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Mixing of magmas from enriched and depleted mantle sources in the northeast Pacific: West Valley segment, Juan de Fuca Ridge

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Abstract

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Smcn∼0.3, 87Sr/86Sr=0.70235–0.70242, and 206Pb/204Pb=18.22–18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Smcn∼1.8, 87Sr/86Sr=0.70245–0.70260, and 206Pb/204Pb=18.73–19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with “plums” of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the “plums”, initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the “plums” and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the “plums” could evolve to their more evolved isotopic composition after a period of 1.5–2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a less-depleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as “plums” throughout the upper mantle, transported from depth by mantle plumes.

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Authors and Affiliations

  1. Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Dr., K1S 5B6, Ottawa, ON, Canada

    Brian L. Cousens

  2. Ocean Drilling Program, 1000 Discovery Drive, 77845, College Station, TX, USA

    James F. Allan

  3. Ottawa-Carleton Geoscience Centre, Department of Geology, University of Ottawa, 770 King Edward, K1N 6N5, Ottawa, ON, Canada

    Matthew I. Leybourne

  4. Department of Geological Sciences, University of British Columbia, 6339 Stores Road, V6T 1Z4, Vancouver, BC, Canada

    R. L. Chase

  5. Department of Geology, Acadia University, B0P 1X0, Wolfville, NS, Canada

    Nancy Van Wagoner

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Cousens, B.L., Allan, J.F., Leybourne, M.I. et al. Mixing of magmas from enriched and depleted mantle sources in the northeast Pacific: West Valley segment, Juan de Fuca Ridge. Contr. Mineral. and Petrol. 120, 337–357 (1995). https://doi.org/10.1007/BF00306512

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