John Ludden

ARCHEAN CONTINENTAL ASSEMBLY IN THE SOUTHEASTERN SUPERIOR PROVINCE OF CANADA

Between 1988 and 1993, seismic reflection and refraction surveys were acquired across the medium- to high-grade Opatica plutonic gneiss belt, the low-grade Abitibi greenstone belt, and the Pontiac metasedimentary belt, all of which form part of the late Archean Superior Province. Shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 Ma of various exotic terranes against a backstop provided by the Opatica belt. This rapid southward growth of the Archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65-km depth in the upper mantle below the Opatica belt. In contrast to the mainly orthogneissic Opatica and Pontiac belts, the midcrust of the Abitibi belt comprises metasedimentary and igneous rocks, plus imbricated units of unknown affinity. Relict midcrustal accretionary complexes of substantial size, whieh are indicative of primary suture zones, are interpreted near the northern and southern limits of the Abitibi belt. An interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous. Evidence of large-scale extension appears to be confined to the Southern Volcanic Zone of the Abitibi, which developed into a half graben as the original suture zone was reactivated in extension. Unusually high seismic P wave velocities, 7.5–8.2 kms−1, are present in the lower 8 km of the Abitibi crust, and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust. Crustal xenolith studies suggest that this process may be linked to early Proterozoic magmatism.

Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry

Abstract Multiple-collector inductively coupled plasma mass spectrometry has been used for the precise measurement of the isotopic composition of Se in geological samples. Se is chemically purified before analysis by using cotton impregnated with thioglycollic acid. This preconcentration step is required for the removal of matrix-interfering elements for hydride generation, such as transitional metals, and also for the quantitative separation of other hydride-forming elements, such as Ge, Sb, and As. The analyte is introduced in the plasma torch with a continuous-flow hydride generation system. Instrumental mass fractionation is corrected with a “standard-sample bracketing” approach. By use of this new technique, the minimum Se required per analysis is lowered to 10 ng, which is one order of magnitude less than the amount needed for the N-TIMS technique. The estimated external precision calculated for the 82 Se/ 76 Se isotope ratio is 0.25‰ (2σ), and the data are reported as delta notation (‰) relative to our internal standard (MERCK elemental standard solution). Measurements of Se isotopes are presented for samples of standard solutions and geological reference materials, such as silicate rocks, soils, and sediments. The Se isotopic composition of selected terrestrial and extraterrestrial materials are also presented. An overall Se isotope variation of 8‰ has been observed, suggesting that Se isotopes fractionate readily and are extremely useful tracers of natural processes.

A discontinuity in mantle composition beneath the southwest Indian ridge

The composition of mid-ocean-ridge basalt is known to correlate with attributes such as ridge topography and seismic velocity in the underlying mantle, and these correlations have been interpreted to reflect variations in the average extent and mean pressures of melting during mantle upwelling. In this respect, the eastern extremity of the southwest Indian ridge is of special interest, as its mean depth of 4.7 km (ref. 4), high upper-mantle seismic wave velocities and thin oceanic crust of 4–5 km (ref. 6) suggest the presence of unusually cold mantle beneath the region. Here we show that basaltic glasses dredged in this zone, when compared to other sections of the global mid-ocean-ridge system, have higher Na8.0, Sr and Al2O3 compositions, very low CaO/Al2O3 ratios relative to TiO2 and depleted heavy rare-earth element distributions. This signature cannot simply be ascribed to low-degree melting of a typical mid-ocean-ridge source mantle, as different geochemical indicators of the extent of melting are mutually inconsistent. Instead, we propose that the mantle beneath ∼1,000 km of the southwest Indian ridge axis has a complex history involving extensive earlier melting events and interaction with partial melts of a more fertile source.

Hf‐Nd input flux in the Izu‐Mariana subduction zone and recycling of subducted material in the mantle

In subduction zones, two major mass fluxes compete: the input flux of altered oceanic crust and sediments subducted into the mantle and the output flux of magma that forms the volcanic arc. While the composition and the amount of material erupted along volcanic arcs are relatively well known, the chemical and isotopic composition of the subducted material (altered oceanic crust and sediments) is poorly constrained and is an important factor in the mass balance calculation. Ocean Drilling Program Leg 185 in the Western Pacific used systematic sampling of the altered basaltic basement and sediment pile and the creation of composite mixtures to quantify the total chemical flux subducted at the Izu-Mariana margin. Here, we report Hf and Nd isotopic compositions of materials recovered from this Leg. The Hf and Nd isotopic compositions of altered basalts from Hole 801C are indistinguishable from those of recent unaltered Pacific mid-ocean ridge basalt, suggesting that hydrothermal alteration had no effect on either isotopic systems. The complete Site 1149 sedimentary pile has a weighted average ɛNd of −5.9 and ɛHf of +4.4, values similar to those of Fe-Mn crusts and nodules. Therefore, the Hf and Nd isotopic compositions of the sediments collected at Site 1149 indicate minimal contributions from continental detrital material to the rare earth elements and high field strength elements. However, the Hf isotopic budget of the oldest sediments is more influenced by continental material than the younger sediments, despite the large distances to continental masses 130 Ma ago. In the Izu subduction zone, we calculate a sedimentary input of less than about 2% in the volcanic lava source. In contrast, at least 85% of the sedimentary Nd and Hf are recycled into the mantle to affect its general composition. Assuming that sediments have been recycled in a similar manner into the mantle for millions of years, large chemical heterogeneities must be produced in the mantle. In addition, the depletion of the mantle due to the extraction of continental crust must be partly counterbalanced by the injection of vast quantities of enriched sedimentary material.

Isotopic portrayal of the Earth’s upper mantle flow field

It is now well established that oceanic plates sink into the lower mantle at subduction zones, but the reverse process of replacing lost upper-mantle material is not well constrained. Even whether the return flow is strongly localized as narrow upwellings or more broadly distributed remains uncertain. Here we show that the distribution of long-lived radiogenic isotopes along the world’s mid-ocean ridges can be used to map geochemical domains, which reflect contrasting refilling modes of the upper mantle. New hafnium isotopic data along the Southwest Indian Ridge delineate a sharp transition between an Indian province with a strong lower-mantle isotopic flavour and a South Atlantic province contaminated by advection of upper-mantle material beneath the lithospheric roots of the Archaean African craton. The upper mantle of both domains appears to be refilled through the seismically defined anomaly underlying South Africa and the Afar plume. Because of the viscous drag exerted by the continental keels, refilling of the upper mantle in the Atlantic and Indian domains appears to be slow and confined to localized upwellings. By contrast, in the unencumbered Pacific domain, upwellings seem comparatively much wider and more rapid.

Os isotopic systematics in mantle xenoliths; age constraints on the Canadian Cordillera lithosphere

Abstract The 187 Os / 188 Os ratios of lherzolites from eight xenolith suites from the Canadian Cordillera show a correlation with Al2O3 and heavy rare earth elements (HREE). The best interpretation of these correlations appears to be ancient melt depletion followed by a long period of radiogenic ingrowth. The 187 Os / 188 Os–Lu correlation is used to calculate an Os model age of 1.12±0.26 Ga for the lithospheric mantle throughout the Canadian Cordillera. This single melting age suggests that the mantle lithosphere now underlying the entire Canadian Cordillera may have formed by melting events closely spaced in time. This is consistent with seismic evidence of the extension of crustal basement under much of the Canadian Cordillera that is independent of the upper-crustal terranes overlying it. Indeed, this Proterozoic Os model age for the mantle contrasts with the younger formation ages (Nd model ages and U–Pb ages of zircons) of most crustal terranes of the region which are around 0.5 Ga. Early Proterozoic basement is exposed only in southeastern British Columbia and has the same age (1.9 to 2.3 Ga) as the ancestral North American crust, but is older than the Os model age of the mantle lithosphere underlying the Canadian Cordillera. The Canadian Cordilleran mantle is thus probably not a simple extension of the North American cratonic lithosphere beneath the adjacent mobile orogenic belt of the Canadian Cordillera. The difference in age between the formation of the Canadian Cordillera upper-crust and the formation of the underlying mantle suggests that this mantle lithosphere does not represent the mantle roots of the crustal terranes overlying it. Instead, these crustal terranes were thrust onto the mantle lithosphere during Canadian Cordillera orogeny. This contrasts strongly with Archean cratonic zones and Early Proterozoic belts where oldest crustal rocks and mantle may have the same formation age.

Isotopic and trace element constraints on the genesis of the Faeroe lava pile

Abstract Basaltic lavas from the Faeroe Islands form three stratigraphic series which define two geochemical groups. Both the lower and middle series are LREE enriched ((La/Yb) e.f. : 2–3) and are characterized by convex LREE profiles; in contrast, the upper series comprises both depleted ((La/Yb) e.f. : 0.45–0.6) and enriched lavas. This twofold geochemical division is also evident from the incompatible trace elements such as Zr, Nb, Hf and Ta and the compatible trace elements Cr, Ni, Sr and Y. Nd-Sr-Pb isotopic measurements show that the basalts are contaminated by crustal materials, implying the presence of Precambrian sialic basement underneath the Faeroes block, a conclusion supported by geophysical data [35,36]. The uncontaminated end-members, for the LREE-depleted basalts ( 87 Sr/ 86 Sr) 0 ∼ 0.7026 and eNd 0 + 10 and for the LREE-enriched basalts ( 87 Sr/ 86 Sr) 0 ∼ 0.7034 and eNd 0 + 9, require two different mantle source regions thus posing serious problems for petrogenetic models such as dynamic partial melting which have been proposed for the Faeroes. We interpret the LREE-depleted basalts as partial melts of the oceanic asthenosphere whilst the LREE-enriched basalts may result either from the partial melting of deep mantle blobs or of the subcontinental lithosphere during upwelling of the asthenosphere.

