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Item 182W and HSE constraints from 2.7 Ga komatiites on the heterogeneous nature of the Archen mantle(Elsevier, 2018-03-02) Puchtel, I.S.; Blichert-Toft, J.; Touboul, M.; Walker, Richard J.While the isotopically heterogeneous nature of the terrestrial mantle has long been established, the origin, scale, and longevity of the heterogeneities for different elements and isotopic systems are still debated. Here, we report Nd, Hf, W, and Os isotopic and highly siderophile element (HSE) abundance data for the Boston Creek komatiitic basalt lava flow (BCF) in the 2.7 Ga Abitibi greenstone belt, Canada. This lava flow is characterized by strong depletions in Al and heavy rare earth elements (REE), enrichments in light REE, and initial e143Nd = +2.5 ± 0.2 and initial e176Hf = +4.2 ± 0.9 indicative of derivation from a deep mantle source with time-integrated suprachondritic Sm/Nd and Lu/Hf ratios. The data plot on the terrestrial Nd-Hf array suggesting minimal involvement of early magma ocean processes in the fractionation of lithophile trace elements in the mantle source. This conclusion is supported by a mean mu142Nd = -3.8 ± 2.8 that is unresolvable from terrestrial standards. By contrast, the BCF exhibits a positive 182W anomaly (mu182W = +11.7 ± 4.5), yet is characterized by chondritic initial g187Os = +0.1 ± 0.3 and low inferred source HSE abundances (35 ± 5% of those estimated for the present-day Bulk Silicate Earth, BSE). Collectively, these characteristics are unique among Archean komatiite systems studied so far. The deficit in the HSE, coupled with the chondritic Os isotopic composition, but a positive 182W anomaly, are best explained by derivation of the parental BCF magma from a mantle domain characterized by a predominance of HSE-deficient, differentiated late accreted material. According to the model presented here, the mantle domain that gave rise to the BCF received only ~35% of the present-day HSE complement in the BSE before becoming isolated from the rest of the convecting mantle until the time of komatiite emplacement at 2.72 Ga. These new data provide strong evidence for a highly heterogeneous Archean mantle in terms of absolute HSE abundances and W isotopic composition, and also indicate slow mixing, on a timescale of at least 1.8 billion years. Additionally, the data are consistent with a stagnant-lid plate tectonic regime in the Hadean and Archean, prior to the onset of modern-style plate tectonics.Item Oceanic and continental lithospheric mantle in the 1.95 Ga Jormua Ophiolite Complex, Finland: implications for mantle and crustal evolution(Oxford, 2023-10-31) Finlayson, Valerie; Haller, Mitchell; Day, James M.D.; Ginley, Stephen; O'Driskoll, Brian; Kontinen, Asko; Hanski, Eero; Walker, Richard J.The ca. 1.95 Ga Jormua Ophiolite Complex (JOC), Finland, is a rare Paleoproterozoic ophiolite that preserves a record of diverse upper mantle materials and melting processes. Meter-scale grid sampling of four JOC outcrops, as well as non-grid samples, permits evaluation of meter- to kilometer-scale mantle heterogeneity within the JOC. Significant heterogeneity is observed between the four grids, and also among a number of the non-grid samples examined. Variations in the concentrations of fluid-mobile elements are particularly large among different samples and locations. New whole-rock major, lithophile trace, and highly siderophile element data (HSE: Os, Ir, Ru, Pt, Pd, Re), including 187Re-187Os isotopic data, for serpentinized harzburgites indicate the presence of two distinct compositional types and probable modes of origin within the JOC. This is consistent with prior findings. Type 1 is similar to modern refractory abyssal-type mantle. Type 2 is more highly refractory than Type 1, and most likely represents samples from sub-continental lithospheric mantle (SCLM). Type 1 mantle is moderately heterogeneous with respect to chemical and Os isotopic compositions at both the meter and kilometer scales. By contrast, Type 2 mantle is considerably more homogeneous than Type 1 grids at the meter scale, but is more heterogeneous at the kilometer scale. The median initial γOs value for Type 1 mantle, calculated for 1.95 Ga, is ~-2.0 (where γOs is the % deviation in 187Os/188Os relative to a chondritic reference calculated for a