Anthony J. Irving

Osmium isotopic compositions of mantle xenoliths: A global perspective

0.0008 (level of confidence 95%) was defined on the basis of 117 spinel-bearing xenoliths from this work and data from the literature, including data for massif peridotites. The 187 Os/ 188 Os ratio of the PUM is similar to the range of compositions defined by ordinary and enstatite chondrites, not carbonaceous chondrites. Spinel-bearing mantle peridotites sampled by volcanism and peridotite massifs appear to have been extracted from a common fertile source (PUM) between 1 and 2 Ga ago. These peridotites now form part of the subcontinental lithospheric mantle underlying continental crust of similar or greater formation age. Copyright

Trace element abundances in megacrysts and their host basalts - Constraints on partition coefficients and megacryst genesis

Abstract In an effort to obtain information about mineral/melt trace element partitioning during the high pressure petrogenesis of basic rocks, we determined rare earth and other trace element abundances in megacrysts of clinopyroxene, orthopyroxene, amphibole, mica, anorthoclase, apatite and zircon, and in their host basalts. In general, the ranges of mineral/melt partition coefficients established from experimental partitioning studies and phenocryst/matrix measurements overlap with the ranges of megacryst/host abundance ratios. Our data for Hf, Sc, Ta and Th partitioning represent some of the only estimates available. Consideration of phase equilibria, major element partitioning and isotopic ratios indicate that most of the pyroxene and amphibole megacrysts may have been in equilibrium with their host magmas at high pressures (mostly 10–25 kb). In contrast, it is unlikely that mica, anorthoclase, apatite and zircon megacrysts formed in equilibrium with their host basalts; instead, we conclude that they were precipitated from more evolved magmas and have been mixed into their present host magmas. Consequently, the trace element abundance ratios for megacryst/host should not be interpreted as partition coefficients, but only as guides for understanding trace element partitioning during high pressure petrogenesis. With this caveat, we conclude that the megacryst/ host trace element abundance data indicate that mineral/melt partition coefficients in basaltic systems during high pressure fractionation are not drastically different from partition coefficients valid for low pressure fractionation.

Depletion and enrichment history of subcontinental lithospheric mantle: An Os, Sr, Nd and Pb isotopic study of ultramafic xenoliths from the northwestern Wyoming Craton

Abstract Elemental and isotopic compositions of spinel peridotite, pyroxenite and glimmerite xenoliths in Eocene minette dikes from the Highwood Mountains and Eagle Buttes, Montana, U.S.A., reveal a prolonged, yet episodic, history of melt removal and addition within the shallow lithospheric mantle of the Archean Wyoming Craton or its modified margin. Ancient, but highly variable, enrichment in incompatible elements is indicated by extreme Sr, Nd and Pb isotopic compositions ( 87 Sr 86 Sr = 0.705 to 1.02 ; ϵ Nd = −9 to −43; 206 Pb 204 Pb = 15.8 to 23.2 ). Very low 187 Os 188 Os (0.110 or less), corresponding to Re depletion model ages ( T RD ) of 2.7 to 2.9 Ga in some of the peridotites reflects melt removal during the Archean. At least one product of Archean melt migrating through this mantle section is preserved as a websterite xenolith that gives 2.7 Ga model ages for both the SmNd and ReOs isotopic systems. The majority of xenoliths, however, define PbPb and SmNd isochrons of mid-Proterozoic age and have ReOs T RD ages of 2 Ga or less. The mid-Proterozoic age could reflect either the time of formation of these peridotites in the shallow mantle or a time of severe overprinting of the incompatible element budget of pre-existing material by interaction with migrating fluids and/or melts. Glimmerite veins within one harzburgite sample yield 1.8 Ga monazite UPb ages and probably represent the products of crystallization of the fluid/melt responsible for the incompatible element enrichment. The material introduced in the Proterozoic was derived from much older, presumably Archean, crustal materials as shown by marked negative Eu anomalies in many samples and highly evolved initial Sr and Nd isotopic compositions. The data highlight the complex chemical evolution experienced by mantle lithosphere and suggest a coupling between the timing of processes affecting the lithospheric mantle and those recorded in the overlying crustal section.

Petrology of the Chilliwack batholith, North Cascades, Washington: generation of calc-alkaline granitoids by melting of mafic lower crust with variable water fugacity

Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO2), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of fH2O variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu*=0.84–0.50) are modeled by melting with fH2O<1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu* =1.0–0.54) require residual amphibole in the source and are modeled by melting with fH2O=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (87Sr/86Sri =0.7033–0.7043; εNd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (87Sr/86Sri =0.7034–0.7040; εNd(0)=+3.3 to +6.9). The fH2O variations during melting may reflect additions of H2O to the lower crust from crystallizing basaltic magmas having a range of H2O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800°C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.

