Kari M. Cooper

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Rapid remobilization of magmatic crystals kept in cold storage

The processes involved in the formation and storage of magma within the Earth’s upper crust are of fundamental importance to volcanology. Many volcanic eruptions, including some of the largest, result from the eruption of components stored for tens to hundreds of thousands of years before eruption. Although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes, they remain relatively poorly known. Eruptions of crystal-rich magma are often suggested to require the mobilization of magma stored at near-solidus conditions; however, accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures. What has been lacking in this debate is clear observational evidence linking the thermal (and therefore physical) conditions within a magma reservoir to timescales of storage—that is, thermal histories. Here we present a method of constraining such thermal histories by combining timescales derived from uranium-series disequilibria, crystal sizes and trace-element zoning in crystals. At Mount Hood (Oregon, USA), only a small fraction of the total magma storage duration (at most 12 per cent and probably much less than 1 per cent) has been spent at temperatures above the critical crystallinity (40–50 per cent) at which magma is easily mobilized. Partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near-solidus temperatures is common. Magma storage at low temperatures indicates that, although thermobarometry calculations based on mineral compositions may record the conditions of crystallization, they are unlikely to reflect the conditions of most of the time that the magma is stored. Our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption.

Re-examination of crystal ages in recent Mount St. Helens lavas: implications for magma reservoir processes ☆

U-series data for recent Mount St. Helens lavas suggest that crystallization preceded eruption by more than 0.5 ka but are complicated by possible evidence of crystal recycling and/or addition of radium to the liquid after crystallization. We report new ion and electron microprobe trace- and major-element data for plagioclase and pyroxene in these recent Mount St. Helens lavas and use these data to reassess 226Ra–230Th crystal ages by taking into account differences in the partitioning behavior of radium and barium and the effects of impurities in mineral separates. Revised 226Ra–230Th model crystallization ages are ∼2–4 ka for plagioclase (with the exception of the 1982 dacite) and ∼0.15–5.7 ka for pyroxene. In contrast to previous interpretations, no late-stage addition of Ra to the liquid after precipitation of the minerals is required. The variability of Ba concentrations measured in plagioclase is too large to be consistent with progressive crystallization from the same liquid or with diffusive re-equilibration of xenocrysts with a new host liquid. Ba heterogeneity limits the residence time of the crystals in a magma at high temperatures and also suggests that in most cases Ra–Th ages have not been significantly modified by Ra diffusion into or out of the crystals. High (226Ra)/Ba in plagioclase in the 1982 dacite relative to the host liquid likely reflects crystallization processes that precluded bulk crystal–liquid chemical equilibrium. One possibility is that of growth entrapment of surface enrichments during rapid crystallization, which could lead to less discrimination between Ra and Ba than predicted by calculated bulk partition coefficients. 226Ra–230Th crystal ages for the Castle Creek andesite and basalt that are younger than 230Th–238U ages of the same crystals could be explained by mixing of crystals into melts with different 230Th/232Th ratios, by combinations of older and younger crystal growth within the same magma, or, for the basalt, by diffusion of Ra into crystals at high magmatic temperatures. Average plagioclase ages in most of the samples of >2 ka imply that some significant mass fraction of the crystals in each flow was present simultaneously beneath the volcano. This observation could be consistent either with simultaneous storage of physically distinct magma batches or with incorporation of a population of similarly aged crystals into each successive magma batch.

Crystal and magma residence at Kilauea Volcano, Hawaii: 230Th-226Ra dating of the 1955 east rift eruption

Abstract Previous estimates of crustal storage time of magmas at Kilauea Volcano, Hawaii, range from a few years to a few thousand years, leading to considerable uncertainty in the time scales of processes of magmatic storage and differentiation. We present a new approach for determining minimum magma residence times which involves dating phenocrysts in a magma using 226Ra–230Th disequilibria, and apply this approach to the early phase of the 1955 east rift eruption at Kilauea. When fractionation of Ra from Ba (a proxy for initial Ra in the crystals) during crystal growth is considered along with the effects of inclusions in the minerals, the data are consistent with co-precipitation of plagioclase and clinopyroxene from a melt represented by the groundmass at a mean age of 1000+300−400 a. Unless a significant fraction (>30%) of the crystals are remnants from an earlier batch of evolved magma in the system, these data constrain the minimum magmatic residence time to be ∼550 yr, considerably longer than most previous estimates of storage time at Kilauea as well as those for some other basaltic systems. For the temperature interval of augite+plagioclase growth in the early 1955 magma, a maximum constant cooling rate of 0.1°C/yr (1×10−5°C/h) is derived from the minimum magmatic residence time of 550 yr. The total magma storage time would be >2500 yr if this cooling rate applied to the entire thermal history of the magma, although a more complex cooling history where cooling rates were more rapid early in the storage history is permissive of a total residence time which is not much longer than 550 yr. The disparate estimates of magma residence at Kilauea may reflect the uncertainties in the methods of estimation in addition to true variations in storage time for different batches of magma. More work is necessary in order to determine whether a long residence time is characteristic of rift zone lavas and/or of Kilauean lavas in general.

