Finlay M. Stuart

High 3He/4He ratios in picritic basalts from Baffin Island and the role of a mixed reservoir in mantle plumes.

The high 3He/4He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earths accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earths mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic 3He/4He ratios yet recorded. A strong correlation between 3He/4He and 87Sr/86Sr, 143Nd/144Nd and trace element ratios demonstrate that the 3He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest 3He/4He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a 3He-recharged depleted mantle (HRDM) reservoir may be the principal source of high 3He/4He in mantle plumes, and may explain why the helium concentration of the ‘plume’ component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure.

Apatite (U–Th)/He age constraints on the development of the Great Escarpment on the southeastern Australian passive margin

The southeast Australian margin, like other high elevation passive margins, is characterised by a steep escarpment that separates a dissected coastal plain from a low relief inland plateau. Quantitative constraints on the generation of escarpments can be provided by apatite (U–Th)/He ages. Here we use a coast-perpendicular traverse across the coastal lowlands, escarpment and plateau to test the three prevailing models of SE Australian escarpment formation, namely retreat into a downwarped rift shoulder, escarpment retreat and down-wearing on high elevation rift shoulder with flexural rebound. Apatites from the coastal plain have He ages of between 87 and 112 Ma, suggesting that the coastal lowlands developed very rapidly after rifting and continental break-up at 85–100 Ma. The He age data are inconsistent with the erosion of a downwarped rift margin, and cannot be explained by a constant post-break-up rate of lateral escarpment retreat across the coastal plain or by constant down-wearing. The data require either rapid escarpment retreat or rapid in-place excavation of the escarpment soon after break-up, in response to rifting and the lowering of base levels on the margin of the new continent at break-up, followed by a period of landscape stability and low erosion. Combined with the existing apatite fission track record, the He data are consistent with erosion of 3–4 km within a maximum of 28 Myr of break-up, at a minimum vertical erosion rate of 130 m Myr−1 along the coast. The rapid denudation period across the coastal plain in this region took less than 48 Myr (from the coast to the escarpment base), which corresponds to an average vertical erosion rate of 45 m Myr−1. This is equivalent to a mean escarpment retreat rate of 5–10 km Myr−1. Apatite He ages from the plateau (183–247 Ma) indicate that the highlands remained stable throughout continental break-up, experiencing average erosion rates of less than 10 m Myr−1 since the late Palaeozoic/early Mesozoic.

Age and source constraints for the giant Muruntau gold deposit, Uzbekistan, from coupled Re-Os-He isotopes in arsenopyrite

The Muruntau gold deposit, Uzbekistan, is one of the largest gold deposits known worldwide, but its origin remains controversial. We used Re-Os arsenopyrite geochronology to precisely determine the age of main-stage gold mineralization at Muruntau to be 287.5 ± 1.7 Ma, which overlaps the emplacement of proximal post-tectonic granitoid magmatism. Additionally, we suggest that arsenopyrite growth may have occurred over an interval of at least 2 m.y. Os initial ratios derived from arsenopyrite were coupled with He isotopic data from fluid inclusions within arsenopyrite to constrain the source of ore metals and fluids. Muruntau arsenopyrite yields relatively unradiogenic initial Os (0.37 ± 0.27) and elevated 3He/4He ratios (0.23–0.33 R a) relative to purely crustal Os-He reservoirs. These data suggest the presence of a mantle-derived component in the ore system that was probably introduced during the generation of the granitoid magmas. These new timing and source constraints provide important new insight into the generation of this giant gold deposit, and they necessitate reexamination of genetic models for Muruntau and potentially other giant “orogenic gold” deposits worldwide.

Multiphase development of the Atacama Planation Surface recorded by cosmogenic 3He exposure ages: Implications for uplift and Cenozoic climate change in western South America

The Atacama Planation Surface is an extensive west-dipping surface developed between 16°S and 27°S along the Pacific margin of South America. It is considered to have formed between 16 and 7.5 Ma and to have important chronostratigraphic significance. Here we present new cosmogenic 3He exposure dates of boulders on the planation surface using pyroxene and amphibole. Exposure ages display good within- and between-site consistency and range from 22 to 1.2 Ma, with ages of ca. 14.6, 7, and 3 Ma recurring at more than one site on the planation surface. The 14.6 Ma peak records the cessation of the main period of planation surface development, but, contrary to popular opinion, the younger ages reflect subsequent modification of the planation surface by alluvial activity. Comparison with other climate proxies for western South America suggests that since 14.6 Ma, a predominantly hyperarid climate, interspersed with short-lived phases of more intense runoff driven by global climate change, has prevailed. The longevity and composite nature of the Atacama Planation Surface suggest that regionally extensive planation surfaces may have a multiphase history, are unlikely to have any chronostratigraphic significance, and cannot be used to reconstruct uplift histories.

Constraints on mantle plumes from the helium isotopic composition of basalts from the British Tertiary Igneous Province

Helium isotopes have been measured in olivines from igneous rocks spanning the history of the Palaeocene volcanic centre at the Isle of Skye, northwest of Scotland. 3He/4He in olivine phenocrysts from basalts from the Skye Lava Field are up to 22 times the atmospheric ratio which is consistent with their origin from the proto-Iceland mantle plume despite being erupted 1300 km from the plume axis. The 3He-anomaly of the Palaeocene starting plume in the North Atlantic appears to have been up to three times greater, for the same distance from the plume axis, than the modern Iceland plume. The Pb isotope and trace element (ΔNb) composition of the least crustally contaminated Skye magmas resemble North Atlantic MORB rather than Iceland basalts. The proto-Iceland plume head appears to have been dominated by upper mantle material with a small deep mantle He contribution rather than undiluted deep mantle material as predicted in the classic plume model.

A diode laser system for heating minerals for (U‐Th)/He chronometry

We have developed a diode laser (25 W, 808 nm) system for He extraction from minerals for (U-Th)/He chronometry. The laser beam is delivered via a 600 μm fiber cable and focused using a binocular microscope. Temperatures necessary for He release from apatite (500–600°C) and zircon (1100–1300°C) encapsulated in Pt-foil tubes are attained by heating to 0.5 W for 30 s and 1.25–2.5 W for 20 min, respectively, using a defocused beam. Heating at these powers does not result in measurable U and/or Th loss from apatite, as noted by the preservation of the distinct Th/U in multiple splits of two different Durango apatite crystals. Analyses of Durango and the California Institute of Technology internal standard apatite 97MR22 yield (U-Th)/He ages of 32.8 ± 1.8 Ma (1σ, n = 11) and 4.6 ± 0.5 (1σ, n = 5), respectively, well within accepted ages. The (U-Th)/He age and Th/U of five Fish Canyon Tuff zircon aliquots yield 29.3 ± 2.2 Ma (1σ) and 0.6 ± 0.03, respectively, and are indistinguishable from ages produced by resistance furnace He extraction. Heating of unencapsulated minerals shows that the diode laser couples well with optically opaque minerals (e.g., hornblende, biotite, muscovite, garnet) and basalt groundmass, suggesting that diode lasers offer a cheap, small, low-maintenance alternative to Nd:YAG and Ar ion lasers for 40Ar/39Ar, cosmogenic noble gas, and stable isotope studies.

The ARGUS multicollector noble gas mass spectrometer: Performance for 40Ar/39Ar geochronology

We describe a new high-sensitivity multicollector noble gas mass spectrometer (ARGUS) that has been specifically designed for simultaneous collection of all Ar isotopes and hence is ideally suited for 40Ar/39Ar geochronology. The instrument uses a detector housing that holds five Faraday collectors. One collector is equipped with a 1011 ohm resistor (for 40Ar), and the remaining four are fitted with 1012 ohm resistors (for 36Ar to 39Ar). ARGUS has a mass resolution of 225–250 and a measured sensitivity of 7 × 10−14 mol/V at 200 μA trap current. During the course of a 1 month run cycle 145 air calibrations yielded a weighted average 40Ar/36Ar of 300.67 ± 0.07 (2σ, eight data points rejected, and mean square weighted deviation = 1.18). The ages of three mineral standards (Taylor Creek Rhyolite sanidine, Heidelberg biotite, and Alder Creek sanidine) are within error of the accepted ages, and uncertainties for two are improvements on previously published data. ARGUS decreases analytical time while allowing more measurements to be made at higher precision compared to standard peak-jumping single-collector mass spectrometers.

An investigation of closure temperature of the biotite Rb-Sr system: The importance of cation exchange

Abstract Factors controlling closure in the biotite Rb-Sr system were investigated in a detailed study of an amphibolite-facies metacarbonate from the central Swiss Alps. Oxygen isotope data suggest that the rock cooled as a closed system. Calcite-dolomite thermometry temperatures of ∼450°C to 500°C and feldspar thermometry temperatures of ∼300°C to 400°C provide evidence of extensive Ca-Mg and Na-K exchange during cooling. Biotite in the sample is 90 mol.% phlogopite and has high Rb (∼900 ppm) compared to Sr (∼0.3 ppm), giving precise Rb-Sr ages. Carefully separated and sized phlogopite shows a range of Rb-Sr ages that do not simply decrease with grain size as predicted by current models of closure temperature. Rb-Sr ages decrease from 18.1 Ma to 16.6 Ma with a decrease in mean grain diameter from 1.16 mm to 0.74 mm, but grains with mean diameter of 0.54 mm show an increase again to 17.6 Ma. This contrasts with Ar-Ar data for single phlogopites, which do show a decrease in age with decreasing grain size. The Rb-Sr age pattern is due to Rb-loss during cooling, which is most pronounced in the finest fraction. The phlogopites are restricted to a 2-cm-thick layer in calcite marble; 87Sr/86Sr of the calcite decreases away from the phlogopite band over 4 cm, indicating that the calcite was moving towards Sr-isotope equilibration with the phlogopites over this distance and that the phlogopite was not equilibrating with an “infinite reservoir.” Ion microprobe traverses across grains of different minerals reveal systematic core-rim variations in major and trace element concentrations. In particular, Sr decreases from calcite core to rim, but increases from core to rim in K-feldspar, whereas Rb decreases from core to rim in phlogopite but also increases from core to rim in K-feldspar. These gradients are interpreted as indicating the direction of transport of elements during cooling as a result of cation exchange reactions; calcite and phlogopite were sources for Sr and Rb, respectively, whereas K-feldspar acted as a sink for both elements. This chemical equilibration was taking place at the same time as isotopic equilibration during cooling, and was equally important in controlling the apparent ages recorded by the mica grains. In contrast, closure temperature calculations for geochronological systems based on classic Dodson-type models assume parent and daughter element concentrations are homogeneous across grains and do not change with time, only isotopic exchange is modeled. Closure in mica Rb-Sr systems will depend both on the factors that control isotopic exchange (grain size, mode, 87Sr diffusion coefficients) and those that control chemical exchange (grain size, mode, Rb and Sr diffusion coefficients, Rb and Sr contents of phases and their partition coefficients).

