Re-Os Isotope and Highly Siderophile Element Systematics of 89 Ma Gorgona Komaiites

Gorgona Island, Colombia contains one of the few examples of Phanerozoic komatiites, and thus these rocks been extensively studied as an analogue to their Archean counterparts. Previous work [1] has shown that three distinct mantle sources are necessary to produce the degree of isotopic diversity shown by Gorgona rocks. Notably, Pb isotope systematics show that komatiites on the eastern and western shores of the island sample distinct sources [2], and Os isotope systematics show that komatiites from the southern part of the island sample a radiogenic source [3]. In order to further constrain the nature of the mantle source region of Gorgona komatiites, new Re-Os isotope, and lithophile trace and highly siderophile element (Re, Os, Ir, Ru, Pt, and Pd) abundance data for komatiites from all three regions of the island are presented here.

Initial ¹⁸⁷Os/¹⁸⁸Os ratios confirm that southern komatiites sample a radiogenic source with a γ¹⁸⁷Os value of +11.8±0.4, and show that eastern and western komatiites are identical to one another with a chondritic γ¹⁸⁷Os value of +0.85±0.79. The eastern and western komatiites are also identical to each other with respect to the other HSEs, and the correlation of Pd and Pt with MgO in these two suites allows us to show that their mantle source region contained 77±7% of the bulk silicate earth (BSE) Pt+Pd concentration [4]. In contrast, the southern komatiite source region was depleted in Pt+Pd, with 46±10% of the BSE concentration. Both the chondritic Os isotopic systematics of the eastern and western komatiites and the radiogenic systematics of the southern komatiites conflict with all groups' elevated ɛ¹⁴³Nd values (9.7±0.9) [1] and low (La/Sm)_N ratios (0.29±0.06), which require previous episodes of melt depletion in their source. A mantle melting model demonstrates that it is possible to decouple these signatures via small amounts of melt extraction early in the history of the Gorgona mantle source.

[1] Kerr, 2005 [2] Dupre and Echeverria, 1984 [3] Walker et al. 1999 [4] Becker et al. 2006