William F. McDonough

Re–Os evidence for replacement of ancient mantle lithosphere beneath the North China craton

Abstract Re–Os data for peridotite xenoliths carried in Paleozoic kimberlites and Tertiary alkali basalts confirm previous suggestions that the refractory and chemically buoyant lithospheric keel present beneath the eastern block of the North China craton (and sampled by Paleozoic kimberlites) is indeed Archean in age and was replaced by more fertile lithospheric mantle sometime after the Paleozoic. Moreover, lithospheric mantle beneath the central portion of the craton (west of the major gravity lineament) formed during the last major Precambrian orogeny, around 1900 Ma ago. This age is significantly younger than the overlying crust (2700 Ma), suggesting that the original Archean lithosphere was replaced in the Proterozoic. The timing of lithospheric replacement in the eastern block of the North China Craton is constrained only to the Phanerozoic by the Re–Os results. Circumstantial geologic evidence suggests this new lithosphere is Jurassic or Cretaceous in age and formed after collision of the Yangtze and North China cratons in the Triassic, an event that was also responsible for the subduction and uplift of ultrahigh-pressure metamorphic rocks. Collectively, these data suggest that lithosphere replacement occurred in response to two continent collisional events widely separated in time (∼1900 and ∼220 Ma). Coupled with observations from other Archean cratons we suggest that wholesale replacement of lithospheric mantle (±lower crust) may require large-scale continental collision.

Thermal structure, thickness and composition of continental lithosphere

Abstract Global compilations of surface heat flow data from stable, Precambrian terrains show a statistically significant secular change from 41±11 mW/m 2 in Archean to 55±17 mW/m 2 in Proterozoic regions far removed from Archean cratons. Using the tectonothermal age of the continents coupled with average heat flow for different age provinces yields a mean continental surface heat flow between 47 and 49 mW/m 2 (depending on the average, non-orogenic heat flow assumed for Phanerozoic regions). Compositional models for bulk continental crust that produce this much or more heat flow (i.e., K 2 O>2.3–2.4 wt%) are not consistent with these observations. More rigorous constraints on crust composition cannot be had from heat flow data until the relative contributions to surface heat flow from crust and mantle are better determined and the non-orogenic component of heat flow in the areally extensive Phanerozoic regions (35% of the continents) is determined. We calculate conductive geotherms for 41 mW/m 2 surface heat flow to place limits on the heat production of Archean mantle roots and to evaluate the significance of the pressure–temperature ( P – T ) array for cratonic mantle xenoliths. Widely variable geotherms exist for this surface heat flow, depending on the values of crustal and lithospheric mantle heat production that are adopted. Using the average K content of cratonic peridotite xenoliths (0.15 wt% K 2 O, assuming Th/U=3.9 and K/U=10,000 to give a heat production of 0.093 μW/m 3 ) and a range of reasonable crustal heat production values (i.e., ≥0.5 μW/m 3 ), we calculate geotherms that are so strongly curved they never intersect the mantle adiabat. Thus the average cratonic peridotite is not representative of the heat production of Archean mantle roots. Using our preferred estimate of heat production in the cratonic mantle (0.03 wt% K 2 O, or 0.019 μW/m 3 ) we find that the only geotherms that pass through the xenolith P–T data array are those corresponding to crust having very low heat production ( 2 O). If the lithospheric mantle heat production is higher than our preferred values, the continental crust must have correspondingly lower heat production (i.e., bulk crustal K, Th and U contents lower than that of average Archean granulite facies terrains), which we consider unlikely. If the xenolith P–T data reflect equilibration to a conductive geotherm, then Archean lithosphere is relatively thin (150–200 km, based on intersection of the P–T array with the mantle adiabat) and the primary reason for the lower surface heat flow in Archean regions is decreased crustal heat production, rather than the insulating effects of thick lithospheric roots. On the other hand, if the xenolith P–T points result from frozen-in mineral equilibria or reflect perturbed geotherms associated with magmatism, then the Archean crust can have higher heat producing element concentrations, lithospheric thickness can range to greater depths and the low surface heat flow in Archean cratons may be due to the insulating effects of thick lithospheric roots. An uppermost limit for Archean crustal heat production of 0.77 μW/m 3 is determined from the heat flow systematics.

