Pb Isotope Arrays Research Articles

Constraints on the sources of ore metals in Mississippi Valley-type deposits in central and east Tennessee, USA, using Pb isotopes

The Mississippi Valley-Type (MVT) Central and East Tennessee (TN) districts contain lead-zinc deposits that occur in the Early Ordovician carbonates of the Knox Group. Although both districts share similar host rock, have similar temperatures of formation, and typically fill open spaces of collapse breccias or replace their host carbonates, previous studies may suggest that these ores did not form from the same mineralizing fluids and may have different Pb sources. Nu Plasma MC-ICP-MS Pb isotopic analyses on sulfides from Central and East TN were conducted and the metal sources evaluated. Pb isotope ratios from the Young Mine in the East TN District plot within ranges defined by other East TN District samples from previous studies. Generally, Pb isotope ratios or ores from the Elmwood Mine in the Central TN District are more radiogenic and plot between samples from the southern Appalachians and Illinois-Kentucky districts. It appears that the thick, black, metal-rich shales of the Appalachian Basin have been the source of the Pb-Zn and ore fluids of MVT deposits that formed the East TN deposits. When East TN Pb data from this study are compared to Central TN Pb data, Central TN Pb is commonly more radiogenic, suggestive of a mineralizing fluid with different compositions. Lead isotopic signature of Central TN ores may imply potential mixing between end-member sources originating from the Southern Appalachians and Illinois basins.Lead isotopes in MVT and carbonate-hosted base metal deposits in US, South America, and Europe display two basic types of Pb-isotopic signatures. The classic MVT deposits in the mid-continent US (SE and Central Missouri, Illinois-Kentucky, and Upper Mississippi Valley districts) have highly radiogenic Pb-isotopic compositions and they generally display linear Pb-isotopic arrays. In contrast, deposits from Europe (Irish-type, Silesian-type, Alpine-type, and Cévennes region), the Appalachians, and Andean-type deposits have lower Pb-isotopic values. The European deposits (except for the Irish-type) have a restricted range of isotopic ratios, which suggest a single mineralizing event of relatively short duration. It appears that basement rocks or sedimentary rocks derived from the basement contributed Pb to many MVT and carbonate-hosted base-metal deposits worldwide. Hydrothermal leaching of radiogenic Pb from the crystalline basement rocks may have occurred during formation of these ore deposits. High 238U/204Pb values for several districts also suggest a supracrustal origin of Pb.

Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc

The fate of crustal material recycled into the convecting mantle by plate tectonics is important for understanding the chemical and physical evolution of the planet. Marked isotopic variability of Mo at the Earth's surface offers the promise of providing distinctive signatures of such recycled material. However, characterisation of the behaviour of Mo during subduction is needed to assess the potential of Mo isotope ratios as tracers for global geochemical cycles. Here we present Mo isotope data for input and output components of the archetypical Mariana arc: Mariana arc lavas, sediments from ODP Sites 800, 801 and 802 near the Mariana trench and the altered mafic, oceanic crust (AOC), from ODP Site 801, together with samples of the deeper oceanic crust from ODP Site 1256. We also report new high precision Pb isotope data for the Mariana arc lavas and a dataset of Pb isotope ratios from sediments from ODP Sites 800, 801 and 802. The Mariana arc lavas are enriched in Mo compared to elements of similar incompatibility during upper mantle melting, and have distinct, isotopically heavy Mo (high 98Mo/95Mo) relative to the upper mantle, by up to 0.3 parts per thousand. In contrast, the various subducting sediment lithologies dominantly host isotopically light Mo. Coupled Pb and Mo enrichment in the Mariana arc lavas suggests a common source for these elements and we further use Pb isotopes to identify the origin of the isotopically heavy Mo. We infer that an aqueous fluid component with elevated [Mo], [Pb], high 98Mo/95Mo and unradiogenic Pb is derived from the subducting, mafic oceanic crust. Although the top few hundred metres of the subducting, mafic crust have a high 98Mo/95Mo, as a result of seawater alteration, tightly defined Pb isotope arrays of the Mariana arc lavas extrapolate to a fluid component akin to fresh Pacific mid-ocean ridge basalts. This argues against a flux dominantly derived from the highly altered, uppermost mafic crust or indeed from an Indian-like mantle wedge. Thus we infer that the Pb and Mo budgets of the fluid component are dominated by contributions from the deeper, less altered (cooler) portion of the subducting Pacific crust. The high 98Mo/95Mo of this flux is likely caused by isotopic fractionation during dehydration and fluid flow in the slab. As a result, the residual mafic crust becomes isotopically lighter than the upper mantle from which it was derived. Our results suggest that the continental crust produced by arc magmatism should have an isotopically heavy Mo composition compared to the mantle, whilst a contribution of deep recycled oceanic crust to the sources of some ocean island basalts might be evident from an isotopically light Mo signature.

