Ronald M. Farquhar

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.

Dating galenas by means of their isotopic constitutions—II

Abstract Recently a rather large number of analyses of common lead samples has become available, and these in turn have led to a better understanding of the precise nature of the physical and geological processes resulting in the observed variations of lead isotope ratios. Specifically, there now appear to be three classes of leads, each of which has lead isotope ratios that vary in a remarkably simple pattern. These are meteoritic leads, anomalous leads, and leads from certain conformable lead ore deposits. The meteoritic leads and conformable leads both appear to have developed in surroundings where thorium and uranium had very similar properties, and in both cases the thorium to uranium ratio corresponds to a present value of 3 · 73 ± 0·03. This is taken to be evidence that both classes developed under predominantly reducing conditions. In contrast, the presence of thorium to uranium ratio present during the production of anomalous leads is extremely variable, suggesting an oxidizing environment. It has been suggested that all vein leads may be anomalous to some degree and therefore should be interpreted with this possibility in mind. The methods for dating galenas proposed previously by the present writers are re-examined in view of these developments and it is shown that they may lead to more explicit information on the age and history of lead ores than has usually been obtained.

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.