Patrick E. Smith

Single crystal 40Ar‐39Ar dating of a Late Quaternary paroxysm on Kos, Greece: Concordance of terrestrial and marine ages

The late Quaternary plinian eruption forming the Kos Plateau Tuff on the island of Kos, Greece, was one of the largest in the Mediterranean. A precise date for this eruption would significantly improve the chronology of the deep-sea tephra layer W-3, because reliable correlation of W-3 with the KOS Plateau Tuff has been previously established. We have dated sanidine from the eruption at 161.3±1.1 ka using the laser-fusion single crystal 40Ar-39Ar technique. This age represents a significant refinement over both conventional and highly precise (Cassignol method) K-Ar ages, and the 40Ar-39Ar age is shown to be in remarkably good agreement with the age of W-3 inferred from recent astronomically calibrated oxygen isotope chronologies.

First successful 40Ar-39Ar dating of glauconies: Argon recoil in single grains of cryptocrystalline material

Potassium-argon dating of glaucony, one of the few widely occurring authigenic materials in the sedimentary record, has provided a large percentage of the dates used for the calibration of the geologic time scale. This material has been considered unsuitable for 40 Ar- 39 Ar dating because of the large losses of neutron-induced 39 Ar during or subsequent to irradiation of samples. We have measured recoiled 39 Ar and 37 Ar from single grains of glaucony (100-350 μm in diameter). Irradiations were done both in air and in vacuo by using a microampoule technique of encapsulation. The observed in vacuo behavior of recoiled atoms can be readily understood by a simple model of their interaction with the network of very fine laminae composing the grains. When the microampoule technique is used, the integrated 40 Ar- 39 Ar ages and their associated precision compare very favorably with conventional K-Ar results. Because glauconies are inherently heterogeneous populations, owing to their complex evolution, it is anticipated that single-grain 40 Ar- 39 Ar dating may lead to significant refinements in the geologic time scale, especially for the Cenozoic.

The Edge of Time: Dating Young Volcanic Ash Layers with the 40Ar-39Ar Laser Probe

Argon-40-argon-39 single-crystal dating of young (5000 to 30,000 years ago) volcanic ash layers erupted from the Mono Craters, California, shows that the method can yield meaningful ages in Holocene tephra. Because of ubiquitous xenocrystic contamination, the data do not form isochrons but plot in wedge-shaped regions on an argon isotopic diagram. The upper boundary of the region is an isochron matching the 14C-derived age of the eruption. Such contamination-related patterns may be common in dating young materials by the single-crystal method. Argon dating by this method can help refine the time scale of physical and biological evolution over the past 100,000 years.

Zircon Lu-Hf systematics and the evolution of the Archean crust in the southern Superior Province, Canada

A combined Lu-Hf and U-Th-Pb isotopic study was made of 25 zircons and 2 whole rocks from the late Archean crust (2,888-2,668 Ma) in the southern Superior Province, Canada. The relative abundances of U, Th, Lu and Hf in zircons from the low grade Michipicoten and Gamitagama greenstone belts show variable patterns which in part reflect the bulk compositional differences of their parent rocks. Zircons from the high grade lower crustal regions adjacent to these belts (Kapuskasing Structural Zone) are distinguished from the low grade zircons by their strong depletions of Lu and Hf. The low Hf contents imply that the growth of metamorphic zircon involves a significant fractionation of the Zr/Hf ratio.Initial Hf isotope ratios for Hf in zircons from the low grade rocks are correlated with silica enrichment of their host rocks. eHf varies from +9.2 to −1.3 and data from similar rock types exhibit correlations of eHf with time. Whole rock basalt analyses yield eHf values of +8.7 and +11.3 suggesting their derivation from a depleted mantle. The basalt data fall on an evolution trend which implies that differentiation from a chondritic mantle occurred at 3,100-2,900 Ma. Low eHf values (−1.3 to +1.4) for rhyolites and granites are consistent with a derivation involving remelting of old crust similar to a 2,888 Ma granite with eHF of +0.5. Significantly higher values (+1.4 to +3.9) are found in zircons from 2,748-2,682 Ma dacites and tonalites suggesting that their parent rocks had higher Lu/Hf ratios. This may indicate that their parent rocks were mafic. However, there is some evidence that the possible lower crustal source reservoirs of these rocks may have undergone processes early in their histories which increased their Lu/ Hf ratios. This would give rise to the higher eHf values observed in their derivatives.

