Explore the vast range of titles available.

Mount St. Helens (MSH) volcano, in the southern Washington Cascades arc, has produced dominantly dacitic to andesitic magmatic products over the last 300 ka. Basaltic to basaltic andesitic magmas erupted only during the relatively brief (ca. 2100-1800 yr B.P.) Castle Creek period from vents separated by no more than a few kilometers. They provide a unique perspective on the evolution of this volcano. Despite close temporal and spatial proximity, these mafic magmas define two distinct compositional lineages: (1) low-K tholeiites (LKT) and (2) basalts of \"oceanic island\" or intraplate affinity (or IPB). Both lack typical arc geochemical signatures and appear to derive from distinct mantle sources, neither of which has been significantly modified by slab-derived fluid or melt components. No true calc-alkalic basalts have erupted from MSH despite its obvious arc setting. Each lineage includes derivative lavas that range from ∼7 to 5 wt% MgO and ∼49 to 55 wt% SiO2, and both are slightly porphyritic with dominantly olivine and plagioclase, minor spinel, and trace clinopyroxene in some but not all samples. With respect to incompatible elements (e.g., K, La, Nb, Th, etc.), compositional trends for the two lineages are dramatically different and inconsistent with simple fractional crystallization processes. The data instead suggest that each lineage was produced dominantly by mixing between distinct parental LKT and IPB basaltic magmas and material of intermediate composition roughly similar to average MSH andesite. Mineralogical characteristics of macrocrysts in MSH basalts indicate that they do not represent equilibrium assemblages. Olivine compositions and textures in some samples implicate accumulation of crystals formed from multiple magmas, and evidence for magma mixing is reinforced by the rare presence of \"blebs\" of rhyolitic glass. These assemblages of crystals presumably are derived from different magmas and/or older MSH magmatic products (including crystal mush zones) within crustal conduit systems. Extrapolation of compositional trends (\"mixing arrays\") to higher MgO content implicates the involvement of three types of parental magma: primitive LKT as well as distinct nepheline- and hypersthene-normative IPBs (or ne-IPB and hy-IPB), variants of which have erupted repeatedly from monogenetic vents in this sector of the Cascades. Such magmas are interpreted to form from distinct lherzolitic mantle sources (less fertile, with lower clinopyroxene content for LKT) at depths on the order of 80 (LKT) and 50-60 (IPBs) kilometers, under near-anhydrous conditions, in response to decompression rather than flux-melting. We also report a set of self-consistent estimates of temperature, pressure, water content, magma density, and weight fraction of \"andesitic\" mixing component for samples of each lineage. These parameters are highly correlated and serve to constrain the structure of the magma feeder system beneath MSH. A dynamic continuum of melt compositions is likely present, controlled principally by temperature and density gradients within the system. We envisage that during Castle Creek time the most primitive basaltic magmas formed distinct reservoirs in the deep crust, with the ne-IPB variant near 28 km depth and the LKT variant near 23 km. More silicic members of these lineages appear to have evolved at depths between ∼20-15 km. We suggest that reservoir depths were controlled mainly by magma density that, in turn, is largely determined by the degree of mixing with \"andesitic\" components at crustal depths. This configuration implies a vertical magmatic plexus with connections extending well into the upper mantle. The sharp chemical distinction between the LKT and IPB mixing arrays suggests that the respective feeder systems were isolated and rarely interacted despite their close proximity. Finally, it appears that the presence of large, complex, and long-lived conduit systems beneath stratovolcanoes can act as \"magma traps,\" within which deeper-seated (mantle) inputs are prone to modification by interaction with stored magmas and their differentiation products. In contrast, the occurrence of relatively primitive basalts from monogenetic vents distal from stratovolcanoes implies that diverse basaltic magmas ascend beneath virtually the entire arc segment and that the true complexity of this \"mantle wind\" is locally masked by modifications within the crust.

