Large‐scale tectonic cycles in Europe revealed by distinct Pb isotope provinces (original) (raw)
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The new Lyon database of Pb isotopes in European and Circum-Mediterranean ores
2015
We have compiled Pb isotope compositions from the literature of >6800 European sulphide ores and several dozen granitic K-feldspars. The compilation includes the Oxford OXALID database, which has been used widely in archaeology, the national Swedish, Norwegian, and Finnish databases, and samples from North Africa and Turkey. The database is restricted exclusively to TIMS and MC-ICP-MS data and to samples with Pb measurements to ensure 3D Pb isotope space. All the data have been geolocated sample by sample using information from the original literature and Google Earth. The Pb/Pb, Pb/Pb, and Pb/Pb data and their literature sources are listed in an excel spreadsheet together with calculated geochemically and geologically informative parameters, which are the Pb model ages (Tmod) and the model U/Pb (),Th/Pb, and Th/U () ratios. To best visualize the data, they have been averaged over 0.5° degree cells and plotted on geographic maps using public-domain GMT software. Each map of eit...
Missing Lead and High 3He/4He in Ancient Sulfides Associated with Continental Crust Formation
Major terrestrial reservoirs have Pb isotopes more radiogenic than the bulk silicate Earth. This requires a missing unradiogenic Pb reservoir, which has been argued to reside in the lower continental crust or dissolved in the core. Chalcophile element studies indicate that continent formation requires the formation of sulfide-bearing mafic cumulates in arcs. Because Pb, but not U, partitions into sulfides, we show that continent formation must have simultaneously generated time-integrated unradiogenic Pb reservoirs composed of sulfide-bearing cumulates, now recycled back into the mantle or stored deep in the continental lithosphere. The generation of such cumulates could also lead to coupled He-Pb isotopic systematics because 4 He is also produced during U-Th-Pb decay. Here, we show that He may be soluble in sulfide melts, such that sulfide-bearing cumulates would be enriched in both Pb and He relative to U and Th, ''freezing'' in He and Pb isotopes of the ambient mantle at the time of sulfide formation. This implies that ancient sulfide-bearing cumulates would be characterized by unradiogenic Pb and He isotopes (high-3 He/ 4 He). These primitive signatures are usually attributed to primordial, undifferentiated mantle, but in this case, they are the very imprint of mantle differentiation via continent formation. T he geochemical behavior of Pb in the Earth is not fully understood. Most upper crustal and upper mantle reservoirs have Pb isotopes more radiogenic than bulk Earth estimates, manifested by silicate reservoirs plotting to the right of the geochron on a 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb diagram. This imbalance, known as the ''Pb paradox'', means that a missing reservoir with unradiogenic Pb is required to balance the radiogenic Pb 1. One solution is that the missing unradiogenic Pb resides in the present-day lower continental crust 2. However, most lower crustal xenoliths are not sufficiently unradiogenic to satisfy this mass balance 3,4,5. Another hypothesis, motivated by experimental studies, is that Pb dissolved into the Earth's core early in Earth's history 6,7. If Pb indeed segregated into the core, the U-Pb systematics of the bulk silicate Earth would indicate that the core formed 80-140 Myr after planetary accretion 6,7 , but this core formation age is much later than the ,30 Myr after accretion based on 182 Hf-182 W isotopic systematics, which is uniquely sensitive to core formation 8,9 (unlike the U-Pb system, which can be fractionated by processes unrelated to core formation). Alternatively, this young U-Pb age of the bulk silicate Earth, 80-140 Myr after Earth's accretion 6,7 , has been used to argue for a late accretion of volatile elements, such as Pb, to the Earth 10,11. Due to these inconsistencies, additional hypotheses should be explored. Sulfide minerals are another potentially important carrier of Pb 12. A recent study by Lee et al. 13 showed that pyroxenite cumulates formed during the differentiation of arc magmas are enriched in sulfides and therefore enriched in strongly chalcophile (sulfide-loving) elements like Cu. The bulk continental crust is felsic in composition because a large amount of mafic cumulates were formed and then removed from the continental crust by delamination 14,15. Lee et al. 13 showed that the Cu content of the continental crust is depleted relative to basaltic magmas and even the mantle, which implies that the complementary mafic cumulates should be sulfide-bearing. The amount of S, in the form of sulfide, which must be missing from the continental crust, can be evaluated by considering the compositional effects on sulfide solubility during magmatic differentiation. Assuming the initial building blocks of continents are basalts and that almost all basalts are sulfide-saturated in their mantle source regions, the initial S contents of these juvenile basalts will range from 1600-2000 ppm 16. Because sulfide solubility decreases substantially with increasing SiO 2 and decreasing FeO contents, by the time the basalts evolve to andesitic compositions typical of bulk continental crust, the S content at sulfide solubility has fallen to ,200 ppm 16 (solubility may rise with further magmatic evolution if oxygen fugacities rise, but it is the andesitic compositions that are of interest here as such compositions most closely match the average composition of the
European Journal of Mineralogy, 2012
The isotopic compositions of lead in copper-bearing hydrothermal sulphide deposits from the central-eastern Southalpine domain were analysed using a Multi-Collector-ICP-MS. The data were combined with existing lead isotope data (ore lead) for hydrothermal polymetallic deposits from the same area and compared with the isotopic compositions of potential lead sources. Copper and polymetallic pre-Variscan (Late Ordovician-Early Silurian) stratiform, post-Variscan (Permian to Triassic) vein, and stratabound sediment-hosted (Late Permian to Early Triassic) deposits, are characterised by highly variable ratios of radiogenic to non-radiogenic lead, but show very similar, high, time-integrated m (¼ 238 U/ 204 Pb) and W (¼ 232 Th/ 204 Pb) values. A progressive relative increase in radiogenic lead is observed from (i) pre-Variscan deposits to (ii) post-Variscan sulphide-rich veins in the Variscan metamorphic basement and in the lower units of the Early Permian volcanic sequence, to (iii) post-Variscan sulphide-rich and fluorite-rich veins in the upper units of the Early Permian volcanic sequence, to (iv) post-Variscan fluorite-rich veins cutting the overlying Late Permian sediments and mid-Triassic mafic dikes. The dominant lead sources for all these deposits were Cambrian-Devonian (meta) sediments of the Variscan basement. Contributions from Permian and Triassic igneous rocks were of minor importance, if any, even for vein deposits which were evidently related to Permian magmatism. The isotopic compositions of some of the Permian vein deposits are consistent with, although they do not unequivocally prove, remobilization of metals from the pre-Variscan stratiform deposits. Stratabound deposits in the Late Permian sandstones and, possibly, those in the Early Triassic carbonates also received a major lead input from the Variscan metasediments, with a variable additional contribution from the host Permian sediments. Deposits in Triassic magmatic rocks are displaced to slightly lower m and W values, suggesting lead contribution from Triassic magmatism. The high m and W values of the deposits studied here are consistent with regional isotopic patterns of Pb-Zn-rich deposits in more northerly and easterly sectors of the Eastern Alps (Austroalpine, eastern Southalpine) and of several circum-Mediterranean Pb-Zn and polymetallic deposits of Paleozoic to Triassic age (Sardinia, Betic Cordillera) or derived from remobilisation of Paleozoic deposits (Tuscany). This isotopic uniformity suggests that an isotopic province characterized by the dominance of old (Early Proterozoic to Archean) detrital source material extended across a relatively wide portion of the former north-Gondwanan margin.
A miner's perspective on Pb isotope provenances in the Western and Central Mediterranean
Journal of Archaeological Science, 2020
The conventional approach to ore provenance studies of ancient silver coins and artifacts has been to first analyze and then try to match them to published data about mining districts, a difficult task given our incomplete knowledge of these. While literary sources are useful to identify possible provenances, they potentially bias interpretations proper because of a variety of limitations of their time. Archeological evidence in the form of mining shafts, galleries, spoil heaps, and tools also provides a tangible and reliable record of mining, but dating such activity can be problematic and the record is inconsistent. Here we propose a new approach driven by Pb isotopic data rather than numismatic groups. Statistical analysis of Pb isotopic data is used to identify ore-defined isotopic clusters. This new method is based on an algorithm that predicts the number of isotopic clusters necessary to fulfill the simple condition that variance within isotopic clusters is minimized whereas inter-cluster variance is maximized. Since each cluster reflects a discrete geological episode within a particular environment broadly datable to a specific Pb model age, it can be identified as a potential source exploited by ancient miners. We explore the potential of this method in two examples using data from coins and ores respectively. In the first example, Roman Republican silver coins form three 'end-member' clusters sourced in mining districts with Cenozoic, Mesozoic, and Paleozoic Pb model ages. The example demonstrates how sources of silver used to mint coinage of the Roman Republic shifted within 50 years of the end of the Second Punic War in 201 BCE. In the second example using Aegean galena samples, Pb isotopes distinguish components with model ages datable to the Hercynian basement, the recent Aegean tectonic province, and Cyprus, noting that significant silver mining districts may remain unidentified in either Spain or the Aegean world. We further clarify a number of potential analytical issues and advocate that users of Pb isotopes for tracing archeological artifacts measure all four lead isotopes and inspect the 12 proposed isotope combinations in order to select those that provide the best representation of the data. We also emphasize that full advantage should be taken of the geologically informed parameters (model age and Th/U/Pb relationships) to identify the geological context of metal sources.
