High‐Ca boninites from the active Tonga Arc

Abstract

We report the first known occurrence of high‐Ca boninites within an active submarine island arc, at Volcano A within the Tonga Arc. Both the whole rock and a population of melt inclusions (in Fo86–92 olivines) from a dredged satellite cone have compositions classified as high‐Ca boninite. All samples from Volcano A, however, may be related to parental boninites, given the similarity in their rare earth element patterns and their coherency along a similar liquid line of descent. The primary high‐Ca boninite liquids were generated in the mantle wedge by high cumulative degrees of melting (>∼24%) at typical mantle wedge temperatures (<1300°C) driven by an influx of slab‐derived fluid (>4 wt % H2O in primary liquids). We propose a two‐stage model for generating primary boninite liquids at Volcano A: (1) melting of fertile peridotite within the Lau back‐arc basin, followed by (2) remelting of this residual peridotite with slab‐derived fluid beneath the Tonga Arc. The occurrence of high‐Ca boninites at Volcano A is related to the relative location and duration of back‐arc spreading. Here, the Eastern Lau Spreading Center has been processing mantle for ∼1 Ma, and corner flow circulation brings mantle from the back‐arc melting regime into the arc melting regime at a rate that is a significant fraction (>30%) of the convergence rate. On the basis of Si6.0 and Ti6.0 relationships, we argue that a significant portion of the central Tonga Arc near Volcano A, as well as several other arc volcanoes with active back‐arc basins, are also erupting basaltic andesites with boninite parentage.

Inter-element fractionation of highly siderophile elements in the Tonga Arc due to flux melting of a depleted source

“…Available data for the Tonga Arc indicate that sulphur and other volatiles have been strongly affected by syn-eruptive degassing. Arc whole-rocks contain <50 µg/g S, compared with up to 2000 µg/g S for olivine-hosted melt inclusions (Cooper et al, 2010). Hence, whole-rock S contents do not provide information on pre-eruptive magma evolution and sulphide precipitation and so additional S data have not been obtained.…”

“…Whole-rock sulphur concentrations unfortunately do not shed light on the likelihood of pre-eruptive sulphide undersaturation because volatile loss during degassing has depleted S in all arc lavas (Cooper et al, 2010) and probably many FSC samples. Some very high S contents have been found in melt inclusions from Tongan Arc magmas (~2000 µg/g; Cooper et al, 2010) but as S solubility increases with increasing oxygen fugacity (fO 2 ) due to the presence of SO 4 2-(e.g. Jugo, 2009), sulphide saturation is not necessarily attained at such concentrations.…”

“…Ewart and Hawkesworth, 1987;Hawkins, 1995) also provides the opportunity to investigate the effects of previous melt extraction on HSE behaviour during mantle melting. In places along the arc system, Si-and Mg-rich boninitic magmas -the only known present-day examples of boninitic volcanism on Earth (Cooper et al, 2010) -have been produced by melting of a highly-depleted, fluid-fluxed mantle source (Crawford et al, 1989). Here we report concentrations of highly siderophile elements (Os, Ir, Ru, Pt, Pd, Re) for boninitic, basaltic and basaltic andesite lavas from the central and northern sections of the Tonga Arc, the Fonualei Spreading Centre (FSC) and the Mangatolu Triple Junction (MTJ).…”

Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc

“…Major element concentrations were determined on the same acid digests used 154 for ICP-MS and silica content was calculated by sum deficit (see Wade et al, 2005 for details). Cooper et al (2010). 158 For radiogenic isotope analysis approximately 100 mg of powder was dissolved in a HF/HNO 3 /HCl 159 mixture followed by complete dissolution of the samples in 6M HCl.…”

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

“…It also seems unlikely that a mantle plume was involved in their petrogenesis (Falloon and Danyushevsky, 2000;Parman and Grove, 2004) since there is no evidence for this in the area. Furthermore, it is important to point out that the lack of a positive Zr spike in the Taishan SHMBs excludes anomalously high mantle temperatures as a primary cause for their petrogenesis, as would be expected if a plume was involved (Bédard, 1999;Falloon and Danyushevsky, 2000;Cooper et al, 2010).…”