Upper Mantle Pollution during Afar Plume–Continental Rift Interaction

New Pb, Sr, Nd, Hf, and He isotope data for Quaternary basalts, erupted from Debre Zeyit, Butajira, and the Wonji Fault Belt of the Main Ethiopian Rift, show systematic mixing relationships involving three distinct mantle sources. The Pb, Sr, Nd, and Hf isotopic arrays converge in a specific region of isotopic multi-space where they define the composition of the Afar mantle plume (centered about (206)Pb/(204)Pb = 19 center dot 5, (87)Sr/(86)Sr = 0 center dot 7035, epsilon(Nd) = +4 center dot 6, epsilon(Hf) = +9 center dot 3, (3)He/(4)He > 15 R(A)). This plume end-member has an identical composition to that observed previously in oceanic basalts. The distinct isotopic arrays for the various volcanic areas in the Main Ethiopian Rift vary spatially in a systematic manner, and may be viewed as pseudo-binary mixing arrays. This further suggests that the Afar mantle plume interacts with the local continental lithosphere and upper mantle asthenosphere (mid-ocean ridge basalt-like source) through an ordered sequence of mixing events. Simple mixing models require that the mass proportions of continental lithosphere and upper mantle involved in magma generation must be nearly constant within each volcanic area, but that the proportion of plume material decreases regularly with distance southwestward along the Main Ethiopian Rift, away from the central axis of the plume. This systematic behavior means that continental lithosphere can become detached and mixed into the shallow mantle prior to the flow of upwelling plume material beneath the developing rift system. Detachment and mixing into the asthenosphere during continental rift evolution is an important process for producing the range of ambient upper mantle compositions sampled by mid-ocean ridge volcanism away from island hotspots.

Geochemical variations at intraplate hot spots caused by variable melting of a veined mantle plume

Three-dimensional geodynamic models of plume-lithosphere interaction were used to explore the causes of spatial patterns of magmatic compositions at intraplate hot spots. The models couple mantle flow, heat transfer, and the melting of multiple components present in the mantle as small blobs or veins with different solidi and composition. Predicted magma compositions evolve from having a strong signature from the deepest-melting component in the early stages of volcanism to a strong signature from the shallowest-melting component in the later stages. This compositional trend arises by progressive melt extraction of the different components and a horizontal displacement of their melting zones due to shear flow associated with plate motion. When three or more components are present, the composition of a volcano evolves along arrays in isotope space that trend toward mixed compositions of the components rather than the components themselves. Models explain the average Pb isotope trends in the Hawaiian Scientific Drilling Program core at Mauna Kea volcano. Observed scatter about the average trends and the distinction between the Kea and Loa subchains are explained by spatial variability in the relative proportions of the components in the mantle. Monte Carlo simulations show that linear Pb isotope arrays are unlikely to originate from nonsystematic, binary mixing if the scale of the magma capture zone is much larger than the scale of isotopic heterogeneity. However, systematic sampling by progressive melt extraction naturally generates such linear arrays if the capture zone is large compared to the scale of heterogeneity.

Mechanism and timing of Pb transport from subducted oceanic crust and sediment to the mantle source of arc lavas

