The trace element and Sr-Nd-Pb isotope geochemistry of Juan Fernandez lavas reveal variable contributions from a high-3He/4He mantle plume

Abstract

The Juan Fernandez Islands in the southeastern Pacific are an atypical linear volcanic chain that exhibits a considerable range in 3 He/ 4 He ratios (8 to 18 R A, where R A is the 3 He/ 4 He ratio of air), but limited ranges of 87 Sr/ 86 Sr and 143 Nd/ 144 Nd. Here we report new trace element abundance data and Sr-Nd-Pb isotope data for mafic lavas previously analyzed for their 3 He/ 4 He and He contents from the two main islands of Robinson Crusoe and Alexander Selkirk. Lavas from these islands have been previously grouped based on geochemical and petrological classification into Group I and III basalts, and Group II basanites. In general, samples have overlapping Sr-Nd-Pb isotope compositions that suggest a common, albeit slightly heterogeneous mantle source. In detail, the Group I and III tholeiitic and alkalic basalts have nearly identical incompatible trace element patterns, whereas the Group II basanites show elevated incompatible trace element abundances. Major and incompatible trace element modeling indicates that Group III basalts (3 He/ 4 He = 7.8-9.5 R A) from younger Alexander Selkirk Island were produced by the highest degree of partial melting (> 10%) of a common mantle source, followed by Group I basalts (13.6-18.0 R A) and Group II basanites (11.2-12.5 R A) from older Robinson Crusoe Island. The 206 Pb/ 204 Pb of Group I basalts and Group II basanites are slightly more radiogenic and limited in range (19.163 to 19.292) compared with those of Group III (18.939 to 19.221). The Group I and II lavas from Robinson Crusoe are consistent with an origin from the so-called focus zone (FOZO) mantle component, whereas the Alexander Selkirk basalts additionally contain contributions from a less-enriched or relatively depleted mantle component. Juan Fernandez lavas reveal limited ranges of Sr-Nd-Pb isotopes but variable 3 He/ 4 He as their parental magmas originated mainly from the FOZO component with high 3 He/ 4 He (> 9 R A) and variably polluted with a depleted component with lower 3 He/ 4 He (ca. 8 R A). Contributions from high-3 He/ 4 He mantle sources to ocean island basalts can therefore vary both spatially and temporally, over meter to kilometer lengths and hundred to millionyear time scales, and may not be strongly correlated to radiogenic lithophile isotope systematics.

40Ar/39Ar Geochronological Constraints on the Age Progression Along the Juan Fernández Ridge, SE Pacific

“…Local differences cannot be explained only by changes in the degree of partial melting and thus subtle variations in the enriched source are needed. These mantle heterogeneities could also explain the low values of 3 He/ 4 He in Alejandro Selkirk compared to the higher values reported for Robinson Crusoe (Farley et al, 1993;Truong et al, 2018), the latter usually interpreted as evidence of an origin in the deep mantle (Hahm et al, 2009) and one of the most compelling probes of a mantle plume involvement in JFR. Despite the internal variation, the narrow field of isotopic values implies a relatively stable source beneath JFR, which is in contrast to the large variation observed in coeval chains on the Nazca Plate like the Easter Seamount Chain (Ray et al, 2012), where a mixture of enriched and depleted sources has been proposed (e.g., Kingsley and Schilling, 1998;Simons et al, 2002).…”

“…We here focus on the shield stage, and because samples come from the upper slope of seamounts and from the subaerial sections of Alejandro Selkirk and Robinson Crusoe islands, they most likely represent the late shield stage. Summarizing the main geochemical features of the shield stage based on recent results (Reyes et al, 2017;Truong et al, 2018) and our ongoing research, JFR is formed by basic lavas highly enriched in incompatible elements (LIL and HFSE) compared to MORB, probably due to a more enriched mantle source and smaller degrees of melting. This geochemical enrichment is more marked in Robinson Crusoe Island and Alpha Guyot, and extends to higher La/Sm and Nb/Zr ratios compared to O'Higgins Guyot and Alejandro Selkirk Island (Figure 4).…”

“…Robinson Crusoe Island (915 masl) and the nearby Santa Clara (375 masl) are part of the same edifice. Their advanced erosion exposes a continuous sequence of shiled stage lavas (Baker et al, 1987;Farley et al, 1993;Reyes et al, 2017;Truong et al, 2018). Submerged marine abrasion terraces (at ∼ 200 and ∼ 500 mbsl; Astudillo, 2014) and sedimentary layers of marine origin now uplifted at 70 masl indicate a complex history of vertical displacement (Sepúlveda et al, 2015).…”

“…Published Sr-Nd-Pb isotopic ratios suggest source enrichment with respect to depleted mantle source of MORB (Farley et al, 1993;Truong et al, 2018), including those from the East Pacific Rise and North Chile Ridge and excluding the extensive participation of the lithospheric mantle. Local differences cannot be explained only by changes in the degree of partial melting and thus subtle variations in the enriched source are needed.…”

Marine Carbonates in the Mantle Source of Oceanic Basalts: Pb Isotopic Constraints

“…The limestone samples were broken into cm-sized fragments, washed with distilled water, dried, and pulverized in an agate mortar and pestle. About 50 mg of the limestone powders were completely dissolved using a mixed, ultra-pure HF:HNO 3 solution in Teflon beakers and then analyzed for bulk limestone Pb and Sr isotopic composition following the established sample purification and thermal ionization mass spectrometry procedure at the Scripps Institution of Oceanography 58 . To analyze the Pb and Sr isotopic composition of carbonate fractions, ca.…”