specified time). This isotopic composition is consistent with a moderate, long-term decrease in Re/Os relative to the estimate for Primitive Mantle, prior to JOC formation. The similarity in this γOs value to the value for the modern abyssal mantle, as well as the initial values for several Phanerozoic ophiolites suggests that the upper mantle achieved a Re/Os ratio similar to the chondritic reference by ~2 Ga, then evolved along a subparallel trajectory to the chondritic reference since then. For this to occur, only limited Re could have been permanently removed from the upper mantle since at least the time the JOC formed. A localized secondary metasomatic event at ~2 Ga, concurrent with the estimated obduction age for the JOC and subsequent Svecofennian Orogeny, affected the HSE systematics of some Type 1 samples. By contrast, late Archean Os TRD model ages for Type 2 rocks indicate a depletion event superimposed upon the long-term Re depletion of the abyssal mantle. This event was established no later than ~2.6 Ga and may have occurred during a period of significant, well-documented crustal production in the Karelia craton at ~2.7 Ga.Item In Search of Late-Stage Planetary Building Blocks(Elsevier, 2015-07-03) Walker, Richard J.; Bermingham, Katherine; Liu, Jingao; Puchtel, Igor S.; Touboul, Mathieu; Worsham, Emily A.Genetic contributions to the final stages of planetary growth, including materials associated with the giant Moon-forming impact, late accretion, and late heavy bombardment are examined using siderophile elements. Isotopic similarities between the Earth and Moon for both lithophile and siderophile elements collectively lead to the suggestion that the genetics of the building blocks for Earth, and the impactor involved in the Moon-forming event were broadly similar, and shared some strong genetic affinities with enstatite chondrites. The bulk genetic fingerprint of materials subsequently added to Earth by late accretion, defined as the addition of ~0.5 wt.% of Earth's mass to the mantle, following cessation of core formation, was characterized by 187Os/188Os and Pd/Ir ratios that were also similar to those in some enstatite chondrites. However, the integrated fingerprint of late accreted matter differs from enstatite chondrites in terms of the relative abundances of certain other HSE, most notably Ru/Ir. The final ≤0.05 wt.% addition of material to the Earth and Moon, believed by some to be part of a late heavy bombardment, included a component with much more fractionated relative HSE abundances than evidenced in the average late accretionary component. Heterogeneous 182W/184Wisotopic compositions of some ancient terrestrial rocks suggest that some very early formed mantle domains remained chemically distinct for long periods of time following primary planetary accretion. This evidence for sluggish mixing of the early mantle suggests that if late accretionary contributions to the mantle were genetically diverse, it may be possible to isotopically identify the disparate primordial components in the terrestrial rock record using the siderophile element tracers Ru and Mo.Item High-Precision Tungsten Isotopic Analysis by Multicollection Negative Thermal Ionization Mass Spectrometry Based on Simultaneous Measurement of W and 18O/16O Isotope Ratios for Accurate Fractionation Correction(American Chemical Society, 2016-01-11) Trinquier, Anne; Touboul, Mathieu; Walker, Richard J.Determination of the 182W/184W ratio to a precision of ±5 ppm (2σ) is desirable for constraining the timing of core formation and other early planetary differentiation processes. However, WO3 − analysis by negative thermal ionization mass spectrometry normally results in a residual correlation between the instrumental-mass-fractionation-corrected 182W/184W and 183W/184W ratios that is attributed to mass-dependent variability of O isotopes over the course of an analysis and between different analyses. A second-order correction using the 183W/184W ratio relies on the assumption that this ratio is constant in nature. This may prove invalid, as has already been realized for other isotope systems. The present study utilizes simultaneous monitoring of the 18O/16O and W isotope ratios to correct oxide interferences on a per-integration basis and thus avoid the need for a double normalization of W isotopes. After