William P. Leeman

Basic magmatism associated with Late Cenozoic extension in the western United States: Compositional variations in space and time

Widespread basic magmatism across much of the western United States in the late Cenozoic followed the cessation of subduction along the Pacific coast. This volcanism accompanied lithospheric extension, block faulting and regional uplift In an attempt to assess the relative contribution of asthenosphere and mantle lithosphere to magmas across the western United States we have analyzed, for major and trace elements, a suite of 750 basic (MgO>4%) lava samples from all the major volcanic fields in the region. The data were divided into seven sets representing the main tectonomagmatic provinces: Basin and Range (BR), Western Great Basin, Transition Zone (TZ), Colorado Plateau, Snake River Plain, Southern Rocky Mountains, and Great Plains. It was further divided into relatively recent ( 5 Ma) subsets on the basis of field relations and K-Ar data. The 5 Ma) subset shows no great differences between the BR and the other tectonomagmatic provinces; all have high La/Nb and Ba/Nb. Crustal contamination alone cannot be responsible for these variations. We conclude that many of the magmas have inherited their chemical and isotopic characteristics from a lithospheric mantle source enriched by fluids expelled from a subducted slab. Pelagic sediment, returned to the mantle by subduction, is a possible agent for fluids rich in Ba, radiogenic Sr and unradiogenic Nd, but very poor in Nb. At least some of this enrichment must have accompanied the formation of the Proterozoic crust. It appears that subduction-enriched lithospheric mantle was involved in the generation of all extension-related basic magmas across the western United States until relatively recently. Only in the younger BR and parts of the TZ have asthenosphere-derived magmas, uncontaminated by lithosphere, reached the surface. These observations conflict with models in which uplift and extension are caused by the replacement of mantle lithosphere by asthenosphere. Triey are best explained by the progressive erosion of the lithospheric manue over a plume currently located beneath the Southern Rocky Mountains. Supplemental data are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B91-001;

Cross-Arc Geochemical Variations in the Kurile Arc as a Function of Slab Depth

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Fractionation of Trace Elements by Subduction-Zone Metamorphism — Effect of Convergent-Margin Thermal Evolution

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.

High-Magnesian Andesite from Mount Shasta: A product of Magma Mixing and Contamination, not a Primitive Mantle Melt

Differential chemical/isotopic alteration during forearc devolatilization can strongly influence the cycling of volatile components, including some trace elements, in subduction zones. The nature and magnitude of this devolatilization effect are likely to be strongly dependent on the thermal structure of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at ≤45 km depths) of rock packets metamorphosed along successively lower-T prograde P-T paths in a rapidly cooling, newly initiated subduction zone, affords a unique evaluation of the effects of varying prograde P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting rocks. In the Catalina Schist, the most extensive devolatilization occurred in metasedimentary rocks which experienced prograde P-T paths encountering the epidote-blueschist facies (>350°C at 9 to 12 kbar) or higher-T conditions; such rocks are depleted in ‘fluid-mobile’ elements such as N, B, Cs, As, and Sb relative to protoliths. Removal of these elements resulted in changes in B/(Be, Li, La, Zr), Cs/Th, Rb/Cs, As/Ce, Sb/Ce, and Creduced/N, and increases in δ15N and δ13C. The relative susceptibilities of the “fluid-mobile” elements to loss along increasingly higher-T P-T paths can be categorized. Boron and Cs show the greatest susceptibility to low-T removal by fluids, showing >50% depletion in even lawsonite-blueschist-facies metasedimentary rocks which experienced relatively low-T prograde metamorphic paths. In rocks which experienced higher-T paths, As and Sb (likely in sulfides) show the greatest depletions (>90%); N, Cs, and B (largely in micas) occur at ∼25% of protolith contents in even partially melted amphibolite-facies rocks. Variations in B/Be, Cs/Th, As/Ce, and Sb/Ce among arcs from differing convergent-margin thermal regimes, and conceivably some cross-arc declines in these ratios, are compatible with evidence from the Catalina Schist for varying degrees of element removal as a function of prograde thermal history. In relatively cool subduction zones (e.g., Kuriles, Marianas, Aleutians, southern Alaska) with thermal regimes similar to that which formed the low-grade units of the Catalina Schist (and blueschist-facies rocks in the Franciscan Complex), forearc devolatilization is less profound, B, Cs, As, Sb, and N are more likely to be deeply subducted, and enriched in arc lavas, and significant devolatilization occurs at the blueschist-to-eclogite transition. High-grade units could reflect thermal evolution analogous to that of relatively warm subduction zones (e.g., Cascadia) and back-arcs in which arc lavas are depleted in B, Cs, As, and Sb due to prior removal by forearc devolatilization. The results of this study also imply less efficient recycling of these elements during the warmer Archean subduction which resulted in greater slab melting and production of abundant trondhjemite-tonalite magmatic suites.

