Geochemistry of near-EPR seamounts: importance of source vs. process and the origin of enriched mantle component

Abstract

Niu and Batiza [Earth Planet. Sci. Lett. 148 (1997) 471^483] show that lavas from the seamounts on the flanks of the East Pacific Rise (EPR) between 5 ‡ and 15 ‡N vary from extremely depleted tholeiites to highly enriched alkali basalts. The extent of depletion and enrichment exceeds the known range of seafloor lavas in terms of the abundances and ratios of incompatible elements. New Sr^Nd^Pb isotope data for these lavas show variations ( 87 Sr/ 86 Sr = 0.702362^0.702951; 206 Pb/ 204 Pb = 18.080^19.325 and 143 Nd/ 144 Nd 0.512956^0.513183) larger than observed in lavas erupted on the nearby EPR axis. These isotopic ratios correlate with each other, with the abundances and ratios of incompatible elements, with the abundances of measured major elements such as MgO, CaO, Na 2 O and TiO 2 contents, and with the abundances and ratios of major elements corrected for crystal fractionation to Mg# = 0.72 (Ti 72 , Al 72 , Fe 72 , Ca 72 , Na 72 , and Ca 72 /Al 72 ). These coupled correlations and the spatial distribution of seamounts require an EPR mantle source that has long-term ( s 1 Ga) lithological heterogeneities on very small scales [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471^483]. Mid-ocean ridge basalt (MORB) major element systematics are, to a great extent, inherited from their fertile sources, which requires caution when using major element data to infer melting conditions. The significant correlations in elemental and isotopic variability (defined as RSD% = 1c/ meanU100) between seamount and axial lavas suggest that both seamount and axial volcanisms share a common heterogeneous mantle source. We confirm previous interpretations [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471^483; Niu et al., J. Geophys. Res. 104 (1999) 7067^7087] that the geochemical variability of lavas from the broad northern EPR region results from melting-induced mixing of a two-component mantle with the enriched (easily melted) component dispersed as physically distinct domains in a more depleted (refractory) matrix prior to the major melting events. The data also allow the conclusion that recycled oceanic crust cannot explain elevated abundances of elements such as Ba, Rb, Cs, Th, U, K, Pb, Sr etc. in enriched MORB and many ocean island basalts. These elements will be depleted in recycled oceanic crust that has passed through subduction zone dehydration reactions. We illustrate that deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting these and other incompatible elements as a result of metasomatism taking place at the interface between the low velocity zone and the cooling and thickening oceanic lithosphere. ß 2002 Elsevier Science B.V. All rights reserved.

Seamounts

“…Seamounts formed in mid-plate settings are called "oceanic intraplate volcanoes." Intraplate volcanoes that are formed near spreading centers, i.e., on young and thus thin lithosphere, consist primarily of tholeiitic magmas (e.g., Niu et al, 2002). Most intraplate seamounts, however, are emplaced on older and therefore thicker lithosphere.…”

“…Plate cracking formed by extension can not only facilitate the rise of melts through the lithosphere but also can trigger asthenospheric upwelling causing decompression melting (O'Connor et al 2015). Because seamounts are generally formed by lower degrees of partial melting and over a larger range of melting than normal mid-ocean ridge basalts (MORB), their igneous rocks exihibit a much larger spectrum of chemical compositions than MORB, since they preferentially sample fertile anomalies in the asthenospheric mantle (e.g., Niu et al, 2002;Hoernle et al, 2011). Seamount volcanism has played an important role in our understanding of mantle dynamics, chemical heterogeneity of the mantle, and magmatism in general.…”

Temporal and petrogenetic constraints on volcanic accretionary processes at 9-10 degrees north East Pacific Rise

“…the ubiquitous presence of heterogeneous mantle domains over short length scales (e.g., Castillo et al, 2000;Fornari et al, 1988;Hekinian et al, 1989;Langmuir et al, 1986;Mahoney et al, 1994;Niu et al, 1997;Niu et al, 1999;Niu et al, 2002;Prinzhofer et al, 1989;Reynolds et al, 1992;Reynolds and Langmuir, 2000;Zindler et al, 1984). While the isotopic and chemical heterogeneity of the present-day mantle almost certainly represents the continuous processes of differentiation and remixing, the exact origin of these enriched mid-ocean ridge mantle domains and the genesis of E-MORB are still vigorously debated.…”

“…Enriched source compositions for E-MORB are generally argued to derive from either melt metasomatism of peridotite, either cryptically or in the form of fine dikes or veins (e.g., Donnelly et al, 2004;Galer & O'Nions, 1986;Niu et al, 2002) or from formation of garnet pyroxenite or eclogite veins through stretching and thinning of subducted oceanic crust (e.g., Allegre & Turcotte, 1986;Ben Othman & Allegre, 1990;Hirschmann & Stolper, 1996;Prinzhofer et al, 1989). In all models positing a metasomatic origin, a common requirement is that an earlier episode of low-degree melting of some other enriched mantle source must occur to generate the elevated highly incompatible trace element abundances (Donnelly et al, 2004;Niu et al, 2002).…”

“…However, long-term chemical heterogeneities in the oceanic mantle have long been known to exist at different length scales (e.g., Hart et al, 1973;Hedge & Peterman, 1970;Tatsumoto, 1966;White & Schilling, 1978;Zindler & Hart, 1986;Sims & Hart, 2006). Even far from the influence of any hot spot, such as at the East Pacific Rise (EPR), the adjacent and seemingly coeval occurrence of enriched (E-MORB), normal (N-MORB) and very depleted MORB (D-MORB) attests to the ubiquitous presence of heterogeneous mantle domains over short length scales (e.g., Castillo et al, 2000;Fornari et al, 1988;Hekinian et al, 1989;Langmuir et al, 1986;Mahoney et al, 1994;Niu et al, 1997;Niu et al, 1999;Niu et al, 2002;Prinzhofer et al, 1989;Reynolds et al, 1992;Reynolds & Langmuir, 2000;Zindler et al, 1984). While the isotopic and chemical heterogeneity of the present-day mantle almost certainly represents the continuous processes of differentiation and remixing, the exact origin of these enriched mid-ocean ridge mantle domains and the genesis of E-MORB are still vigorously debated.…”