normalization of W isotope ratios to a pair of W isotopes, following the exponential law, no residual W−O isotope correlation is observed. However, there is a non-ideal mass bias residual correlation between 182W/iW and 183W/iW with time. Without double normalization of W isotopes and on the basis of three or four duplicate analyses, the external reproducibility per session of 182W/184W and 183W/184W normalized to 186W/183W is 5−6 ppm (2σ, 1−3 μg loads). The combined uncertainty per session is less than 4 ppm for 183W/184W and less than 6 ppm for 182W/184W (2σm) for loads between 3000 and 50 ng.Item Widespread tungsten isotope anomalies and W mobility in crustal and mantle rocks of the Eoarchean Saglek Block, northern Labrador, Canada: Implications for early Earth processes and W recycling(Elsevier, 2016-08-15) Liu, Jingao; Touboul, Mathieu; Ishikawa, Akira; Walker, Richard J.; Pearson, D. GrahamWell-resolved 182W isotope anomalies, relative to the present mantle, in Hadean–Archean terrestrial rocks have been interpreted to reflect the effects of variable late accretion and early mantle differentiation processes. To further explore these early Earth processes, we have carried out W concentration and isotopic measurements of Eoarchean ultramafic rocks, including lithospheric mantle rocks, metakomatiites, a layered ultramafic body and associated crustal gneisses and amphibolites from the Uivak gneiss terrane of the Saglek Block, northern Labrador, Canada. These analyses are augmented by in situ W concentration measurements of individual phases in order to examine the major hosts of W in these rocks. Although the W budget in some rocks can be largely explained by a combination of their major phases, W in other rocks is hosted mainly in secondary grain-boundary assemblages, as well as in cryptic, unidentified W-bearing ‘nugget’ minerals. Whole rock W concentrations in the ultramafic rocks show unexpected enrichments relative, to elements with similar incompatibilities. By contrast, W concentrations are low in the Uivak gneisses. These data, along with the in situ W concentration data, suggest metamorphic transport/re-distribution of W from the regional felsic rocks, the Uivak gneiss precursors, to the spatially associated ultramafic rocks. All but one sample from the lithologically varied Eoarchean Saglek suite is characterized by generally uniform ∼+11 ppm enrichments in 182W relative to Earth’s modern mantle. Modeling shows that the W isotopic enrichments in the ultramafic rocks were primarily inherited from the surrounding 182W-rich felsic precursor rocks, and that the W isotopic composition of the original ultramafic rocks cannot be determined. The observed W isotopic composition of mafic to ultramafic rocks in intimate contact with ancient crust should be viewed with caution in order to plate constraints on the early Hf–W isotopic evolution of the Earth’s mantle with regard to late accretionary processes. Although 182W anomalies can be erased via mixing in the convective mantle, recycling of 182W-rich crustal rocks into the mantle can produce new mantle sources with anomalous W isotopic compositions that can be tapped at much later times and, hence, this process should be considered as a mechanism for the generation of 182W-rich rocks at any subsequent time in Earth history.Item Age, genetics, and crystallization sequence of the group IIIE iron meteorites(Elsevier, 2023-06-14) Chiappe, Emily M.; Ash, Richard; Walker, Richard J.Chemical and isotopic data were obtained for ten iron meteorites classified as members of the IIIE group. Nine of the IIIE irons exhibit broadly similar bulk siderophile element characteristics. Modeling of highly siderophile element abundances suggests that they can be related to one another through simple crystal-liquid fractionation of a parent melt. Our preferred model suggests initial S, P, and C concentrations of approximately 12 wt%, 0.8 wt %, and 0.08 wt%, respectively. The modeled IIIE parent melt composition is ~4 times more enriched in highly siderophile elements than a non-carbonaceous (NC) chondrite-like parent body, suggesting a core comprising ~22% of the mass of the parent body. Although chemically distinct from the other IIIE irons, formation of the anomalous IIIE iron Aletai can potentially be accounted for under the conditions of this model