New high-precision (double-spike) Pb isotope analyses of lavas from Tafahi and Niuatoputapu, the northernmost islands of the Tonga–Kermadec Island Arc, are used to examine the source of Pb in these samples, and the relative timing of Pb addition from the subducting oceanic crust and subducting sediment. Lavas from these islands have distinctive, radiogenic Pb isotope compositions, which are inherited from the basaltic crust of the subducting Louisville Seamount Chain on the Pacific Plate. The subducting oceanic plate and the overlying upper mantle beneath northern Tonga therefore have very different Pb isotope compositions, allowing the proportion of Pb derived from each of these sources, and from subducting sediment to be estimated. We show that between 42% and 90% of the Pb in northern Tonga lavas is derived from the basaltic crust of the subducting Louisville Seamount Chain. The dominant source of mantle Pb in arc lavas (at least in northern Tonga), is therefore subducted basaltic oceanic crust, rather than the overlying mantle wedge. The orientation of the Pb isotope arrays constrain the relative timing of Pb addition from these different sources, and show that sediment Pb must be mixed with the upper mantle before Pb from the subducted oceanic crust is added during a separate event. This observation is consistent with the results of experimental studies, which suggest that altered oceanic crust and sediment are likely to lose Pb by dehydration or melting at different depths. Melting of sediment at depths > 120 km, followed by migration of these melts to shallower levels within the overlying mantle, where Pb-bearing fluids derived from dehydration of oceanic crust trigger mantle melting, could explain the observed mixing relationships. Mass balance calculations show that the Pb flux into the arc magma source corresponds to the amount of Pb contained in the uppermost 67 to 143 m of the subducting basaltic crust of the Louisville Seamount Chain. If this minimum estimate is representative of subduction zones worldwide, where 7 km thick oceanic crust containing 0.5 ppm Pb is subducted, then the average Nd/Pb ratio of the oceanic crust that is recycled into the deep mantle is decreased by at least 3.5% as a result of subduction.

Modeling lead isotopic heterogeneity in mid-ocean ridge basalts

We have investigated the evolution of the U–Th–Pb system in a 4-reservoir model with an isolated mantle of bulk silicate Earth composition, a depleted mantle, and a two-layered continental crust. These models not only quantify the mean composition of the reservoirs, but also explicitly track the creation, evolution and destruction of heterogeneities (sub-reservoirs) within the reservoirs. The heterogeneities are sampled statistically during melting that produces basalts, and the model results are directly compared with observed Pb-isotope arrays. The resolution of the Pb paradox and a satisfactory fit to the MORB 207Pb– 206Pb pseudo-isochron require the removal of a large fraction (∼ 50%) of lead from subducted oceanic crust. The pseudo-isochron slope represents a mixing line between the “background matrix” (average of small length scale sub-reservoirs which appear in roughly the same relative proportion in all samples) in the MORB source and the larger-scale sub-reservoirs (which appear in varying proportion in different samples). The background matrix is similar in isotopic composition to the apparent components FOZO and C, and hence these components need not be in the lower mantle. The kappa conundrum only presents a problem if the mantle is treated as a closed system.

Subducted upper and lower continental crust contributes to magmatism in the collision sector of the Sunda-Banda arc, Indonesia

Pb isotopes in igneous rocks from the Banda-Sunda arc show extreme along-arc variations, which correspond to major lithologic changes in crustal components entering the subduction system. An increase in 2 0 6 Pb/ 2 0 4 Pb ratios toward the zone of collision with the Australian continent reflects input of subducted upper-crustal material; maximum values coincide with anomalously radiogenic 3 He/ 4 He ratios that have been earlier attributed to the involvement of the continental margin. The collision zone is further characterized by 2 0 8 Pb/ 2 0 4 Pb ratios that are higher for a given 2 0 7 Pb/ 2 0 4 Pb value than observed in the noncollisional sectors, and in the central part of the collision zone, the 2 0 6 Pb/ 2 0 4 Pb ratios are lower than the most radiogenic values in the adjacent areas. We propose that these Pb isotope signatures reflect input of subducted lower crust, mobilized as a result of slabwindow formation during arc-continent collision. Variations in Pb isotopes in the collision zone are solely determined by variations in the nature and proportions of different subducted components. The Pb isotope arrays in the noncollision area may be dominated by slab components as well and could reflect mixing between subducted oceanic crust and entrained sediments, rather than between subarc mantle and subducted sediments. Our new interpretation of the Indonesian Pb isotope data does not call for involvement of ocean-island basalt (OIB)-type mantle or Australian subcontinental lithospheric mantle, as has been suggested previously.

