Ocean Ridges

Article | Published: 28 January 2019

• Sung-Hyun Park, 
• Charles H. Langmuir, 
• Kenneth W. W. Sims, 
• Janne Blichert-Toft, 
• Seung-Sep Kim, 
• Sean R. Scott, 
• Jian Lin, 
• Hakkyum Choi, 
• Yun-Seok Yang & 
• Peter J. Michael 

Nature Geosciencevolume 12, pages206–214 (2019) | Download Citation

Abstract

The mantle sources of mid-ocean ridge basalts beneath the Indian and Pacific oceans have distinct isotopic compositions with a long-accepted boundary at the Australian–Antarctic Discordance along the Southeast Indian Ridge. This boundary has been widely used to place constraints on large-scale patterns of mantle flow and composition in the Earth’s upper mantle. Sampling between the Indian and Pacific ridges, however, has been lacking, especially along the remote 2,000 km expanse of the Australian–Antarctic Ridge. Here we present Sr, Nd, Hf and Pb isotope data from this region that show the Australian–Antarctic Ridge has isotopic compositions distinct from both the Pacific and Indian mantle domains. These data define a separate Zealandia–Antarctic domain that appears to have formed in response to the deep mantle upwelling and ensuing volcanism that led to the break-up of Gondwana 90 million years ago, and currently persists at the margins of the Antarctic continent. The relatively shallow depths of the Australian–Antarctic Ridge may be the result of this deep mantle upwelling. Large offset transforms to the east may be the boundary with the Pacific domain.

Vanadium isotope compositions of igneous rocks have the potential to constrain variations of physico-chemical conditions such as oxidation states during magmatism. Here, we present V isotope data for 27 fresh lavas (ranging from basaltic to dacitic compositions) from mid-ocean ridges, 31 altered basalts and gabbros from IODP site 1256 near the East Pacific Rise (EPR), and 2 back arc basin basalts (BABB). Our analyses of fresh mid-ocean ridge basalt (MORB) provide new constraints on the V isotope composition of MORBs, i.e. ‰δ51V=−0.84±0.02‰ (2SE, n=22). In addition, the mean δ51V of MORBs from individual segments is correlated with the mean ridge depth and Na_8.0 of the segment, which might reflect the effect of melting extent on V isotope fractionation during mantle melting.

The mafic profile of intact altered oceanic crust (AOC) from the IODP site 1256 has δ51V ranging from −1.01 to ‰−0.77‰, similar to that of fresh MORBs, suggesting that V isotope fractionation is limited during alteration of oceanic crust. These results also indicate the V isotopic homogeneity of the bulk oceanic crust with average δ51V of ‰−0.85±0.02‰ (2SE, n=53), which is unaffected by ocean water and hydrothermal fluid alteration. Our results provide a guideline for application of V isotopes into studies of low and high temperature geochemical processes.

The evolved lavas (basaltic andesites, andesites, and dacites) from the East Pacific Rise (EPR) show apparent shifts towards heavy δ51V values with increasing degree of differentiation, which can be explained by the crystal–liquid fractionation during crystallization with an inferred isotope fractionation factor of Δ51Vmineral-melt=−0.15×106/T2. The enrichment of ⁵¹V with increasing differentiation degree for the 9°N Overlapping Spreading Center (OSC) lavas is consistent with direction of the isotope shift observed in lavas from Anatahan Island (Northern Mariana Arc) and Hekla Volcano (Iceland), but the magnitude (0.3‰) is much smaller than that (2‰) reported in Prytulak et al. (2017). Modeling of V isotope fractionation between mineral and melt shows that variations in redox condition are important for controlling V isotope fractionation, but insufficient to explain the dramatically different Δ51V_mineral-melt between 9°N OSC lavas and Anatahan/Hekla suites. More studies are necessary for better understanding of mechanisms of V isotope fractionation during magmatism.

The concentration of carbon in primary mid‐ocean ridge basalts (MORBs), and the associated fluxes of CO₂ outgassed at ocean ridges, is examined through new data obtained by secondary ion mass spectrometry (SIMS) on 753 globally distributed MORB glasses. MORB glasses are typically 80–90% degassed of CO₂. We thus use the limited range in CO₂/Ba (81.3 ± 23) and CO₂/Rb (991 ± 129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO₂ concentrations for ridges that have Ba and/or Rb data. When combined with quality‐controlled volatile‐element data from the literature (n = 2,446), these data constrain a range of primary CO₂ abundances that vary from 104 ppm to 1.90 wt%. Segment‐scale data reveal a range in MORB magma flux varying by a factor of 440 (from 6.8 × 10⁵ to 3.0 × 10⁸ m³/year) and an integrated global MORB magma flux of 16.5 ± 1.6 km³/year. When combined with CO₂/Ba and CO₂/Rb‐derived primary magma CO₂ abundances, the calculated segment‐scale CO₂ fluxes vary by more than 3 orders of magnitude (3.3 × 10⁷ to 4.0 × 10¹⁰ mol/year) and sum to an integrated global MORB CO₂ flux of  × 10¹² mol/year. Variations in ridge CO₂ fluxes have a muted effect on global climate; however, because the vast majority of CO₂degassed at ridges is dissolved into seawater and enters the marine bicarbonate cycle. MORB degassing would thus only contribute to long‐term variations in climate via degassing directly into the atmosphere in shallow‐water areas or where the ridge system is exposed above sea level.

Plain Language Summary

Estimated CO₂ contents of primary mid‐ocean ridge basalts (MORB), calculated on a segment‐by‐segment basis, vary from 104 ppm to 1.9 wt%. CO₂‐enriched MORB are present in all ocean basins, in particular, in the Atlantic Ocean basin, which is younger and more likely to contain admixed material from recent subduction compared to the much older Pacific Ocean basin. CO₂ fluxes at individual ridge segments vary by 3 orders of magnitude due primarily to large variability in primary CO₂ content. This study provides the most detailed and accurate estimate to date of the integrated total flux of CO₂ from mid‐ocean ridges of  × 10¹² mol/year.