HELIUM AND ARGON ISOTOPE SYSTEMATICS OF THE CENTRAL LAU BASIN AND VALU FA RIDGE - EVIDENCE OF CRUST MANTLE INTERACTIONS IN A BACK-ARC BASIN

We report helium and argon isotope analyses of fresh Lau Basin volcani c glasses from six sites within the central basin (approximately 18-de grees-S), six localities along the Valu Fa Ridge (21-degrees-22.4-degr ees-S), and a single site from the northeastern basin (approximately 1 5-degrees-S). Central basin basalts have He-3/He-4 ratios (R) between 8.2-8.5 R(A) (R(A) = air He-3/He-4), Ar-40/Ar-36 ratios significantly greater than atmosphere (up to 4900), and He-4 contents from 3.5-9.4 X 10(-6) cm3 STP/g, similar to N-MORBs worldwide. These results are con sistent with trace element and other (radiogenic) isotope data on thes e samples which indicate derivation from a depleted mantle source regi on. In contrast, evolved lavas from the Valu Fa Ridge all have R/R(A) < MORB and show evidence of mixing with a component rich in radiogenic helium. There is a clear relationship between the He-3/He-4 ratios of these samples and their chemistry: basaltic andesites have 6.02 < R(C )/R(A) < 7.65, andesites have lower ratios (2.37-2.65 R(A)), and a dac ite has the lowest value of the entire sample suite (1.19 R(A)). All l avas have Ar-40/Ar-36 ratios similar to the atmospheric value and low helium concentrations, from 3-11 x 10(-8) cm3 STP/g, or between 30 and 300 times less than the central basin basalts. Although the helium is otope results of the Valu Fa lavas mirror the shift to more radiogenic values of other isotope systems (e.g., Pb-206/Pb-204, Sr-87/Sr-86) wh ich indicate addition of subducted sediment to these magma sources, we find no evidence that the radiogenic helium has a mantle or slab deri vation or is in any way coupled to these other tracers. Instead, the m ost plausible mechanism to explain its incorporation into the Valu Fa lavas is by assimilation of old Lau crust in the near-surface environm ent by previously degassed magma. We argue that this mechanism has gen eral applicability and can explain a number of hitherto apparently par adoxical geochemical features of some back-arc and ocean ridge lavas s uch as their high volatile and LIL element contents with low rare gas concentrations, and their mantle He-3/He-4 ratios with (hydrated ocean ic) crustal D/H values. The realisation that the helium and argon syst ematics of the Valu Fa lavas are controlled by crust/mantle interactio ns has important implications for distinguishing between a number of m odels proposed for the formation of such evolved lavas, and we show th at fractional crystallisation processes can most readily account for t he low concentrations of, and systematic trends in, the mantle-derived helium and argon component of these lavas. In addition, because pre-e xisting crust in the Lau Basin must be old and/or altered enough to su pply the radiogenic helium and atmospheric-like Ar-40/Ar-36 component to the Valu Fa lavas, the occurrence of crust/mantle interactions impl ies that old (forearc) crust may have been trapped within the Lau Basi n: such a scenario has a clear bearing on ideas of the tectonic develo pment of the basin. Finally, because of the potential of crust/mantle interactions to modify He-3/He-4 and Ar-40/Ar-36 ratios of mantle-deri ved melts, we assess the implications for using He and Ar tracers to c haracterise mantle sources in arcs, back-arcs, and spreading ridges, a nd consider the consequences for the combined use of rare gases with o ther (radiogenic) isotopic tracers of magma provenance at such setting s. The basaltic andesite from the northeastern basin may also be influ enced by the same kind of crustal interaction as the Valu Fa lavas as it falls within the He-3/He-4 range (6.9 R(A)) of the other basaltic a ndesites. Interestingly, other helium isotope studies indicate that th is part of the basin is characterised by a wide range in He-3/He-4 rat ios, from MORB values up to 22 R(A). The low He-3/He-4 ratio of the ba saltic andesite, therefore, serves to illustrate the possible effects of magma chamber processes on the rare gas and other volatile characte ristics of hotspot lavas: an observation which is important not only f or this part of the Lau Basin but for other localities worldwide.