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2013
Day, JMD.  2013.  Hotspot volcanism and highly siderophile elements. Chemical Geology. 341:50-74.   10.1016/j.chemgeo.2012.12.010   AbstractWebsite

Hotspot volcanic rocks are formed under conditions that differ from conventional plate tectonic boundary magmatic processes and are compositionally distinct from mid-oceanic ridge basalts. Hotspot volcanic rocks include - but are not limited to - ocean island basalts (OIB), continental flood basalts (CFB), komatiites, oceanic plateau and some intraplate alkaline volcanic rocks. Studies of the highly siderophile element (HSE) geochemistry of hotspot volcanic rocks have provided new perspectives into mantle convection, mantle heterogeneity, core-mantle interactions, crustal and mantle lithospheric recycling, melting processes and crust-mantle interactions. The HSE, comprising Os, Ir, Ru, Rh, Pt, Pd, Re and Au, have strong affinities for metal and sulphide relative to silicate. These elements also have variable partitioning behaviour between highly compatible Os, Ir, Ru and Rh relative to compatible Pt and Pd and to moderately incompatible Re and Au during melting and crysta! The HSE can be utilised to understand sub-aerial volcanic degassing and crustal assimilation processes in hotspot volcanic rocks such as CFB and OIB, as well as for quantitative assessment of fractional crystallisation. Mantle melting studies have highlighted the strong control of sulphide in the mantle prior to exhaustion of S and generation of Os Ir Ru metal alloys at similar to>25% partial melting; a behaviour of the HSE that is fundamental to understanding terrestrial hotspot volcanism. Perhaps the most exciting utility of the HSE, however, lies in their ability to reveal both short- and long-term fractionation processes acting on hotspot volcanic sources from inter-element HSE fractionations and Os-187/Os-188-Os-186/Os-188 systematics. The growing database for HSE abundances and Os-187/Os-188 in hotspot volcanic rocks is consistent with their generation from a heterogeneous upper mantle generated by melt differentiation and recycling of crust and mantle lithosphere d! The HSE provide geochemical evidence for how lithological and chemical heterogeneities are sampled within the mantle. Modeling of HSE abundances and Os isotopes show that large apparent recycled contributions (50% to 90%) in some OIB can be explained by the preferential melting of volumetrically minor (<10%) pyroxenite in their sources. Preferential melting of more fusible materials in the mantle also explains why low-degree partial melts, such as alkali basalts and basanites, may exhibit more extreme isotopic variations than tholeiites or komatiites, which likely contain a higher contribution from peridotite in a hybridised pyroxenite-peridotite mantle source. High-precision Os-188/Os-188 data for hotspot volcanism are limited, but the combined variations in long-term Re/Os and Pt/Os retained in some mantle sources may reflect either the long-term fractionation of Re and Pt from Os between the inner and outer core, or ancient sulphide segregation and lithological variati! Study of the HSE in hotspot volcanic rocks from Solar System bodies also informs on planetary-scale processes, indicating that Earth, the Moon, Mars and fully differentiated asteroids all have HSE abundances in their mantles that are higher than expected from low-pressure metal-silicate partitioning. Furthermore, the HSE are in broadly chondritic-relative abundances for these planetary bodies, at similar to 0.0002 (Moon), to similar to 0.007 (Mars), to similar to 0.009 (Earth) x carbonaceous chondrite Ivuna (CI) composition. The timing of addition of the HSE to planetary bodies preserved in their magmas and volcanic products is consistent with Solar-System-wide late accretion no later than 3.8 Ga for Earth, and even earlier based on evidence from the Moon (similar to 4.4 Ga), Mars (similar to 4.5 Ga) and asteroids (>4.56 Ga). (C) 2013 Elsevier B.V. All rights reserved.DELMON.A, 1972, AMERICAN JOURNAL OF SCIENCE, V272, P805

2011
Day, JMD, Hilton DR.  2011.  Origin of (3)He/(4)He ratios in HIMU-type basalts constrained from Canary Island lavas. Earth and Planetary Science Letters. 305:226-234.   10.1016/j.epsl.2011.03.006   AbstractWebsite

New helium isotope and abundance measurements are reported for olivine and clinopyroxene phenocrysts from HIMU-type (high-mu=elevated (238)U/(204)Pb) lavas and xenoliths spanning the stratigraphies of El Hierro and La Palma, Canary Islands. Some pyroxene phenocrysts have suffered post-eruptive modification, either by less than 1% assimilation of crustal-derived He, or by closed-system ageing of He. Olivine phenocrysts record mantle source (3)He/(4)He compositions, with the average (3)He/(4)He for La Palma olivine (7.6 +/- 0.8R(A), where R(A) is the atmospheric (3)He/(4)He ratio of 1.38 x 10(-6)) being within uncertainty of those for El Hierro (7.7 +/- 0.3R(A)), and the canonical mid-ocean ridge basalt range (MORB: 8 +/- 1R(A)). The new helium isotope data for El Hierro and La Palma show no distinct correlations with whole-rock (87)Sr/(86)Sr, (143)Nd/(144)Nd, (187)Os/(188)Os, or Pb isotopes, but (3)He/(4)He ratios for La Palma lavas correlate with (18)O/(16)O measured for the same phenocryst populations. Despite limited (3)He/(4)He variations for El Hierro and La Palma, their He-O isotope systematics are consistent with derivation from mantle sources containing distinct recycled oceanic basaltic crust (El Hierro) and gabbroic lithosphere (La Palma) components that have mixed with depleted mantle, and a high-(3)He/(4)He component (>9.7R(A)) in the case of La Palma. The new data are consistent with models involving generation of compositionally and lithologically (e.g., pyroxenite, eclogite, peridotite) heterogeneous mantle sources containing recycled oceanic crust and lithosphere entrained within upwelling high-(3)He/(4)He mantle that has been severely diluted by interaction with depleted mantle. We propose that the noble gas systematics of HIMU-type lavas and ocean island basalts (OIB) in general, are most simply interpreted as being controlled by the most gas-rich reservoir involved in mixing to generate their mantle sources. In this scenario, HIMU and enriched mantle (EM) sources are dominated by depleted mantle, or high-(3)He/(4)He mantle, because recycled crust and lithosphere have low He concentrations. Consequently, high-(3)He/(4)He OIB would predominantly reflect derivation from a less depleted mantle source with sub-equal to higher He contents than depleted mantle. The available coupled He-O isotope systematics measured for OIB lavas are consistent with this hypothesis. (C) 2011 Elsevier B.V. All rights reserved.

2010
Day, JMD, Pearson DG, Macpherson CG, Lowry D, Carracedo JC.  2010.  Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands. Geochimica Et Cosmochimica Acta. 74:6565-6589.   10.1016/j.gca.2010.08.021   AbstractWebsite

Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (delta(18)O(olivine) = 5.17 +/- 0.08 parts per thousand; (87)Sr/(86)Sr = 0.7029 to 0.7031; epsilon(Nd) = +5.7 to +7.1; (187)Os/(188)Os = 0.1481 to 0.1750; (206)Pb/(204)Pb = 19.1 to 19.7; Pd/Ir = 6 +/- 3) versus those from La Palma (delta(18)Oo(livine) = 4.87 +/- 0.18 parts per thousand; (87)Sr/(86)Sr = 0.7031 to 0.7032; epsilon(Nd) = +5.0 to +6.4; (187)Os/(188)Os = 0.1421 to 0.1460; (206)Pb/(204)Pb = 19.5 to 20.2; Pd/Ir = 11 +/- 4) are revealed from the dataset. Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and USE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIM U-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (>= 110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 +/- 0.05 Os, 0.17 +/- 0.16 Ir, 0.34 +/- 0.32 Ru, 2.6 +/- 2.5 Pt, 1.4 +/- 1.2 Pd, 0.39 +/- 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai'ian tholeiites. The HIMU affinities of the lavas, in conjunction with the low delta(18)O(olivine) and high (206)Pb/(204)Pb for La Palma, and elevated (187)Os/(188)Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic (206)Pb/(204), (187)Os/(188)Os, elevated Re/Os and Pt/Os, and low-delta(18)O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (similar to 50 km). (C) 2010 Elsevier Ltd. All rights reserved.