Christopher D. Coath

Prolonged residence times for the youngest rhyolites associated with Long Valley Caldera:230Th—238U ion microprobe dating of young zircons

Abstract We describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238U—230Th disequilibrium. Our results support independent inferences that zircon can remain suspended for long periods (> 100 ka) in the convecting potions of the magma from which it crystallizes. Because the crystallization ages date when the magma cooled to its zircon saturation temperature, even when the zircons have long magmatic residence times,238U—230Th zircon dating can be used to constrain the thermochemical evolution of silicic magmas. 238U—230Th ages have been determined for individual zircons from rhyolites associated with the Long Valley magmatic system of eastern California. The samples are from Deer Mountain, an 115 ± 3 ka low-silica moat rhyolite, and from the coarsely porphyritic, low-silica rhyolite of South Deadman dome, one of the ∼ 0.6 ka Inyo domes. Previous investigations have suggested that the two lavas were derived from the same magma reservoir. A few of the zircon model ages, calculated with respect to the isotopic characteristics of the whole rocks, are within error of that for eruption of the Deer Mountain rhyolite. However, the majority of zircons from both lavas cluster around an age of ∼ 230 ka. This common interval of zircon nucleation and growth, for petrologically similar lavas, suggests that the younger Inyo dome lava may have tapped the same magma body from which the Deer Mountain rhyolite erupted more than 100 ka before. On the other hand, most of the zircon model ages are younger than previous episodes of silicic volcanism in the Long Valley Caldera, suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of Long Valley caldera. Zircon saturation temperatures for the rhyolites studied (795–810°C) are the same as those obtained from coexisting Fe—Ti oxides (809 ± 4°C), showing that the magma cooled to

Origin of Nucleosynthetic Isotope Heterogeneity in the Solar Protoplanetary Disk

Stable-isotope variations exist among inner solar system solids, planets, and asteroids, but their importance is not understood. We report correlated, mass-independent variations of titanium-46 and titanium-50 in bulk analyses of these materials. Because titanium-46 and titanium-50 have different nucleosynthetic origins, this correlation suggests that the presolar dust inherited from the protosolar molecular cloud was well mixed when the oldest solar system solids formed, but requires a subsequent process imparting isotopic variability at the planetary scale. We infer that thermal processing of molecular cloud material, probably associated with volatile-element depletions in the inner solar system, resulted in selective destruction of thermally unstable, isotopically anomalous presolar components, producing residual isotopic heterogeneity. This implies that terrestrial planets accreted from thermally processed solids with nonsolar isotopic compositions.

The oxygen isotopic composition of the sun inferred from captured solar wind

The Sun is highly enriched in the most abundant isotope of oxygen, oxygen-16, relative to most other planetary materials. All planetary materials sampled thus far vary in their relative abundance of the major isotope of oxygen, 16O, such that it has not been possible to define a primordial solar system composition. We measured the oxygen isotopic composition of solar wind captured and returned to Earth by NASA’s Genesis mission. Our results demonstrate that the Sun is highly enriched in 16O relative to the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the average isotopic composition of the solar system for elements heavier than lithium, we conclude that essentially all rocky materials in the inner solar system were enriched in 17O and 18O, relative to 16O, by ~7%, probably via non–mass-dependent chemistry before accretion of the first planetesimals.

Mass-independent isotope effects in Archean (2.5 to 3.8 Ga) sedimentary sulfides determined by ion microprobe analysis

We report sulfur isotope anomalies with 33 S, the deviation from a mass-dependent fractionation line for the three-isotope system ( 34 S/ 32 S vs. 33 S/ 32 S), ranging up to 2‰ within individual Archean sedimentary sulfides from a variety of localities. Our measurements, which are made in situ by multicollector secondary ion mass spectrometry, unequivocally corroborate prior bulk measurements of mass-independent fractionations (MIF) in sulfur and provide additional evidence for an anoxic atmosphere on the Earth before 2 Ga. This technique also offers new opportunities for exploring ancient sulfur metabolisms preserved in the rock record. The presence of MIF sulfur in sulfides from a 3.8-Ga Fe-rich quartzite from Akilia (island), West Greenland, is consistent with a marine sedimentary origin for this rock. Copyright

Low temperature replacement of monazite in the Ireteba granite, Southern Nevada: geochronological implications

