[B-CG05] Decoding the history of Earth: From Hadean to the present

[BCG05-P04] Mantle sources of Phanerozoic komatiite from Gorgona Island, Colombia: Implication from osmium isotope heterogeneity

*Ihara Yuta¹, Akira Ishikawa¹, Kenji Shimizu², Tetsuya Yokoyama¹ (1.Tokyo Institute of Technology, 2.JAMSTEC)

Keywords:Komatiite, Large igneous provinces, Osmium isotope, Mantle plume

Gorgona Island, off the coast of Colombia, is known for the locality for occurring young and fresh komatiites (~90 Ma) and is thought to be the part of the Caribbean-Colombian Oceanic Plateau (CCOP) [1]. Since most of komatiites found in Archean and Proterozoic greenstone belts were highly influenced by subsequent tectonic activities and alterations, it is difficult to constrain their erupted settings and compositions of mantle sources partly because of ambiguous relations to neighboring igneous rocks. Thus, Gorgona Island komatiites are important resource for understanding generation of komatiite magma because the CCOP can be regarded as melting products of a plume rising from the deep mantle, and their tectonic modifications and alterations are minimal. Previous studies revealed that the Gorgona komatiites display significant variations in ¹⁸⁷Os/¹⁸⁸Os ratios ranging from 0.1286 to 0.1509 and other isotope systematics, suggesting that heterogeneous magma sources played a role on the generation of komatiites [1, 2]. However, its relation to other igneous rocks in Gorgona Island such as basalts, picrites, and mafic-ultramafic cumulates (gabbro, olivine gabbro, wehrlite, and dunite) still remains unclear. In this study, we analyzed the Os isotopic compositions of all types of igneous rocks found in Gorgona and examined their genetic relationships to the komatiite. Although their major element variations tend to follow a single crystal fractionation trend, two distinct parental magmas could be required for the observed variations in TiO₂ and Os contents: (1) main magma that formed komatiites, basalts and mafic-ultramafic cumulates, and (2) a TiO₂ and Os-poor magma responsible for the formation of picrites. When ¹⁸⁷Os/¹⁸⁸Os variations are viewed separately in lithological types, komatiites (0.1285-0.1314), picrites (0.1276-0.1317), and mafic-ultramafic cumulates (0.1270-0.1366) overlap each other and the low values previously obtained for komatiites and picrites [2]. Further studies on estimating the initial ¹⁸⁷Os/¹⁸⁸Os values and extents of trace element variations could be important for constraining their origins and genetic relationships between the two magmas.



[1] Kerr, A.C. (2005) Lithos 84, 77-101. [2] Walker et al. (1999) Geochimica et Cosmochimica Acta 63,713-728.