals that individual ash and dust bands can be traced for up to 10 km, and that the same sequence of tephra and dust bands can be recognized throughout a single geographic area, irrespective of local ice flow conditions. The tephra and dust layers appear to have been deposited as stratigraphic layers rather having been emplaced by shear at the base of the ice sheet, based on their consistent orientation, coherent stratigraphy and the nature of their contacts with adjacent ice. They have sharp lower contacts interpreted as depositional surfaces, and diffuse upper contacts where mixing occurred with later snow. No significant shearing or brittle deformation the dust and tephra section was observed.

Petrographic observations show that many of the layers are primary tephra, ranging from basaltic glass fragments to trachytic pumice and shards. Tephra range in size from <2 to 250 mm, and appear fresh and unabraded in many layers. Potential source volcanoes are up to 500 km away for the coarser tephras. Because of their ease of access and sampling, blue ice areas offer an alternative to deep ice cores in the reconstruction of regional and possible global volcanic records. ⁴⁰Ar/³⁹Ar dating of large tephra samples, which can be readily collected from blue ice areas, offers a means of establishing a chronology which may extend back to 300 ka or more. Furthermore, dated layers found in blue ice areas may be geochemically correlated with tephra in deep ice

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Abstract: Mt. Erebus is a 3794 m high active phonolite volcano on Ross Island, Antarctica. Anorthoclase phonolite lava flows are exposed around the flanks and infill a major caldera. The summit cone is mainly composed of anorthoclase phonolite bombs erupted over the last 20 years. Mechanical disintegration of the bombs has left a lag of anorthoclase crystals up to 10 cm in length. In the bombs and lavas anorthoclase is abundant (~30-40%) and is often riddled with melt (glass) inclusions trapped during rapid growth. The high K2O content (3-4%), abundance and large size potentially makes the anorthoclase ideal for ⁴⁰Ar/³⁹Ar dating.

Conventional K/Ar ages of anorthoclase, glass and whole rock samples from Erebus ranged from 0.94 to 0.15 Ma. Historically erupted anorthoclase have an age of 0.44 Ma, indicating excess ⁴⁰Ar is a significant problem.

⁴⁰Ar/³⁹Ar age spectrum analyses on pure and impure (1 to 5% glass contaminated) anorthoclase were obtained on summit (1984 eruption) samples. A clear correlation between apparent age and degree of glass contamination is observed for duplicate analyses of summit phenocrysts. Nearly pure anorthoclase gave total gas ages of 68 and 41 ka whereas glass contamination of about 1% yielded total gas ages of 184 and 165 ka and 5% contamination produced total gas ages of 283 and 276 ka. Isochrons for each of the summit samples yield no apparent linear correlation and therefore suggest a heterogeneous trapped ⁴⁰Ar/³⁶Ar composition.

Mt. Erebus glass typically contains ~1600 ppm Cl. The measurement of 38Ar formed from a nuclear reaction on 37Cl provides a measure for sample Cl during age analysis. As the anorthoclase is presumed Cl-free, the glass content is directly proportional to the Cl signal. Unfortunately, there is not a straight-forward relationship between Cl and excess ⁴⁰Ar for the summit samples, thus it may not be possible to quantitatively correct for excess argon.

Geologically older samples from the flanks were also analyzed in duplicate (non-contaminated versus glass contaminated). Unlike the historic samples, the flank samples yield nearly identical apparent ages for both pure (isochron age = 88±2 ka) and glass contaminated (isochron age = 84±9 ka) anorthoclase. The glass causes a significant drop in radiogenic yield as well as a large increase in chlorine. However, there appears to be minimal excess argon associated with the glass for these samples. Therefore we believe that the apparent ages from the flank samples are geologically relevant.

Additional, older samples were also analyzed. Although minor amounts of glass were found to be present within these samples, the apparent ages were all less than 100 ka. Because glass enriched duplicates were not analyzed for these samples the presence of excess argon is difficult to assess. These ages, however, do represent a maximum age that are substantially younger than previously determined conventional K/Ar ages. Therefore, we conclude that while excess argon resides in the glass, the presence of glass itself does not confirm the existence of excess argon. However analysis of nearly clean versus glass contaminated samples does provide a technique whereby non-anorthoclase excess argon may be verified.

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