The Ireteba pluton is a relatively homogeneous, ∼64 Ma (zircon ion probe age) two-mica granite that was intruded by two 16 Ma Miocene plutons at depths ranging from 5 to 13 km. Deeper levels of the Ireteba and Miocene plutons were ductilely deformed at 15–16 Ma. At shallow levels remote from the Miocene plutons, the Ireteba granite appears to have experienced little Miocene heating and deformation. Monazites from different portions of the pluton reflect the different histories experienced by the host rock. Irregularly shaped (patchy) zones with high huttonite component (ThSiO4) are widespread in monazite at deep levels adjacent to Miocene plutons but less common in shallow-level rock; monazite grains with extensive replacement generally have irregular, embayed surfaces. In undeformed rocks distant from the Miocene plutons, monazites are less modified and more nearly euhedral, though fine networks of replacement veins are common and irregular rims are evident in some grains. Secondary monazite from these samples is poorer in huttonite. Ion probe Th–Pb dating yields 60–65 Ma ages for magmatic and some replacement zones in monazite from the shallow samples, and veins yield apparent ages as young as mid-Tertiary. Monazites from deep samples yield a few 55–65 Ma ages for remnant magmatic zones and abundant Miocene ages for replacement zones (∼14–18 Ma). These data demonstrate extensive Miocene replacement of magmatic monazite, especially at deep levels near Miocene plutons, and they suggest an early replacement episode as well. Both events were probably related to influxes of fluid; the first may have been associated with initial solidification of the Ireteba pluton and the second with the Miocene plutons and/or extensional deformation. Ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at T as low as 400°C or less.

Carbon isotopic composition of individual Precambrian microfossils.

Ion microprobe measurements of carbon isotope ratios were made in 30 specimens representing six fossil genera of microorganisms petrified in stromatolitic chert from the approximately 850 Ma Bitter Springs Formation, Australia, and the approximately 2100 Ma Gunflint Formation, Canada. The delta 13C(PDB) values from individual microfossils of the Bitter Springs Formation ranged from -21.3 +/- 1.7% to -31.9 +/- 1.2% and the delta 13C(PDB) values from microfossils of the Gunflint Formation ranged from -32.4 +/- 0.7% to -45.4 +/- 1.2%. With the exception of two highly 13C-depleted Gunflint microfossils, the results generally yield values consistent with carbon fixation via either the Calvin cycle or the acetyl-CoA pathway. However, the isotopic results are not consistent with the degree of fractionation expected from either the 3-hydroxypropionate cycle or the reductive tricarboxylic acid cycle, suggesting that the microfossils studied did not use either of these pathways for carbon fixation. The morphologies of the microfossils suggest an affinity to the cyanobacteria, and our carbon isotopic data are consistent with this assignment.

In situ U-Pb ages of zircons from the Bishop Tuff: No evidence for long crystal residence times

Ion microprobe U-Pb isotope analyses of zircons from fallout and intercalated ash-flow deposits of the early, most chemically evolved Bishop Tuff (east-central California) yield a weighted mean age of 823 ± 11 ka (mean square of weighted deviates, MSWD = 0.8; n = 22). A similar mean age of 839 ± 36 ka (MSWD = 0.6, n = 15) obtained on unpolished zircon rims suggests that the difference between these ages and that of 760 ka for the Bishop Tuff obtained by recent Ar isotope determinations may be largely analytical in origin. Considerations based only on the effect of 230 Th- 234 U disequilibria on the U-Pb ages yield an apparent maximum mean crystal residence time for zircon of ∼100 k.y. The zircon residence times are significantly shorter than those inferred from previous estimates for the timing of phenocryst growth and suggest that, once evolved, the early Bishop Tuff rhyolite was not stored for long before erupting.

U–Pb isotopic behaviour of zircon during upper-amphibolite facies fluid infiltration in the Napier Complex, east Antarctica

