Colin G. Macpherson

Oxygen isotope composition of mantle peridotite

Abstract The oxygen isotopic composition of 76 samples of olivine in spinel-, garnet- and diamond-facies peridotites, and as syngenetic inclusions within diamond has been determined by laser fluorination (LF). δ18OV-SMOW values of olivine are almost invariant, averaging 5.18 ± 0.28‰ (2 S.D.) with an overall range of 4.8–5.5‰. Coexisting clinopyroxenes display positive Δ18Ocpx-olivine fractionations averaging 0.4‰ that are consistent with isotopic equilibrium at normal mantle temperatures. The bulk compositions of spinel-, garnet- and diamond-facies mantle are similar, with a calculated bulk mantle δ18O of +5.5‰. Olivine inclusions in diamonds of Archaean age yield δ18O values that are indistinguishable from olivine in post-Archaean mantle and the LF data do not provide evidence for gross spatial or temporal δ18O variation in peridotitic mantle. There is no evidence that mantle hydration processes impart distinctive oxygen isotope characteristics, other than a subtle isotopic disequilibrium disturbing pyroxenes relative to olivine.

Amphibole “sponge” in arc crust?

Pressure-temperature-time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes, and hence the compositions of liquid fractionates. Here we use La/Yb and Dy/Yb versus SiO 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma. Production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid-lower crust, where amphibole is stable. Because amphibole is rarely a phenocryst phase, we term this “cryptic amphibole fractionation.” If this process is as widespread as our investigation suggests, then (1) amphibole cumulates may act as an effective filter for water dissolved in mantle-derived magmas; (2) amphibole cumulates may act as a fertile source of intracrustal melts and fluids; and (3) recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle.

Resolution of the effects of crustal assimilation, sediment subduction, and fluid transport in island arc magmas: Pb Sr Nd O isotope geochemistry of Grenada, Lesser Antilles