Re–Os constraints on harzburgite and lherzolite formation in the lithospheric mantle: a study of northern Canadian Cordillera xenoliths

Osmium isotope data from lherzolite and harzburgite xenoliths of the northern Canadian Cordil- lera provide constraints on the genesis and age of the lithospheric mantle in a typical off-cratonic continental setting. The 187 Os/ 188 Os ratios of the lherzolites show a positive correlation with Al 2O3 and heavy rare earth elements (HREE), which probably reflects Re/Os fractionation during various degrees of mantle melting, followed by a long period of radiogenic ingrowth. These observations are consistent with melting during the Proterozoic. Harzburgite Os isotopic ratios, however, plot above the regional correlation of the lherzolites. A positive correlation between their Os isotopic ratios and 1/Os concentrations suggests that they are the end result of the introduction of metasomatic agents with low Os contents, but high 187 Os/ 188 Os ratios, into the lithospheric mantle. These fluids or melts may have originated from a region of anomalously slow mantle detected seismically (Frederiksen et al., 1998) below harzburgite-rich xenolith localities (Shi et al., 1998). Alternatively, the radiogenic Os-bearing metasomatic agents may have been related to subduction processes along the western margin of the Canadian Cordillera, as has been previously suggested to explain the high Os isotopic ratios of xenoliths from the northern US Cordillera (Brandon et al., 1996). Copyright

Archaean crustal growth and tectonic processes: a comparison of the Superior Province, Canada and the Dharwar Craton, India

Abstract We present a comparison of the processes involved in the tectonic evolution of two Archaean cratons, the Superior Province of Canada and the Dharwar Craton of India. These two cratons exhibit distinct map patterns, the Superior Province being dominated by elongate belts, while the Dharwar Craton is characterized by dome and basin features. We suggest that certain tectonic processes operating in the Phanerozoic, such as enhanced mantle plume activity, subduction of young (warm) oceanic crust, and faster than usual accretion of crust, may have been the norm during the Archaean. In the Superior Province rapid crustal growth occurred, largely due to horizontal tectonic forces. Models analagous to modern plate tectonics are applicable, but the rates of convergence and accretion exceeded those normal for the present day. Accreted crust was warm and subject to more ductile deformation than in modern accretionary zones. These accreted arc, ocean-floor and ocean-plateau fragments would have been underlain by a thick refractory, buoyant, warm lithospheric root that was rapidly underplated (or imbricated) below the recently accreted terranes. In the Dharwar craton a major thermal event appears to characterize its evolution at 2.5 Ga. Reheating of the lower and middle crust in response to magmatism and metamorphism, resulted in diapirism and growth of crust in a vertical sense. The southern Superior Province’s evolution reflects accretion at the margins of a protocraton, while the Dharwar craton’s tectonic environment may reflect plume impact and incipient rifting in the centre of an Archaean craton.

Antimony isotope variations in natural systems and implications for their use as geochemical tracers

Abstract Multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has been used for the precise measurement of Sb isotopic composition in geological samples, as well as Sb(III) and Sb(V) species in aqueous samples. Sb is chemically purified prior to analysis by using cation-exchange resin and cotton impregnated with thioglycollic acid (TCF). Purification through cation-exchange resin is required for the removal of matrix interfering elements such as transitional metals, whereas TCF is required for the separation of other hydride-forming elements such as Ge and As. The analyte is introduced in the plasma torch using a continuous flow hydride generation system. Instrumental mass fractionation is corrected with a “standard-sample bracketing” approach. Using this technique, the minimum Sb required per analysis is as low as 10 ng for an estimated external precision calculated for the 123 Sb/ 121 Sb isotope ratio of 0.4 e units (2 σ ). Sb isotope fractionation experiments reported here indicate strong fractionation (9 e units) during Sb(V) reduction to Sb(III). Seawater, mantle-derived rocks, various environmental samples, deep-sea sediments and hydrothermal sulfides from deep-sea vents have been analyzed for their Sb isotope composition. We define a continental and oceanic crust reservoir at 2±1 e units. Seawater e 123 Sb values do not vary significantly with depth and yield a restricted range of 3.7±0.4 e units. Sb deposited in hydrothermal environments has a significant range of Sb isotopic composition (up to 18 e units). These variations may reflect not only contributions from different Sb-sources (such as seawater and volcanic rocks), but also kinetic fractionation occurring at low temperature in aqueous media through the reduction of seawater-derived Sb(V) in more reducing environment. Our results suggest that Sb isotopes can be extremely useful tracers of natural processes and may be useful as paleoredox tracers in oceanic systems.