Lithium isotopic composition of Central American Volcanic Arc lavas: implications for modification of subare mantle by slab-derived fluids

Li contents and isotopic compositions were determined for a suite of well-c haracterized basaltic lavas from the Central American Volcanic Are (CAVA). Variable Li/Y (0.2-0.5), Li/Sc (0.1-0.4), and delta(6)Li values (+ 2.6 to - 7.7 parts per thousand) attest to significant compositional heterogeneity in the subarc mantle. Within specific are segments, these parameters correl ate strongly with each other and with a number of other constituents (e.g., K, Rb; Ba, B/La, Be-10/Be-9, Sr-87/Sr-86; U/Ce and Th-230/Th-232, among ot hers); these correlations are particularly strong for Nicaragua samples. Co upling of this particular set of constituents is best explained in terms of addition of 'subduction components' to the subarc mantle. Moreover, their selective enrichment with respect to relatively fluid-immobile incompatible elements signifies the dominance of fluid vs. silicate melt transport of s lab components to the subarc mantle. Several interesting nuances are reveal ed by the Li data. First, although Li and B are strongly correlated in both Costa Rica and Nicaragua, there are systematic along-strike variations in Li/B that are consistent with these elements having different 'fluid releas e patterns' from subducted slab segments. For example, Li/B is highest in C osta Rica where auxiliary evidence indicates higher subduction zone tempera tures; apparently B is preferentially depleted and Li retained in the slab under warmer conditions. The same relations are reflected in Li/Be-10 and o ther subduction tracer systematics, all of which point to larger subduction contributions below Nicaragua. Yet, even Nicaragua lavas vary widely in le vels of subduction enrichment. High-Ti basalts from Nejapa are the least en riched and have the highest delta(6)Li (1.4 to 2.6 parts per thousand); the se values are greater than in fresh MORE (ca. - 4 parts per thousand) and a re not easily explained by additions of subducted Li because most oceanic c rustal rocks and marine sediments have lower delta(6)Li than MORE (with typ ical values between - 8 and - 20 parts per thousand). Thus, it appears the Nejapa data may be representative of isotopically Light mantle domains. Rel atively light delta(6)Li values in an undepleted spinel Iherzolite ($11.3 p arts per thousand) from Zabargad Is. (Red Sea) and in primitive backarc bas alts (- 1.6 to - 0.5 parts per thousand) from Lau Basin support this conclu sion. Considering representative fluid and mantle endmember compositions, t he CAVA data are consistent with limited (up to a few percent) additions of slab-derived fluids to a heterogeneous mantle containing variably depleted and enriched domains to form the respective magma sources. In our view, th e subarc mantle is heterogeneous on a small scale, but some are sectors cle arly received greater slab inputs than others. (C) 1999 Elsevier Science B. V. All rights reserved.