The role of hydrothermal fluids in the production of subduction zone magmas: Evidence from siderophile and chalcophile trace elements and boron

It has been proposed that high-Mg andesites (HMAs) from the Mount Shasta area may represent near-primary mantle melts, carrying signatures of slab melt interaction with the Cascadia mantle wedge. We present strong evidence that their formation involved mixing of dacitic and basaltic magmas and entrainment of ultramafic crystal material, and thus they cannot represent primitive magmas. The rocks contain (1) low-Mg# (65–72) clinopyroxene (cpx) and orthopyroxene (opx) phenocryst cores containing dacitic melt inclusions, and (2) high-Mg# opx and olivine xenocrysts, all of which are rimmed by euhedral overgrowths of cpx or opx similar in Mg# (87) to skeletal olivine phenocrysts. Textural relations indicate that ultramafic xenocrysts reacted with dacitic liquid, after which the contaminated magma mixed with basaltic liquid to produce a hybrid HMA bulk composition. High Mg, Cr, and Ni derive from the latter inputs, whereas high Sr/Y and overall adakite affinity is inherited from the dacite end member, which is arguably crustal in origin. We suggest that open system processes may be more important in the petrogenesis of HMAs than generally recognized, and that their magnesian compositions do not necessarily imply that they are primitive mantle melts.

Boron geochemistry of the Central American Volcanic Arc: Constraints on the genesis of subduction-related magmas

Abstract In order to evaluate the processes responsible for the enrichments of certain siderophile/ chalcophile trace elements during the production of subduction-related magmas, representative lavas from seven subduction zones have been analyzed for Pb, As, Sb, Sn, W, Mo, Tl, Cu, and Zn by inductively coupled plasma-mass spectrometry (ICP-MS), radiochemical epithermal neutron activation analysis (RENA), and atomic absorption (AA). The siderophile/chalcophile elements are compared to the highly fluid-mobile element B, the light rare earth elements (LREEs), U, and Th in order to place constraints on their behavior in subduction zones. Boron, As, Sb, and Pb are all enriched in arc lavas and continental crustal rocks more so than expected assuming normal magmatic processes (melting and crystallization). Tin, W, and Mo show little evidence of enrichment. Correlations of Pb/Ce, As/Ce, and Sb/Ce with B/La are statistically significant and have high correlation coefficients (and, more importantly, slopes approaching one) suggesting that Pb, As, and Sb behave similarly to B (i.e., that they are fluid-mobile). In addition, across-arc traverses show that B/La, As/Ce, Pb/Ce, and Sb/Ce ratios decrease dramatically with distance towards the back-arc basin. W/Th, Tl/La, Sn/Sm, and Mo/Ce ratios and Cu and Zn concentrations have much less systematic across-arc variations and correlations with B/La are not as strong (and in some cases, not statistically significant) and the regression lines have much lower slopes. Mixing models between upper mantle, slab-derived fluid, and sediment are consistent with a fluid-derived component in the arcs displaying extra enrichments of B, Pb, As, and Sb. These observations imply efficient mobilization of B, Pb, As, Sb, and possibly Tl into arc magma source regions by hydrothermal fluids derived from metamorphic dehydration reactions within the slab. Tin, W, and Mo show little, if any, evidence of hydrothermal mobilization. Copper appears to be slightly enriched in arc lavas relative to mid-ocean ridge basalts (MORBs) whereas Zn contents of arc lavas, MORB, ocean island basalts (OIBs), and continental crustal samples are similar suggesting that the bulk partition coefficient for Zn is approximately equal to one. However, Zn contents of the upper mantle are lower than these reservoirs implying an enrichment of the source region in Zn prior to melting. These nonigneous enrichments have implications not only for arc magma genesis but also for continental crust formation and crust-mantle evolution. The mobility of Pb, As, Sb, and B in hot, reducing, acidic hydrothermal fluids may be greatly enhanced relative to the large-ion lithophile elements (LILEs; including U) as a result of HS−, H2S, OH−, or other types of complexing. In the case of Pb, continued transport of Pb from subducted slabs into arc magma source regions throughout Earth history coupled with a U fluxing of the mantle a the end of the Archean may account for the depletion of Pb in the upper mantle, the low U/Pb of most arc volcanics and continental crustal rocks, and provide an explanation for the Pb- Paradox (Hofmann et al., 1986; McCulloch, 1993; Miller et al., 1994) . Recycled slabs will then retain high U/Pb ratios upon entering the deep mantle and may eventually become incorporated into the source regions of many OIBs; some with HIMU (high 238U/204Pb) signatures.