Understanding the factors that contribute to U–Pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages. Modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of U–Pb systematics in zircon might be achieved under subsolidus conditions. We present a field example from the Napier Complex, east Antarctica, in which a Palaeoproterozoic (2450–70 Ma) zircon population interacted locally with an Early Palaeozoic (498±1.7 Ma) aqueous fluid at upper-amphibolite facies conditions. Conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of U and Pb in zircons (both normal and reverse discordance) with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration. In contrast, ion probe ‘drilling’ or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of U–Pb systematics, to depths of ∼0.2 μm, with near-surface ages consistent with the timing of fluid influx at ∼498 Ma. Although zircon underwent some radiogenic Pb redistribution during fluid interaction, infiltrating fluids resulted in minimal grain-scale isotopic modification of zircon. Based on ion probe depth profiling results, we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses, in this case, is controlled by an analytical mixture of reset and/or recrystallised zircon along penetrative micro-fracture networks with that of adjacent unaffected zircon. We also suggest that the observed reverse discordance is genuine, resulting from localised intra-grain net accumulations of radiogenic Pb. We conclude that the isotopic response of zircon, in this case, is controlled by the interaction of an aqueous metamorphic fluid, of low to moderate salinity, resulting in sub-micrometre depth scale isotopic modification at natural crystal faces and along penetrative micro-fracture networks, and that grain-scale isotopic modification was negligible. Therefore, we urge caution when considering regional chronological interpretations that appeal to significant zircon isotopic resetting caused exclusively by metamorphic fluid interaction at upper-amphibolite facies conditions.

U–Pb geochronology from Tonagh Island, East Antarctica: implications for the timing of ultra-high temperature metamorphism of the Napier Complex

Abstract Ion microprobe U–Pb zircon geochronology of an orthopyroxene-bearing felsic orthogneiss from central Tonagh Island, Enderby Land, East Antarctica provides insight into the chronological-metamorphic evolution of the Archaean Napier Complex, the details of which have been the source of debate for over two decades. The orthogneiss crystallised at 2626±28 Ma, predating peak, ultra-high temperature (UHT) metamorphism and development of an intense regional S1 gneissosity. Two subsequent episodes of zircon growth/resetting can be identified. A minor period of zircon growth occurred at 2546±13 Ma, the regional significance and geological nature of which is unclear. This was followed by an episode of abundant zircon growth, as mantles on ∼2626 Ma cores and as anhedral grains, partly characterised by high Th/U (>1.2), at ∼2450–2480 Ma. This age coincides with both lower and upper concordia intercept ages from other U–Pb zircon studies, and several Rb–Sr and Sm–Nd whole-rock isochron ages from the Napier Complex. We conclude that UHT metamorphism occurred at ∼2450–2480 Ma, and find no compelling evidence that UHT occurred much earlier as has been postulated. The zircon U–Pb data from this study also indicates a lower intercept age of ∼500 Ma, which coincides with the emplacement of Early Palaeozoic pegmatite swarms and synchronous infiltration of aqueous fluids into the southwestern regions of the Napier Complex.

Sulfur isotopic compositions of individual sulfides in Martian meteorites ALH84001 and Nakhla: implications for crust–regolith exchange on Mars

Atmospheric chemical reactions on Mars have been invoked to explain non-mass-dependent Δ33S anomalies (Δ33S=δ33S−0.516δ34S) reported from bulk analyses of Martian meteorites. To explore this signature in detail, a new ion microprobe multi-collector technique was developed to obtain precise in situ 32S, 33S and 34S measurements of individual sulfide grains from Martian meteorites ALH84001 (>4.0 Ga) and Nakhla (1.3 Ga). This technique permits high-precision simultaneous measurement of multiple isotopes to uniquely evaluate Δ33S at the grain scale (<30 μm). Our data reveal resolvable non-mass-dependent Δ33S anomalies in two separate ALH84001 pyrite grains (Δ33S=−0.74±0.39‰ and −0.51±0.38‰, 2σ); none were detectable in Nakhla pyrrhotite (total range in Δ33S=−0.4±0.5‰ to −0.07±0.5‰, 2σ). Our results might reflect a difference in how these meteorites exchanged sulfur with the Martian regolith and/or differences in their sources (atmospheric versus meteoritic) of anomalous sulfur. Nebular heterogeneities in sulfur isotope composition are indicated by Δ33S anomalies preserved in, for example, the ureilites. The Δ33S anomalies in ALH84001 pyrite could suggest that early (pre-4 Ga) additions of a meteoritic component carried isotopically anomalous sulfur to the Martian regolith, and was stored there as seen in the detection of Δ33S anomalies from bulk measurements of Nakhla. Therefore, meteoritic contributions should also be considered in addition to atmospheric effects when explaining the large non-mass-dependent anomalies seen in Martian meteorites. These studies provide insight into how hydrothermal systems have facilitated exchange between volatile reservoirs on Mars, a planet that lacks efficient crustal recycling mechanisms and preserves ancient (and anomalous) Δ33S signatures.