ANOMALOUSLY NUCLEOGENIC NEON IN NORTH CHILE RIDGE BASALT GLASSES SUGGESTING A PREVIOUSLY DEGASSED MANTLE SOURCE

Fresh basalt glasses from the North Chile Ridge (NCR) in the southeast ern Pacific have Ne isotopic compositions distinctly different from ty pical mid-ocean ridge basalts (MORB). In a three-isotope plot of Ne-20 /Ne-22 vs. Ne-21/Ne-22, the NCR data define a correlation line with a slope smaller than that of the MORE correlation line, i.e. their Ne co mposition is more nucleogenic than that of MORE. He-3/He-4 ratios are slightly lower than the MORE average, whereas in a few stepwise heatin g fractions very high Ar-40/Ar-36 ratios up to 28,000 are found. One m odel to explain the data assumes contamination of the NCR mantle sourc e by material from the continental or oceanic crust, but in addition t o difficulties with quantitatively reconciling the noble gas patterns with such a model it seems unable to account for some geochemical char acteristics of NCR basalts reported earlier [Bach et al., Earth Planet . Sci. Lett. 142 (1996) 223-240], such as depletions in highly incompa tible elements and unradiogenic Sr isotope compositions. Therefore we favor the scenario of a mantle source which was depleted and degassed previously, possibly as a residue from mantle melting beneath the sout hern East Pacific Rise that was transported to the NCR and melted agai n. The time during which such a depleted reservoir would have to be se parated from the MORE mantle is estimated at similar to 10-100 Ma base d on U/Th-Ne systematics, in reasonable agreement with the time scale deduced from the formation history of the NCR and the temporal evoluti on of the southeast Pacific. (C) 1998 Elsevier Science B.V. All rights reserved.