Chemical and chronologic complexity in the convecting upper mantle: Evidence from the Taitao ophiolite, southern Chile

Abstract

Exposure of the ca. 6 Ma Taitao ophiolite, Chile, located ∼50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ 18O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ 18O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by 87Sr/ 86Sr and 143Nd/ 144Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial 187Os/ 188Os ratios that range widely from 0.1168 to 0.1288 ( γ Os = −8.0 to +1.1), averaging 0.1239 ( γ Os = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg # of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions. In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ 18O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial 187Os/ 188Os ratios, calculated for 6 Ma, that range from 0.126 ( γ Os = −1) to as high as 0.561 ( γ Os = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in 187Os/ 188Os between MORB and abyssal peridotites. The collective results of this study suggest that this >500 km 3 block of the mantle underwent at least two stages of melting. The first stage occurred at ∼1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes.