Maryjo Brounce

A serpentinite-hosted ecosystem in the Southern Mariana Forearc

Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures and vent communities occur in different tectonic settings. The discovery of the Lost City hydrothermal field in the Mid-Atlantic Ridge has stimulated interest in the role of serpentinization of peridotite in generating H2- and CH4-rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments. Abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system in the southern Mariana forearc were discovered during a DSV Shinkai 6500 dive in September 2010. We named this system the “Shinkai Seep Field (SSF).” The SSF appears to be a serpentinite-hosted ecosystem within a forearc (convergent margin) setting that is supported by fault-controlled fluid pathways connected to the decollement of the subducting slab. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.

The effect of primary versus secondary processes on the volatile content of MORB glasses: An example from the equatorial Mid-Atlantic Ridge (5°N-3°S)

We report microanalysis of volatile and trace element compositions, as well as Fe3+/ΣFe ratios, from 45 basaltic glasses from cruise RC2806 along the equatorial Mid-Atlantic Ridge. The along-strike variations in volatiles result from the complex geodynamical setting of the area, including numerous transform faults, variations in ridge depth, melting degree, and source composition. The strongest gradient is centered on 1.7°N and encompasses an increase of H2O, Cl, and F contents as well as high F/Zr ratio spatially coincident with radiogenic isotope anomalies. We interpret these variations as source enrichment due to the influence of the nearby high-μ-type Sierra Leone plume. South of the St. Paul fracture zone, H2O and F contents, as well as H2O/Ce and F/Zr ratios, decrease progressively. This gradient in volatiles is consistent with progressive dilution of an enriched component in a heterogeneous mantle due to the progressive increase in the degree of melting. These two large-scale gradients are interrupted by small-scale anomalies in volatile contents attributed to (1) low-degree melts preferentially sampling enriched heterogeneities near transform faults and (2) local assimilation of hydrothermal fluids in four samples from dredge 16D. Finally, 20 RC2806 samples described as “popping rocks” during collection do not show any difference in volatile content dissolved in the glass or in vesicularity when compared to the RC2806 “nonpopping” samples. Our observations lead us to question the interpretation of the CO2 content in the highly vesicular 2πD43 “popping rock” as being representative of the CO2 content of undegassed mid-ocean ridge basalt.

Temporal evolution of mantle wedge oxygen fugacity during subduction initiation

Arc basalts have a higher proportion of Fe in an oxidized state (Fe^(3+)) relative to Fe^(2+) compared to mid-oceanic ridge basalts (MORBs), likely because slab-derived fluids oxidize the mantle wedge where subduction zone magmas originate. Yet, the time scales over which oxygen fugacity of the mantle wedge changes during subduction initiation and margin evolution are unknown. Fe speciation ratios show that magmas produced during the early stages of subduction in the Mariana arc record oxygen fugacities ∼2× more oxidized than MORB. Mantle wedge oxygen fugacity rises by ∼1.3 orders of magnitude as slab fluids become more involved in melt generation processes, reaching conditions essentially equivalent to the modern arc in just 2–4 m.y. These results constrain existing models for the geochemical evolution of the mantle wedge and suggest that oxidation commences upon subduction initiation and matures rapidly in the portions of the mantle wedge that produce melts. This further implies that sulfide or other reduced phases are not present in the mantle wedge in high enough abundance to prevent oxidation of the magmas that form upon subduction initiation. The arc mantle source is oxidized for the majority of a subduction zone’s lifetime, influencing the mobility of multivalent elements during recycling, the degassing of oxidized volcanic volatiles, and the mechanisms for generating continental crust from the immediate onset of subduction.

The Fina Nagu volcanic complex: Unusual submarine arc volcanism in the rapidly deforming southern Mariana margin

In the Mariana convergent margin, large arc volcanoes disappear south of Guam even though the Pacific plate continues to subduct and instead, small cones scatter on the seafloor. These small cones could form either due to decompression melting accompanying back-arc extension or flux melting, as expected for arc volcanoes, or as a result of both processes. Here, we report the major, trace, and volatile element compositions, as well as the oxidation state of Fe, in recently dredged, fresh pillow lavas from the Fina Nagu volcanic chain, an unusual alignment of small, closely spaced submarine calderas and cones southwest of Guam. We show that Fina Nagu magmas are the consequence of mantle melting due to infiltrating aqueous fluids and sediment melts sourced from the subducting Pacific plate into a depleted mantle wedge, similar in extent of melting to accepted models for arc melts. Fina Nagu magmas are not as oxidized as magmas elsewhere along the Mariana arc, suggesting that the subduction component responsible for producing arc magmas is either different or not present in the zone of melt generation for Fina Nagu, and that amphibole or serpentine mineral destabilization reactions are key in producing oxidized arc magmas. Individual Fina Nagu volcanic structures are smaller in volume than Mariana arc volcanoes, although the estimated cumulative volume of the volcanic chain is similar to nearby submarine arc volcanoes. We conclude that melt generation under the Fina Nagu chain occurs by similar mechanisms as under Mariana arc volcanoes, but that complex lithospheric deformation in the region distributes the melts among several small edifices that get younger to the northeast.

Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth’s oxygenation

Significance Volcanic degassing at Mauna Kea volcanoes decreases the oxidation state of both Fe and S in the magmas, consistent with recent results from Kilauea volcano. The least degassed magmas from Mauna Kea are more oxidized than midocean ridge basalt magmas, possibly due to plate tectonic cycling of material from the oxidized surface to the deep mantle, which is then returned to the surface as a component of buoyant plumes. If the degassing trend observed for Mauna Kea applies to basaltic systems more broadly, decompression does not lead to significant changes in the reducing capacity of the exsolved gases and thus cannot explain the rise of oxygen at the Archean/Proterozoic boundary. The behavior of C, H, and S in the solid Earth depends on their oxidation states, which are related to oxygen fugacity (fO2). Volcanic degassing is a source of these elements to Earth’s surface; therefore, variations in mantle fO2 may influence the fO2 at Earth’s surface. However, degassing can impact magmatic fO2 before or during eruption, potentially obscuring relationships between the fO2 of the solid Earth and of emitted gases and their impact on surface fO2. We show that low-pressure degassing resulted in reduction of the fO2 of Mauna Kea magmas by more than an order of magnitude. The least degassed magmas from Mauna Kea are more oxidized than midocean ridge basalt (MORB) magmas, suggesting that the upper mantle sources of Hawaiian magmas have higher fO2 than MORB sources. One explanation for this difference is recycling of material from the oxidized surface to the deep mantle, which is then returned to the surface as a component of buoyant plumes. It has been proposed that a decreasing pressure of volcanic eruptions led to the oxygenation of the atmosphere. Extension of our findings via modeling of degassing trends suggests that a decrease in eruption pressure would not produce this effect. If degassing of basalts were responsible for the rise in oxygen, it requires that Archean magmas had at least two orders of magnitude lower fO2 than modern magmas. Estimates of fO2 of Archean magmas are not this low, arguing for alternative explanations for the oxygenation of the atmosphere.