Lead isotopes combined with trace element data represent a powerful tool for non-ferrous metal provenance studies. Nevertheless, unconsidered geological factors and archaeological data, as well as ignored analytical procedures, may substantially modify the interpretation of the isotopic and trace element signature obtained as a potential ore candidate. Three archaeological examples, accompanied by high-resolution lead isotopic measurements (MC-ICP-MS), are presented here to discuss the above-mentioned criticisms and to propose some solutions. The first example deals with prehistoric/historical gold/silver-mining activity from Romania (the Baia Borşa and Roşia Montanȃ ore deposits). The second one regards the lead/silver metallurgical activity from the Mont-Lozère massif (France) during medieval times. The third example focuses on the comparison between two batches of lead isotope data gathered on Roman lead ingots from Saintes-Maries- using different SRM 981 Pb values. geochronology: more details are reported elsewhere . We only recall that three stable Pb isotopes out of four, 206 Pb, 207 Pb and 208 Pb, are radiogenic (time dependent) and that they are produced by the radioactive decay of 238 U, 235 U and 232 Th, respectively. The 204 Pb isotope is stable (time independent) and its abundance has been constant since the Earth's formation. The abundances of the four lead isotopes are approximately 52.4%, 22.1%, 24.1% and 1.4% for 208 Pb, 207 Pb, 206 Pb and 204 Pb respectively. Since the introduction of multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) in the mid-1990s, it has been possible to measure several isotopes simultaneously, with a high mass resolution (Rehkämper and Halliday 1998). These new spectrometers permit notable advances in geochemistry, and more recently in archaeology too. The presentday precise and accurate measurements improve the tracing by increasing the analytical precision by a factor of 10 over that obtainable with routine thermal ionization mass spectrometry (TIMS), and comparable to the precision of TIMS with double or triple spiking. MC-ICP-MS also allows the measurements of new isotopic tracers such as Cu, Fe, Sn and so on (called 'non-traditional isotopes') . The isotopic data measured nowadays by MC-ICP-MS are more significant and relevant, and they can be more accurately interpreted.
Lead isotope behaviour in a polyphased Pb-Zn ore deposit: Les Malines (Cévennes, France)
Mineralium Deposita, 1991
Les Malines Mine (C6vennes, France) provides an example of Pb-Zn ore bodies that underwent a polyphased evolution. On the Pb-Pb diagram, experimental points of the Les Malines ore types always plot in the central part of the field defined for the C6vennes metallogenic province. This homogeneity, the similarities with the Pb isotopic compositions of the surrounding rocks and the comparison with Pb isotopes evolution of whole rocks and minerals of the neighbouring continental lands, which emerged during Triassic and Liassic times, rule out the participation of extraneous Pb during the successive concentration stages and agree with an initial metal stock transformed and mobilized in a Pb closed system. Hercynian K-feldspars are the main source of the Pb preconcentrated during Triassic times. Most of the C6vennes Pb-Zn ore bodies could derive from this Triassic metal stock; nevertheless, local Pb isotopic anomalies suggest hydrothermal sources for some Pb-Zn ore bodies located along the C6vennes fault.
Geological constraints on the use of lead isotopes for provenance in archaeometallurgy
Archaeometry, 2020
The first half of this article is a brief critical history of the use of lead isotopes for inferring the geological provenance of archaeological materials, with emphasis on non-ferrous metals. The second half examines variation in the lead isotopic ratios of oxide and sulphide ore minerals in selected regions of the world, and relates these to the geological histories of ore formation in each region. This exercise shows that in regions where most ore deposits are of similar geological age-as in the Andes, Europe, and the circum-Mediterranean-provenance analysis with lead isotopes is inherently difficult because geographically distant sources often exhibit similar isotopic ratios. Conversely, regions with many periods of ore formation-like southern Africa-appear to be very promising regions for future studies of provenance with lead isotopes. The wider implication of our exploratory survey is that archaeologists should consider carefully the range and clustering of geological lead isotopic ratios in their regions of interest before investing large sums of money in lead isotopic analysis of artefacts.
