R. A. Cliff

The evolution of excess argon in alpine biotites — A40Ar-39Ar analysis

Alpine biotites containing excess40Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of40Ar,39Ar,37Ar and36Ar and also displayed on a correlation plot of36Ar/40Ar vs.39Ar/40Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80–90%39Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing36Ar/40Ar with increasing temperature of heating step with three samples having a negative correlation of36Ar/40Ar vs.39Ar/40Ar and one a positive correlation; (3) there appear to be two36Ar components, one released at high temperatures and correlated with radiogenic40Ar and one released at low temperatures which is not correlated with radiogenic40Ar; and (4) there is no significant effect of neutron irradiation on the release of40Ar and36Ar. Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high40Ar/36Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.

Isotopic dating in metamorphic belts

Isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions. Of these, temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed. The central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated, in order to illustrate the factors which control closure temperatures and limit their application in practice. The possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined, taking examples involving K-Ar, Rb-Sr, Sm-Nd and U-Pb analyses. The indirect dating of metamorphism via structurally dated plutons is briefly considered, and the problems of interpreting both discordant U-Pb zircon ages and Rb-Sr whole rock isochrons in metamorphosed granites are discussed. The value of near-concordant zircon ages is emphasized. The extent of isotopic equilibration during metamorphism, specifically of Sr-isotopes in metasediments, is shown not to be a simple function of metamorphic grade. The flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently. The concluding discussion briefly examines existing data for the Dalradian and indicates the potential for further progress using both well-established and novel approaches.

Characterization of the St Helena magma source

Summary Combined Sr-Nd-Pb isotope and trace-element data for St Helena are interpreted in terms of changing thermal and chemical fluxes impinging on and interacting with the base of the lithosphere over a period of 6 Ma. The data reveal the existence of two geochemically distinct components in the St Helena source region: (i) a HIMU (high U/Pb) component which has extremely radiogenic lead isotopes (206Pb/204Pb >20.8) with 87Sr/86Sr and 143Nd/144Nd ratios displaced below the mantle array; (ii) a component with less radiogenic lead and strontium isotopic compositions and more radiogenic neodymium compositions. Coupled trace-element and isotope variations are evident during the activity of each volcano. During shield development an increase in incompatible-trace-element enrichment occurs. This is coupled to a decrease in 143Nd/144Nd, whilst strontium and lead isotope ratios become progressively more radiogenic with time. The time-dependent variations are thought to be consistent with high-level processes occurring at the base of, or within, the lithosphere. A decrease in the signature of the depleted component with time is shown to be the result of a decreasing thermal flux acting on the base of the lithosphere. As a consequence the degree of partial melting of the depleted component (which may reside in the lithosphere or asthenosphere) is reduced, increasing the signature of the HIMU component in the erupted magmas.

Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints

Abstract The Sr-, Nd-, and Pb-isotopic compositions of Late Tertiary to Recent mafic alkaline rocks of the Cape Verdes archipelago vary from 143 Nd 144 Nd = 0.512606–0.513045 (ϵ Nd = −0.5 to +8.1) , 87 Sr 86 Sr = 0.702922–0.703934 , and 206 Pb 204 Pb = 18.743–19.881 . The variation of 207 Pb 204 Pb with 206 Pb 204 Pb in Cape Verdes lavas coincides with data for MORB, Hawaii, and Iceland, while 208 Pb 204 Pb vs. 206 Pb 204 Pb is an oblique, positive trend, i.e., 208Pb is higher in samples from the southern Cape Verdes islands which have lower 206 Pb 204 Pb . At least three isotopically distinct components, including depleted upper mantle, are required in Cape Verdes magma sources. Samples from the southern islands have less radiogenic Nd and Pb and more radiogenic Sr relative to northern islands. Isotopic and trace element characteristics of an enriched source component (EM) present only in the southern Cape Verdes suggest an origin as recycled subcontinental mantle or lithosphere, except that this component in the Cape Verdes had a significant long-term depletion of U relative to Th. The isotopic and trace element characteristics of a component predominant in northern Cape Verdes magma sources with relatively radiogenic Pb and Nd (HIMU) suggest an origin for this component as ancient recycled oceanic crust. The trace element and isotopic variations, the large scale isotopic heterogeneity, the intra-island stratigraphic variations, the apparent geographic age progression of magmatism, and the geophysical constraints are best explained by variable partial melting of a heterogeneous plume (HIMU + depleted mantle) in the northern Cape Verdes, and mixing of plume-derived melts with lithospheric (EM) melts in the southern Cape Verdes. Alternatively, an EM plume produces magma supplied to the southern Cape Verdes, and the northern Cape Verdes, located off the plume track, are supplied by relatively lower degrees of melting at the plume margin which has entrained pre-existing HIMU components from a heterogeneous (DM + HIMU) upper mantle.