Isotopic and trace element composition of the upper mantle beneath a young continental rift: Results from Kilbourne Hole, New Mexico

Abstract Clinopyroxenes (cpx) separated from discrete spinel lherzolite xenoliths from Kilbourne Hole, New Mexico, are compositionally and isotopically heterogeneous. On a Nd-Sr isotope correlation diagram, the cpx plot largely within the mantle array, from near Bulk Earth to depleted MORB values. None of the bulk xenoliths are equivalent to primitive mantle; all have undergone one or more depletion events and some have been enriched in incompatible elements. The lherzolites as well as the cpx show significant variations in incompatible element ratios including Sm/ Nd and Sr/Nd; moreover, 147 Sm 144 Nd and 143 Nd 144 Nd ratios of the cpx are positively correlated and suggestive of an 0.4 Ga fractionation event. Cpx from spinel pyroxenite dikes and lherzolite wallrocks of composite xenoliths are relatively homogeneous (isotopically) and similar to OIB and some MORB. The wallrocks are isotopically equilibrated with the pyroxenites or nearly so, and have negative Nd model ages; we infer that the pyroxenite-forming event caused enrichment in incompatible elements in the contiguous wallrock. The pyroxenite parent magma was probably a primitive basanite characterized by low Hf/Sm and Ti/Sm ratios relative to primitive mantle as a consequence of residual garnet. Our data confirm the presence of a MORB-related component in the mantle beneath Kilbourne Hole. This component is fertile with respect to basaltic constituents, is relatively LREE-depleted, and is isotopically similar to MORB; it is probably derived from asthenosphere. The isotopic heterogeneity of the discrete lherzolites requires a second, enriched component characterized by relatively low 143 Nd 144 Nd and high 87 Sr 86 Sr ratios and unsupported 143 Nd 144 Nd ratios. Old but disparate Sr and Nd model ages require that MORB-related spinel lherzolites have had a complicated history and differentiated from primitive mantle more than 1 b.y. ago. Two plausible models, one involving more than one depletion event and the second involving mixing of mantle components depleted at distinct times, can explain the common observation that Sr model ages are older than Nd model ages.

Strontium, neodymium, and lead isotopic evidence for the interaction of post-suhduction asthenospheric potassic mafic magmas of the Highwood Mountains, Montana, USA, with ancient Wyoming craton lithospheric mantle

The Eocene potassic mafic rocks of the Highwood Mountains in Montana, USA, share many petrographic, major element, and trace element characteristics with potassic rocks erupted in Recent arcs, including Italy, Indonesia, and western Mexico. However, isotopic compositions of the Highwood samples (radiogenic 87Sr86Sr of 0.707 to 0.709, unradiogenic ϵNd of −11 to −20, unradiogenic 206Pb204Pb of 16 to 18) are very different from those of their more modern counterparts, and, as for most other magmas emplaced into the Archean/Proterozoic Wyoming Province, must reflect the influence of ancient, geochemically extreme lithologies in their petrogenesis. The most primitive Highwood minettes and leucitites (8–14 wt% MgO) have high K20 (4.6 to 8.2 wt%) and Ba (2000–5000 ppm), yet are relatively depleted in TiO2, Nb, and Ta. Although the Highwood magmas ascended through thick Precambrian crust, their very high trace element contents coupled with their primitive compositions indicate that crustal assimilation was negligible. Instead, it is proposed that the distinctive isotopic and trace element characteristics of the Highwood magmas were acquired by assimilation of lithospheerc mantle by ascending asthenospheric melts. Alternative models suggesting derivation of these and other Wyoming Province magmas by direct melting of lithospheric mantle are rejected on the basis of thermal constraints and the extreme isotopic compositions of mantle xenoliths, including a glimmerite-veined harzburgite, sampled by one of the Highwood minettes. The isotopic and trace element systematics can be modeled by mixing one or more ancient metasomatized mantle components with a dominantly asthenospheric component that has ϵNd near or greater than zero (as observed for many Wyoming Province kimberlitic-alnoitic magmas and for Recent potassic arc magmas that have not traversed ancient lithosphere). The voluminous Eocene mafic magmatism throughout central Montana may have been triggered by foundering and southwestward rollback of the subducted, low-angle Farallon plate as convergence slowed. By analogy with their occurrence in modern arcs, potassic magmas could have been generated by decompression melting within convectively upwelling portions of phlogopite-bearing asthenospheric wedge that had been metasomatized by earlier (Cretaceous) slab-derived fluids. It is possible that their ascent to the surface was facilitated by preferential channeling into pre-existing vein networks, resulting in enhanced assimilation of ancient, isotopically extreme, mica-pyroxene-rich metasomes. The rare, younger (28−2 Ma)lamproitic magmas of the province likely reflect a larger contribution from veined lithospheric mantle than is evident in the minettes. Nevertheless, we propose that an important component in their petrogenesis is asthenospheric mantle modified by subduction-related, potassic metasomatism that preceded their eruption by 50–80 Ma.