Lead isotopes combined with trace element data represent a powerful tool for non-ferrous metal provenance studies. Nevertheless, unconsidered geological factors and archaeological data, as well as ignored analytical procedures, may substantially modify the interpretation of the isotopic and trace element signature obtained as a potential ore candidate. Three archaeological examples, accompanied by high-resolution lead isotopic measurements (MC-ICP-MS), are presented here to discuss the above-mentioned criticisms and to propose some solutions. The first example deals with prehistoric/historical gold/silver-mining activity from Romania (the Baia Borşa and Roşia Montanȃ ore deposits). The second one regards the lead/silver metallurgical activity from the Mont-Lozère massif (France) during medieval times. The third example focuses on the comparison between two batches of lead isotope data gathered on Roman lead ingots from Saintes-Maries- using different SRM 981 Pb values. geochronology: more details are reported elsewhere . We only recall that three stable Pb isotopes out of four, 206 Pb, 207 Pb and 208 Pb, are radiogenic (time dependent) and that they are produced by the radioactive decay of 238 U, 235 U and 232 Th, respectively. The 204 Pb isotope is stable (time independent) and its abundance has been constant since the Earth's formation. The abundances of the four lead isotopes are approximately 52.4%, 22.1%, 24.1% and 1.4% for 208 Pb, 207 Pb, 206 Pb and 204 Pb respectively. Since the introduction of multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) in the mid-1990s, it has been possible to measure several isotopes simultaneously, with a high mass resolution (Rehkämper and Halliday 1998). These new spectrometers permit notable advances in geochemistry, and more recently in archaeology too. The presentday precise and accurate measurements improve the tracing by increasing the analytical precision by a factor of 10 over that obtainable with routine thermal ionization mass spectrometry (TIMS), and comparable to the precision of TIMS with double or triple spiking. MC-ICP-MS also allows the measurements of new isotopic tracers such as Cu, Fe, Sn and so on (called 'non-traditional isotopes') . The isotopic data measured nowadays by MC-ICP-MS are more significant and relevant, and they can be more accurately interpreted.
The significance of galena Pb model ages and the formation of large Pb-Zn sedimentary deposits
Chemical Geology, 2021
In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the former from ores and the latter separated from granites. All samples are from Spain and the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, a gap which is well outside the uncertainties of ~30 Ma assigned to the model. The histogram of the new high-precision Pb isotope data shows prominent peaks of galena Pb model ages at 94±38 Ma and 392±39 Ma. When the data are consolidated with literature data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 395±40 (Middle Devonian), 90±34 Ma (Middle Cretaceous), and 613±42 Ma (Neoproterozoic), with two minor peaks at 185+26 Ma (Jurassic) and 313±41 (Upper Carboniferous). To a large extent, the model ages centered around these peaks correspond to distinct localities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan-African tectonic events, which are the main tectonic episodes that shaped Iberian geology, but rather match well-known global oceanic anoxic events. It is argued that surges of metals weathered from continental surfaces scorched during anoxic events accumulated and combined in anoxic water masses with unoxidized marine sulfide released by submarine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope compositions require that galenas from black shales are the source of the final ores. The Revised manuscript with no changes marked Click here to view linked References 2 sulfides were later remobilized by large-scale convective circulation of basinal and hydrothermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas and do not correlate with magmatic emplacement ages. It is suggested that they instead reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb isotopes can be used as a regional provenance tool, such an approach requires that the data are considered in a fully 3-dimensional space.
Archaeometry, 2013
Lead isotopes combined with trace element data represent a powerful tool for non-ferrous metal provenance studies. Nevertheless, unconsidered geological factors and archaeological data, as well as ignored analytical procedures, may substantially modify the interpretation of the isotopic and trace element signature obtained as a potential ore candidate. Three archaeological examples, accompanied by high-resolution lead isotopic measurements (MC-ICP-MS), are presented here to discuss the above-mentioned criticisms and to propose some solutions. The first example deals with prehistoric/historical gold/silver-mining activity from Romania (the Baia Borşa and Roşia Montanȃ ore deposits). The second one regards the lead/silver metallurgical activity from the Mont-Lozère massif (France) during medieval times. The third example focuses on the comparison between two batches of lead isotope data gathered on Roman lead ingots from Saintes-Maries-de-laMer , using different SRM 981 Pb values.