Faulted terrace risers place new constraints on the late Quaternary slip rate for the central Altyn Tagh fault, northwest Tibet

The active, left-lateral Altyn Tagh fault defines the northwestern margin of the Tibetan Plateau in western China. To clarify late Quaternary temporal and spatial variations in slip rate along the central portion of this fault system (85°–90°E), we have more than doubled the number of dated offset markers along the central Altyn Tagh fault. In particular, we determined offset-age relations for seven left-laterally faulted terrace risers at three sites (Kelutelage, Yukuang, and Keke Qiapu) spanning a 140-km-long fault reach by integrating surficial geologic mapping, topographic surveys (total station and tripod–light detection and ranging [T-LiDAR]), and geochronology (radiocarbon dating of organic samples, 230 Th/U dating of pedogenic carbonate coatings on buried clasts, and terrestrial cosmogenic radionuclide exposure age dating applied to quartz-rich gravels). At Kelutelage, which is the westernmost site (37.72°N, 86.67°E), two faulted terrace risers are offset 58 ± 3 m and 48 ± 4 m, and formed at 6.2–6.1 ka and 5.9–3.7 ka, respectively. At the Yukuang site (38.00°N, 87.87°E), four faulted terrace risers are offset 92 ± 12 m, 68 ± 6 m, 55 ± 13 m, and 59 ± 9 m and formed at 24.2–9.5 ka, 6.4–5.0 ka, 5.1–3.9 ka, and 24.2–6.4 ka, respectively. At the easternmost site, Keke Qiapu (38.08°N, 88.12°E), a faulted terrace riser is offset 33 ± 6 m and has an age of 17.1–2.2 ka. The displacement-age relationships derived from these markers can be satisfied by an approximately uniform slip rate of 8– 12 mm/yr. However, additional analysis is required to test how much temporal variability in slip rate is permitted by this data set.

Changes in magma storage conditions following caldera collapse at Okataina Volcanic Center, New Zealand

Large silicic volcanic centers produce both small rhyolitic eruptions and catastrophic caldera-forming eruptions. Although changes in trace element and isotopic compositions within eruptions following caldera collapse have been observed at rhyolitic volcanic centers such as Yellowstone and Long Valley, much still remains unknown about the ways in which magma reservoirs are affected by caldera collapse. We present 238U–230Th age, trace element, and Hf isotopic data from individual zircon crystals from four eruptions from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand, in order to assess changes in trace element and isotopic composition of the reservoir following the 45-ka caldera-forming Rotoiti eruption. Our data indicate that (1) mixing of magmas derived from crustal melts and mantle melts takes place within the shallow reservoir; (2) while the basic processes of melt generation likely did not change significantly between pre- and post-caldera rhyolites, post-caldera zircons show increased trace element and isotopic heterogeneity that suggests a decrease in the degree of interconnectedness of the liquid within the reservoir following collapse; and (3) post-caldera eruptions from different vents indicate different storage times of the amalgamated melt prior to eruption. These data further suggest that the timescales needed to generate large volumes of eruptible melt may depend on the timescales needed to increase interconnectedness and achieve widespread homogenization throughout the reservoir.

Vapor transfer prior to the October 2004 eruption of Mount St. Helens, Washington

Dome lavas from the 2004 eruption of Mount St. Helens show elevated Li contents in plagioclase phenocrysts at the onset of dome growth in October 2004. These cannot be explained by variations in plagioclase-melt partitioning, but require elevated Li contents in coexisting melt, a fact confirmed by measurements of Li contents as high as 207 µg/g in coexisting melt inclusions. Similar Li enrichment has been observed in material erupted prior to and during the climactic May 1980 eruption, and is likewise best explained via pre-eruptive transfer of an exsolved alkali-rich vapor phase derived from deeper within the magma transport system. Unlike 1980, however, high Li samples from 2004 show no evidence of excess (210Pb)/(226Ra), implying that measurable Li enrichments may occur despite significant differences in the timing and/or extent of magmatic degassing. Diffusion modeling shows that Li enrichment occurred within ∼1 yr before eruption, and that magma remained Li enriched until immediately before eruption and cooling. This short flux time and the very high Li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption. Overall, observation of a high Li signature in both 1980 and 2004 dacites indicates that Li enrichment may be a relatively common phenomenon, and may prove useful for petrologic monitoring of Mount St. Helens and other silicic volcanoes. Lithium diffusion is also sufficiently rapid to constrain vapor transfer on similar time scales to short-lived radionuclides.