Precise elemental and isotope ratio determination by simultaneous solution nebulization and laser ablation-ICP-MS: application to U–Pb geochronology

Abstract We have developed a procedure for precise in situ elemental and isotope ratio measurements by simultaneous solution nebulization and laser ablation inductively coupled plasma mass spectrometry, which can be applied to isotope and element ratio determinations (e.g., 6 Li / 7 Li , 10 B / 11 B , Ca/Sr and others) covering the entire mass range. Using a quadrupole mass spectrometer, our procedure yields precision of ≤2.0% (all errors are 2 sigma of the standard error) for 206 Pb / 238 U and 207 Pb / 206 Pb and ≤3% for 207 Pb / 235 U in neo-Proterozoic or older zircons and baddeleyite with U contents ≥65–270 ppm. Importantly, this is accomplished without the use of an external calibration standard. We nebulize a solution containing known amounts of natural Tl and a 235 U spike simultaneously with ablation of an unknown accessory phase. This allows precise mass discrimination correction of Pb/Pb as well as Pb/U in the ablated signal. Laser-induced elemental fractionation of Pb from U is observed to be a near function of the number of laser pulses (crater depth) and is inversely exponentially correlated with spot size. These systematics allow us to correct for elemental fractionation. Spots with diameters ≥150 μm show no appreciable Pb/U fractionation, whereas for 35 μm spots U becomes progressively depleted relative to Pb, with a factor of four increase in Pb/U over a 2-min ablation period. For the Harvard standard zircon, 91 500, we obtain a 206 Pb / 238 U age of 1061±4 Ma and a 207 Pb / 206 Pb age of 1074±8 Ma (TIMS age: 1065 Ma for 206 Pb / 238 U , [Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Ober, F., von Quant, A., Roddick, J.C., Spiegel, J., 1995. Three natural zircon standards for U–Th–Pb, Lu–Hf, trace element and REE analyses. Geostand. Newsl. 19, 1–23]); for the SHRIMP zircon standard, SL13, we obtain a 206 Pb / 238 U age of 578±10 Ma and a 207 Pb / 206 Pb age of 595±13 Ma (TIMS age: 572 Ma, [Claoue-Long, J.C., Compston, W., Roberts, J., Fanning, C.M., 1995. Two carboniferous ages: A comparison of SHRIMP zircon dating with conventional zircon ages and 40 Ar / 39 Ar analysis. In: Geochronology Time Scales and Global Stratigraphic Correlation. SEPM Special Publication, pp. 3–21], 206 Pb / 238 U age from SHRIMP: 580–565 Ma, [Compston, W., 1999. Geological age by instrumental analysis: The 29th Halmond Lecture. Mineralogical Magazine 63, 297–311]). The Phalaborwa baddeleyite is strongly reverse discordant yielding an upper intercept age of 2057±8 Ma (TIMS age: 1 Ma, [Reischmann, T., 1995. Precise U/Pb age determination with baddeleyite (ZrO2), a case study from the Phalaborwa igneous complex, South Africa. S. Afr. J. Geol. 98, 98]; 2059.8 Ma, [Heaman, L.M., LeCheminant, A.N., Paragenesis and U–Pb systematics of baddeleyite (ZrO2). Chemical Geology 110, 95–126]) and a lower intercept at ∼0 Ma. These results demonstrate that LA-ICP-MS is capable of dating accessory phases with precision and accuracy comparable to SHRIMP.

Tracking the budget of Nb and Ta in the continental crust

Abstract High precision trace element data are reported for representative samples of the upper continental crust: 11 loess samples and 22 shale samples (PAAS) previously used by Taylor and McLennan to define the rare earth element (REE) content of the upper crust. Our results confirm the REE concentrations of Taylor and McLennans [Taylor, S.R., McLennan, S.M., 1985. The continental crust: its composition and evolution. Blackwell, Oxford, 312 pp.] estimate of the upper continental crust but suggest substantial revisions for Nb and Ta, in agreement with recent work of Plank and Langmuir [Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chem. Geol. 145, 325–394.]. From our data, the upper continental crust has average Nb=11.5±2.6 ppm (instead of 25 ppm) and Ta=0.92±0.12 ppm (instead of 2.2 ppm), which translates into a bulk crust Nb=8 ppm, Ta=0.7 ppm, La/Nb=2.2, and Nb/Ta=12–13. These revisions confirm the crustal subchondritic Nb/Ta and superchondritic La/Nb ratios and reinforce the observation that the continental crust and the Depleted Mantle are not strictly complementary: an additional Nb- and Ta-rich reservoir having superchondritic Nb/Ta is required to balance the Silicate Earth. Using the continental crusts La/Nb ratio to estimate the proportions of crustal growth in convergent margin and intraplate settings, we suggest a plume contribution of only between 5 and 20% to the continents, even lower than previous estimates.