Abstract New lead, strontium, neodymium, and oxygen isotope and chemical data are reported for forty-three samples from the picrite to low-CaO andesite differentiation series (the M-series) of Grenada, Lesser Antilles island arc. Revised isotopic data for the low-Mg, high-Ca C-series basalts show tight correlations with MgO that confirm published interpretations of C-series evolution by coupled assimilation-fractional crystallisation (AFC). δ18O determined by laser fluorination on phenocryst augite increases from mantle values (+5.6) to the highest values observed in Grenada (+6.2) over the small observed range in 143Nd/144Nd, consistent with AFC. The most magnesian C-series basalts can be generated by about 16% olivine fractionation from picrites similar in major elements to M-series picrites, but with greater large ion lithophile element (LILE) enrichment and more MORB-like isotopic ratios. The strong enrichment in LILE relative to rare earth elements (REE), and in REEs relative to Nb and Zr, implies fluid transport of these elements from subducted altered ocean crust. This is consistent with constant 87Sr/86Sr around 0.7045, but lower 143Nd/144Nd and higher Δ207Pband Th/U than MORB require a small (ca. 0.2%) additional subducted sediment contribution. Some LILE (Pb, Rb) are not strongly enriched and Ce/Pb ratios are close to those of MORB. This may be due to release of a substantial part of the slab Pb at shallower depth than the zone of magma generation. Grenada high-silica andesites (>58% SiO2) lie on extensions of C-series AFC correlations, reflecting assimilation with amphibole-dominated fractionation from C-series parents. AFC does not lead to strongly radiogenic Pb in the andesites despite an inferred contaminant With 206Pb/204Pb > 20. This is because Pb is much more incompatible than Nd due to apatite and hornblende fractionation, resulting in progressively slower 206Pb/204Pb increase as magmatic Pb contents rise. M-series picrites and basic andesites display evidence for three-component mixing on Sr Nd isotope, Nd Pb isotope, and Pb/Nd ratio— 143Nd/144Nd diagrams. One endmember has isotopic compositions similar to the uncontaminated C-series lavas, although it has less LILE enrichment from subduction fluid. Two groups of M-series picrites are recognized with lower and higher La/Y; these have respectively higher (ca. 0.51290) and lower (ca. 0.51280) 143Nd/144Nd, but similar ranges in 87Sr/86Srand206Pb/204Pb. The sample with the most extreme low 143Nd/144Nd (0.51256) has the lowest 206Pb/204Pb in the M-series: all aspects of the chemistry and isotope systematics of this sample are consistent with generation from a mantle source with a relatively high contribution (ca. 2%) from subducted local seafloor sediment (206Pb/204Pb ≈ 19.2). A progressively greater contribution from this sediment can successfully explain the neodymium isotopic shift from the C-series through the low-La/Y M-series picrites to the high-La/Y M-series picrites. The extent of fluid modification of the mantle source, as monitored by LILE enrichment relative to LREE, broadly decreases as the subducted sediment component increases. Basic andesites of the M-series also display low- and high-La/Y groups, and can be generated from the picrites by amphibole-dominated fractional crystallisation. The highest 206Pb/204Pb,87Sr/86Sr, and Pb/Nd in Grenada are found in low-La/Y basic andesites and suggest that fractionation of low-La/Y picrite was accompanied by some 6% high-level assimilation of crust with high Sr/Ndand206Pb/204Pb > 20. This hypothesis is strongly supported by the presence of 18O-enriched quartz xenocrysts of crustal origin in two of the three basic andesites. Smaller amounts (2–5%) of the same crustal component can satisfactorily model the variation of Pb/Nd,206Pb/204Pb,and87Sr/86Sr at constant 143Nd/144Nd in the picrites, an interpretation supported by correlations between isotope ratios and MgO in the high-La/Y picrites. AFC relationships in the C-series can be modelled using a similar crustal assimilant, but it must have much lower Pb/Nd ratios. The C-series assimilant cannot be M-series magma products, since M-series Pb/Nd ratios are too high, and M-series magmas themselves only achieve high 206Pb/204Pb through crustal assimilation. Lead isotope compositions of Grenada magmas, even picrites, are largely controlled by high-level crustal assimilation, mostly through AFC processes. Since these magmas include some of the most magnesian lavas found in arcs worldwide, we recommend caution in interpreting the lead isotope chemistry of arc magmas elsewhere to be the result of mantle and subduction processes.

Tectonic setting of Eocene boninite magmatism in the Izu-Bonin-Mariana forearc.

Abstract Middle Eocene boninites were simultaneously generated over a large region during the early history of the Izu–Bonin–Mariana (IBM) arc. However, widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions. This suggests that an additional tectonic or thermal factor influenced the generation of the IBM boninite suite. Regional uplift, ocean island basalt-style magmatism and high heat-flow also characterised the northern Philippine Sea plate (PSP) at the start of the Middle Eocene. These features are similar to those observed in large volume basaltic volcanic provinces such as the early Tertiary North Atlantic and suggest that the IBM boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the PSP. The reconstructed Middle Eocene location of the IBM arc and West Philippine Basin lies close to the present day Manus Basin where petrological and geochemical evidence indicate the presence of a mantle plume. A calculated plume track linking these locations through time also passes close to the Eauripik Rise, an aseismic ridge on the Caroline Plate, during the Oligocene and Miocene. Therefore, we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western Pacific from the Middle Eocene until the present day.

High-precision oxygen isotope microanalysis of ferromagnesian minerals by laser-fluorination

