Geochemical constraints on the source of EPR MORBs

Abstract:

Geochemical variations of mid-ocean ridge basalts (MORBs) have long been attributed to the presence of compositionally distinct crustal and mantle components in their sources. The oxygen isotope composition of MORBs was proposed to be a valuable tracer of subducted crustal materials [1] since ¹⁸O/¹⁶O ratios are strongly fractioned during low-temperature water-rock interactions (e.g. seafloor alteration). Here, we present a study of combined oxygen isotopes, radiogenic isotopes, and trace elements on volcanic lavas sampled at both ridge axes and off-axis seamounts in the northern East Pacific Rise (EPR; 5-15°N). The chemical composition of these lavas ranges from normal-MORB to enriched-MORB, and the newly analyzed ¹⁸O/¹⁶O ratios of hand-picked fresh glasses from these 53 lavas were measured using laser-fluorination technique, with a typical precision of ±0.08‰ (1σ).

Our results show that the δ¹⁸O_SMOW values of EPR glasses span ～0.64‰ (from 5.08‰ to 5.72‰), with an average value of 5.53(±0.12, 1σ) ‰, which is comparable to results from previous survey of global MORBs (5.52±0.11‰) [1]. Two andesitic glasses show unusually lower δ¹⁸O values (5.08‰ and 5.27‰, respectively) and exhibit excess Cl addition ([Cl]>3000ppm), probably indicating an assimilation of seawater-derived component (e.g., saline brines or altered oceanic crust). Previous studies [2, 3, 4, 5] have shown the correlated variations of trace element ratios (e.g. La/Sm, and K₂O/TiO₂ element ratios) and radiogenic isotope compositions (e.g. ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, Pb isotope ratios), suggesting the presence of at least two components in the source of EPR MORBs, an enriched component and depleted component. Our results show that δ¹⁸O values of these glasses lack statistically-significant correlations with other geochemical indicies in the same sample, suggesting that (1) the source of EPR MORBs has a uniform oxygen isotope composition, (2) the enriched components in the sources cannot be directly derived from recycled upper oceanic crustal rocks/sediments.

References: [1] Cooper et al. (2009), G-3,10. [2] Niu & Batiza (1997), EPSL, 148, 471-483. [3] Niu et al. (1999), JGR, 104, 7067-7087.[4] Niu et al. (2002), EPSL, 199, 327-345.[5] Castillo et al. (2000), G-3, 1.