Bruce F. Schaefer

Boron and oxygen isotope evidence for recycling of subducted components over the past 2.5 Gyr

Evidence for the deep recycling of surficial materials through the Earth’s mantle and their antiquity has long been sought to understand the role of subducting plates and plumes in mantle convection. Radiogenic isotope evidence for such recycling remains equivocal because the age and location of parent–daughter fractionation are not known. Conversely, while stable isotopes can provide irrefutable evidence for low-temperature fractionation, their range in most unaltered oceanic basalts is limited and the age of any variation is unconstrained. Here we show that δ18O ratios in basalts from the Azores are often lower than in pristine mantle. This, combined with increased Nb/B ratios and a large range in δ11B ratios, provides compelling evidence for the recycling of materials that had undergone fractionation near the Earth’s surface. Moreover, δ11B is negatively correlated with 187Os/188Os ratios, which extend to subchondritic values, constraining the age of the high Nb/B, 11B-enriched endmember to be more than 2.5 billion years (Gyr) old. We infer this component to be melt- and fluid-depleted lithospheric mantle from a subducted oceanic plate, whereas other Azores basalts contain a contribution from ∼3-Gyr-old melt-enriched basalt. We conclude that both components are most probably derived from an Archaean oceanic plate that was subducted, arguably into the deep mantle, where it was stored until thermal buoyancy caused it to rise beneath the Azores islands ∼3 Gyr later.

Deep mantle plume osmium isotope signature from West Greenland Tertiary picrites

Picrites from Nuussuaq Peninsula and Qeqertarssuaq (Disko) Island, West Greenland, preserve trace element and isotopic signatures reflecting the composition of the Icelandic plume head. Os isotope ratios are low (187Os/188Os(i)=0.1272–0.1371) in terms of global plume related magmatism, and this is coupled with anomalously high Os abundances and radiogenic 143Nd/144Nd(i) isotopes. Crustally contaminated basalts within the West Greenland sequences possess Os and Nd isotopic signatures consistent with mixing between initial plume head compositions and two discrete types of West Greenland continental crust. One crustal component is of a local sedimentary origin, and the other is typical of ancient felsic crust. The low Os isotopic signatures of the picritic units are considered to be those of the initial mantle plume. The fact that such low Os isotope ratios occur in sequences with high 3He/4He, and the non-systematic variation in Os isotopes with indices of fractionation/accumulation and Pb isotopes, argue against mixing with depleted MORB mantle or ancient subcontinental lithospheric mantle. The high Os concentrations in the picrites are attributed to high degrees of partial melting (>25%) of mantle containing no residual sulphide. This is consistent with models for plume heads in which anomalous mantle temperatures initiate melting at high pressures generating large degrees of partial melting. Unradiogenic Os and radiogenic Nd components in plume-related CFB magmatism may preserve contributions from a reservoir which is sampled only occasionally in young oceanic basalts. Such a reservoir shares Os isotopic features with the primitive upper mantle (PUM), which may also be manifest in the ‘Kea’ component of Hawaiian magmatism. Therefore, portions of the West Greenland continental flood basalt province arguably represent the first direct sampling of unmodified plume head material derived from the lower mantle or lower portions of the upper mantle.

Palaeoproterozoic Kimban mobile belt, Eyre Peninsula: Timing and significance of felsic and mafic magmatism and deformation

Within the Kimban mobile belt on the eastern margin of the Gawler Craton, two separate tectono‐thermal and up to four separate magmatic events can be recognised. The first (inferred) magmatic event involved underplating of the mafic, lower crustal protolith of the Donington Granitoid Suite at 2100–2000 Ma. Emplacement, around 1850 Ma, of the Donington and Colbert Granitoid Suites and the mafic, syn‐plutonic Jussieu dykes, together forming the Lincoln Batholith, comprise the second magmatic event. Donington Suite emplacement was accompanied by high‐temperature meta‐morphic conditions, indicated by syn‐intrusive migmatitic shearing in host rock of the Lincoln Batholith and by the generation of voluminous felsic selvages around the Jussieu dykes. This first tectonother‐mal event in the Lincoln Batholith is associated with D1 in Hutchison Group metasediments. The mafic Tournefort dykes, emplaced during the third magmatic event, are structurally and compositionally distinct from the Jussieu dykes. Their time o...