Direct radiometric age determination of carbonate diagenesis using U-Pb in secondary calcite

Abstract Carbonate rocks and corals from the Middle Devonian Lucas formation, Ontario have 238 U/ 204 Pb (μ) values ranging from 132 to 559 and give concordant, apparent 206 Pb/ * 238 U and 207 Pb/ * 235 U ages in approximate agreement with the primary age of sedimentation. A carbonate rock from one part of the section gives an anomalously young age and contains patches of secondary calcite spar. This spar is predominantly pale purple under cathodoluminescence (CL) and contains 250–600 ppb U and 15–22 ppb Pb. The μ values range from 2 × 10 3 –10 4 , making the Pb very radiogenic. Apparent 206 Pb/ * 238 U ages range from 208 to 249 Ma and 232 Th/ 238 U (κ) values are from 0.005 to 0.040. The younger ages are correlated with an increase in orange CL and higher apparent κ values. The age scatter is attributed to mixing of a subordinate calcite spar which infiltrated the older spar. The young calcite is bright orange under CL, has 7200 ppb U, μ of 6.2 × 10 4 , a κ of 0.23 and is dated at 45 Ma. The age of the older spar has been determined by a sequential dissolution technique. The resulting U-Pb data can be plotted as U-Pb “age spectra” and yield lplateau” dates ranging from 231.5 ± 1.4 to 244.9 ± 1.6 Ma and an integrated 206 Pb/ * 238 U age of 238.2 ± 1.1 Ma. Also, the sequential dissolution increases the μ values progressively to a5 × 10 4 making these ages independent of the initial Pb correction. The occurrence of two young ages of diagenetic calcite in these strata is consistent with the idea of recurrent fluid activity along fracture zones in subsurface sediments in the Michigan Basin. This study shows that it is now possible to date secondary carbonate diagenesis by U-Pb and by inference to determine precisely the timing of activity on ancient fracture zones in carbonate rocks.

Single-crystal 40Ar-39Ar dating of pyrite: No fool's clock

Single crystals of dispersed trace pyrite from two igneous rocks were dated by using the 40 Ar- 39 Ar laser probe method, and the resulting ages were compared to step-heating ages of associated amphibole and phlogopite. The isochron ages for 0.1–0.3 mm cubes of pyrite agree well with the respective amphibole ages of these igneous samples. Preliminary step-heating analyses of single pyrite crystals yield internal isochrons with indistinguishable, although less precise, ages. Both pyrite and amphibole are significantly older than associated phlogopite in one sample, which has undergone postcrystallization alteration. These results suggest that pyrite can give reliable and precise 40 Ar- 39 Ar ages even in the presence of subsequent alteration. Given the ubiquity of pyrite in many geologic environments, this technique has great promise for application to the dating of ores, sediments, and hydrocarbon migration, all of which have been very difficult to date directly.

Lasing in the Holocene: extending the 40Ar39Ar laser probe method into the 14C age range

Abstract Dating methods applicable to the last 50,000 yr, such as 14 C, require absolute calibrations, which are difficult to obtain over much of this interval. Extension of the 40 Ar 39 Ar isochron technique to ages 14 C dating. We have dated the Mono craters, California, at 12,560 ± 470 yr, using laser fusion to generate an isochron from 63 sanidine crystals from 5 sites. The near-atmospheric initial isotopic ratios and the agreement in ages from all sites suggest that no significant excess argon is present. It thus appears that, particularly with improved instrumentation and techniques currently under development, most of the 14 C age range is accessible to 40 Ar 39 Ar laser dating.