Uranium-series chronology of Gorda Ridge volcanism: new evidence from the 1996 eruption

Abstract We present new uranium-series plagioclase and glass data for the 1996 eruption of the North Gorda Ridge. The glass data provide a more accurate estimate of ‘zero-age’ disequilibria for use in external isochron dating than was previously available. Furthermore, plagioclase–glass 226Ra–230Th disequilibria delimit the degree of initial fractionation of radium from barium during crystal growth, with effective DRa/DBa ∼0.25–0.5. These data are inconsistent with the common assumption that DRa=DBa but are qualitatively consistent with theoretical model predictions that radium and barium should be fractionated during crystallization, with DRa/DBa∼0.2. In more detail, differences between model predictions and data could be explained by an extreme combination of model and data uncertainties, but more likely suggest suppression of efficient fractionation during rapid crystallization. We also assess the extent to which use of barium as an analog for radium would result in underestimating 226Ra–230Th disequilibria produced during melting. Effects of a lower value of DRa on most melting models are small, with a slight increase in the porosity or melt fraction for a given value of 226Ra/230Th. The new plagioclase data also indicate that plagioclase accumulation and assimilation in the crustal reservoir would have only a negligible effect on mantle-derived 226Ra–230Th disequilibria. External isochron ages from U–Th, U–Pa, and Th–Ra data calculated using initial disequilibria from the 1996 sample are concordant, with one exception. Anomalously young ages for off-axis samples most likely reflect volcanism up to 1 km off-axis. Radium ages for near-axis dredge samples for other areas of the North Gorda Ridge generally range from 2000 to 4000 yr, similar to calculated steady-state eruptive periodicity of 3000 yr for this ridge segment. However, comparison of radium excesses for the 1996 eruption and a nearby older lava suggests that recent volcanism at this site occurs with relatively short-lived (

Downhole variation of lithium and oxygen isotopic compositions of oceanic crust at East Pacific Rise, ODP Site 1256

Bulk rock lithium and oxygen isotope compositions from ODP Site 1256 were analyzed to investigate the seawater circulation in the upper oceanic crust formed at the East Pacific Rise (EPR). The upper extrusive basalts have ?18O values from +6.1‰ to +9.2‰, reflecting alteration of oceanic crust by seawater at low temperatures (<200–250°C). Bulk rocks from the sheeted dike complex and plutonic section have overall lower ?18O values (+3.0‰–+5.5‰). In the sheeted dike complex bulk rock ?18O values gradually decrease with depth, and then increase toward the fresh MORB ?18O value after reaching a minimum of +3.0‰ at ?1350 m below seafloor (mbsf). The entire sampled crust is dominated by rocks with low lithium contents relative to fresh MORBs except for a few localized Li enrichment. The upper volcanic zone is characterized by a spread of ?7Li from low to high values relative to average unaltered MORB values (?7Li = +3.4 ± 1.4‰). The presence of rocks with low ?7Li values in the upper crust most likely indicates zones of upwelling of relatively hot (?200–250°C) hydrothermal fluids. In the sheeted dike complex, bulk rock ?7Li values show wide range of variation, but exhibit a general trend from enriched to depleted values at ?1280 mbsf and then return to that for fresh MORB within the upper tens of meters of the plutonic section at the bottom of the after reaching a minimum at ?1350 mbsf (?7Li = ?1.6‰). The downhole pattern of ?7Li principally reflects variations in water-rock ratio (w/r) together with a downhole increase of temperature. Seawater flow in the upper volcanic zone is likely to be channeled with generally small but variable w/r ratios. The w/r ratios increase rapidly with depth in the lower volcanic section into the sheeted dike complex indicating water dominated pervasive hydrothermal flow due to intensive upwelling of hydrothermal fluids.

Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals

Taupo Volcanic Zone magma spent more than 90% of its life deep and crystalline before rapid shallow accumulation and eruption. Quick eruption after a long bake Minerals such as zircon can record the storage conditions of magma before volcanic eruption. Rubin et al. combined traditional 238U-230Th dating with lithium concentration profiles in seven zircons from the Taupo supervolcanic complex in New Zealand to determine magma storage conditions. The zircons spent more than 90% of their lifetime in an uneruptible, mostly crystalline, and deep magmatic reservoir. The zircons were eventually transported to hotter, shallower, and eruptible magma bodies, where they spent only decades to hundreds of years before eruption. The result suggests a two-stage model for magmatic systems with large thermal variations. Science, this issue p. 1154 Silicic volcanic eruptions pose considerable hazards, yet the processes leading to these eruptions remain poorly known. A missing link is knowledge of the thermal history of magma feeding such eruptions, which largely controls crystallinity and therefore eruptability. We have determined the thermal history of individual zircon crystals from an eruption of the Taupo Volcanic Zone, New Zealand. Results show that although zircons resided in the magmatic system for 103 to 105 years, they experienced temperatures >650° to 750°C for only years to centuries. This implies near-solidus long-term crystal storage, punctuated by rapid heating and cooling. Reconciling these data with existing models of magma storage requires considering multiple small intrusions and multiple spatial scales, and our approach can help to quantify heat input to and output from magma reservoirs.