Nile River sediment fluctuations over the past 7000 yr and their key role in sapropel development

The provenance pattern of Nile River sediments can be used as a proxy for paleoclimatic changes in East Africa. The 87Sr/86Sr ratios are particularly appropriate for such provenance investigations, because the White Nile drains predominantly crystalline basement rocks, whereas the Blue Nile and Atbara flow off the Ethiopian Highlands, which consist of Tertiary volcanic rocks. A high-resolution profile of 87Sr/86Sr and Ti/Al ratios from a well-dated core in the Nile Delta shows a close correspondence with known changes in Nile flow over the past 7000 yr. At times of higher river flow there was markedly decreased input of Blue Nile–derived and total sediment. This change was caused by northward movement of the Inter Tropical Convergence Zone, resulting in increased vegetative cover in the Ethiopian Highlands due to higher rainfall and a longer wet season. This inverse relationship between Nile River flow and sediment flux may have had important implications in the development of agricultural technology in ancient Egypt. The marked minimum in 87Sr/86Sr at 4200–4500 yr B.P. is coincident with the end of the Old Kingdom in Egypt and provides independent evidence that demise of the Old Kingdom might have been associated with an extended period of catastrophic low floods. During the Quaternary and late Neogene, there was periodic deposition of organic-rich sediments (sapropels) in the eastern Mediterranean that represent important indicators of major environmental change. Evidence from the Ti/Al ratio suggests that the pattern of erosion and sediment supply from the Nile catchment observed in this study also occurred throughout much of the Neogene and Quaternary. The reduced inputs of Blue Nile sediment during times of sapropel formation contributed to the increased primary productivity by reducing the amount of phosphate removed on particles and to the observed change to N limitation in the eastern Mediterranean, which are important characteristics of sapropel deposition.

Sm?Nd and Pb isotopic study of mafic rocks associated with early Proterozoic continental rifting: the Perpohja schist belt in northern Finland

The Peräpohja schist belt in northern Finland rests unconformably on Archaean granitoids, and marks the early stages of Proterozoic crustal evolution in the Fennoscandian (Baltic) shield. 2440 Ma old layered mafic intrusions predate the supracrustal , and ca. 2200 Ma old sills of the gabbro-wehrlite association intrude the lowest quartzites and volcanics (Runkaus) of the sequence. The Sm-Nd mineral isochron of the Penikat layered intrusion gives an age of 2410±64 Ma. The initial ɛNd-values of the Penikat intrusion (ɛNd(2440) = −1.6) and the Runkausvaara sill (ɛNd(2200) ≈ 0) suggest that these mafic magmas were contaminated by older crustal material. The Sm-Nd and Pb isotopic results on the 2.44–2.2 Ga old Runkaus volcanics indicate mobility of Pb, fractionation of Sm/Nd during “late” greenschist facies metamorphism, and crustal contamination. The Pb-Pb data provide an age of 1972±80 Ma with a high initial 207Pb/204Pb ratio (μ1 = 8.49), while scattered Sm-Nd data result in an imprecise age of 2330±180 Ma, with an initial ɛNd-value of about zero. Secondary titanite gives an U-Pb age of ca. 2250 Ma. The Jouttiaapa basalts, in contrast, ascended from the mantle without interaction with older crust. These LREE depleted tholeiites mark a break in continental sedimentation, and yield a Sm-Nd age of 2090±70 Ma. Their initial ɛNd = + 4.2 ±0.5 implies that the subcontinental early Proterozoic mantle had been depleted in LREE for a long period of time. The first lava flows are strongly depleted in LREE, suggesting that their source was significantly more depleted than the source of mid-ocean ridge basalts today.

Dating deformation using Rb-Sr in white mica: Greenschist facies deformation ages from the Entrelor shear zone, Italian Alps

Ages of deformation have been obtained by Rb-Sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation. Since white micas commonly define deformation fabrics in medium-grade metamorphic rocks, these ages can be directly related to structural geometries with regional context. This direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust. It does require that the dated minerals attained isotopic equilibrium with the dominant Sr reservoir at temperatures lower than the closure temperature. This resetting was apparently achieved during dynamic recrystallization of white micas in greenschist-facies metasediments and metagranitoid units in the western Alps. The results suggest that the Sr isotopic composition of the new mica is buffered by the coexisting high-Sr phases (calcite, feldspar or epidote) via the grain boundary network. High-strain rocks from the Entrelor shear zone system of the western Alps have yielded indistinguishable white mica Rb-Sr ages along 30 km of individual and kinematically linked shear zones. The age of the back-thrusting event is constrained at 34±1Ma, the age yielded by the younger generation of synkinematically crystallized white micas. This event was short-lived, involving at least 20 km of shortening in ∼1 m.y. or less. An earlier, variably overprinted component, dating from 38 to 37 Ma, has been identified in the mica fabric, but its kinematic significance is uncertain. This method of dating strain fabrics offers a powerful tool for tectonic studies, since isotopic resetting can be directly linked to structural geometries, microstructural textures, and PT conditions. It allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust.