Age and metasomatic alteration of the Mt Neill Granite at Nooldoonooldoona Waterhole, Mt Painter Inlier, South Australia

Quartz feldspar augen gneisses, quartz augen schists and trondhjemites outcrop at Nooldoonooldoona Waterhole in the southwestern corner of the Proterozoic Mt Painter Inlier, northern Flinders Ranges, South Australia. These rocks were previously interpreted as having different origins and ages. However, we argue that all rock types were the result of deformation and strong metasomatic alteration of one common precursor: the Mt Neill Granite. Our conclusion is based on field observations that show that the different lithologies grade into each other and that intrusive contacts are lacking. Whole rock major and trace element analyses also point to a common protolith. Finally, Pb/Pb dating of magmatic zircons gave the same ca 1576 Ma age for the different rock types. Our findings necessitate a re evaluation of the published regional geology and lithostratigraphy of the Mt Painter Inlier. They also indicate that extreme care should be taken in the classification and genetic interpretation of rocks that have experienced extensive metasomatic alteration, which is common in many high grade terrains in Australia.

Re-Os isotope characteristics of postorogenic lavas: Implications for the nature of young lithospheric mantle and its contribution to basaltic magmas

Re-Os isotopes have been measured on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Plateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinental lithospheric mantle peridotites typically have subchondritic 187 Os/ 188 Os; however, the postorogenic lavas are characterized by radiogenic 187 Os/ 188 Os ratios (0.139‐0.559). Simple modeling shows that only very large degrees of melt depletion (>25%‐30%) can lower source Re/Os ratios sufficiently to permit timeintegrated development of subchondritic 187 Os/ 188 Os ratios. Such processes may have been largely restricted to the older Precambrian, and the peridotite component of the postorogenic lavas source was probably depleted by <25%. The more radiogenic values may reflect increasing contributions from metasomatic components or possibly crustal contamination. Our findings imply the need for caution in the use of Os isotopes as a diagnostic tracer of subcontinental lithospheric mantle contributions to lavas erupted through younger Proterozoic and Phanerozoic lithosphere.

Assessing uncertainty in the integration of aeromagnetic data and structural observations in the Deering Hills region of the Musgrave Province

Integrated aeromagnetic and field-based structural analysis provides a method for establishing a constrained regional-scale tectonic model where insufficient outcrop, remoteness or restricted access precludes widespread structural mapping. In this method the different scales of observation lead to uncertainty in integrating the observations. An integrated analysis of the Deering Hills region of the Musgrave Province shows that this uncertainty is largely dependent on the magnetic data resolution, the scale of deformation, and mineralogy. In the Deering Hills, four deformation events are defined, each with different structural character. These differences in character result in different levels of uncertainty in integrating these observations with aeromagnetic data. D1 was only evident at small scales in outcrop, and therefore any correlation with aeromagnetic data is inherently uncertain. However, well-defined relative timing to D2, and the parallel nature of an S1 foliation and regional D1 structures identified in the aeromagnetic data permitted the derivation of a moderately reliable regional model indicating northwest to southeast directed shortening at a deep crustal level during the Musgravian Orogeny. This resulted in a pervasive gneissic foliation, and a regional array of northeast-trending reverse-shear zones and tight to isoclinal upright folds. A higher confidence regional model was derived for the second deformation event (D2), which was identified at the regional scale in outcrop and could be directly correlated with features in the aeromagnetic data. North–south-directed crustal shortening, either in the late Musgravian Orogeny or during deformation ca 1060 Ma resulting in east-southeast- and southeast-trending reverse- and dextral-reverse shear zones and southeast-trending recumbent isoclinal nappes. For the third and fourth deformation events, links could not be made between field observations and aeromagnetic data, and the regional models for these events are low in confidence. The regional setting of D3 is not defined, and D4 is interpreted to represent the development of shear zones during north–south compression in the Petermann Orogeny.

New insights into the size and timing of the Lawn Hill impact structure: relationship to the Century Zn – Pb deposit

The Lawn Hill circular structure in northwest Queensland contains unambiguous evidence of an extraterrestrial impact, including planar deformation features in quartz, impact diamonds, widespread shatter cone formation and impact melt breccia in the Mesoproterozoic basement. The question of its relevance to ore genesis is investigated because the world-class Century Zn – Pb deposit is situated at the conjunction of the 100+ km Termite Range Fault and the previously defined margin of the impact structure. The impact structure is considered to be a 19.5 km wide feature, this constrained in part by the outer margin of an annulus of brecciated and highly contorted limestone. New evidence is presented indicating impact into this Cambrian limestone, including: (i) ‘dykes’ of brecciated Cambrian limestone extending hundreds of metres into the Mesoproterozoic basement; (ii) highly contorted bedding in the limestone annulus compared with essentially undeformed limestone away from the impact site; as well as (iii) a 1 Mt megaclast of Mesoproterozoic Century-like ore suspended in the limestone. Through aerial photograph analysis, large-scale convoluted flow structures within the limestone are identified, and these are interpreted to indicate that parts of the Cambrian sequence may have been soft or only semi-consolidated at the time of impact. This highly contorted limestone bedding is suggested to represent slump-filling of an annular trough in response to impact-induced partial liquefaction of a sediment veneer. The age of impact is therefore considered to be concurrent with limestone formation during the Ordian to early Templetonian, at 520 – 510 Ma. Formation of the Century deposit is found to be unrelated to impact-generated hydrothermal activity, although some minor hydrothermal remobilisation of metals occurred. However, there was macro-scale remobilisation of gigantic ore fragments driven by impact-induced lateral and vertical injection of limestone into the Proterozoic sediments. The limestone-filled annular trough surrounds a 7.8 km diameter central uplift, consistent with formation of a complex crater morphology.

Potassic alteration and veining and the age of copper emplacement at Mount Isa, Australia

Preliminary 187Re/188Os dating of whole rocks and sulphide separates from the Mount Isa copper orebody has generated an isochron age of 1367 ± 80 Ma (MSWD=49; n=6). This age is approximately 150 myr younger than published biotite 40Ar/39Ar ages previously assumed to date the copper-forming event at ca. 1523 Ma. These older ages are from rocks in which biotite is likely to be metamorphic rather than hydrothermal in origin. Unambiguous potassic alteration related to copper formation is characterised by biotite replacement of metabasalt (brownstones) and potassium feldspar replacement of meta-tuffs. Previous 40Ar/39Ar dating of biotite from brownstone yields 1352 Ma to 1385 Ma ages, while 87Rb/86Sr dating of potassium feldspar altered tuffs gives 1323 Ma. Muscovite from the Buck Quartz Fault, considered a conduit for copper mineralising fluids, yields an 40Ar/39Ar age of 1324 Ma. We suggest that these ages more accurately reflect the age of copper emplacement, whereas the older 40Ar/39Ar ages more likely relate to cooling from peak metamorphism.

The accuracy and precision of in situ Re-Os isotopic measurements by laser ablation MC-ICPMS

Goldschmidt Conference Abstracts 2009 Spring residence times: Role in weathering rates F.A.L. P ACHECO 1 AND C.H. V AN DER W EIJDEN 2 Department of Geology and Centre for Chemistry, UTAD, Vila Real, Portugal (fpacheco@utad.pt) Utrecht University, The Netherlands (chvdw@geo.uu.nl) Estimation of plagioclase (Pl) weathering rates (W Pl = ([Pl]/t)×(Q/A Pl )) at the watershed scale of springs requires the prior evaluation of a number of parameters which include the mole fractions of Pl ([Pl]) and their fracture surface areas (A Pl ), the residence times of springs (t) and their annual discharge (Q). An atempt to relate the weathering of plagioclase to mixtures rich in halloysite and to quantify the W Pl for a number of very small spring watersheds from the Vila Pouca de Aguiar region (VPA, North of Portugal) is documented in [1]. In this paper we take a step further by focusing our attention on adjusting the previously used advective flow equation and introducing hydraulic turnovers for the assessment of t. Now, the advective flow equation (t = (n e /K)(F 2 /D h )) replaces the average watershed depth (D) by the average depth of the saturated aquifer (D h ), whereas hydraulic turnovers assign t = V h n e /Q. V h is the saturated volume of the aquifer characterized by an effective porosity n e and a hydraulic conductivity K, and F is the average lateral path from the recharge area to the spring site. The evaluation of n e , K, F and Q has been addressed by [1]. The D h of the VPA springs could be related to their isotopic composition ( 87 Sr/ 86 Sr) and to annual precipitation (P): D h = [( 87 Sr/ 86 Sr) spring – ( 87 Sr/ 86 Sr) rain ] / (5.62×10 –7 P – 4.66×10 –4 ). The corresponding V h ’s were determined from the total watershed volumes (V) as calculated by a terrain modeling software: V h = V×(D h /D). The plagioclase log rates (Figure 1) are: –12.4±1.8 (adjusted flow equation) and –13.5±1.1 (turnover times). Relative to the former results, there is a decrease in the average log rates, by 0.2 in the first case and 1.4 in se second case. Number of cases Methane oxidation rates by AMS M. P ACK 1 *, M. H EINTZ 2 , W.S. R EEBUR G H 1 , S.E. T RUMBORE 1 , D.L. V ALENTINE 2 AND X. X U 1 University of California Irvine, Irvine, CA 92697 (*correspondence: mpack@uci.edu) University of California Santa Barbara, Santa Barbara, CA 93106 (mbheintz@umail.ucsb.edu, valentine@geol.ucsb.edu) In the marine environment methane (CH 4 ) oxidation consumes up to 84% of the CH 4 produced and mitigates the release of CH 4 , a potent green house gas, to the atmosphere [1]. The microbialy mediated process is an important sink in the global CH 4 budget, yet it remains poorly quantified because only a small number of direct oxidation rate measurements are available. Traditional oxidation rate measurements use regulated levels of radiotracers ( 14 C- and 3 H-CH 4 ) in conjunction with scintillation counting and come with certain limitations: safety and contamination factors restrict the measurements to isotope vans, and radioisotope use may not be permitted in foreign venues and may complicate shipping. We have developed a rate measurement that utilizes non- regulated levels of 14 C-CH 4 tracer (<50nCi/g) [2] in conjunction with accelerator mass spectrometry (AMS). The high sensitivity of AMS allows for a 10 3 reduction in tracer activity which relaxes complications with tracer shipping, handling and waste disposal. Together with ease of performance, this method could provide a larger sample throughput and therefore a better quantification of the marine CH 4 oxidation sink. Further, it allows for easy quantification of the fraction of CH 4 taken up in microbial biomass as well as the fraction oxidized, thereby providing important information about the activity of methanotrophs in the ocean. Our rate measurements compared to 3 H-CH 4 rate measurements on water from the same Niskin bottles are generally consistent. The two measurements are similar when ambient rates are high, but diverge when rates are low. [1] Reeburgh (2007) Chem. Rev. 107, 486-513. [2] Vogel (2000) Nucl. Instrum. Methods Phys. Res. B 172, 885-891. Turnover times Flow equation Times A983 Log (W Pl ) Figure 1: Log rates of plagioclase. [1] Pacheco F.A.L., Van der Weijden C.H. (2008). Geochimica et Cosmochimica Acta, v. 72, no. 17, Page A715.