Osmium isotope evidence for a heterogeneous 3He/4He mantle plume beneath the Juan Fernandez Islands

Abstract

Mantle plume models have been widely applied to explain the formation of ocean island basalts (OIB), with high-3 He/ 4 He in their lavas being explained by sampling of a primitive deep mantle source. The Juan Fernandez Islands have 3 He/ 4 He (7.8-18 R A ) similar to or higher than in mid-ocean ridge basalts (MORB; 8 ± 1 R A ) and have been used to both support and refute the mantle plume hypothesis. Ambiguity regarding the origin of the Juan Fernandez Islands primarily originates from interpretation of mantle source signatures between the lava series from the two main islands, Robinson Crusoe and Alexander Selkirk. To examine this issue, we report new whole-rock and olivine separate 187 Os/ 188 Os ratios and major-, trace-, and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances. The HSE and trace element abundances in Juan Fernandez main shield lavas can be explained by up to 30% olivine removal, together with spinel crystallization at 1-5 kbar, whereas Robinson Crusoe rejuvenated lavas can be reproduced by higher-pressure fractional crystallization (up to 10 kbar). An assemblage of 30 modal % olivine and 1-5 modal % spinel, combined with the additional contribution of primary melt trapped in olivine inclusions reproduces the range of HSE compositions observed in Juan Fernandez Archipelago olivine grains. Ratios of 187 Os/ 188 Os for Juan Fernandez lavas are generally less radiogenic than global OIB and show no correlation with indices of fractionation, indicating that they reflect mantle source compositions. Younger basanite lavas from Robinson Crusoe represent rejuvenated volcanism dominantly from a depleted lithospheric mantle source ( 187 Os/ 188 Os < 0.13) mixed with a high-3 He/ 4 He component from the main shield building stage. These lavas are similar in origin and composition to other Pacific rejuvenated lavas (e.g., Samoa, Hawaii). Robinson Crusoe main shield lavas are from a high-3 He/ 4 He (>18 R A ) and enriched mantle source ( 187 Os/ 188 Os = 0.1312) similar to the 'C' or 'FOZO' component, whereas Alexander Selkirk lavas are consistent with a dominant contribution from a depleted, low-3 He/ 4 He (<10 R A ) mantle component. The mantle sources of the shield lavas yield subtle variations in Sr-Nd-Os-Pb isotope space and have no clear variations with relative or absolute abundances of the HSE or trace elements. These results are consistent with a heterogeneous mantle plume model, with initial eruption of lavas from a primitive high-3 He/ 4 He mantle source $4 million years ago (Ma) to form Robinson Crusoe Island, which also led to 3 He enrichment of the oceanic lithosphere, followed by eruption of Alexander Selkirk lavas from a more depleted mantle source at $2 Ma. Juan Fernandez lavas show that mantle heterogeneity preserved in OIB can occur over short-timescales (<2 Ma) and can impact lithospheric compositions, leading to eruption of rejuvenated lavas with unusual isotopic characteristics.

Tracing the Origin and Magmatic Evolution of the Rejuvenated Volcanism in Santa Clara Island, Juan Fernández Ridge, SE Pacific

“…Previous petrological studies suggest that volcanism along the JFR is related to the melting of a slightly heterogeneous mantle plume with apparently low potential temperatures, high 3 He/ 4 He isotopic ratios in some cases, and a well-developed age progression [24][25][26][27][28][29], ranging from ~9.26 Ma in the east (Guyot O'Higgins) to ~0.62 Ma in the west (Friday Seamount) [29]. Shield volcanoes at the JFR are predominantly composed of transitional basalts, picrites, and meimequites with a Sr-Nd-Pb isotopic signature close to the common FOZO (FOcal ZOne) mantle endmember, with additional contributions from enriched mantle 1 (EM1) and high µ (HIMU) components [28] (see [31][32][33][34][35] for more general information on mantle components).…”

“…Regarding the mantle source, Ref. [27] proposes the influence of metasomatized lithosphere based on Os isotopes, although the signature is very similar to the shield and shows a significant range of variability.…”

“…This type of source has been proposed for Samoa [7], the Selvagen [22], Canary [21] Cook-Austral [55], Fieberling-Guadalupe [56], and Hawaii [15,16]. For JFR, a depleted lithospheric mantle source was proposed for the rejuvenated lavas based on Os isotopes, polluted by the high 3 He/ 4 He signature from the main shield stage [27]. However, the reported 187 Os/ 188 Os ratios (0.134-0.122) are not entirely conclusive; in fact, they are partially within the range of Os isotope ratios reported for the shield stage of JFR originating from a mantle plume (Robinson Crusoe: 0.143-0.128 and Alejandro Selkirk: 0.138-0.126).…”

“…At most, they could indicate slight variations in the depth or degree of melting [65]. This suggests a source for the rejuvenated volcanism similar to that which generated the shield stage, characterized as a slightly heterogeneous mantle plume [24][25][26][27][28].…”

“…In the Nazca Plate, SE Pacific, rejuvenated volcanism has only been reported on the Juan Fernández Ridge (JFR) [8,9,23], whose shield stage was formed by a slightly heterogeneous mantle plume [24][25][26][27][28]. Rejuvenated volcanism has been reported for Robinson Crusoe Island [8,23] and O'Higgins Guyot [9].…”