Boron depletion during progressive metamorphism: Implications for subduction processes

Boron contents were measured in representative Quaternary lavas from the Central American Volcanic Arc to evaluate along-strike variations in subduction processes. Despite the significant range in B concentrations (~2–37 ppm) in the mafic lavas, B/La ratios vary in a systematic fashion along the arc; higher values (> 1) are typical between Guatemala and northern Costa Rica, whereas low values (most <0.5) typify central Costa Rica and western Panama. B/La is highly correlated with 10Be/9Be (r2 = 0.94, excluding one sample) and appears to be a useful indicator of subduction contributions to the magma sources. If enrichments of both B and 10Be are proportional to the flux of subducted sediment, along-strike variations in B/La suggest at least a twofold variation in this flux with maximum values below western Nicaragua and minimum values below Costa Rica and western Panama where the Cocos Ridge is being subducted. These data may also reflect significant differences in thermal state of the descending slab, which in turn differentially affects release patterns of fluids and fluid-mobile trace elements, and possibly melting processes beneath different parts of the arc. The following scenario is suggested to explain the geochemical results. Beneath the northwestern part of the arc steep subduction of older, relatively cold slab favors more efficient subduction of fluid components to depths beneath the volcanic front. The released fluids carry fluid-mobile elements to the overlying mantle, which upon melting produces calcalkalic magmas. Shallow subduction of warmer slab beneath the southeastern part of the arc favors shallow release of fluids and limits fluid-related metasomatism of sub-arc mantle beneath the volcanic front. Under such conditions, B/La and Ba/La ratios in the sub-arc mantle vary little from values seen in oceanic island basalts. Magmas in this part of the arc nevertheless display the highest La/Yb and lowest Ba/La and B/La ratios, which are consistent with prior light rare-earth-element enrichment in the source, significantly lower degrees of melting, or a combination thereof. Because some of the largest volcanoes occur in Costa Rica, and magma flux there is nearly an order of magnitude higher than elsewhere in the arc, source enrichment is considered to be the more plausible explanation. It is proposed that Quaternary magma production below Costa Rica involved lithospheric sources containing trapped or stored melt components, but this enrichment process is unlikely to have involved typical arc magmas or subduction fluids because we see no B-enrichment.

BBe systematics in subduction-related metamorphic rocks: Characterization of the subducted component

Systematic differences in trace element compositions of volcanic arc versus intraplate magmas are commonly attributed to involvement of subducted sediments and/or seawater-altered oceanic crust in arc magma genesis. The exact contributions from these materials is poorly understood. Metamorphic processes may significantly modify subducted oceanic slabs before they reach subarc depths. The nature and magnitude of such compositional modifications, as exemplified by the element boron, are investigated in metamorphic suites having protoliths analogous to subducted marine sediments and basalts. Our results indicate that in addition to protolith type, B content depends on metamorphic temperature. Greenschist and amphibolite facies metasediments generally have B contents at least a factor of two lower than in equivalent unmetamorphosed sediments. Moreover, four low-pressure pelitic to semipelitic metasediment suites display progressive loss of B with increasing peak metamorphic temperature (ca. 200–750°C). Metabasalts of greenschist and higher grades also have consistently lower B contents (</ 10 ppm) than estimated for altered oceanic crust. Granulites and migmatites are invariably depleted in B (typically < 3 ppm). Most subducted B initially resides in marine sediment and altered oceanic crust (i.e., within the uppermost kilometer). Our observations suggest that B may be extracted by fluids released by devolatilization reactions upon progressive heating during subduction. The relatively high B contents observed in many arc basalts imply that slab temperatures at subarc depths commonly do not exceed conditions at which B is systematically depleted. Otherwise, subducted slabs would retain insufficient quantities of B to balance its inventory in arc magmas. Using the temperature dependence of B depletion estimated from metamorphic suites, maximum temperatures are unlikely to exceed ∼ 800–900°C for uppermost portions of subducted slabs beneath arcs which show typical B enrichments. In contrast, arc basalts commonly have eruptive temperatures approaching the dry liquidus (ca. 1300°C) of subducted oceanic crust. By the time such temperatures are reached in subducted slabs, it is likely that H2O, B and other fluid-mobile elements are strongly depleted by metamorphic processes. Thus, slab melting does not appear to be a viable process for generating typical arc basalts. A more plausible scenario that is commonly invoked for production of arc basaltic magmas involves migration of aqueous fluid and fluid-mobile elements from the slab to hotter regions in the mantle wedge. Such fluids could metasomatize portions of the mantle wedge and induce melting to produce B-rich arc magmas.

Olivine/liquid distribution coefficients and a test for crystal-liquid equilibrium

The mobility of B and Be in H2O-rich fluids and felsic silicate liquids produced during metamorphism of subducted oceanic slab and sediments has been investigated through analysis of subduction-zone metamorphic rocks of the Catalina Schist, California. In metasedimentary rocks, B/Be and the range in B/Be decrease with increasing metamorphic grade (mean = 72, std. dev. = 41 for lowest-grade lawsonite-albite fades rocks; mean = 21, std. dev. = 11 for higher-grade greenschist and epidote-amphibolite facies equivalents). This decrease to more uniformly low B/Be may be attributed to the preferential removal of B in H2O-rich fluids produced by devolatilization reactions over the approximate temperature interval of 350–600°C. Metamafic rocks do not show pronounced decrease in B/Be with increasing metamorphic grade; however, all metamafic samples have B/Be< 30, lower than values for many altered seafloor basalts. In amphibolite-grade exposures, felsic leucosomes and pegmatites reflecting partial melting have low B/Be similar to their metasedimentary and metamafic hosts, which presumably experienced prior reduction in B/Be during lower temperature devolatilization. This evidence for B and Be mobility during high-P/T metamorphism complements studies of B-Be systematics in arc volcanic rocks in further characterizing mechanisms by which slab-derived elements can be added to the source regions of arc lavas. Before subducted mafic and sedimentary rocks reach Wadati-Benioff zone depths beneath arcs ( 80–150 km), the B/ Be of these rocks is likely to have decreased to <30. Thus, highly fractionated, slab-derived hydrous fluids may be necessary to generate the high-B/ Be signatures observed in many arcs (B/Be of up to ~200). The B-Be data, together with previously presented stable isotope data for the Catalina Schist, demonstrate that subduction-zone metamorphic processes are capable of homogenizing presubduction variability in the concentrations of particularly “fluid-mobile” elements in rocks and may, through mixing, produce fluids which trend toward uniform trace element and isotopic compositions. These homogeneous fluids could infiltrate parts of the mantle wedge and contribute to the characteristic trace element and isotopic signatures of arc-magma source regions. In hotter subduction zones ( e.g., involving subduction of young, hot oceanic lithosphere ), silicate melts derived from previously devolatilized sedimentary and mafic rocks may contribute relatively low-B/Be signatures to arc source regions. Thus, significant variations, among arcs, in the ranges of B/Be observed in front-rank volcanoes (e.g., for the Bismarck arc, B/Be = 20–190; for the Cascades arc, B/ Be < 5) may be related in part to varying thermal structure, which could govern both the B/Be of hydrous fluids and the relative proportions of hydrous fluid and silicate melt derived from the subducted slab and sediments.

Strontium, neodymium and lead isotopic compositions of deep crustal xenoliths from the Snake River Plain: evidence for Archean basement

We present major and trace element data for olivines and rapidly quenched groundmass material separated from eight samples of basaltic rock. From these data apparent olivine/liquid distribution coefficients (D) for the elements Mg, Mn, Ni, Co, Cr, Sc, Na, Sm, and Ca have been calculated. Petrographic and electron microprobe studies indicate that olivines in most of these samples are but slightly zoned and groundmass glasses are essentially uniform in composition; these data suggest that the co-existing olivine-groundmass pairs are in equilibrium. In addition, olivine crystallization temperatures were calculated using experimental calibrations of lnD vs.1/T for the elements Mg, Fe, Mn, Ni, and Co. A high degree of concordancy for the resulting temperatures, based on five different elements for most samples, also suggests that olivine-groundmass equilibrium was obtained. We conclude that the apparent olivine/liquidDs derived from this study are representative of equilibriumDs, but emphasize that variations in temperature, and possibly bulk composition, strongly affect suchDs in natural magmas.