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Author's Personal Copy Author's personal copy Chemical Geology 271 (2010) 70–85 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Chemical variations and regional diversity observed in MORB Ricardo Arevalo Jr. ⁎, William F. McDonough Department of Geology, University of Maryland, College Park, MD, 20742, USA article info abstract Article history: An assemblage of MORB analyses (n =792 samples), including a suite of new, high-precision LA-ICP-MS Received 14 April 2009 measurements (n =79), has been critically compiled in order to provide a window into the chemical Received in revised form 4 December 2009 composition of these mantle-derived materials and their respective source region(s), commonly referred to Accepted 17 December 2009 as the depleted MORB mantle (DMM). This comprehensive MORB data set, which includes both “normal- ” fi b “ ” ≥ Editor: D.B. Dingwell type (N-MORB, de ned by (La/Sm)N 1.00) and enriched-type samples (E-MORB, (La/Sm)N 1.00), defines a global MORB composition that is more enriched in incompatible elements than previous models. A Keywords: statistical evaluation of the true constancy of “canonical” trace element ratios using this data set reveals that MORB during MORB genesis Ti/Eu, Y/Ho and Ce/Pb remain constant at the 95% confidence-level; thus, the ratios DMM recorded in MORB (Ti/Eu=7060±1270, 2σ; Y/Ho=28.4±3.6, 2σ; Ce/Pb=22.2±9.7, 2σ)mayreflect the Basalt composition of the DMM, presuming the degree of source heterogeneity, component mixing and conditions Trace element of melting/crystallization of the DMM are adequately recorded by global MORB. Conversely, Ba/Th, Nb/U, Concentration ratio Zr/Hf, Nb/Ta, Sr/Nd, and Th/U are shown to fractionate as a function of MORB genesis, and thus these ratios do not faithfully record the composition of the DMM. Compared to samples from the Pacific and Indian Oceans, MORB derived from Atlantic ridge segments are characterized by statistically significant (≥95% confidence-level) enrichments in both highly incompatible elements (e.g., light REE, TITAN group elements, Sr, Ba, Pb, Th, and U) as well as less incompatible elements (e.g., heavy REE), indicating: i) a prominent recycled source component; ii) variable proportions of pyroxenite in the Atlantic source region; and/or, most likely iii) smaller degrees of melting and/or greater extents of fractional crystallization due to slower ridge spreading rates. Conversely, PacificMORBhasthe most depleted regional signatures with regard to highly incompatible elements (e.g., Ba, Pb, Th, and U), likely due to faster ridge spreading rates. Indian Ocean MORB exhibit limited variation in incompatible element enrichments/depletions but are generally the most depleted in more compatible elements (e.g., Ti, Cr, Sc, and heavy REE), potentially due to distinct source characteristics or deep source melting in the garnet field. Atlantic, Pacific and Indian MORB can also be distinguished by trace element ratios, particularly Ce/Pb and Th/U, which is distinct at the N99% confidence-level. Global MORB, and by inference the DMM, are characterized by enrichments in Y/Ho and depletions in Th/U relative to the chondritic ratios, and are complementary to the continental crust. However, the median of global MORB and the bulk continental crust both have sub-chondritic Ti/Eu and Nb/Ta ratios, suggesting an under-represented Ti- and Nb-rich reservoir in the Earth, potentially refractory, rutile-bearing eclogite at depth in the mantle. © 2009 Elsevier B.V. All rights reserved. 1. Introduction arc processes, and the thermal evolution of the planet. As opposed to major element compositions, which exhibit limited diversity and The chemical compositions of the individual components that primarily reflect the source lithology, trace element abundances span constitute the silicate Earth (SE), including the largely depleted source orders of magnitude and provide a perspective into mantle processes of mid-ocean ridge basalts (MORB), the enriched mantle domain(s) as well as source compositions because mantle phases incorporate frequently observed in ocean island basalts (OIB), and the incompat- and exclude trace elements with much greater selectivity than major ible element-rich continental crust, are important parameters for elements. Thus, trace elements hold the key to understanding the models of oceanic and continental crust generation, subduction and evolution of the SE and its major source reservoirs. crustal recycling, mantle source mixing, intraplate volcanism, island The chemical and isotopic compositions of mantle derivatives, such as MORB, OIB and island arc volcanics, reflect the compositions of their respective source regions but also the extent of mixing between dif- ⁎ Corresponding author. Tel.: +1 301 405 6248; fax: +1 301 405 3597. ferent source components, degree of melting and/or fractional crystal- E-mail address: [email protected] (R. Arevalo). lization (Stracke and Bourdon, 2009), in addition to the tectonic 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.12.013 Author's personal copy R. Arevalo Jr., W.F. McDonough / Chemical Geology 271 (2010) 70–85 71 environment, crustal contamination and potential post-eruptive alter- yses of both depleted and enriched MORB endmembers. Following the ation. Mid-ocean ridge basalts, which are predominantly tholeiitic in protocol established by Arevalo and McDonough (2008), external cali- composition (e.g., Engel et al., 1965; Melson et al., 1976), provide an bration techniques were implemented in order to maximize precision ideal case study of the modern mantle as these melts represent adia- and accuracy as well as account for any potential non-spectral matrix batic upwelling and decompression of the ambient upper mantle (e.g., effects; two USGS basaltic glasses (BIR-1G and BCR-2G; c.f., Jochum et al., McKenzie and O'Nions, 1991), and therefore these samples exemplify a 2005a)andfive MPI-DING silicate glasses (BM90/21-G, ML3B-G, StHs6/ relatively simple melting history. Additionally, the average global rate of 80-G, KL2-G, and T1-G; c.f., Jochum et al., 2000, 2005b), which together magma emplacement is between about 26 and 34 km3 yr−1, of which span between 100.5 and 103.1 orders of magnitude in concentration for 75% is generated by mid-ocean ridge volcanism (Crisp, 1984); thus, all elements measured here, were used as standard reference materials MORB samples are generally fresh (b1 Ma), abundant and accessible. for external calibration (see Supplemental materials). Spectral matrix The MORB source region, or depleted MORB mantle (DMM), is largely effects, particularly isobaric interferences from potential diatomic sol/liq depleted in incompatible trace elements (Di b1), those elements oxides, were limited by implementing a standard tuning procedure that preferentially partition into melt phases over residual solids, that maximized the elemental signal (based on 43Ca and 232Th spectra) relative to more compatible trace elements. The depleted composition and minimized oxide production (232Th16O/232Th≤0.15%). The typical of the DMM has long been identified as a complementary geochemical external reproducibility for our concentration measurements, which signature compared with the incompatible element enriched continen- include new data for Sc, Cr, Sr, Y, Zr, Nb, Ba, the rare-earth elements tal crust (e.g., Hofmann, 1988; Sun and McDonough, 1989), with several (REE; from La to Lu), Hf, Ta, and Pb were determined to be ≤3% (2σm) notable exceptions (e.g., Nb and Ta; McDonough, 1990). for four replicate analyses of each sample. The abundances of P, K, Ti, W, Multiple studies have attempted to constrain the chemical com- Th, and U were originally reported by Arevalo and McDonough (2008) position of the DMM, including models based on upper mantle melts and Arevalo et al. (2009). (i.e., MORB; Salters and Stracke, 2004) and residues (i.e., abyssal A comprehensive collection of trace element data for a more com- peridotites; Workman and Hart, 2005), as well as inferences based on plete global set of MORB was manually compiled from a number of cosmochemical arguments (i.e., super-chondritic 142Nd/144Nd ob- reliable references with established, high-precision and demonstrably served in terrestrial samples; Boyet and Carlson, 2006); however, the accurate methods of analysis (c.f., replicate measurements of standard absolute depletion of this mantle volume remains unsettled due to reference materials, such as BCR-1, BHVO-1 and VG-2) in order to difficulties in modeling the incompatible element budget of the DMM, complement the new MORB data presented here. The inclusive data set which is a consequence of the wide variance in incompatible ele- (n=792 samples), which encompasses a wide geographic distribu- ment concentrations observed in MORB and off-axis seamounts (e.g., tion (Fig. 1) and includes samples from the Atlantic (n=342), Pacific Zindler et al., 1984; Graham et al., 1988; Hofmann, 1988; Sun and (n=259) and Indian (n=191) Oceans, also consist of: i) LA-ICP-MS McDonough, 1989; Graham et al., 1996; Niu and Batiza, 1997; trace element measurements for a global spectrum of MORB reported by Hofmann, 2003), as well as upper mantle peridotites and xenoliths Sun et al. (2003, 2008); ii) the PetDB MORB data set from Salters and (e.g., Jagoutz et al., 1979; Nixon et al., 1981; McDonough and Frey, Stracke (2004), which was filtered to include only samples with b55 wt. 1989; McDonough and Sun, 1995; Niu, 2004). %SiO2, smooth REE patterns and eruption depths in excess of 2000 m; Trace element concentrations in MORB provide one way to iii) ICP-MS, isotope dilution (ID-) thermal ionization mass spectrometry constrain the composition of the DMM, as these mantle derivatives (TIMS) and ID-ICP-MS measurements of MORB samples from the Gorda generally represent mafic melts of their source. Highly incompatible Ridge (Davis et al., 2008)andon-axis(Sims et al., 2002; Hall et al., 2006) elements, though, commonly show skewed frequency distributions in and off-axis lavas from the East PacificRise(Sims et al., 2003); and, iv) terrestrial samples (Ahrens, 1954; McDonough, 1990) and thus pose a Indian MORB analyses from Mahoney et al.
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