The Triangle Shearzone, Zimbabwe, revisited: new data document an important event at 2.0 Ga in the Limpopo Belt

Abstract The Limpopo Belt in Southern Africa has been used to demonstrate that modern-style continent-continent collision operated during the Late Archaean (2.6–2.7 Ga). We have studied the age and PT conditions of strike-slip tectonism along the important right-lateral Triangle Shearzone. Our results substantiate existing Proterozoic metamorphic mineral age data of prior uncertain significance. Using the Pb Pb and Sm Nd garnet chronometers and the Ar Ar step heating technique for amphibole, we have dated pre- and syn-tectonic metamorphic minerals at 2.2 and 2.0 Ga. Thus the Triangle Shearzone can now be regarded as an important Proterozoic suture. Examination of corresponding high-grade PT conditions, reaching ∼ 800°C at 9 kbar, indicates a clockwise metamorphic evolution with pronounced isothermal uplift. Although the evidence that thrusting of the Marginal Zones of the Limpopo Belt over the adjoining cratons occurred during the Late Archaean clearly remains, it is now very uncertain to which event the various PT paths obtained in the Limpopo Belt may be assigned. Therefore the question of whether the 2.6–2.7 Ga tectonism fits on its own a modern-style continental collision model remains open and has to be reassessed.

The characterisation of Saharan dusts and Nile particulate matter in surface sediments from the Levantine basin using Sr isotopes

Abstract The provenance of sediments within the Levantine basin of the eastern Mediterranean was studied using 87 Sr/ 86 Sr isotopic ratios, together with major elements. Measurements were made on the detrital fraction of surface sediments, and the two most important sources of detrital matter to the region. Saharan dust was characterised by an 87 Sr/ 86 Sr isotopic ratio in the range 0.7160–0.7192. There was a small systematic decrease in 87 Sr/ 86 Sr from west to east which is interpreted as due to a change in the balance of aeolian source material. The Nile particulate matter had a 87 Sr/ 86 Sr isotopic ratio in the range 0.7057–0.7071. 87 Sr/ 86 Sr isotopic signatures together with Sr concentration were used to calculate the contributions made by Saharan dust and Nile particulate matter to the surface sediments of the Levantine basin. It was shown that Nile-derived sediment was dispersed widely across the Levantine basin enabling the area affected by retention of sediment behind the Aswan dam to be determined.

Kinematic reworking and exhumation within the convergent Alpine Orogen

Abstract Kinematic data from the internal zones of the Western Alps indicate both top-to-SE and top-to-NW shearing during synkinematic greenschist facies recrystallisation. Rb/Sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 °C. The data indicate an initial phase of accretion and foreland-directed thrusting at ca. 60 Ma followed by almost complete reworking of thrust-related deformation by SE-directed shearing. This deformation is localised within oceanic units of the Combin Zone and the base of the overlying Austroalpine basement, and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike. The timing of deformation in this shear zone spans 9 Ma from 45 to 36 Ma. The SE-directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy, and such structures have been interpreted in the past as backthrusts in response to ongoing Alpine convergence. However, on a regional scale, the top-to-SE deformation is related to crustal extension, not shortening, and is coincident with exhumation of eclogites in its footwall. During this extension phase, deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking. Top-NW kinematics preserved within the Combin Zone show a range of ages. The oldest (48 Ma) may reflect the final stages of emplacement of Austroalpine Units above Piemonte oceanic rocks prior to the onset of extension. However, much of the top-to-NW deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone. 40Ar/39Ar data from the region are complicated due to the widespread occurrence of excess 40Ar in eclogite facies micas and partial Ar loss during Alpine heating. Reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 Ma. These ages are slightly younger than Rb/Sr deformation ages and suggest that cooling below ca. 350 °C occurred after juxtaposition of the units by SE-directed extensional deformation. Our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the Alpine Orogen. To constrain the palaeogeographic and geodynamic evolution of the Alps requires that these data be integrated with data from the more external zones of the orogen. Complexity such as that described is unlikely to be restricted to the Western Alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens. Recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens.