Elisabeth Widom

The nature of metasomatism in the sub-arc mantle wedge: evidence from Re-Os isotopes in Kamchatka peridotite xenoliths

Abstract We have performed Re–Os isotope measurements on a suite of 21 Kamchatka mantle xenoliths including 19 harzburgites and two lherzolites, from the northern arc front (Valovayam Volcano), the southern arc front (Avachinsky Volcano), and behind the arc front in the south (Bakening Volcano). Os and Re concentrations vary from 0.02 to 8.2 and 0.003 to 0.437 ppb, respectively, and 187Re/188Os varies from 0.004 to 3.811. 187Os/188Os ratios range from 0.1226 to 0.1566. Regional variations in Re–Os isotope signatures are apparent, with peridotites from Avachinsky exhibiting the least radiogenic Os isotope signatures and lowest Re/Os ratios, and those from Bakening the most radiogenic Os and highest Re/Os. Peridotites from Valovayam span the distinct compositional fields defined by the Avachinsky and Bakening peridotites. All of the Kamchatka peridotites are, however, characterized by radiogenic 187Os/188Os compared to non-arc continental peridotites with comparable Re abundances or Re/Os ratios. The relatively radiogenic Os isotope signatures in the Kamchatka peridotites cannot easily be explained by contamination of the xenoliths by their host lavas, as this process would result in Re/Os ratios higher than observed in the xenoliths. In situ radiogenic ingrowth of high Re/Os mantle followed by recent Re depletion also cannot explain the observed radiogenic Os signatures in the Kamchatka peridotites, as the time required for radiogenic ingrowth would be significantly greater than the age of the lithospheric terranes that make up the respective regions of Kamchatka. The radiogenic Os isotope signatures in the Kamchatka peridotites are instead attributed to metasomatism of the Kamchatka sub-arc mantle wedge by radiogenic slab-derived fluids and melts. The regional variations in Re–Os isotope signatures are consistent with previous petrographic and geochemical studies of the Kamchatka mantle xenoliths that reveal multistage metasomatic histories resulting from interaction of the mantle wedge with a variety of slab-derived fluids and melts, including silicic slab-melt metasomatism associated with subduction of relatively hot, young (∼15–25 Ma) oceanic crust in the northern arc front, hydrous slab-fluid metasomatism associated with subduction of colder, old (∼100 Ma) oceanic crust in the southern arc front, and carbonate-rich slab-melt metasomatism in the southern segment behind the arc front, where the slab is deeper. Positive correlations between 187Os/188Os, La/Sm, and Ru/Ir in Avachinsky harzburgites support a model in which high fO2, Cl-rich, hydrous slab fluids transport LREE, Ru, and radiogenic Os into the mantle wedge beneath the southern arc front. Re is either not transported, or is not retained in the mantle during fluid–mantle interaction. Relatively higher Re and more radiogenic Os (but low Os abundances) in the Valovayam and Bakening peridotites indicate that both scavenging of mantle Os as well as exchange with radiogenic slab-derived Os, and incorporation of Re, occurs during interaction of the mantle wedge with oxidized, adakitic, and carbonate-rich slab melts. Similar ranges of Re–Os isotope signatures in peridotites from Avachinsky, Japan and Lihir, and from Valovayam and the Cascades, respectively, suggest that the age (temperature) and depth of subducting oceanic crust influences the Re–Os composition of metasomatized sub-arc mantle.

Abundance and distribution of PGE and Au in the island-arc mantle: implications for sub-arc metasomatism

Ultramafic xenoliths from a veined mantle wedge beneath the Kamchatka arc have non-chondritic, fractionated chondrite-normalized platinum-group element (PGE) patterns. Depleted (e.g., low bulk-rock Al2O3 and CaO contents) mantle harzburgites show clear enrichment in the Pd group relative to the Ir group PGEs and, in most samples, Pt relative to Rh and Pd. These PGE signatures most likely reflect multi-stage melting which selectively concentrates Pt in Pt–Fe alloys while strongly depleting the sub-arc mantle wedge in incompatible elements. Elevated gold concentrations and enrichment of strongly incompatible enrichment (e.g., Ba and Th) in some harzburgites suggest a late-stage metasomatism by slab-derived, saline hydrous fluids. Positive Pt, Pd, and Au anomalies coupled with Ir depletions in heavily metasomatized pyroxenite xenoliths probably reflect the relative mobility of the Pd and Ir groups (especially Os) during sub-arc metasomatism which is consistent with Os systematics in arc mantle nodules. Positive correlations between Pt, Pd, and Au and various incompatible elements (Hf, U, Ta, and Sr) also suggest that both slab-derived hydrous fluids and siliceous melts were involved in the sub-arc mantle metasomatism beneath the Kamchatka arc.

Oxygen isotope signatures in olivines from São Miguel (Azores) basalts: implications for crustal and mantle processes

Oxygen isotope ratios were measured in olivines from eight Sao Miguel basalt lavas. With one exception (4.57‰), the olivines are indistinguishable from one another with an average δ18O of 4.92±0.03‰ (1σ). This value is slightly lower than that characteristic of upper mantle peridotite and MORB olivines (5.2±0.2‰). Assimilation of ≥10–20% of high-temperature altered lower oceanic crust or 4–9% hydrothermally altered volcanic edifice rocks could produce the low δ18O signatures in the Sao Miguel olivines; both of these assimilation models are permitted by the trace element and radiogenic isotope variations in the Sao Miguel basalts. However, the limited variation in δ18O despite eruption of the basalts through compositionally and tectonically variable lithosphere, and the lack of correlation of δ18O with olivine forsterite content, are more easily explained if the olivine δ18O signatures are inherited from their mantle source. If the δ18O signatures reflect mantle source compositions, then the relatively low and uniform δ18O signatures allow constraints to be placed on the origin of the mantle sources beneath Sao Miguel. Extreme variations in radiogenic isotope signatures have previously been attributed to two component source mixing between a predominant Azores plume source with mild HIMU-like characteristics, and an EMII-type mantle with very radiogenic Sr. The low δ18O signatures in the Sao Miguel basalt olivines suggest that the predominant Azores plume source contains >10% hydrothermally altered recycled oceanic crust. The limited variation in δ18O is consistent with a component of recycled sediment in the Sao Miguel EMII-type source, although, unlike the case for other EMII OIB (e.g. Samoa and Society), the relatively low δ18O signatures in Sao Miguel restrict any involvement of recycled sediment to <2% of a relatively low δ18O and very radiogenic Sr or high Rb/Sr sediment. Involvement of several percent metasomatized subcontinental lithospheric mantle could alternatively produce the EMII-type Sr–Nd–Pb isotope signatures without significantly affecting the plume-related low δ18O signatures. The Sao Miguel δ18O data are thus consistent with mixing between a low δ18O Azores plume source with a component of subducted, hydrothermally altered lower oceanic crust, and either minor recycled sediment or localized EMII-rich delaminated subcontinental lithospheric mantle. The latter could have been introduced into the lithosphere or shallow asthenosphere during opening of the Atlantic ocean basin.

Sources of ocean island basalts: A review of the osmium isotope evidence

The Os isotope system provides insight into the origin of mantle source heterogeneity which is complementary to that provided by the incompatible element isotope systems of Sr, Nd and Pb. The Os isotope system is both an extremely sensitive tracer of crustal contamination and the only isotope system which can clearly distinguish between contamination of magmas in the crust vs. contamination in the lithospheric mantle. Os isotopes therefore provide important constraints regarding whether basalts are recording plume or lithospheric signatures. Basalts inferred to be recording mantle plume signatures indicate that the plume sources contain an enriched component which is more radiogenic in Os than primitive upper mantle. While this might be attributed to recycled crustal material, the expected correlations between Os and Pb isotopic signatures are largely absent. An alternative possibility is that mantle plumes contain a component of lower mantle which is radiogenic in Os. The Os isotopic compositions of plumes may be further enriched in some cases by the addition of recycled crustal components which also produces distinctive signatures in the incompatible element isotope systems. HIMU basalts, which have both extremely radiogenic Os and Pb isotopic signatures, can be produced by the addition of 15–25% recycled oceanic crust to a plume source already slightly enriched in Os due to a radiogenic lower mantle component. The origins of EMI and EMII mantle sources are currently less well constrained by Os isotopic signatures, but might be attributed to recycling of oceanic crust plus pelagic sediment, and metasomatized subcontinental lithospheric mantle, respectively.

Earth science: Ancient mantle in a modern plume

Osmium isotopes record evidence for 2.5-billion-year-old mantle beneath the Azores. The origin of this ancient mantle has implications for the nature and timescale of mantle convection.

Intrinsic conditions of magma genesis at the Lunar Crater Volcanic Field (Nevada), and implications for internal plumbing and magma ascent

Abstract The northern part of the Lunar Crater Volcanic Field (central Nevada, U.S.A.) contains more than 100 Quaternary basaltic cones and maars and related eruptive products. We focused on four informal units of different ages and locations in the field to test the compositional variability and magma ascent processes within the time span of an individual eruption and the variability between very closely spaced volcanoes with different ages. Based in whole-rock chemistry, mineral chemistry and the calculation of intrinsic properties (pressure, temperature, and oxygen fugacity) we found that individual magma batches were generated in the asthenospheric mantle from a heterogeneous garnet lherzolite/olivine websterite source by ~3-5% partial melting. Each magma batch and temporary deep reservoir was a separate entity rather than part of a continuous long-lived reservoir. Magmas ascended relatively fast, stalled and crystallized in the uppermost several kilometers of the mantle near the base of the crust and some also stalled at mid-crustal levels with minor or no geochemical interaction with surrounding rocks. Our data also suggest that volcanoes erupting within certain time windows had similar source characteristics and ascent processes whether they were located within a few hundred meters of each other or were separated by many kilometers.

Lunar Crater volcanic field (Reveille and Pancake Ranges, Basin and Range Province, Nevada, USA)

The Lunar Crater volcanic field (LCVF) in central Nevada (USA) is dominated by monogenetic mafic volcanoes spanning the late Miocene to Pleistocene. There are as many as 161 volcanoes (there is some uncertainty due to erosion and burial of older centers); the volumes of individual eruptions were typically ∼0.1 km 3 and smaller. The volcanic field is underlain by a seismically slow asthenospheric domain that likely reflects compositional variability relative to surrounding material, such as relatively higher abundances of hydrous phases. Although we do not speculate about why the domain is in its current location, its presence likely explains the unusual location of the LCVF within the interior of the Basin and Range Province. Volcanism in the LCVF occurred in 4 magmatic episodes, based upon geochemistry and ages of 35 eruptive units: episode 1 between ca. 6 and 5 Ma, episode 2 from ca. 4.7 to 3 Ma, episode 3 between ca. 1.1 and 0.4 Ma, and episode 4, ca. 300 to 35 ka. Each successive episode shifted northward but partly overlapped the area of its predecessor. Compositions of the eruptive products include basalts, tephrites, basanites, and trachybasalts, with very minor volumes of trachyandesite and trachyte (episode 2 only). Geochemical and petrologic data indicate that magmas originated in asthenospheric mantle throughout the lifetime of the volcanic field, but that the products of the episodes were derived from unique source types and therefore reflect upper mantle compositional variability on spatial scales of tens of kilometers. All analyzed products of the volcanic field have characteristics consistent with small degrees of partial melting of ocean island basalt sources, with additional variability related to subduction-related enrichment processes in the mantle, including contributions from recycled ocean crust (HIMU source; high-µ, where µ = 238 U/ 204 Pb) and from hydrous fluids derived from subducted oceanic crust (enriched mantle, EM source). Geochemical evidence indicates subtle source heterogeneity at scales of hundreds of meters to kilometers within each episode-scale area of activity, and temporary ponding of magmas near the crust-mantle boundary. Episode 1 magmas may have assimilated Paleozoic carbonate rocks, but the other episodes had little if any chemical interaction with the crust. Thermodynamic modeling and the presence of amphibole support dissolved water contents to ∼5–7 wt% in some of the erupted magmas. The LCVF exhibits clustering in the form of overlapping and colocated monogenetic volcanoes that were separated by variable amounts of time to as much as several hundred thousand years, but without sustained crustal reservoirs between the episodes. The persistence of clusters through different episodes and their association with fault zones are consistent with shear-assisted mobilization of magmas ponded near the crust-mantle boundary, as crustal faults and underlying ductile deformation persist for hundreds of thousands of years or more (longer than individual episodes). Volcanoes were fed at depth by dikes that occur in en echelon sets and that preserve evidence of multiple pulses of magma. The dikes locally flared in the upper ∼10 m of the crust to form shallow conduits that fed eruptions. The most common volcanic landforms are scoria cones, agglomerate ramparts, and ‘a‘ā lava fields. Eruptive styles were dominantly Strombolian to Hawaiian; the latter produced tephra fallout blankets, along with effusive activity, although many lavas were likely clastogenic and associated with lava fountains. Eroded scoria cones reveal complex plumbing structures, including radial dikes that fed magma to bocas and lava flows on the cone flanks. Phreatomagmatic maar volcanoes compose a small percentage of the landform types. We are unable to identify any clear hydrologic or climatic drivers for the phreatomagmatic activity; this suggests that intrinsic factors such as magma flux played an important role. Eruptive styles and volumes appear to have been similar throughout the 6 m.y. history of the volcanic field and across all 4 magmatic episodes. The total volume and time-volume behavior of the LCVF cannot be precisely determined by surface observations due to erosion and burial by basin-fill sediments and subsequent eruptive products. However, previous estimates of a total volume of 100 km 3 are likely too high by a factor of ∼5, suggesting an average long-term eruptive flux of ∼3–5 km 3 /m.y.