Geochemistry of xenolithic eclogites from West Africa, Part I: A link between low MgO eclogites and Archean crust formation

Oxygen isotope, mineral trace element, and measured and reconstructed whole rock compositions are reported for low MgO (6 -13 wt.% MgO in the whole rock) eclogite xenoliths from the Koidu kimberlite complex, Sierra Leone. The d 18 O values of garnet (4.7- 6.8‰), determined by laser fluorination on clean mineral separates, extend beyond the range for mantle peridotites. All low MgO eclogites have reconstructed trace element patterns that are depleted in Ba, Th, U, and light rare earth element (LREE), with jadeite-rich samples having more variable trace element patterns than jadeite-poor samples. These observations, coupled with low SiO2 contents, and Nb-rich but LREE-depleted reconstructed whole rock compositions, suggest the low MgO eclogites are remnants of altered oceanic crust that was partially melted during subduction. Partial melting of a mafic protolith at high pressure (leaving a garnet-bearing residue) is the preferred model to explain the origin of Archean tonalite-trondhjemite-granodiorite (TTG) suites, which make up large portions of the crust in Archean cratons. We therefore suggest that the Koidu low MgO eclogites may be residues from Archean continental crust formation. Copyright

CONTRASTING OLD AND YOUNG VOLCANISM IN RURUTU ISLAND, AUSTRAL CHAIN

Abstract We present new geochemical data (major and trace element, and Sr, Nd and Pb isotopic compositions) for volcanic rocks from Rurutu Island in Polynesia. The rocks are divided into two age groups: 13–10.8 Ma old basalts derived from the plume that is now active beneath the Macdonald seamount, and 1.8–1.1 Ma old volcanics that were produced during a younger volcanic event. Marked chemical differences exist between the two generations of lavas. Typical HIMU isotopic compositions are found in the old lavas but not in the young lavas, and unusual trace element fractionations (Ce/Pb, Zr/Hf and Ti/Eu) are restricted to the young lavas. These results are interpreted in a model in which the composition of the Quaternary lavas results from the interaction between magmas from an ascending young plume and the oceanic lithosphere under the island. The contaminating component in the lithosphere is thought to be a residual carbonatite liquid from the first volcanic episode. This contaminant has isotopic compositions similar to those of the first volcanic event but very high trace-element concentrations characterized by significant negative Ti and Pb anomalies. This model raises questions about the use of isotopic and trace element data from islands located downstream from another plume to understand the general composition and origin of the source of plume volcanism. However, it does not put into question the existence of extreme isotopic compositions (HIMU, EM I and EM II) in the mantle seurces of oceanic island basalts.

Geochemistry of xenolithic eclogites from West Africa, part 2: origins of the high MgO eclogites

Abstract Oxygen isotope, mineral trace element, and measured and reconstructed whole-rock compositions are reported for the high MgO eclogite xenolith suite (16 to 20 wt% MgO in the whole rock) from the Koidu Kimberlite complex, Sierra Leone. In contrast to the previously published data for low MgO eclogites (6 to 13 wt% MgO) from this area, high MgO eclogites equilibrated at higher temperatures (1080 to 1130°C vs. 890 to 930°C) have only mantlelike δ18O and show variable degrees of light rare earth element (REE) enrichment. Analyses of multiple mineral generations suggest that the heterogeneous REE patterns of the high MgO eclogites reflect variable degrees of metasomatic overprinting. High MgO and Al2O3 contents of the eclogites suggest a cumulate origin, either as high-pressure (2 to 3 GPa) garnet–pyroxene cumulates or low-pressure ( 4 GPa).

ReOs and UPb geochronological constraints on the eclogite–tonalite connection in the Archean Man Shield, West Africa

Abstract Mantle-derived eclogite xenoliths and tonalite–trondhjemite–granodiorites (TTG) that occur in the Man Shield, West Africa, sample different levels of Archean lithosphere. Chemical and oxygen isotope systematics indicate that low MgO eclogites from the Koidu kimberlite are ancient remnants of subducted oceanic crust that may have been involved in regional Archean crust formation. ReOs whole rock isotopic data for these eclogites scatter about a line with slope corresponding to an Archean age of 3.44±0.76 Ga (2σ), with ReOs model ages ranging from 1.4 to 3.9 Ga. This wide range of model ages overlaps with the age range for crust formation and metamorphism in the Man Shield. In situ UPb ages of zircons from crustal rocks have been measured by laser ablation ICP-MS. A tonalitic gneiss has discordant zircons with rare old cores (∼3.6 Ga) and an upper concordia intercept at 2890±9 Ma (2σ). Zircons from a mafic lower crustal granulite xenolith are concordant at 2686±32 Ma. Our results, together with previously published ages for Man Shield rocks, indicate an early Archean crust formation event followed by major crustal growth at 2.9–3.0 Ga and a last major metamorphic event at 2.7 Ga. These data show that the eclogites and the continental crust of the West African Craton overlap in time of formation (but only at the very broad age uncertainty provided by the eclogite ReOs results). They are permissive of Archean crustal growth by melting of the protoliths of the materials now sampled as the Koidu eclogite xenoliths. If so, this suggests that Archean crustal growth in the Man Shield occurred in a convergent margin setting.

Supernova Sources and the 92Nb-92Zr p-Process Chronometer

We report new Zr isotope evidence for live (92)Nb (mean life: tau&d1;92Nb=52 Myr) within the early solar system resulting in &parl0;92Nb&solm0;93Nb&parr0;initial approximately 10-3. The meteoritic minerals rutile and zircon have, respectively, very high and very low Nb/Zr ratios and are ideal for exploring the (92)Nb-(92)Zr chronometer. Rutiles exhibit high positive straightepsilon92Zr ( approximately 14-36) while a zircon has a negative straightepsilon92Zr ( approximately -4), as would be expected if (92)Nb was live in the early solar system. The meteoritic rutiles appear to be young, with apparent times of formation of approximately 80-220 Myr subsequent to the origin of the solar system. The initial (92)Nb/(92)Mo for the solar system is broadly compatible with a model of uniform production if the (92)Nb/(92)Mo production ratio for Type II supernova (SNII) sources with neutrino-driven winds is used. Data for all the now extinct p-process nuclides ((92)Nb, (97)Tc, and (146)Sm) are consistent with these isotopes being derived by uniform production from SNII sources and a free decay interval of approximately 10 Myr. Consideration of a range of models indicates that the average p-process production ratio of (92)Nb/(92)Mo needs to be at least in the range of 0.06-0.25.

A gravimetric K2OsCl6 standard: Application to precise and accurate Os spike calibration

The Re-Os isotopic system is currently limited as a chronometer because of the lack of accurate gravimetric standards for Os spike calibration and uncertainty of the 187Re half-life, which is also dependent on the accuracy of Os spike calibration. We demonstrate that the concentration of an Os spike can be calibrated accurately to ±0.2%. The accuracy of this calibration was achieved by using a high-purity, stoichiometric, and anhydrous K2OsCl6 standard. We show that (NH4)2OsCl6, the standard material used by most other laboratories, is less reliable for Os spike calibration because it is hygroscopic and decomposes during heating. Nebulization and ionization in a plasma at 6500 to 8000 K does not discriminate Os of different oxidation states and chemical forms, allowing Os isotopic ratios in spike-normal mixtures to be equilibrated “on-line” and reproduced at the ∼0.02% level. Multiple isotope dilution measurements reproduce Os concentrations at the 0.04% level. The Os isotopic compositions of the standards determined by multiple collector inductively coupled plasma mass spectrometry are in excellent agreement with that of negative thermal ionization mass spectrometry. A strong linear relationship between instrumental mass fractionation factors (β) for Os and Ir in MC-ICP-MS allows us to determine the isotopic composition of the Os spike with high precision and accuracy. Application of such an accurately calibrated spike is anticipated to reduce the uncertainty of 187Re half-life significantly, thereby increasing the accuracy of the 187Re-187Os chronometer.