Precise Re–Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater

Rhenium and osmium isotope and abundance data have been obtained on precisely-located samples from three suites of immature, organic-rich mudrocks from Jurassic coastal outcrops in England, The data provide accurate whole-rode ages of 207 +/- 12 Ma, 181 +/- 13 Ma and 155 +/- 4.3 Ma for suites of Hettangian, Toarcian (exaratum Subzone) and Kimmeridgian (sensu anglico, wheatleyensis Subzone) samples. These new Re-Os ages are indistinguishable, within the assigned analytical uncertainties, from interpolated depositional ages estimated from published geological timescales, and establish the importance of the Re-Os dating technique for chronostratigraphic studies. Early-diagenetic pyrite nodules possess levels of Re and Os which are similar to 1-2 orders of magnitude lower than in the enclosing organic-rich mudrocks, indicating that these elements had already been removed from sediment pore waters at the time of nodule formation. Thus the Re-Os isotope system in these organic-rich mudrocks has been closed since, or from very soon after, the time of sediment deposition. Because most of the Re (98+%) and Os (95-99.8+%) in the mudrocks is shown to be hydrogenous, the Os-187/Os-188((i)) of the samples is interpreted to be that of contemporaneous seawater. The data thereby provide the first estimates of the Os isotope composition of Jurassic seawater. During the earliest Jurassic (Hettangian), the seawater Os-187/Os-188 ratio was extremely unradiogenic (similar to 0.15); it had increased to similar to 0.8 at the end of the Early Jurassic (Toarcian) similar to 20 Ma later, while in the Late Jurassic (Kimmeridgian) the seawater Os-187/Os-188 ratio was similar to 0.59. The most likely explanation for the unradiogenic Os isotope composition of Hettangian seawater is that the contribution of unradiogenic Os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic Os from the continents at that time. This interpretation is in Line with observations suggesting that global weathering rates were low in the Hettangian, and that increased hydrothermal and volcanic activity preceded the break-up of Pangea. The Re/Os ratios of Hettangian mudrocks (and by inference, of contemporaneous seawater) are similar to those of mudrocks deposited at later times during the Jurassic, and argues against the unradiogenic Os in Hettangian seawater being derived from extraterrestrial meteoritic sources

Stalagmite luminescence and peat humification records of palaeomoisture for the last 2500 years

Recent research has suggested that both raised and blanket bogs can provide proxy climate signals from variations in peat humification. In particular, oceanic margin sites have provided sensitive records that demonstrate century scale variations in humification. However, previous research has not compared records of peat humification with other terrestrial palaeoclimate proxies. Here, two records of climate change from an oceanic marginal site in NW Scotland are analysed. One, from a blanket bog, is derived from peat humification and covers the period 2100–100 BP. A second, from two stalagmites in a cave overlain by the bog, is derived from stalagmite luminescence wavelength variations for the samples deposited over 2500–0 BP. Both peat humification and stalagmite luminescence records demonstrate 90–100 year oscillations in bog wetness, that are attributed to variations in rainfall intensity or totals over this time period. It is argued that this is probably generated by a southward shift of the path of northern hemisphere depression tracks, possibly linked to variations in solar output

Not all supercontinents are created equal:Gondwana-Rodinia case study

The geologic records associated with the formation of the supercontinents Rodinia and Gondwana have markedly different seawater Sr and zircon Hf isotopic signatures. Rodinia-related (Grenville-Sveconorwegian-Sunsas) orogens display significantly less enriched crustal signatures than Gondwana-related (Pan-African) orogens. Seawater Sr isotope ratios also exhibit a more pronounced crustal signal during the span of the Gondwana supercontinent than at the time of Rodinia. Such isotopic differences are attributed to the age and nature of the continental margins involved in the collisional assembly, and specifically to the depleted mantle model ages, and hence the isotope ratios of the material weathered into the oceans. In our preferred model the isotopic signatures of Rodinia-suturing orogens reflect the closure of ocean basins with dual subduction zones verging in opposite directions, analogous to the modern Pacific basin. This would have resulted in the juxtaposition of juvenile continental and island arc terrains on both margins of the colliding plates, thus further reworking juvenile crust. Conversely, the assembly of Gondwana was accomplished primarily via a number of single-sided subduction zones that involved greater reworking of ancient cratonic lithologies within the collisional sutures. The proposed geodynamic models of the assembly of Rodinia and Gondwana provide a connection between the geodynamic configuration of supercontinent assembly and its resulting isotopic signature.</p

