Anderson
Molly J.
Anderson
Molly J.
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ArticleExtreme heterogeneity in mid-ocean ridge mantle revealed in lavas from the 8 degrees 20 ' N near-axis seamount chain(American Geophysical Union, 2020-12-14) Anderson, Molly ; Wanless, V. Dorsey ; Perfit, Michael R. ; Conrad, Ethan ; Gregg, Patricia M. ; Fornari, Daniel J. ; Ridley, W. IanLavas that have erupted at near‐axis seamounts provide windows into mid‐ocean ridge mantle heterogeneity and melting systematics which are not easily observed on‐axis at fast‐spreading centers. Beneath ridges, most heterogeneity is obscured as magmas aggregate toward the ridge, where they efficiently mix and homogenize during transit and within shallow magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of near‐axis seamounts and volcanic ridges perpendicular to the East Pacific Rise. We assess the chemistry of near‐ridge mantle using a ∼200 km‐long chain at ∼8°20′N. High‐resolution bathymetric maps are used with geochemical analyses of ∼300 basalts to evaluate the petrogenesis of lavas and the heterogeneity of mantle feeding these near‐axis eruptions. Major and trace element concentrations and radiogenic isotope ratios are highly variable on <1 km scales, and reveal a continuum of depleted, normal, and enriched basalts spanning the full range of ridge and seamount compositions in the northeast Pacific. There is no systematic compositional variability along the chain. Modeling suggests that depleted mid‐ocean ridge basalt (DMORB) lavas are produced by ∼5%–15% melting of a depleted mid‐ocean ridge (MOR) mantle. Normal mid‐ocean ridge basalts (NMORB) form from 5% to 15% melting of a slightly enriched MOR mantle. Enriched mid‐ocean ridge basalts (EMORB) range from <1% melting of 10% enriched mantle to >15% melting of 100% enriched mantle. The presence of all three lava types along the seamount chain, and on a single seamount closest to the ridge axis, confirms that the sub‐ridge mantle is much more heterogeneous than is commonly observed on‐axis and heterogeneity exists over small spatial scales.
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ArticleRelative timing of off‐axis volcanism from sediment thickness estimates on the 8°20’N seamount chain, East Pacific Rise(American Geophysical Union, 2022-08-06) Fabbrizzi, Andrea ; Parnell-Turner, Ross ; Gregg, Patricia M. ; Fornari, Daniel J. ; Perfit, Michael R. ; Wanless, V. Dorsey ; Anderson, MollyVolcanic seamount chains on the flanks of mid-ocean ridges record variability in magmatic processes associated with mantle melting over several millions of years. However, the relative timing of magmatism on individual seamounts along a chain can be difficult to estimate without in situ sampling and is further hampered by Ar40/Ar39 dating limitations. The 8°20’N seamount chain extends ∼170 km west from the fast-spreading East Pacific Rise (EPR), north of and parallel to the western Siqueiros fracture zone. Here, we use multibeam bathymetric data to investigate relationships between abyssal hill formation and seamount volcanism, transform fault slip, and tectonic rotation. Near-bottom compressed high-intensity radiated pulse, bathymetric, and sidescan sonar data collected with the autonomous underwater vehicle Sentry are used to test the hypothesis that seamount volcanism is age-progressive along the seamount chain. Although sediment on seamount flanks is likely to be reworked by gravitational mass-wasting and current activity, bathymetric relief and Sentry vehicle heading analysis suggest that sedimentary accumulations on seamount summits are likely to be relatively pristine. Sediment thickness on the seamounts' summits does not increase linearly with nominal crustal age, as would be predicted if seamounts were constructed proximal to the EPR axis and then aged as the lithosphere cooled and subsided away from the ridge. The thickest sediments are found at the center of the chain, implying the most ancient volcanism there, rather than on seamounts furthest from the EPR. The nonlinear sediment thickness along the 8°20’N seamounts suggests that volcanism can persist off-axis for several million years.
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ArticleSubmarine deep‐water lava flows at the base of the western Galápagos Platform(John Wiley & Sons, 2018-10-25) Anderson, Molly ; Wanless, V. Dorsey ; Schwartz, Darin M. ; McCully, Emma ; Fornari, Daniel J. ; Jones, Meghan R. ; Soule, Samuel A.To investigate the initial phases of magmatism at the leading edge of the upwelling mantle plume, we mapped, photographed, and collected samples from two long, deep‐water lava flows located at the western base of the Galápagos Platform using the remotely operated vehicle Hercules. Lavas were recovered from four areas on the seafloor west of Fernandina volcano, including the western flow fronts of two deep‐water flows, heavily sedimented terrain between the two flows, and the eastern, shallower end of one flow. The sediment cover and morphologies are distinct between the western flow fronts and the eastern region based on seafloor imagery, suggesting that the long lava flows are not a single eruptive unit. Major and trace element concentrations reveal both tholeiitic and alkalic compositions and support the interpretation that multiple eruptive units comprise the deep‐water flows. Alkalic lavas have higher [La/Sm]N ratios (2.05–2.12) and total alkali contents (5.18–5.40) compared to tholeiitic lavas, which have [La/Sm]N ratios ranging from 1.64 to 1.68 and total alkali contents ranging from 3.07 to 4.08 wt%. Radiogenic isotope ratios are relatively homogeneous, suggesting a similar mantle source. We use petrologic models to assess three alternative mechanisms for the formation of the alkalic magmas: (1) high‐pressure crystallization of clinopyroxene, (2) mixing of high silica and mafic magmas, and (3) variable extents of melting of the same mantle source. Our modeling indicates that the alkalic samples form from lower extents of melting compared to the tholeiitic lavas and suggests that the deep‐water alkalic lavas are analogous to the initial, preshield building phase observed south of Hawaii and at the base of Loihi Seamount.
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ArticleConstraints on near-ridge magmatism using 40Ar/39Ar geochronology of enriched MORB from the 8°20' N seamount chain(Elsevier, 2023-06-29) Anderson, Molly K. ; Perfit, Michael R. ; Morgan, Leah E. ; Fornari, Daniel J. ; Cosca, Michael A. ; Wanless, Virginia DorseyOur understanding of the spatial-temporal-compositional relationships between off-axis magmatism and mid-ocean ridge spreading centers is limited. Determining the 40Ar/39Ar ages of mid-ocean ridge basalt (MORB) lavas erupting near mid-ocean ridges (MOR) has been a challenge due to the characteristically low K2O contents in incompatible element-depleted normal MORB (NMORB). High-precision 40Ar/39Ar geochronology is used here to determine ages of young, basaltic lavas erupted along the 8°20' N seamount chain west of the East Pacific Rise (EPR) axis that have a range of incompatible element enrichments (EMORB) suitable for 40Ar/39Ar geochronology (e.g., K2O contents > 0.3 wt%). 40Ar/39Ar ages were determined in 29 well-characterized basalts sampled using HOV Alvin and dredging. Detailed geochronology and geochemical analyses provide important constraints on the timing, distribution, and origins of lavas that constructed this extensive volcanic lineament relative to magmatism beneath the adjacent EPR axis. Seamount eruption ages are up to ∼1.6 Ma younger than the underlying lithosphere, supporting a model of prolonged off-axis magmatism for at least 2 Myrs at distances as great as ∼90 km from the ridge axis. Increasing geochemical heterogeneity with eruption distance reflects the diminishing effect of sub-ridge melt focusing. The range of geochemically distinct lavas erupted at given distances from the ridge highlights the dynamic nature of the near-ridge magmatic environment over Myr timescales. Linear ridge-like (EPR-parallel) morphotectonic features erupt the youngest and most incompatible element-enriched lavas of the entire seamount chain, indicating there is a recent change in the influence of mantle heterogeneity and off-axis melt metasomatism on the near-ridge lithospheric mantle. Changes in seamount morphologies are attributed to counter-clockwise rotation and southward migration of the nearby Siqueiros transform over the last few million years.