U-Pb systematics and alteration trends of Pennsylvanian-aged aragonite and calcite

Abstract Carbonate allochems from the mid-Pennsylvanian Buckhorn fauna of the Boggy Formation, Oklahoma, USA, consist of primary aragonite preserved in nautiloids and of secondary low-Mg calcite present in crinoids and corals. Unaltered nautiloids contain 11–38 ppb U and 23–422 ppb Pb and have low 238 U 204 Pb (μ) values from 4.1–14.8. They give a 238U-206Pb isochron age of 311 ± 48 Ma (2σ), which is in agreement with the assigned stratigraphic age (mid-Desmoinsian; ~305 Ma) of the Boggy Formation sediments. Their initial lead isotope ratios are 206 Pb 204 Pb = 19.11 ± 0.08 , 207 Pb 204 Pb = 15.70 ±0.04 and 208 Pb 204 Pb ~ 38.7 and reflect an upper crustal source for Pb in mid-Pennsylvanian seawater. These values are similar to those of coals of this age. The contents of both U and Pb in the Buckhorn nautiloids are significantly higher than those found in the modern nautilus Nautilus pompilius. These enrichments are attributed to bitumen which was added to the carbonates during diagenesis. The diagenetic transformation of aragonite to low-Mg calcite in the Buckhorn nautiloids was apparently associated with a net loss of Pb, thereby increasing theirμ values. The secondary carbonates of crinoids contain 265–352 ppb U, 136–262 ppb Pb and their μ values range from 68–111; consequently, their present-day uranogenic lead isotopic ratios are higher than those of the Buckhorn nautiloids. The U-Pb data for some diagenetic low-Mg components indicate an age of ~280 Ma suggesting an early timing for the post-depositional alteration process. The apparent initial lead isotope ratios of crinoids are variable and some are significantly higher than the coexisting aragonite ( 206 Pb 204 Pb = 20.50 , 207 Pb 204 Pb = 15.73 ). This very radiogenic signature is similar to those of Mississippi Valley-type ore deposits and both types may share a similar genesis.

Under the volcano: A new dimension in Ar‐Ar dating of volcanic ash

A significant part of the history of the Greek island of Kos has been decoded from a handful of volcanic ash by argon-argon laser probe dating of single crystals of the minerals sanidine, plagioclase and quartz. On a 3-D argon isotope plot, the data define six straight lines, corresponding to four distinct volcanic events from 161 to 1728 ka. All four events produced plagioclase crystals, whose ages and Ca/K ratios are consistent with the known evolution of the rest of the Aegean arc. Sanidine appeared only in the final eruption at 161 ka. The joint occurrence of quartz and plagioclase at a 925 ka event implies that quartz did not reside for millennia in the magma, as has been suggested for quartz from the Bishop Tuff, California. The step-heated cooling age of a schist fragment from the ash provides the first radiometric evidence for ∼250 Ma crust under the volcano.

Assessment of the U-Th-Pb system in two Archean metabasalts: Deciphering the complex histories of sulphides and silicates using acid leaching methods

Abstract A U-Th-Pb study of Archean metabasalts from two greenstone belts in the eastern Wawa Subprovince of the Canadian shield indicated variable disturbances had occurred in both whole rock systems. Changes in the Pb content appear to predominate over loss of parent elements, and meaningful Pb isochron ages could not be obtained from either of the metabasalts. Detailed leaching experiments on the rocks and analysis of associated disseminated sulphides indicate that the Pb isotopic compositions of the whole rocks are dominated by updated, but for the most part cogenetic, sulphides. Model ages for Pb released by sulphides and metabasalt acid leaches suggest that the sulphide Pb has been updated or remobilized during discrete (thermal ?) episodes. The validity of the inferred events is supported by ages indicated by other isotopic systems for nearby rocks. The silicate residues of the acid leached volcanics give well-defined Pb isochron ages. The Gamitagama belt metabasalt has a Pb isochron age of 2694 ± 54 Ma . Zircons from this rock unit have been dated by U-Pb at 2691 Ma, demonstrating that the acid leaching technique on whole rocks can isolate residues which give meaningful ages. Sulphides in this metabasalt appear to be affected by an event at 2.55 Ga. A similar rock from the Michipicoten belt has undergone a multistage history and yields a Pb isochron age of 2761 ± 36 Ma , consistent with a zircon U-Pb date for overlying felsic metavolcanic rocks. Later events affecting this basalt occurred at approximately 2.4, 2.2 and 1.6 Ga. The results show that through acid leaching, the primary ages of metabasalts and their later overprinting histories can be approximated. Initial Pb ratios for these metabasalts have been estimated. It appears that both depleted and enriched mantle reservoirs contributed to volcanism in this area. Acid leaching, combined with measurement of U and Th contents, appears to be a useful tool for better understanding the Pb isotope systematics of Archean metabasalts.