Evolution and paragenetic context of low δD hydrothermal fluids from the Panasqueira W-Sn deposit, Portugal: new evidence from microthermometric, stable isotope, noble gas and halogen analyses of primary fluid inclusions

Abstract The evolution, paragenetic context and origin of remarkably low δD hydrothermal fluids from the Hercynian Panasqueira W-Sn-Cu(Ag) deposit have been investigated through a combined microthermometric, stable isotope, halogen and noble gas fluid inclusion study. Large variations in δD between −60 ‰ and −134‰ have been observed in primary fluid inclusions from growth zones in a quartz crystal (Pa66) paragenetically constrained to the main sulfide stage (MSS) of ore formation. The same fluids exhibited relatively constant fluid inclusion homogenisation temperatures (254 to 260°C), salinities (7.4 to 8.7 wt.% NaCl equivalent) and calculated fluid δ18O (3.8 to 4.4‰). All the fluids exhibited 40Ar excesses. Mean molar Br/Cl and I/Cl ratios varied from 2.3 × 10−3–4.2 × 10−3 and 8 × 10−4–10 × 10−4 respectively, with the more Br-rich fluids being associated with the more deuterium-depleted fluids. The low palaeolatitude of Panasqueira throughout the main stages of ore formation and the overwhelming predominance of meteoric fluids in the main, late and post-ore mineralising fluids with δD of −40 to −65‰ lends little support for any models involving low δD palaeometeoric water. The limited range of 40Ar∗/Cl ratios (1.1 × 10−5–1.6 × 10−5) also precludes boiling during the MSS or mixing of meteoric water with highly fractionated deuterium-depleted magmatic waters as viable mechanisms for producing the low δD fluids. The high Br/Cl and I/Cl of the MSS fluids indicate significant fluid interactions with organic rich sediments or metasediments, in particular regionally abundant Carboniferous coals or coaly sediments that are inferred to be depleted in deuterium by around 85‰ compared to palaeometeoric water. The large range of δD observed in the MSS fluids may be explained by isotopic exchange of palaeometeoric water with these coals at varying water/rock (wt./wt.) ratios between 0.02 and 0.002 with fluid oxygen isotopic compositions controlled by exchange with the Beira Schists. Such a model is consistent with the requirements that any satisfactory model must take into account viz. (i) the relative constancy of temperature, salinity, 40Ar∗ and fluid δ18O during excursions to low δD; (ii) the high I/Cl and Br/Cl ratios observed; and (iii) the coincidence of these low δD fluids with the onset of precipitation of polymetallic sulfides and phosphates under reducing conditions. The model indicates that (meta)sedimentary sources of ore forming components became increasingly important during the evolution of the Panasqueira Sn-W deposit and accounts in part for the observed progression from oxide-dominated to base-metal sulfide-dominated parageneses.

Cenozoic deformation and exhumation history of the Central Kyrgyz Tien Shan

New low-temperature thermochronological data from 80 samples in eastern Kyrgyzstan are combined with previously published data from 61 samples to constrain exhumation in a number of mountain ranges in the Central Kyrgyz Tien Shan. All sampled ranges are found to have a broadly consistent Cenozoic exhumation history, characterized by initially low cooling rates (<1°C/Myr) followed by a series of increases in exhumation that occurred diachronously across the region in the late Cenozoic that are interpreted to record the onset of deformation in different mountain ranges. Combined with geological estimates for the onset of proximal deformation, our data suggest that the Central Kyrgyz Tien Shan started deforming in the late Oligocene-early Miocene, leading to the development of several, widely spaced mountain ranges separated by large intermontane basins. Subsequently, more ranges have been constructed in response to significant shortening increases across the Central Kyrgyz Tien Shan, notably in the late Miocene. The order of range construction is interpreted to reflect variations in the susceptibility of inherited structures to reactivation. Reactivated structures are also shown to have significance along strike variations in fault vergence and displacement, which have influenced the development and growth of individual mountain ranges. Moreover, the timing of deformation allows the former extent of many intermontane basins that have since been partitioned to be inferred; this can be linked to the highly time-transgressive onset of late Cenozoic coarse clastic sedimentation.