Timothy Harrison

Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga

The long-favored paradigm for the development of continental crust is one of progressive growth beginning at ∼4 billion years ago (Ga). To test this hypothesis, we measured initial 176Hf/177Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. ϵHf (deviations of 176Hf/177Hf from bulk Earth in parts per 104) values show large positive and negative deviations from those of the bulk Earth. Negative values indicate the development of a Lu/Hf reservoir that is consistent with the formation of continental crust (Lu/Hf ≈ 0.01), perhaps as early as 4.5 Ga. Positive ϵHf deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4 to 4.5 Ga and was rapidly recycled into the mantle.

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.

The incorporation of Pb into zircon

Abstract The incorporation of Pb into zircons grown from Pb-rich solutions was evaluated using three different approaches: (1) high-temperature growth of large crystals from Pb-silicate melts; (2) hydrothermal overplating of thin epitaxial layers on substrates of natural zircon; and (3) growth of small, homogeneously nucleated crystals from aqueous fluids. The melt-grown zircons (50–400 μm) were crystallized from PbOSiO2ZrO2 (±P2O5) liquid at atmospheric pressure by cooling from 1430° to 1350°C. In the P2O5 free system, despite 66 wt% PbO in the melt, these zircons contain 3 atom% Pb, with apparent zircon/fluid partition coefficients of 4.2 and 2.6, respectively, for Pb4+ and Pb2+. In contrast to the case of melt-grown zircons, available P is excluded from the aqueous epitaxial zircon, suggesting that charge balance is accomplished by H+ instead. Small (2–5 μm) zircons grown by cooling aqueous solutions (PbO + SiO2 + ZrO2 ± P2O5) from 800°C or 900°C contain ∼ 0.25–0.5 atom% Pb (∼ 2–4 wt% PbO), yielding apparent DPb values of ∼ 0.2–0.3. Available P5+ is incorporated in a 2:1 ratio with Pb2+, suggesting a specific charge-balance mechanism: [2P5+ + Pb2+] = [2Si4+ + Zr4+]. However, Pb enters the zircon even when P is unavailable, so H+ may again play a charge-balancing role. Because of the rapid, polythermal modes of zircon growth and the high Pb content of the experimental systems, the apparent partition coefficients should not be viewed as equilibrium values, but as qualitative indicators of Pb compatibility under various growth circumstances. The overall results are consistent with the low but variable levels of non-radiogenic (common) Pb in natural zircons. The increased compatibility of Pb in fluid-grown, low-temperature zircons suggests a possible fingerprint for zircons from hydrothermal and wet-metamorphic rocks, i.e., high concentrations of common Pb.

Response to Comment on "Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga"

Valley et al. review the lines of evidence on which we drew to conclude that continental crust formed much earlier than previously thought. Their comment contains some misrepresentations that we correct, but new information they provide appears to bolster our hypothesis. Nothing in their comment refutes the presence of continental crust or plate boundary processes prior to 4 billion years ago.

Richter and Harrison receive the Bowen award

At the 1995 Fall Meeting in San Francisco, Calif., Frank Richter and Timothy Mark Harrison were honored with the 1995 N. L. Bowen Award, which is given to individuals for a single outstanding contribution to volcanology, geochemistry, or petrology made during the preceding 5 years. The award citations and Richters and Harrisons responses are given here.