The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP‐2 drill core

We analyzed Pb isotopic compositions of 50 samples from the HSDP‐2 drill hole, covering the time interval 180 to 550 kyr B.P. in the stratigraphic record of Mauna Kea. All analyses were corrected for instrumental bias using a triple‐spike technique. The aims of this study are to document temporal changes in sources contributing to Mauna Kea and to investigate how these may relate to the chemical structure of the Hawaiian plume. Lead isotopic compositions of the lavas have 206Pb/204Pb ratios ranging from 18.41 to 18.63, 207Pb/204Pb from 15.47 to 15.49, and 208Pb/204Pb from 37.97 to 38.22. In 207Pb/204Pb‐206Pb/204Pb space, the samples display a broad linear array, while three distinct arrays are found in 208Pb/204Pb‐206Pb/204Pb space. These arrays can clearly be distinguished by their 208Pb/204Pb ratios and are referred to as “Kea‐lo8,” “Kea‐mid8,” and “Kea‐hi8.” The 206Pb/204Pb isotope ratios exhibit rapid shifts by ∼0.2 over 100 m depth intervals, and jumps from one Pb isotope array to another and back in less than ∼100 m depth. Despite these rapid Pb isotope fluctuations, a particular Pb isotope array dominates over periods of several tens to hundreds of kiloyears. We interpret the Pb isotope arrays found in HSDP‐2 in terms of mixing of end‐members lying along the radiogenic and unradiogenic extensions of the arrays. At the radiogenic extension the three HSDP‐2 arrays converge to a common end‐member. The lower extensions of the arrays diverge in three directions, each with different 208Pb/204Pb ratios. This topology suggests that the HSDP‐2 arrays were produced by mixing of at least four end‐members. The origin of these end‐members was investigated using Monte Carlo simulations of a Pb isotope evolution model. The simulations suggest that the common radiogenic end‐member of the three Pb isotope arrays contains material with elevated μ values and has a relatively young age (<1.5 Ga). Such a signature can be plausibly interpreted in terms of the presence of recycled oceanic crust in the source. The HSDP‐2 Kea‐lo8, Kea‐mid8, and Kea‐hi8 Pb isotope arrays dominate over different time periods and can be related to the displacement of Mauna Kea relative to the plume center over time. The Kea‐lo8 array is present between ∼180 and 370 ka and samples more peripheral parts of the plume, while the Kea‐mid8 and Kea‐hi8 arrays occur in the deeper parts of the core (∼370 to 550 kyr ago), when Mauna Kea was closer to the plume center. Over the time intervals when each array dominates, we derive corresponding “lengths” of materials in the source by integrating the estimated upwelling velocity across the plume. These calculations suggest Pb isotope heterogeneities of at least several tens of kilometers in vertical length within the Hawaiian plume. The Pb isotope arrays may correspond to relatively small‐scale heterogeneities derived from the D″ layer in the lower mantle.

Geographic control on Pb isotope distribution and sources in Indian Ocean Fe-Mn deposits

High-precision Pb isotope data have been obtained on Indian Ocean Fe-Mn deposits with a large geographic coverage. These data reveal a strong geographic control on the distribution of Pb isotopes as well as sources of Pb within this basin. The provinciality of Pb isotopes at the scale of the whole Indian Ocean, as well as that of individual basins, broadly matches the pattern of deepwater flow.The existence of several sources of Pb is best illustrated by the presence of three well-defined Pb isotopic arrays, each of which is confined to clear-cut geographic domains. These arrays imply a dominance of binary mixing of Pb sources within each of these domains. The domains consist of the North-Indian (N-Indian; north of 20°S), Southwest-Indian (SW-Indian; 20°S to 50°S, west of 45°E), South-Indian (S-Indian; 20°S to 50°S, east of 45°E) and Antarctic-Indian (A-Indian; south of 50°S), and clearly exhibit a strong control by latitude. The S-Indian domain and Mozambique Channel samples form a cluster, suggesting that there are more than just two sources contributing in these regions.We show that the N-Indian is dominated by sources of Pb derived from the High Himalayas and the Trans-Himalayan Complex, and most likely inherited from interaction at the water interface with Bengal and Indus Fan sediments. The SW-Indian and A-Indian domains share a common unradiogenic component, represented by circumpolar waters derived from the Pacific Ocean and flowing through the Drake Passage into the Atlantic and Indian Oceans. However, the radiogenic sources of Pb in these two domains are clearly distinct. In the SW-Indian, the radiogenic Pb signal reflects the tongue of North Atlantic Deep Water flowing around the tip of South Africa, while in the A-Indian, the source of radiogenic Pb is less certain, but quite probably originates in the Weddell Sea region.These data illustrate the complexity of the spatial Pb isotopic distribution, and Pb sources, within a single ocean basin at the present day, which should be borne in mind when interpreting long-term radiogenic isotope paleorecords from Fe-Mn crusts.

