Katherine A. Kelley

Water and the Oxidation State of Subduction Zone Magmas

Tracing Mantle Oxidation The chemical composition of the Earths mantle varies with tectonic setting. For example, basaltic melts near subduction zones are more oxidized than magma near divergent plate boundaries. Kelley and Cottrell (p. 605; see the Perspective by Hirschmann) examined melts formed in different tectonic environments, using highly sensitive synchrotron-based analytical methods. The oxidation state of Fe increased with water content and mobile trace elements concentrations. Thus, fluids released from wet subducting plates drive mantle oxidation above subduction zones, which may help to explain the spatial differences in oxygen fugacity of the mantle. Oxidation of Earth’s mantle at subduction zones is caused by fluids released from the melting of subducting plates. Mantle oxygen fugacity exerts a primary control on mass exchange between Earth’s surface and interior at subduction zones, but the major factors controlling mantle oxygen fugacity (such as volatiles and phase assemblages) and how tectonic cycles drive its secular evolution are still debated. We present integrated measurements of redox-sensitive ratios of oxidized iron to total iron (Fe3+/ΣFe), determined with Fe K-edge micro–x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H2O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe3+/ΣFe ratios increase toward subduction zones (at ridges, 0.13 to 0.17; at back arcs, 0.15 to 0.19; and at arcs, 0.18 to 0.32) and correlate linearly with H2O content and element tracers of slab-derived fluids. These observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge.

Mantle melting as a function of water content beneath back-arc basins

Subduction zone magmas are characterized by high concentrations of H_(2)O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H_(2)O concentration of the mantle (H_(2)O_o ) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO_2 as a proxy for F, then use F to calculate H_(2)O_o from measured H_(2)O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H_(2)O or more in their mantle sources and define positive, approximately linear correlations between H_(2)O_o and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°–1500°C), correlate with variations in axial depth and wet melt productivity (∼30–80% F/wt % H_(2)O_o ). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H_(2)O_o are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H_(2)O. These results contrast with those from ridges, which record low H_(2)O_o (<0.05 wt %) and broadly negative correlations between H_(2)O_o and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H_(2)O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings.

Prediction of magmatic water contents via measurement of H2O in clinopyroxene phenocrysts

Water is fundamental to magma genesis, evolution, and eruption. Few direct measurements of magmatic H 2 O exist, however, because rocks found at the surface have extensively degassed upon eruption. Olivine-hosted melt inclusions provide a standard approach to measur ing volatiles in undegassed magma, but many volcanic deposits do not contain melt inclusions large enough for analysis (>30 μm), or olivine at all. Here we use an Al IV -dependent partitioning relationship to calculate magmatic H 2 O from direct measurements of H 2 O in clinopyroxene phenocrysts. We test this approach using phenocrysts from four arc volcanoes (Galunggung, Irazu, Arenal, and Augustine) that span the global range in H 2 O contents as measured in olivine-hosted melt inclusions (from 0.1 to 7 wt% H 2 O). The average and maximum magmatic H 2 O contents calculated from the clinopyroxene measurements agree within 15% of the melt inclusion values for most of the samples. The evolutionary paths recorded in H 2 O-Mg# variations overlap in some clinopyroxene and olivine-hosted melt inclusion populations, and in others, the clinopyroxenes record a larger portion of the liquid line of descent or a different portion of the magma system. Thus, the use of phenocrysts to estimate magmatic H 2 O contents creates a new and powerful tool in igneous petrology and volcanology.

Use of herbal treatments in pregnancy

OBJECTIVE Interest in herbal treatments has increased without data on safety, efficacy, or rates of use in pregnancy. We examined antenatal herbal and natural product use among mothers of nonmalformed infants in 5 geographic centers. STUDY DESIGN We used data on nonmalformed infants from the Slone Epidemiology Centers case-control surveillance program for birth defects to examine rates and predictors of herbal use. Exposures were identified through maternal interview. In addition to overall use, 5 categories based on traditional uses and 2 natural product categories were created; topical products and herbal-containing multivitamins were excluded. RESULTS Among 4866 mothers of nonmalformed infants, 282 (5.8%) reported use of herbal or natural treatments. Use varied by study center and increased with increasing age. CONCLUSION Although rates of use are low, there remains a need for investigation of the safety of these products. Given sparse data on efficacy, even small risks might well outweigh benefits.

Probing the Pacific’s oldest MORB glass: mantle chemistry and melting conditions during the birth of the Pacific Plate

Major element chemistry of basalt from the southern East Pacific Rise (EPR) is different from that of the EPR at the time of the formation of the Pacific Plate at 170 Ma.Glass recovered from Jurassic age (170 Ma) Pacific ocean crust [Bartolini and Larson, Geology 29 (2001) 735^738] at Ocean Drilling Program Hole 801C records higher Fe8 (10.77 wt%) and marginally lower Na8 (2.21 wt%) compared to the modern EPR, suggesting deeper melting and a temperature of initial melting that was 60‡C hotter than today.Trace element ratios such as La/Sm and Zr/Y, on the other hand, show remarkable similarities to the modern southern EPR, indicating that Site 801 was not generated on a hotspot-influenced ridge and that mantle composition has changed little in the Pacific over the past 170 Ma.Our results are consistent with the observation that mid-ocean ridge basalts (MORBs) older than 80 Ma were derived by higher temperature melting than are modern MORBs [Humler et al., Earth Planet. Sci. Lett. 173 (1999) 7^23], which may have been a consequence of the Cretaceous superplume event in the Pacific.Site 801 predates the formation of Pacific oceanic plateaus and 801C basalt chemistry indicates that higher temperatures of mantle melting beneath Pacific ridges preceded the initiation of the superplume.F 2002 Published by Elsevier Science B.