A Non-local Source of Irish Chalcolithic and Early Bronze Age Gold

Lead isotope analyses of 50 Irish Chalcolithic and Early Bronze Age gold artefacts favour a gold source in southern Ireland. However when combined with major element analysis, the artefacts are not consistent with any Irish gold deposit analysed to date. Understanding the lead isotope signatures of ore deposits within a study region allows informed inferences to be drawn regarding the likelihood that an unanalysed ore deposit was exploited in the past. If an Irish gold source is assumed, then the gold is most likely to have originated from deposits hosted by Old Red Sandstone in the Variscan ore field of south-west Ireland. However, based on our current understanding of mineralisation in the region, this scenario is considered unlikely. A non-Irish source for the gold is therefore preferred – a scenario that may favour cosmologically-driven acquisition, ie, the deliberate procurement of a material from distant or esoteric sources. Available geochemical data, combined with current archaeological evidence, favour the alluvial deposits of south-west Britain as the most likely source of the gold.</jats:p

Late Quaternary speleothem pollen in the British isles

As with many terrestrial areas, the British Quaternary sequence is characterised by incomplete, fragmentary records, whose correlation is based on stratigraphic or biostratigraphic techniques due to the lack of radiometric ages beyond the similar to 40 kyr limit of C-14 dating. Speleothems (secondary cave calcite deposits) offer a significant advantage over many sources of palaeoenvironmental information; they can be dated to a high precision and accuracy by uranium-thorium (U-238-Th-230) thermal ionisation and inductively coupled plasma (ICP) mass spectrometry in the time period back to 500 kyr. They may also contain sufficient well-preserved pollen representative of contemporary vegetation above the cave to allow palaeoenvironmental reconstruction. This study adopts the novel approach of combining pollen and thermal ionisation mass spectrometric (TIMS) U-Th dating of British speleothems to produce well-constrained palaeoenvironmental records. We report for the first time precisely dated records of pollen assemblages from speleothems suggesting the presence of thermophilous arboreal species in phases previously considered to have been consistently cool or cold and devoid of trees. Copyright (C) 2007 John Wiley &amp; Sons, Ltd.</p

Late Quaternary speleothem pollen in the British isles

As with many terrestrial areas, the British Quaternary sequence is characterised by incomplete, fragmentary records, whose correlation is based on stratigraphic or biostratigraphic techniques due to the lack of radiometric ages beyond the similar to 40 kyr limit of C-14 dating. Speleothems (secondary cave calcite deposits) offer a significant advantage over many sources of palaeoenvironmental information; they can be dated to a high precision and accuracy by uranium-thorium (U-238-Th-230) thermal ionisation and inductively coupled plasma (ICP) mass spectrometry in the time period back to 500 kyr. They may also contain sufficient well-preserved pollen representative of contemporary vegetation above the cave to allow palaeoenvironmental reconstruction. This study adopts the novel approach of combining pollen and thermal ionisation mass spectrometric (TIMS) U-Th dating of British speleothems to produce well-constrained palaeoenvironmental records. We report for the first time precisely dated records of pollen assemblages from speleothems suggesting the presence of thermophilous arboreal species in phases previously considered to have been consistently cool or cold and devoid of trees. Copyright (C) 2007 John Wiley &amp; Sons, Ltd.</p

Contrasting granite metallogeny through the zircon record:a case study from Myanmar

Granitoid-hosted mineral deposits are major global sources of a number of economically important metals. The fundamental controls on magma metal fertility are tectonic setting, the nature of source rocks, and magma differentiation. A clearer understanding of these petrogenetic processes has been forged through the accessory mineral zircon, which has considerable potential in metallogenic studies. We present an integrated zircon isotope (U-Pb, Lu-Hf, O) and trace element dataset from the paired Cu-Au (copper) and Sn-W (tin) magmatic belts in Myanmar. Copper arc zircons have juvenile ϵHf (+7.6 to +11.5) and mantle-like δ18O (5.2-5.5‰), whereas tin belt zircons have low ϵHf (-7 to-13) and heavier δ18O (6.2-7.7‰). Variations in zircon Hf and U/Yb reaffirm that tin belt magmas contain greater crustal contributions than copper arc rocks. Links between whole-rock Rb/Sr and zircon Eu/Eu∗highlight that the latter can monitor magma fractionation in these systems. Zircon Ce/Ce∗and Eu/Eu∗are sensitive to redox and fractionation respectively, and here are used to evaluate zircon sensitivity to the metallogenic affinity of their host rock. Critical contents of Sn in granitic magmas, which may be required for the development of economic tin deposits, are marked by zircon Eu/Eu∗values of ca. ≤0.08.</p

