He, Sr and Nd isotopic variations in lavas from the Juan Fernandez Archipelago, SE Pacific

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Authors:Farley, K. A.; Basu, A. R.; Craig, H.
Author Affiliations:Primary:
Scripps Institution of Oceanography, La Jolla, CA, United States
Other:
University of Rochester, United States
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 115(1), p.75-87. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1993
Note:In English. 49 refs.; illus., incl. 2 tables, geol. sketch maps
Summary:Helium, Sr and Nd isotopic ratios and major- and trace-element compositions were measured on a suite of lavas from the intra- plate volcanoes of this archipelago off Chile. Lavas from the islands of Mas Afuera and Mas a Teirra and from Monte Alpha and Friday seamounts have Sr and Nd isotopic ratios lying on the low 143Nd/144Nd side of the mantle array. The homogeneity of these tracers suggests the involvement of a restricted range of mantle source compositions throughout Juan Fernandez volcanism. In marked contrast is the large range in 3He/4He, from 7.8 to 18.0 RA. A bimodal 3He/4He distribution on Mas a Tierra is associated with two distinct volcanic lineages; 3He/4He ratios of 14.5-18.0 RA (n=15) occur in alkalic and tholeiitic shield basalts, whereas post-shield basanites range from 11.2 to 13.6 RA (n-12). Elemental and isotopic systematics demonstrate a transition from an enriched (Loihi-like) plume source in the shield lavas to a more MORB-like source in the post-shield volcanic rocks. The transition between these sources is much more pronounced in 3He/4He than in the other isotopic tracers. The predominantly tholeiitic basalts of Mas Afuera have exceptionally uniform isotopic and elemental characteristics; Sr and Nd ratios are similar to those of Mas a Tierra, but 3He/4He ratios are lower and uniform at 8.3 +- 0.5 RA (n-17). The dramatic distinction between Mas Afuera and Mas a Tierra He is surprising, given the great similarity between the two islands in other geological characteristics. Both the Mas Afuera and the Mas a Tierra results show that He records systematic processes not readily apparent from other isotopic or elemental indicators. Neither magma chamber degassing nor local metasomatic events are likely to be responsible. The authors suggest that the observed variations may be attributed to mixing of plume and aesthenospheric sources in which the plume component is characterized by a heterogeneous distribuion of volatiles or has suffered extraction of small- degree partial melts prior to mixing. [P.Br.]
Subjects:Alkaline earth metals; Basalts; He-4/He-3; Helium; Igneous rocks; Isotopes; Lava; Major elements; Mantle; Metals; Nd-144/Nd-143; Neodymium; Noble gases; Ocean-island basalts; Rare earths; Sediments; Sr-87/Sr-86; Stable isotopes; Strontium; Tholeiitic composition; Trace elements; Volcanic rocks; Chile; East Pacific; Pacific Ocean; South America; South Pacific; Southeast Pacific; Juan Fernandez Archipelago; Mas Afuera Island; Masa Tierra Island
Abstract Numbers:94M/2020
Record ID:1995031282
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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