Halogen systematics (Cl, Br, I) in Mid-Ocean Ridge Basalts: A Macquarie Island case study
Introduction
Primitive oceanic basalts provide a means of sampling chemical heterogeneities in the Earth’s mantle (Hofmann, 2003). The absolute and relative elemental or isotopic composition of halogens in oceanic basalts is therefore of great interest. Halogens are concentrated in the Earth’s surface reservoirs of seawater and sediment (Pyle and Mather, 2009). As a result, constraining the elemental or isotopic composition of halogens in the Earth’s mantle could help demonstrate the extent to which surface volatiles (including halogens, water and noble gases) can be recycled to the mantle in subduction zones (cf. Bonifacie et al., 2008, John et al., 2010, Sumino et al., 2010, Kendrick et al., 2011).
Existing data show that basaltic melts have extremely variable Cl concentrations (∼1–4000 ppm; Michael and Cornell, 1998, Kent et al., 2002, Saal et al., 2002). Mantle source enrichment accounts for only a small part of the total variation; however, some of the highest Cl concentrations have been reported for Island-arc and back arc basin basalts (cf. Michael and Cornell, 1998, Kent et al., 2002, Wysoczanski et al., 2006, Sun et al., 2007). Several recent studies have shown Cl isotopes exhibit limited variation in the mantle (cf. Sharp et al., 2007, Bonifacie et al., 2008, Layne et al., 2009, John et al., 2010). However, there is still very little data available for the heavy halogens Br and I (Schilling et al., 1978, Schilling et al., 1980, Deruelle et al., 1992, Jambon et al., 1995). Furthermore, high precision simultaneous-measurements of I with Cl and Br have not been reported previously for suites of related basalt melts.
The strong fractionation of halogens observed during the dehydration of subducting slabs (Kendrick et al., 2011), implies that Br/Cl and I/Cl ratios could exhibit systematic variation between basalt melts formed in different tectonic settings. However, before the Br/Cl and I/Cl ratios of basalt melts can be linked to subduction processes, it is necessary to better document the halogen composition of MORB, investigate whether halogen abundance ratios exhibit detectable fractionation during melt evolution, and provide a means to test if melts have assimilated seawater, brine or altered oceanic crust (e.g. Michael and Schilling, 1989, Michael and Cornell, 1998, Kent et al., 2002).
In this contribution, we report Cl, Br and I data for a well documented suite of enriched Mid-Ocean Ridge Basalt (E-MORB) glasses from Macquarie Island in the southwest Pacific (Fig. 1; Varne et al., 1969, Varne et al., 2000, Kamenetsky et al., 2000, Kamenetsky and Maas, 2002, Kamenetsky and Eggins, 2011). The Macquarie Island glasses have Sr, Nd and Pb isotope signatures that are fairly typical of Pacific MORB, but show strongly variable enrichment in trace elements (Kamenetsky et al., 2000, Kamenetsky and Maas, 2002). The trace element signatures of these melts have been attributed to variable degrees of partial melting and fractional crystallisation (Kamenetsky et al., 2000, Kamenetsky and Maas, 2002). Measurement of Cl, Br and I in these melts therefore enables systematic evaluation of the extent to which partial melting and fractional crystallisation can influence halogen abundance ratios (cf. Schilling et al., 1980).
Finally, the halogen measurements were made using the noble gas method (40Ar–39Ar methodology; Kendrick, 2012), meaning they are obtained simultaneously with 40Ar–39Ar ages and 40Ar/36Ar ratio measurements. Additional helium isotope analyses were undertaken on three of the samples. These data enable us to contrast the behaviour of halogens with noble gases, thereby providing additional constraints on the relative degassing behaviours of two different groups of volatile element.
Section snippets
Macquarie island basalts
The 37 km long and 3–5 km wide Macquarie Island lies at a latitude of 54 °S on the seismically active Macquarie Ridge slow spreading centre which extends for 1200 km from SSW New Zealand (47 °S) to about 57 °S in the Southwest Pacific (Fig. 1). There is general agreement that the basalts formed along this spreading centre in the Miocene (e.g. Varne et al., 1969, Varne et al., 2000, Duncan and Varne, 1988, Kamenetsky et al., 2000, Portner et al., 2010). Previous age constraints include 40Ar–39Ar
Sampling and methodology
Seventeen samples were selected to encompass the full range of MgO and La/Sm compositions previously reported for Macquarie Island basalts (Table 1; e.g. Fig. 2, Fig. 3; Kamenetsky et al., 2000). Chips of fresh basalt glass with maximum dimensions of ∼0.3–1.0 mm were handpicked under a binocular microscope. The high purity glass separates were then washed in distilled water and acetone using an ultrasonic bath and the cleaned separates were re-checked under the binocular microscope. Gram-sized
Results and discussion
The full halogen and 40Ar–39Ar dataset for 17 irradiated samples, and the noble gas data for three non-irradiated samples (47979, GG256 and 47963), are available in the Electronic supplement. The halogen and 40Ar/39Ar data are summarised in Table 1 and representative stepped heating data are presented in Table 2.
Halogens in the MORB mantle
The trace element enrichment of E-MORB magmas is sometimes attributed to the presence of primitive or recycled material (e.g. Arevalo and McDonough, 2010 and references therein). However, Mid-Ocean Ridge Basalts exhibit continuous variation in La/Sm, suggesting that the distinction between N-MORB and E-MORB is somewhat arbitrary (Hofmann, 2003). In the case of the Macquarie Island E-MORB glasses, the presence of primitive or recycled materials is not supported by the current results: for
Summary
Enriched MORB glasses from Macquarie Island have K/Cl, Br/Cl and I/Cl ratios that appear fairly representative of the modern MORB mantle. The new data show that K/Cl, Br/Cl and I/Cl ratios are largely unfractionated during generation of basalt melts by different degrees of partial melting and crystallisation. Furthermore, the halogens are not fractionated by extensive degassing of CO2 and noble gases. Combined Br/Cl, I/Cl and K/Cl analyses can be used to help assess whether seawater
Acknowledgements
Stanislav Szczepanski is thanked for technical assistance in the Melbourne University Noble Gas Laboratory. Maya Kamenetsky is gratefully acknowledged for help with sample preparation. Dr. M.A. Kendrick is the recipient of an Australian Research Council QEII Fellowship (Project No. DP0879451). Dr. V.S. Kamenetsky is the recipient of an Australian Research Council Professorial Fellowship (Project No. DP0555984) and the University of Tasmania “New Star” Proffessorship. Bill McDonough and Ray
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