Generation and preservation of continental crust in the Grenville Orogeny

Detrital zircons from modern sediments display an episodic temporal distribution of U-Pb crystallization ages forming a series of 'peaks' and 'troughs'. The peaks are interpreted to represent either periods of enhanced generation of granitic magma perhaps associated with mantle overturn and superplume events, or preferential preservation of continental crust during global collisional orogenesis. The close association of those peaks with the assembly of supercontinents implies a causal relationship between collisional orogenesis and the presence of zircon age peaks. Here these two end-member models (episodic periodicity of increased magmatism versus selective preservation during collisional orogenesis) are assessed using U-Pb, Hf, and O analysis of detrital zircons from sedimentary successions deposited during the ∼1.3-1.1 Ga accretionary, ∼1.1-0.9 Ga collisional, and &lt;0.9 Ga extensional collapse phases of the Grenville orogenic cycle in Labrador and Scotland. The pre-collisional, accretionary stage provides a baseline of continental crust present prior to orogenesis and is dominated by Archean and Paleoproterozoic age peaks associated with pre-1300 Ma Laurentian geology. Strata deposited during the Grenville Orogeny display similar Archean and Paleoproterozoic detrital populations along with a series of broad muted peaks from ∼1500 to 1100 Ma. However, post-collisional sedimentary successions display a dominant age peak between 1085 and 985 Ma, similar to that observed in modern North American river sediments. Zircons within the post-orogenic sedimentary successions have progressively lower εHf and higher δ18O values from ∼1800 to ∼1200 Ma whereupon they have higher εHf and δ18O within the dominant 1085-985 Ma age peak. Furthermore, the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material. The timing of this dominant age peak coincides with the peak of metamorphism and magmatism associated with the Grenville Orogeny, which is a typical collisional orogenic belt. The change from broad muted age peaks in the syn-orogenic strata to a single peak in the post-orogenic sedimentary successions and in the modern river sediments implies a significant shift in provenance following continental collision. This temporal change in provenance highlights that the source(s), from which detrital zircons within syn-orogenic strata were derived, was no longer available during the later stages of the accretionary and collisional stages of the orogenic cycle. This may reflect some combination of tectonic burial, erosion, or possibly recycling into the mantle by tectonic erosion of the source(s). During continental collision, the incorporated continental crust is isolated from crustal recycling processes operative at subduction margins. This tectonic isolation combined with sedimentary recycling likely controls the presence of the isotopic signature associated with the Grenville Orogeny in the modern Mississippi and Appalachian river sediments. These results imply that zircon age peaks, which developed in conjunction with supercontinents, are the product of selective crustal preservation resulting from collisional orogenesis.</p

Intermontane basins and bimodal volcanism at the onset of the Sveconorwegian Orogeny, southern Norway

The greenschist-facies late Mesoproterozoic Bandak succession in southern Norway consists of interlayered quartzites and meta-volcanic rocks with well preserved depositional and structural relations, which when combined provide important information on the late Mesoproterozoic continental margin of Baltica prior to the assembly of the supercontinent Rodinia. The timing of deposition of the Bandak succession is constrained by previously published tuff ages and new detrital zircon analyses reported here. The lower units of the Bandak succession (the Ofte and Røynstaul formations) display diverse detrital zircon age spectra implying derivation from a wide array of sources. The Morgedal and Gjuve formations have unimodal U-Pb age spectra, suggesting input from a single source and probably accumulation in restricted basins. The overlying Eidsborg Formation displays a wide range of detrital zircon age peaks indicative of input from more varied source regions. Hf and O isotopes in detrital zircons of the Bandak succession indicate derivation from typical Fennoscandian basement rocks with the youngest dominant population (∼1150 Ma) having been derived from sources formed by remelting of the pre-Sveconorwegian juvenile Gothian basement. Whole rock geochemistry and Nd isotopic signatures further imply that the rhyolite of the Dalaå Formation was formed from anatexis of c. 1500 Ma crust. In contrast, mafic volcanic rocks indicate a mantle source that had been previously enriched through subduction, and variable contamination from older continental crust, or melts derived from it (i.e. the Dalaå Formation). The mafic lithologies reveal decreasing amounts of crustal contamination higher in the section, compatible with increasing amounts of extension and a thinner crustal column. Age, sedimentological, and geochemical data for the volcanosedimentary units in the Bandak succession record an episode of intracontinental extension, associated with formation of a series of intermontane basins. We hypothesize that these localized basins were formed at different stages of extensional collapse following a pulse of regional orogenesis. The Ofte and Røynstaul formations accumulated during rifting and were followed by bimodal volcanism within the Morgedal, Dalaå, and Gjuve formations. The Eidsborg Formation, at the top of the succession, records post-extension transgressive sedimentation. This tectonic scenario represents a pre-Sveconorwegian stage of orogenesis that can be related to convergent margin processes. The surface expression of crustal growth during this time is relatively small; however, the volume of a postulated mafic underplate that accreted and intruded the lower crust in the Telemark region at this time is unknown. Crustal growth during syn-convergent extension in a retro-arc region may be significant if volumes of accreted mafic underplate are large