RAPID TIMESCALES FOR MAGMA OCEAN CRYSTALLIZATION ON THE HOWARDITE-EUCRITE-DIOGENITE PARENT BODY

Asteroid 4 Vesta has long been postulated as the source for the howardite-eucrite-diogenite (HED) achondrite meteorites. Here we show that Al-free diogenite meteorites record variability in the mass-independent abundance of 26Mg (26Mg*) that is correlated with their mineral chemistry. This suggests that these meteorites captured the Mg-isotopic evolution of a large-scale differentiating magma body with increasing 27Al/24Mg during the lifespan of the short-lived 26Al nuclide (t 1/2 ~ 730,000 yr). Thus, diogenites and eucrites represent crystallization products of a large-scale magma ocean associated with the differentiation and magmatic evolution of the HED parent body. The 26Mg* composition of the most primitive diogenites requires onset of the magma ocean crystallization within 0.6–0.4 + 0.5 Myr of solar system formation. Moreover, 26Mg* variations among diogenites and eucrites imply that near complete solidification of the HED parent body occurred within the following 2-3 Myr. Thermal models predict that such rapid cooling and magma ocean crystallization could only occur on small asteroids (<100 km), implying that 4 Vesta is not the source of the HED meteorites.

Origin of K-poor leucosomes in a metasedimentary migmatite complex by ultrametamorphism, syn-metamorphic magmatism and subsolidus processes

Two types of stromatic leucosomes are identified in metasedimentary rocks from the Skagit migmatite complex, North Cascades, Washington state, U.S.A. Both types are trondhjemitic and appear similar in outcrop, but, although both contain low abundances of REE, one type consists of leucosomes that are relatively REE-enriched compared to the other, and contains (1) small ( 700°C, water-rich fluid present), inferences about the origin of the above-listed mineralogical and fluid inclusion features, and modeling of leucosome trace element abundances. The second type of leucosome is interpreted to have formed entirely by subsolidus processes (e.g., metamorphic differentiation) because these leucosomes lack features consistent with an origin by partial melting. K-poor (tonalitic/trondhjemitic) leucosomes associated with metasedimentary (biotite-bearing) source rocks may form by water-saturated partial melting or by subsolidus processes. Both general leucosome-forming mechanisms may operate at different times during upper amphibolite facies regional metamorphism. Partial melting may be initiated by syn-metamorphic magmatic activity if crystallizing plutons serve as external sources of the water-rich fluid necessary for ultrametamorphism in the middle crust during orogenesis. Large-scale migmatite complexes such as the Skagit migmatites may form at least in part in response to contact effects of plutonism associated with high-grade metamorphism, so, although migmatite complexes are a volumetrically substantial part of many orogenic belts, they may not themselves represent a significant original source of magma for larger-scale igneous bodies.

Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars

The geochemistry of Martian meteorites provides a wealth of information about the solid planet and the surface and atmospheric processes that occurred on Mars. The degree to which Martian magmas may have assimilated crustal material, thus altering the geochemical signatures acquired from their mantle sources, is unclear. This issue features prominently in efforts to understand whether the source of light rare-earth elements in enriched shergottites lies in crustal material incorporated into melts or in mixing between enriched and depleted mantle reservoirs. Sulphur isotope systematics offer insight into some aspects of crustal assimilation. The presence of igneous sulphides in Martian meteorites with sulphur isotope signatures indicative of mass-independent fractionation suggests the assimilation of sulphur both during passage of magmas through the crust of Mars and at sites of emplacement. Here we report isotopic analyses of 40 Martian meteorites that represent more than half of the distinct known Martian meteorites, including 30 shergottites (28 plus 2 pairs, where pairs are separate fragments of a single meteorite), 8 nakhlites (5 plus 3 pairs), Allan Hills 84001 and Chassigny. Our data provide strong evidence that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet’s history. The signature of mass-independent fractionation observed also indicates that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues, suggesting fundamental differences between the dominant sulphur chemistry in the atmosphere of Mars and that in the atmosphere of Earth.

Correlation of Cerium Anomalies with Indicators of Paleoenvironment

ABSTRACT Among 21 whole-rock samples of the Upper Cretaceous Niobrara Formation from Colorado, the abundance of cerium relative to other rare earth elements (Ce anomaly), the weight percent organic carbon (%Corg), and the intensity of bioturbation all covary. This covariation is provocative because %Corg and intensity of bioturbation track changes in the concentration of oxygen in the local water column at the time of deposition (Savrda and Bottjer 1989). Ce anomalies in apatite-rich fractions of the Maastrichtian Zumaya-Algorta Formation from France and Spain and the Miocene Monterey Formation from California show changes that also may coincide with changes in ancient oxygen levels. Our results for the Niobrara samples are the closest correspondence demonstrated betwe n paleo-redox conditions and Ce anomalies, but we cannot yet determine whether the correspondence reflects a cause-and-effect relationship. Variation in Ce anomalies is influenced by a number of factors, including terrigenous input, depositional environment, and diagenetic conditions. Potential interplay of these factors prevents a unique interpretation of our whole-rock data; dissecting whole-rock Ce anomalies through analysis of isolated sedimentary components, though, is a promising avenue of research.