Analysis of a sugar maple tree core for monitoring environmental uranium contamination

Concentrations and isotopic ratios of metals in tree rings can be employed to resolve temporal changes in contamination, but few studies have explored the behavior of uranium (U). This study measured U abundance and isotopic compositions of sugar maple (Acer saccharum) tree rings near the former Fernald Feed Materials Production Center (FFMPC), a U purification facility in Ohio. U concentrations, 235U/238U, and 234U/238U are generally consistent with known events in FFMPC history. Additionally, the outermost rings have compositions consistent with contemporary soil. These results suggest that sugar maple may be reliable for monitoring past and present environmental U contamination.

Petrology of the Azores Islands

This chapter presents an overview of the petrology of the Azores Archipelago, based on a review of the published literature on the mineralogy, petrology and major element geochemistry of the Azores islands and Formigas islets. In this chapter, we describe the mineralogy, petrology and major element chemistry of xenoliths/enclaves including peridotites, gabbros and syenites, and their roles in the magmatic evolution of the islands in which they occur. Where sufficient temporal data are available, we further describe the petrogenesis of the islands within a geochronological context. This synthesis and comparison of the petrology of each island is further combined with new modeling results for depth of melting and melt evolution paths to better understand the origin of the distinct petrologic characteristics of individual islands and the archipelago as a whole.

Uranium mobility across annual growth rings in three deciduous tree species

Black walnut (Juglans nigra), slippery elm (Ulmus rubra), and white ash (Fraxinus americana) trees were evaluated as potential archives of past uranium (U) contamination. Like other metals, U mobility in annual growth rings of trees is dependent on the tree species. Uranium concentrations and isotopic compositions (masses 234, 235, 236, and 238) were analyzed by thermal ionization mass spectrometry to test the efficacy of using tree rings to retroactively monitor U pollution from the FFMPC, a U purification facility operating from 1951 to 1989. This study found non-natural U (depleted U and detectable 236U) in growth rings of all three tree species that pre-dated the start of operations at FFMPC and compositional trends that did not correspond with known contamination events. Therefore, the annual growth rings of these tree species cannot be used to reliably monitor the chronology of U contamination.