A 12 m.y. record of large rhyolitic eruptions from the Coromandel (CVZ) and Taupo (TVZ) Volcanic Zones of New Zealand is contained in cores retrieved by Leg 181 of the Ocean Drilling Program. Site 1124, located 670 km from the TVZ, has a maximum of 134 macroscopic tephra layers with a total thickness of 13.18 m. These units, along with between 7 and 63 tephras from 3 other sites, were dated by a combination of magnetostratigraphy, biostratigraphy, isothermal plateau fission track determinations, and geochemical correlation with onshore tephra deposits. Additional time control for the last 3 m.y. came from an orbitally tuned, benthic, oxygen isotope profile for Site 1123.Results extend the incomplete terrestrial record of volcanism by placing the first major rhyolitic eruption in the CVZ at c. 12 Ma, c. 1.6-1 m.y. earlier than previously known. Tephras became thicker and more frequent from the late Miocene into the Quaternary-a trend that probably reflected (1) more frequent and intense volcanism and (2) reduced distances between sources and depositional sites on the evolving Australian/Pacific plate system. The passage from CVZ to Quaternary TVZ occurred without a major hiatus in activity, suggesting the transition was gradational. The ensuing TVZ volcanism was more continuous than known previously from the onshore geology. Ash dispersal was primarily eastward, highlighting the dominance of westerly winds since the middle Miocene. Nevertheless, variations in dispersal patterns suggest periodic changes in wind direction/speed and/or ejection of ash beyond the Roaring Forties.

Felsic magmatic systems represent the vast majority of volcanic activity that poses a threat to human life. The tempo and magnitude of these eruptions depends on the physical conditions under which magmas are retained within the crust. Recently the case has been made that volcanic reservoirs are rarely molten and only capable of eruption for durations as brief as 1,000 years following magma recharge. If the "cold storage" model is generally applicable, then geophysical detection of melt beneath volcanoes is likely a sign of imminent eruption. However, some arc volcanic centers have been active for tens of thousands of years and show evidence for the continual presence of melt. To address this seeming paradox, zircon geochronology and geochemistry from both the frozen lava and the cogenetic enclaves they host from the Soufrière Volcanic Center (SVC), a long-lived volcanic complex in the Lesser Antilles arc, were integrated to track the preeruptive thermal and chemical history of the magma reservoir. Our results show that the SVC reservoir was likely eruptible for periods of several tens of thousands of years or more with punctuated eruptions during these periods. These conclusions are consistent with results from other arc volcanic reservoirs and suggest that arc magmas are generally stored warm. Thus, the presence of intracrustal melt alone is insufficient as an indicator of imminent eruption, but instead represents the normal state of magma storage underneath dormant volcanoes.volcano | eruption | arc magma | zircon D etermining the timescale of magma storage and remobilization in the upper crust is key to understanding the tempo and magnitude of volcanic eruptions (1-13). Whether a volcano can erupt is controlled by the recharge rate to the magma reservoir (13) (reservoir in this context refers to the portion of the igneous complex that is potentially eruptible), which in turn determines the duration of the "eruption window" [generally defined as the rheological state during which the subvolcanic reservoir is below ∼60% crystals and hence capable of eruption (4)]. However, estimates for how long this eruption window remains open vary over four orders of magnitude; this suggests either profound problems in assumptions underlying one or more of these estimates or a continuum of physical mechanisms that resist formulation of a unified model for the state of magma reservoirs before eruption (1-13). The preservation of sharp compositional gradients in plagioclase phenocrysts, assumed to have crystallized >10 ka before eruption, has recently been interpreted to indicate that arc volcanic reservoirs characteristically remain in "cold storage" at temperatures below the eruption window, possibly below the solidus, and thus only capable of erupting during brief recharge events (<10 ka) (1). In contrast, zircon dating and heat budget considerations are difficult to reconcile with this scenario; instead, they are consistent with continuously partially molten reservoirs capable of erupting (i.e., with melt portion ≥40%) ov...

No abstract

No abstract

No abstract

Large volume (100-1000 km 3 ), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the nonwelded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25B0.12 Ma), Whakamaru group (0.34B0.03 Ma), Matahina (0.34B0.02 Ma), Chimp (0.33B0.02 Ma), Kaingaroa (0.31B0.01 Ma) and Mamaku (0.23B0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation.The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of 40 Ar/ 39 Ar. Errors as limited as 10-30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34-0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession.

No abstract

No abstract