Occurrences of high-Mg andesite (HMA) in modern volcanic arcs raise the possibility that significant volumes of continental crust could be directly derived from Earth's mantle. Such rocks are commonly associated with subduction of young, warm oceanic lithosphere or occur in areas heated by mantle convection. A relatively rare occurrence near Mt. Shasta in the Cascades volcanic arc has been considered to represent one such primary mantle-derived magma type, from which more evolved andesitic and dacitic magmas are derived. Recognition that the Shasta area HMA is actually a hybrid mixed magma, calls into question this notion as well as the criteria upon which it is based. We report new chemical and mineralogical data for samples of the Shasta HMA that bear on the components and processes involved in its formation. Several generations of pyroxenes and olivines are present along with different generations of oxide minerals and melt inclusions. The most magnesian olivines (Fo93) exhibit disequilibria textures, exotic melt inclusions, and reaction rims of Fo87 composition; these crystals along with spongy, ∼Mg#87 orthopyroxene crystals are interpreted to be xenocrystic and do not signify a primitive mantle derivation. The groundmass is andesitic with moderate MgO content, and melt inclusions of similar compositions are hosted by equilibrium olivine (∼Fo87). The bulk magma (whole rock) is more magnesian, but primarily due to incorporation of mafic minerals and ultramafic xenolith debris. We propose that the exotic crystal and lithic debris in these rocks is derived from (1) dacitic magmas of possible crustal derivation, (2) prograded ultramafic rocks in the underlying crust, and (3) random lithic debris and crystals derived from conduit wall rocks and earlier intruded magmas within the feeder plexus beneath Shasta. The HMA is inferred to represent a mixture between evolved dacitic and primitive basaltic magmas as well as incorporation of xenolithic crystal cargo. There is no compelling evidence that HMA is present in large volumes, and it is not considered to be an important parental liquid to more evolved magmas at Shasta.

The redox state of arc mantle The redox state of the upper mantle beneath arcs is relevant to understanding mantle melting and melt differentiation. Because we have no access to samples from the convective mantle, this question has to be addressed indirectly using information derived from the chemistry of melts. Cin-Ty Lee et al . show that the ratio of zinc to total iron content constrains the valence state of iron in primary arc basalts and their mantle sources. They find that primitive arc magmas have Zn/Fe ratios identical to those of mid-ocean ridge basalts, hinting at a similar iron oxidation state of primary mantle melts in arcs and ridges. The results suggest that the subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. They conclude that the observed higher oxidation states of arc lavas must therefore be, in part, a consequence of shallow-level differentiation processes. Here it is shown that the ratio of zinc to total iron content constrains the valence state of iron in primary arc basalts and their mantle sources. Primitive arc magmas have identical Zn/Fe T ratios (Fe T = Fe 2+ + Fe 3+ ) as mid-ocean-ridge basalts, indicating a similar iron oxidation state of primary mantle melts in arcs and ridges and that the subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. It is concluded that the observed higher oxidation states of arc lavas must therefore be, in part, a consequence of shallow-level differentiation processes. Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle 1 , 2 , 3 , 4 . As a consequence, the sub-arc mantle wedge is widely believed to be oxidized 3 , 5 . The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source 6 , 7 . Here we show that Zn/Fe T (where Fe T = Fe 2+  + Fe 3+ ) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe 2+ and Zn behave similarly, but because Fe 3+ is more incompatible than Fe 2+ , melts generated in oxidized environments have low Zn/Fe T . Primitive arc magmas have identical Zn/Fe T to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/Fe T during early differentiation involving olivine requires that Fe 3+ /Fe T remains low in the magma. Only after progressive fractionation does Fe 3+ /Fe T increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood.

For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes. Subduction of fracture zones is predicted to have local geochemical and physical manifestations in volcanic arcs. Here, the authors show boron enrichment near fracture zones in some arcs and infer the processes occurring there using detailed geodynamic modelling.