A laser-assisted fluorination (LF) technique, modified from the previously published Sharp method, is described for the determination of 18O16O in microgram quantities of ferromagnesian silicate minerals. The system employs a 60-W continuous Nd: YAG laser operating at 1064 nm. Samples are reacted with CIF3 in a 16-position nickel sample holder within a stainless-steel chamber fitted with a sapphire window. The LF system is on-line to the mass spectrometer microinlet. Procedural blanks are almost negligible, typically <0.02 μmol O2, permitting analysis of samples weighing <20 μg. Measured oxygen isotope ratios are very sensitive to oxygen yield. Low oxygen yields are associated with the fluorination reaction and, for the analysis of very small samples, the O2CO2 conversion step. Fluorination yields are related to laser power, beam diameter, quantity of CIF3 admitted to the sample chamber and beam rastering speed. Optimisation of these conditions allows 100 ± 2% primary yields to be obtained for minerals with different fusion temperatures and melt properties. δ18O data for analyses having fluorination yields greater than 95% are reproducible to better than ±0.1‰ (1σ). NBS-30 biotite (quoted δ18O-value + 5.10‰) independently calibrated against NBS-19, gives a mean value of +5.03‰ by LF with an overall reproducibility of ±0.08‰ (1σ, n = 82). Analyses of very small samples weighing less than ∼ 750 μg give δ18O-values that are systematically lower than expected. Replicate single-grain analyses show systematic relationships between sample weight and apparent oxygen yield, with concomitant isotopic fractionation of the δ13C and δ18O of CO2 generated during conversion over hot graphite. Lower oxygen yields, and low δ13C- and δ18O-values are an artifact of the Royal Holloway system caused by enhanced CO production in the graphite reactor at low oxygen pressures. Using pure oxygen gas, a detailed study of the oxygen-carbon dioxide conversion reaction at low oxygen pressures reveals a linear relationship between δ13C and δ18O that is largely independent of reactor operating conditions. The measured δ13C-value of CO2 converted from oxygen released from small (20–750 μg) silicate samples is used to calculate the true δ18O-value of the silicate, corrected for CO production, from a calibration obtained using pure oxygen gas. This procedure permits δ18O analysis of samples weighing as little as 20 μg to better than ±0.2‰.

Pyroxenite-rich mantle formed by recycled oceanic lithosphere: Oxygen-osmium isotope evidence from Canary Island lavas

Plate tectonic processes result in recycling of crust and lithosphere into Earths mantle. Evidence for long-term preservation of recycled reservoirs in the mantle comes from the enriched isotopic character of oceanic island basalt (OIB) lavas. Although recycled constituents can explain much of the geochemical variation in the OIB-source mantle, it has been shown that direct melting of these components would lead to magmas with evolved compositions, unlike OIB. Instead, it has been argued that either metasomatic pyroxene-rich peridotite that has inherited the trace element and isotopic character of subducted materials, or high-temperature intramantle metasomatism of lithosphere can explain OIB compositions. To test these models, we present new oxygen and osmium isotope data for lavas from the Canary Islands of El Hierro and La Palma. These islands have distinct 18O/16O and 187Os/188Os compositions that can be explained through melting of pyroxenite-enriched peridotite mantle containing <10% recycled oceanic lithosphere. We also assess O-Os isotope systematics of lavas from Hawai‘i and the Azores and show that they also conform to addition of distinct recycled oceanic components, including lithosphere and pelagic sediment. We conclude that enriched isotopic signatures of some OIBs are consistent with pyroxenite-rich mantle sources metasomatized by recycled components.

Lapita migrants in The Pacific's oldest cemetery: isotopic analysis at Teouma, Vanuatu

Teouma, an archaeological site on Efate Island, Vanuatu, features the earliest cemetery yet discovered of the colonizers of Remote Oceania, from the late second millennium B.C. In order to investigate potential migration of seventeen human individuals, we measured isotopes of strontium (87Sr/86Sr), oxygen (δ18O), and carbon (δ13C), as well as barium (Ba) and strontium (Sr) concentrations, in tooth enamel from skeletons excavated in the first two field seasons. The majority of individuals cluster with similar isotope and Ba/Sr ratios, consistent with a diet of marine resources supplemented with plants grown on the local basaltic soils. Four outliers, with distinctive 87Sr/86Sr and δ18O, are probably immigrants, three of which were buried in a distinctive position (supine, with the head to the south) with higher Ba/Sr and δ13C, consistent with a terrestrial, nonlocal diet. Among the probable immigrants was a male buried with the crania of three of the locally raised individuals on his chest.

Shifting gender relations at Khok Phanom Di, Thailand : isotopic evidence from the skeletons.

The values for isotopes of strontium, carbon, and oxygen in human tooth enamel from the prehistoric site of Khok Phanom Di (ca. 2100–1500 BC) in Thailand shed light on human mobility and marital residence during a crucial period of subsistence change. Khok Phanom Di was a sedentary coastal community that apparently relied on hunting, gathering, and fishing in the midst of a transition to rice agriculture in the interior. The results of the isotope analyses indicate female immigration and then a marked shift to local strontium isotope signatures among females accompanied by a clear increase in the prestige of female burials. A possible explanation is a shift in the pattern of exogamy with a concomitant change in gender relations. Observation of a very similar transition at Ban Chiang, in northeastern Thailand, suggests the possibility of a regionwide social transition. In the case of Khok Phanom Di, the increasing role of females in producing high‐quality ceramic vessels may have contributed to the change.

Evidence for an 18O-depleted mantle plume from contrasting 18O/16O ratios of back-arc lavas from the Manus Basin and Mariana Trough

Abstract Back-arc basin glasses from the Mariana Trough and Manus Basin display contrasting oxygen isotope characteristics that require differences in their mantle sources. In both basins, the lavas that are most depleted in high field strength elements possess δ18O values of around 6.0‰, that are elevated with respect to mid-ocean ridge basalt (MORB). This characteristic is consistent with a mantle source that has been infiltrated by fluids released from subducted oceanic lithosphere. The nature of the more fertile mantle component differs between the two basins. The lowest δ18O values in the Mariana Trough are similar to MORB and suggest that the ambient upper mantle interacts with a subduction-modified mantle to produce Mariana Trough back-arc basin basalts. Oxygen isotope ratios of basaltic glasses from the Manus Basin display a negative correlation with helium isotope ratios. The subduction-modified component is associated with 3He/4He ratios typical of the upper mantle. Glasses with 3He/4He ratios greater than average MORB, characteristic of a deep mantle plume, have δ18O values that are lower than expected for upper mantle melts. This suggests that the Manus Basin plume taps a reservoir that is 18O-depleted relative to the source of MORB. Two mechanisms are identified that might generate this reservoir. Deep recycling of oceanic crust that has been hydrothermally altered at high temperature may provide large 18O-depleted domains in the deep mantle. Alternatively, we propose that interactions between silicate and iron alloy during the segregation of the Earth’s core may have the potential to generate such reservoirs. Resolution between these mechanisms requires further experimental investigation of oxygen partitioning between silicates and iron alloy. Each of these mechanisms has distinct implications for the origins and dynamics of the Manus Basin plume.

Absence of a high time-integrated 3He/(U+Th) source in the mantle beneath continents

Volcanic rocks from ocean island and continental flood basalt provinces can exhibit 3He/4He ratios greatly in excess of those of mid-oceanic-ridge basalts (MORB). High 3He/4He ratios must indicate derivation from a mantle source with high time-integrated 3He/(U+Th) relative to depleted MORB-source mantle. The location of the high 3He/4He mantle reservoir is a poorly resolved but important issue because of the constraints it places upon the structure and convective style of Earths mantle. It has been proposed that the high 3He/4He reservoir resides in the upper mantle, rather than the lower mantle, because Earth should be volatile poor and highly differentiated, with incompatible elements (such as He) concentrated in the upper mantle and crust. This hypothesis can be tested using continental intraplate alkaline volcanics (CIAV) that are generated at or near the boundary between the conducting lithospheric and convecting asthenospheric mantle. Olivine and clinopyroxene phenocrysts from Cretaceous to Miocene CIAV from Canada, South Africa, and Uganda have 3He/4He ratios more radiogenic than MORB, strongly arguing against a widespread high 3He/4He source in the continental lithosphere or the underlying convecting upper mantle. Combined with a global data set of CIAV and continental lithosphere mantle xenoliths, these results provide no evidence for high 3He/4He in any samples known to originate from this environment. Therefore, volcanic rocks with 3He/4He greater than MORB 3He/4He are likely to sample a mantle source with high time-integrated 3He/(U+Th) that cannot exist within or below the continents. This reservoir is also unlikely to exist within the upper mantle as defined by the 3He/4He distribution in MORB.