Volatile cycling of H2O, CO2, F, and Cl in the HIMU mantle: A new window provided by melt inclusions from oceanic hot spot lavas at Mangaia, Cook Islands

Mangaia hosts the most radiogenic Pb-isotopic compositions observed in ocean island basalts and represents the HIMU (high µ = 238U/204Pb) mantle end-member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end-member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine-hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end-member. We also report major and trace element abundances and Pb-isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances postmantle melting, including diffusive Fe and H2O loss, degassing, and assimilation. H2O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H2O/Ce ratios vary up to ∼200, and are consistent with significant dehydration of oceanic crust during subduction and long-term storage in the mantle. CO2 concentrations range up to 2346 ppm CO2 in the inclusions. Relatively high CO2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 ± 9; 2σ standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 ± 0.028) fall in the range of pristine mantle (0.02–0.08).

Redox Heterogeneity in Mid-Ocean Ridge Basalts as a Function of Mantle Source

Redox Recycling Plate tectonics drive the continuous exchange of material between Earths crust and mantle. Subduction adds crustal materials to the mantle, which influence the composition of erupted lavas at mid-ocean ridges. Because chemical and physical processes in the mantle change over time as a response to the availability of oxygen, the redox state of mid-ocean ridge basalts may trace the history of recycling between crust and mantle. Cottrell and Kelley (p. 1314, published online 2 May) analyzed the relation between the oxidation state of iron in a global suite of mid-ocean ridge basalts and tracers for mantle source composition. Over tectonic time scales, the recycling of reduced carbon in ancient crustal sediments may result in the preservation of more reduced zones in the mantle. Subducted carbon from ancient oceanic crust results in a more reduced mantle. The oxidation state of Earth’s upper mantle both influences and records mantle evolution, but systematic fine-scale variations in upper mantle oxidation state have not previously been recognized in mantle-derived lavas from mid-ocean ridges. Through a global survey of mid-ocean ridge basalt glasses, we show that mantle oxidation state varies systematically as a function of mantle source composition. Negative correlations between Fe3+/ΣFe ratios and indices of mantle enrichment—such as 87Sr/86Sr, 208Pb/204Pb, Ba/La, and Nb/Zr ratios—reveal that enriched mantle is more reduced than depleted mantle. Because carbon may act to simultaneously reduce iron and generate melts that share geochemical traits with our reduced samples, we propose that carbon creates magmas at ridges that are reduced and enriched.

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.

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

Despite the role of the Alaska-Aleutian megathrust as the source of some of the largest earthquakes and tsunamis, the history of its pre–twentieth century tsunamis is largely unknown west of the rupture zone of the great (magnitude, M 9.2) 1964 earthquake. Stratigraphy in core transects at two boggy lowland sites on Chirikof Island’s southwest coast preserves tsunami deposits dating from the postglacial to the twentieth century. In a 500-m-long basin 13–15 m above sea level and 400 m from the sea, 4 of 10 sandy to silty beds in a 3–5-m-thick sequence of freshwater peat were probably deposited by tsunamis. The freshwater peat sequence beneath a gently sloping alluvial fan 2 km to the east, 5–15 m above sea level and 550 m from the sea, contains 20 sandy to silty beds deposited since 3.5 ka; at least 13 were probably deposited by tsunamis. Although most of the sandy beds have consistent thicknesses (over distances of 10–265 m), sharp lower contacts, good sorting, and/or upward fining typical of tsunami deposits, the beds contain abundant freshwater diatoms, very few brackish-water diatoms, and no marine diatoms. Apparently, tsunamis traveling inland over low dunes and boggy lowland entrained largely freshwater diatoms. Abundant fragmented diatoms, and lake species in some sandy beds not found in host peat, were probably transported by tsunamis to elevations of >10 m at the eastern site. Single-aliquot regeneration optically stimulated luminescence dating of the third youngest bed is consistent with its having been deposited by the tsunami recorded at Russian hunting outposts in 1788, and with the second youngest bed being deposited by a tsunami during an upper plate earthquake in 1880. We infer from stratigraphy, 14C-dated peat deposition rates, and unpublished analyses of the island’s history that the 1938 tsunami may locally have reached an elevation of >10 m. As this is the first record of Aleutian tsunamis extending throughout the Holocene, we cannot estimate source earthquake locations or magnitudes for most tsunami-deposited beds. We infer that no more than 3 of the 23 possible tsunamis beds at both sites were deposited following upper plate faulting or submarine landslides independent of megathrust earthquakes. If so, the Semidi segment of the Alaska-Aleutian megathrust near Chirikof Island probably sent high tsunamis southward every 180–270 yr for at least the past 3500 yr.