through the nonequilibrium mixing of an evolved liquid and early formed solid. Cosmic ray exposure-corrected nucleosynthetic Mo, Ru, and W isotopic compositions of four of the bona fide IIIE irons and Aletai indicate that they originated from the non-carbonaceous (NC) isotopic domain. Tungsten-182 isotopic data for the IIIE irons and Aletai yield similar model metal-silicate segregation ages of 1.6 ± 0.8 Myr and 1.2 ± 0.8 Myr, respectively, after calcium aluminum-rich inclusion (CAI) formation. These ages are consistent with those reported for other NC-type iron meteorite parent bodies. The IIIE irons are chemically and isotopically similar to the much larger IIIAB group. Despite some textural, mineralogical, and chemical differences, such as higher C content, the new results suggest they may have originated from a different crystallization sequence on the same or closely-related parent body.Item Geodynamic implications of ophiolitic chromitites in the La Cabaña ultramafic bodies, Central Chile(Taylor & Francis, 2014-07-18) González-Jiménez, José María; Barra, Fernando; Walker, Richard J.; Reich, Martin; Gervilla, FernandoChromitites (>80% volume chromite) hosted in two ultramafic bodies (Lavanderos and Centinela Bajo) from the Palaeozoic metamorphic basement of the Chilean Coastal Cordillera were studied in terms of their chromite composition, platinum group element (PGE) abundances, and Re-Os isotopic systematics. Primary chromite (Cr# = 0.64–0.66; Mg# = 48.71–51.81) is only preserved in some massive chromitites from the Centinela Bajo ultramafic body. This chemical fingerprint is similar to other high-Cr chromitites from ophiolite complexes, suggesting that they crystallized from arc-type melt similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction mantle. The chromitites display enrichment in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), with PGE concentrations between 180 and 347 ppb, as is typical of chromitites hosted in the mantle of supra-subduction zone (SSZ) ophiolites. Laurite (RuS2)-erlichmanite (OsS2) phases are the most abundant inclusions of platinum-group minerals (PGM) in chromite, indicating crystallization from S-undersaturated melts in the subarc mantle. The metamorphism associated with the emplacement of the ultramafic bodies in the La Cabaña has been determined to be ca. 300 Ma, based on K-Ar dating of fuchsite. Initial 187Os/188Os ratios for four chromitite samples, calculated for this age, range from 0.1248 to 0.1271. These isotopic compositions are well within the range of chromitites hosted in the mantle section of other Phanaerozoic ophiolites. Collectively, these mineralogical and geochemical features are interpreted in terms of chromite crystallization in dunite channels beneath a spreading centre that opened a marginal basin above a supra-subduction zone. This implies that chromitite-bearing serpentinites in the metamorphic basement of the Coastal Cordillera are of oceanic-mantle origin and not oceanic crust as previously suggested. We suggest that old subcontinental mantle underlying the hypothetical Chilenia micro-continent was unroofed and later altered during the opening of the marginal basin. This defined the compositional and structural framework in which the protoliths of the meta-igneous and meta-sedimentary rocks of the Eastern and Western Series of the Chilean Coastal Cordillera basement were formed.Item Preservation of Earth-forming events in the W isotopic 1 composition of modern flood basalts(Science Magazine, 2016-05-13) Rizo, Hanika; Walker, Richard J.; Carlson, Richard W.; Horan, Mary F.; Mukhopadhyay, Sujoy; Manthos, Vicky; Francis, Don; Jackson, Matthew G.How much of Earth's compositional variation dates to processes that occurred during planet formation remains an unanswered question. High-precision tungsten isotopic data from rocks from two large igneous provinces, the North Atlantic Igneous Province and the Ontong Java Plateau, reveal preservation to the Phanerozoic of tungsten isotopic heterogeneities in the mantle. These heterogeneities, caused by the decay of hafnium-182 in mantle domains with high hafnium/tungsten ratios, were created during the first ~50 million years of solar system history, indicating that portions of the mantle that formed during Earth’s primary accretionary period have survived to the present.