The crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid-ocean basalts (N-MORB) but some subtle, but distinctive, differences exist. These are largely due to differences in volatiles, Fe oxidation ratio, abundances of large ion lithophile elements (LIL), and some high field strength (HFS) elements. Isotope data for Sr, Pb, Nd, He, and O are variable but broadly are MORB-like. We present data contrasting Mariana Trough (MTB) and Lau Basin basalts (LBB) showing how they differ in certain trace element and isotopic signatures and compare both to N-MORB. Relative to N-MORB and LBB, and MTB are enriched in alkalis, alkaline earths, light rare earth elements (REE), H2O and Fe'''/Fe'', even for ''primitive'' samples, while having MORB-type He (3/4=7.8 to 8.4¿RA), Sr (87/86 mean=0.70293) and Nd (&egr;=+10.2). The Lau Basin is zoned with a certain area having N-MORB characteristics for H2O, LIL, REE, Nd (&egr;=+8.1 to 9.0), and O but a depletion in HFS elements. Sr is more radiogenic (mean 87/86=0.70347) and 3He/4He (3/4=7.2 to 11.0¿RA) is high relative to N-MORB. The Lau Basin has a zone, bordering the adjacent island arcs, which is like MTB. In both back arc basins, relatively primitive (unfractionated) basalts with Mg/100 (Mg+Fe'')>62 are found which resemble postulated N-MORB. Calculated parental magmas (obtained by adding xenocrystic olivine) plot near or on 3 and 4 phase boundaries at 1--1.5 GPa for the OL-DI-SI system. Low P fractional crystallization and magma mixing explain most of the observed rock types. Possible explanation for differences between MTB and LBB include differences in the extent of previous melting episodes of their mantle sources and effects of source-metasomatism through dehydration and partial melting of subducted lithosphere.

The long and complex history of Mariana Trough mantle sources led to a metasomatized mantle which gives rise to the distinctive MTB. In contrast, the Lau Basin is dominated by basalts derived from a source depleted in volatiles, LIL, an HFS elements. A previous melting history is implied. Mixing with a mantle component enriched in 87Sr and 3He may be due to mantle counterflow driven by the subduction process.