High precision lead isotope systematics of lavas from the Hawaiian Scientific Drilling Project

We report Pb isotopic compositions for 35 samples of the volcanoes Mauna Loa and Mauna Kea from the Hawaiian Scientific Drilling Project (HSDP-1) core at Hilo. These data were obtained with an external precision of ∼100 ppm (2 σ ext.) on the ratios 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb by using a Pb triple spike to correct for instrumental mass fractionation. The Pb isotopic compositions in the lower section (1200 to 280 m) of the core sample 200 to 400 ka-old Mauna Kea lavas, and display two well-defined linear arrays in 207Pb/ 204Pb– 206Pb/ 204Pb and 208Pb/ 204Pb– 206Pb/ 204Pb isotope spaces. There is a suggestion that Mauna Loa (0 to 280 m depth) also displays such linear array(s). However, analysis of the Mauna Loa samples is complicated by residual contamination and/or sample heterogeneity. While these latter data exhibit a satisfactory array in 208Pb/ 204Pb vs. 206Pb/ 204Pb, there still remains scatter in 207Pb/ 204Pb– 206Pb/ 204Pb space, making it difficult to assess the true Pb isotope systematics of Mauna Loa. The presence of two linear Pb isotopic arrays in Mauna Kea can be interpreted as either reflecting two parallel isochrons or in terms of binary mixing. If interpreted as isochrons, the 207Pb/ 204Pb– 206Pb/ 204 Pb systematics correspond to an age of ∼1.9 Ga. Comparison of measured Th/U ratios in the lavas and those inferred from Pb isotope systematics strongly suggest that the Pb isotopic arrays reflect binary mixing, and this bears directly on the question of how many distinct components are present in the Hawaiian plume. Most of the new Mauna Kea data lie well outside the mixing-component triangle defined in the literature by the “Kea”, “Loihi”, and “Koolau” components. On the basis of the relationships between Pb isotope ratios in 3D and a principal component analysis of the Mauna Kea Pb isotope dataset, we show here that a three-component mixing model can in principle explain both mixing lines. However, such an explanation requires a highly specific set of mixing conditions in order to produce parallel arrays in Pb isotope space (2D and 3D). Therefore, our preferred interpretation is that the two arrays reflect binary mixing, with four discrete source components involved in the generation of the Kea lavas. Comparison of the Pb isotope characteristics of these lavas with those of East Pacific Rise (EPR) MORB glasses further suggests that EPR-type Pacific lithosphere does not contribute to the source of Kea lavas. The position of samples along the mixing lines does not correlate with stratigraphic height in the core, and therefore the age of the lavas. Rather, it appears as though the relative proportions of the endmembers are controlled by the spatial configuration of these endmembers, and by melting and transport processes in the source itself. The stratigraphic fluctuations of Pb and Sr isotopes contrast with the monotonic decrease of ε Nd and ε Hf values as a function of age. This may in part be explained by differences in analytical precision of isotope measurements relative to the total range of values observed. This analytical resolution is far higher for Pb than for the other radiogenic isotopes. Alternatively, the observed fluctuation may be caused by the mobility of lead (as well as Rb and/or Sr) during the ancient differentiation process that created the differences in parent–daughter ratios.

The Sub-lithospheric Source of North Atlantic Basalts: Evidence for, and Significance of, a Common End-member

Palaeogene basalts from the margins of the North Atlantic often show geochemical variations that are consistent with their parental magmas having interacted with the lithosphere en route to the Earth's surface. These geochemical trends vary depending on the nature of the local lithospheric contaminants. Using examples from the British Tertiary Igneous Province and SE Greenland, we construct coherent contamination trends, which converge on a restricted Pb isotope composition, apparently indicating a common uncontaminated asthenospheric mantle component. Significantly, this composition is also suitable as one end-member of the Pb isotope arrays recorded in Recent Icelandic basalts. We conclude that this composition has been a persistent component of the Iceland plume over 60 my, dominating the mantle contribution to the Palaeocene phase of flood basalt magmatism but constituting only one end-member on Iceland. The Pb isotope composition of this 'North Atlantic end-member' is consistent with, but not necessarily demanding of, a primordial source. Recent evidence suggesting a lower- mantle origin for mantle plumes encourages investigation of whether the geochemical evidence supports that hypothesis. Helium isotope data from Palaeogene North Atlantic basalts support a lower-mantle contribution. However, mixing models suggest that it is untimely that the lower-mantle contribution is large enough to dominate the Sr-Nd-Pb isotope compositions and lithophile trace element signatures of any plume-derived basalts.

