Submarine back‐arc lava with arc signature: Fonualei Spreading Center, northeast Lau Basin, Tonga

Abstract

[1] We present major, volatile, and trace elements for quenched glasses from the Fonualei Spreading Center, a nascent spreading system situated very close to the Tofua Volcanic Arc (20 km at the closest), in the northeast Lau Basin. The glasses are basalts and basaltic andesites and are inferred to have originated from a relatively hot and depleted mantle wedge. The Fonualei Spreading Center shows island arc basalt (IAB) affinities, indistinguishable from the Tofua Arc. Within the Fonualei Spreading Center no geochemical trends can be seen with depth to the slab and/or distance to the arc, despite a difference in depth to the slab of >50 km. Therefore we infer that all the subduction-related magmatism is captured by the back arc as the adjacent arc is shut off. There is a sharp contrast between the main spreading area of the Fonualei Spreading Center (FSC) and its northernmost termination, the Mangatolu Triple Junction (MTJ). The MTJ samples are characteristic back-arc basin basalts (BABB). We propose that the MTJ and FSC have different mantle sources, reflecting different mantle origins and/or different melting processes. We also document a decrease in mantle depletion from the south of the FSC to the MTJ, which is the opposite to what has been documented for the rest of the Lau Basin where depletion generally increases from south to north. We attribute this reverse trend to the influx of less depleted mantle through the tear between the Australian and the Pacific plates, at the northern boundary of the Lau Basin.

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

“…Lau Basin; Pearce et al, 1994;Macpherson and Mattey, 1998). The similarity of arc and FSC magmas is proposed to result from the proximity of the FSC to the arc, and therefore the similar depth-to-the-slab in the region (Keller et al, 2008). In addition, magmatism in the arc at the same latitude as the FSC appears to have ceased, with the melts instead refocused into the FSC.…”

“…As the slab flux is likely to decrease as slab depth increases, due to slab dehydration at shallower levels, the weaker subduction signature is consistent with a greater depth-to-the-slab at the point of melt generation in the MTJ region (~210km, FSC: 130-180km, Keller et al, 2008).…”

“…Boninites are Si-and Mg-rich arc rocks generated by melting of highly-depleted, fluidmetasomatised mantle (Crawford et al, 1989). Keller et al (2008) includes a thorough characterisation of the FSC and concludes that these lavas possess an equivalent, but somewhat reduced, elemental subduction signature to the Tonga Arc sampled immediately to the south. This is unlike most BABB, including the MTJ to the north, which tend to have largely MORB-like major element characteristics, while still exhibiting the effects of a subduction-related flux in the form, for example, of enrichments in LILE (e.g.…”

“…MTJ lavas are more Na-, Ti-and Fe-rich than FSC samples, but contain less K (Sun et al, 2003b;Keller et al, 2008). In terms of mobile and conservative trace elements, the MTJ samples possess a distinct subduction signature with somewhat elevated large-ion lithophile element (LILE)/LREE and LILE/HFSE (high field-strength element) ratios (e.g.…”

Hydrothermal activity in the Northwest Lau Backarc Basin: Evidence from water column measurements

“…This combination of effects is thought to produce elevated mantle temperatures and vigorous mantle flow patterns in the Lau Basin. The rapid extension and elevated temperatures may result in enhanced magmatic and hydrothermal activity in the Lau Basin [German et al, 2006;Keller et al, 2008;Embley et al, 2009;Kim et al, 2009;Baker et al, 2011;Resing et al, 2011]. In this paper we examine whether the rapid extension accommodated by the NWLSC is expressed by elevated magmatic activity as identified by contemporaneous hydrothermal activity.…”