The United States established an academy for educating future army officers at West Point in 1802. Why, then, did it take this maritime nation forty-three more years to create a similar school for the navy? The Long Road to Annapolis examines the origins of the United States Naval Academy and the national debate that led to its founding. Americans early on looked with suspicion upon professional military officers, fearing that a standing military establishment would become too powerful, entrenched, or dangerous to republican ideals. Tracing debates about the nature of the nation, class identity, and partisan politics, William P. Leeman explains how the country's reluctance to establish a national naval academy gradually evolved into support for the idea. The United States Naval Academy was finally established in 1845, when most Americans felt it would provide the best educational environment for producing officers and gentlemen who could defend the United States at sea, serve American interests abroad, and contribute to the nation's mission of economic, scientific, and moral progress. Considering the development of the naval officer corps in relation to American notions of democracy and aristocracy, The Long Road to Annapolis sheds new light on the often competing ways Americans perceived their navy and their nation during the first half of the nineteenth century.

This article concerns application of cathodoluminescence (CL) spectroscopy to volcanic quartz and its utility in assessing variation in trace quantities of Ti within individual crystals. CL spectroscopy provides useful details of intragrain compositional variability and structure but generally limited quantitative information on element abundances. Microbeam analysis can provide such information but is time-consuming and costly, particularly if large numbers of analyses are required. To maximize advantages of both approaches, natural and synthetic quartz crystals were studied using high-resolution hyperspectral CL imaging (1.2–5.0 eV range) combined with analysis via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Spectral intensities can be deconvolved into three principal contributions (1.93, 2.19, and 2.72 eV), for which intensity of the latter peak was found to correlate directly with Ti concentration. Quantitative maps of Ti variation can be produced by calibration of the CL spectral data against relatively few analytical points. Such maps provide useful information concerning intragrain zoning or heterogeneity of Ti contents with the sensitivity of LA-ICPMS analysis and spatial resolution of electron microprobe analysis.

Continents ride high above the ocean floor because they are underlain by thick, low-density, Si-rich, and Mg-poor crust. However, the parental magmas of continents were basaltic, which means they must have lost Mg relative to Si during their maturation into continents. Igneous differentiation followed by lower crustal delamination and chemical weathering followed by subduction recycling are possible solutions, but the relative magnitudes of each process have never been quantitatively constrained because of the lack of appropriate data. Here, we show that the relative contributions of these processes can be obtained by simultaneous examination of Mg and Li (an analog for Mg) on the regional and global scales in arcs, delaminated lower crust, and river waters. At least 20% of Mg is lost from continents by weathering, which translates into >20% of continental mass lost by weathering (40% by delamination). Chemical weathering leaves behind a more Si-rich and Mg-poor crust, which is less dense and hence decreases the probability of crustal recycling by subduction. Net continental growth is thus modulated by chemical weathering and likely influenced by secular changes in weathering mechanisms.

New U-Pb zircon ages are reported for granulite facies crustal xenoliths brought to the surface by mafic lavas in the Snake River Plain. All samples yield Meso-to-Neoarchean ages (2.4–3.6 Ga) that significantly expand the known extent of the Archean Wyoming Craton at least as far west as the west-central Snake River Plain. Most zircon populations indicate multiple growth episodes with complexity increasing eastward, but they bear no record of major Phanerozoic magmatic episodes in the region. To extrapolate this work further west to the inferred craton boundary, zircons from southwestern Idaho batholith granodiorites were also analyzed. Although most batholith zircons record Cretaceous formation ages, all samples have zircons with inherited cores—with some recording Proterozoic ages (approaching 2 Ga). These data enhance our perspectives regarding lithosphere architecture beneath southern Idaho and adjacent areas and its possible influence on Cenozoic magmatism associated with the Snake River Plain–Yellowstone “melting anomaly”.

There were three errors in the original publication [...]

Lavas from transects across the Kurile Islands arc showed geochemical variations related to changes in the compositions of fluids derived from the subducting slab. Enrichment factors for boron, cesium, arsenic, and antimony, all elements with strong affinities for water, decreased across the arc. This decrease is presumably related to losses of water-rich fluids during the dehydration of the subducting plate. Enrichments of potassium, barium, beryllium-10, and the light rare earth elements remained constant; these species may move in silica-rich fluids liberated from the slab at greater depths.