Torfajökull: a radiogenic end-member of the Iceland Pb-isotopic array

Torfajökull is the largest silicic center in Iceland and is located on the eastern branch of Iceland's neovolcanic zone. Torfajökull's location on the Iceland volcanic crust is unique because (1) it is situated behind the tip of a propagating ridge, and (2) several ridge jumps made it possible for lavas to extrude through relatively old basaltic crust (up to 10 Ma). Torfajökull is dominated by large amounts of silicic rocks, with less abundant basaltic and intermediate compositions. Several lavas show evidence of mixing of basaltic and rhyolitic melts. Varying degrees of assimilation of melts from hydrated and metamorphosed basaltic crust into primitive mantle-derived melts have been argued as the key mechanism in generating the variation of rock types and isotopic signatures found in the Icelandic lavas. Torfajökull, with its underlying old basaltic crust, offers the most favourable place to identify the contribution of crustal melts through radiogenic isotope analysis. Our data show that the Torfajökull lavas are among the most radiogenic Sr- and Pb-containing lavas in Iceland, but we find no correlation between major elements and the radiogenic isotope compositions. Basaltic, intermediate and rhyolitic lavas from the Torfajökull silicic center have 87Sr/ 86Sr- and 143Nd/ 144Nd-values of 0.70323–0.70342 and 0.512963–0.512999, respectively. Despite a large variation in SiO 2 concentrations (50%–74%), there are no significant differences in isotopic compositions between the different rock types. Torfajökull lavas plot towards the high 87Sr/ 86Sr end of the Icelandic Sr–Nd-isotope array. The Pb-isotope compositions are even more strikingly uniform ( 206Pb/ 204Pb 19.168–19.308, 207Pb/ 204Pb 15.513–15.587, 208Pb/ 204Pb 38.729–39.066) with no indication of radiogenic enrichment in the silicic lavas. The Pb-isotopic signature of the Torfajökull basaltic and silicic lavas clearly defines the most radiogenic end of the Iceland Pb-isotopic arrays. The radiogenic Sr- and Pb-isotope component of the Iceland isotopic array is well defined as a FoZo (focal zone) mantle component.

Reply to Comment by Y.-F. Zheng on “Pb-isotopic evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite facies transition from the Lewisian complex of north-west Scotland: Implications for Pb-Pb dating”

In a recent paper ( Whitehouse 1989a), Pb-isotopic arrays obtained from a prograde amphibolite to granulite facies transition in the Lewisian Complex were interpreted in terms of different degrees of Pb-isotopic homogenisation and different styles of U Pb fractionation during the 2660 Ma Badcallian metamorphism. Zheng (1990) comments that post-2660 Ma U-Pb disturbance might have contributed significantly to the observed Pb-isotopic arrays, thus requiring a re-evaluation of the proposed model for U-Th-Pb behaviour at 2660 Ma. This comment raises an important point with regard to the general interpretation of Pb- (and other) isotopic data from ancient poly-phase metamorphic terranes, namely the extent to which particular events may be evaluated as significant in multi-stage isotopic evolution or dismissed as insignificant, thus reducing the number of stages and permitting more definitive modelling of earlier events. In this reply, it is argued that, for the samples studied by Whitehouse (1989a), post-Badcallian events are not represented or are unable to generate the observed Pb-isotopic arrays. The modelling of Zheng (1989) for these samples is considered to be inappropriate and misleading. The Pb-isotopic data of Moorbath et al. (1969), for which measured U Pb ratios are available, appear to indicate an U-Pb disturbance at ~900 Ma when reduced using a three-dimensional isochron plane model (ZHENG, 1990). Similar results are obtained using the approach of Gale and Mussett (1973) for reducing three-stage data to two-stage. However, ages obtained from both these methods are highly dependent upon the assumption of initial ratio homogenisation during Badcallian metamorphism. Geochemical considerations discussed by Whitehouse (1989a) and reiterated here question the validity of this assumption and any modelling based upon it.