Fluid-mobile Trace Elements Research Articles

Estimation of temperature rise in a shallow slip zone of the megasplay fault in the Nankai Trough

A megasplay fault branching from the subduction boundary megathrust in the Nankai Trough is thought to have caused large tsunamis associated with past Tonankai earthquakes. Because the temperature recorded in the fault can constrain parameters of seismic slip, we evaluated several temperature proxies (fluid-mobile trace element concentrations, Sr isotopes, magnetic minerals, inorganic carbon content, and Raman spectra of carbonaceous material) in material from a localized slip zone, sampled at Site C0004 by Integrated Ocean Drilling Program Expedition 316. Our results showed that the shallow part of the megasplay fault has not experienced temperatures above 300 °C. We also found that in addition to shear stress and slip distance, slip rate and porosity are effective parameters for frictional heat, as demonstrated by numerical analysis incorporating thermal diffusion. The slip rate estimated through these proxies may indicate that the fault did not slip with high velocity near the seafloor.

Subduction Influence on Oxygen Fugacity and Trace and Volatile Elements in Basalts Across the Cascade Volcanic Arc

Fluids or melts derived from a subducting plate are often cited as a mechanism for the oxidation of arc magmas. What remains unclear is the link between the fluid, oxygen fugacity, and other major and trace components, as well as the spatial distribution of the impact of those fluids. To test the potential effects of addition of a subduction-derived fluid or melt to the sub-arc mantle, olivine-hosted melt inclusions from primitive basaltic lavas sampled from across the central Oregon Cascades (43°-45°N) have been analyzed for major, trace and volatile elements and fO 2 . Oxygen fugacity was determined in melt inclusions from sulfur speciation determined by electron microprobe and from olivine-chromite oxygen geobarometry. The overall range in fO 2 based on sulfur speciation measurements is from <-0·25 log units to + 1·9 log units (ΔFMQ, where FMQ is fayalite-magnetite-quartz buffer). Oxygen fugacity is positively correlated with fluid-mobile trace element and light rare earth element contents in basalts generated by relatively low-degree partial melting. Establishing a further correlation between fO 2 and fluid-mobile trace element abundances with position along the arc requires the basalts to be subdivided into shoshonitic, calc-alkaline, low-K tholeiite and enriched intraplate basalt groups. Melt inclusions from enriched intraplate and shoshonitic lavas show increasing fO 2 and trace element abundances closer to the trench, whereas calc-alkaline melt inclusions exhibit no significant across-arc variations. Low-K tholeiitic melt inclusions record an increase in incompatible trace elements closer to the trench; however, there is no correlated increase in fO 2 . The correlation observed in enriched intraplate and shoshonitic melt inclusions is interpreted to reflect a progressively greater proportion of a fluid-rich, oxidized subduction component in magmas generated nearer the subduction zone. Significantly, calc-alkaline melt inclusions with high ratios of large ion lithophile elements to high field strength elements, characteristic of ‘typical’ arc magmas, have oxidation states indistinguishable from low-K tholeiite and enriched intraplate basalt melt inclusions. The lack of across-arc geochemical variation in calc-alkaline melt inclusions may suggest that these basalts are not necessarily the most appropriate magmas for examining recent addition of a subduction component to the sub-arc mantle. Flux and batch melt model results produce a wide range of predicted amounts of melting and subduction component added to the mantle source; however, general trends characterized by increased melting and proportion of the subduction component from enriched intraplate, to low-K tholeiite, to calc-alkaline are robust. The model results do not require enriched intraplate, low-K tholeiite and calc-alkaline magmas to be produced from the same more fertile mantle source. However, enriched intraplate magmas, in contrast to calc-alkaline and low-K tholeiite magmas, cannot be generated from a depleted mantle source. Flux or batch melting of either the more fertile or depleted mantle sources used to generate the low-K tholeiite, calc-alkaline, and enriched intraplate magmas cannot reproduce shoshonitic compositions, which require a significantly depleted mantle source strongly metasomatized by a subduction component. The potential mantle source for shoshonitic basalts has a predicted fO 2 (after oxidation) from + 0·3 to + 2·4 log units (ΔFMQ) whereas the mantle source for low-K tholeiite, calc-alkaline, and enriched intraplate magmas may range from -1·1 to + 0·7 log units (ΔFMQ).

Iron and lithium isotope systematics of the Hekla volcano, Iceland — Evidence for Fe isotope fractionation during magma differentiation

In this study potential iron isotope fractionation by magmatic processes in the Earth's crust was systematically investigated. High precision iron isotope analyses by MC-ICP-MS were performed on a suite of rock samples representative for the volcanic evolution of the Hekla volcano, Iceland. The whole series of Hekla's rocks results from several processes. (i) Basaltic magmas rise and induce partial melting of meta-basalts in the lower part of the Icelandic crust. The resulting dacitic magma evolves to rhyolitic composition through crystal fractionation. During this differentiation the δ 56/54Fe IRMM-014 values increase successively from 0.051 ± 0.021‰ for the primitive dacites to 0.168 ± 0.021‰ for the rhyolites. This increase can be described by a Rayleigh fractionation model using a constant bulk fractionation factor between all mineral phases (M) and the silicate liquid (L) of Δ 56/54Fe M–L = − 0.1‰. (ii) The basaltic magma itself differentiates by crystal fractionation to basaltic andesite composition. No Fe isotope fractionation was found in this series. All basalts and basaltic andesites have an average δ 56/54Fe IRMM-014 value of 0.062 ± 0.042‰ (2SD, n = 9), identical to mean terrestrial basaltic values reported in previous studies. This observation is consistent with the limited removal of iron from the remaining silicate melt through crystal fractionation and small mineral-melt Fe isotope fractionation factors expected at temperatures in excess of 1050 °C. (iii) Andesites are produced by mixing of basaltic andesite with dacitic melts. The iron isotope composition of the andesites is matching that of the basaltic andesites and the less evolved dacites, in agreement with a mixing process. In the Hekla volcanic suite Li concentrations are positively correlated with indicators of magma differentiation. All Hekla rocks have δ 7Li values typical for the upper mantle and demonstrate the absence of resolvable Li isotope fractionation during crystal fractionation. As a fluid-mobile trace element, Li concentrations and isotopes are a potential tracer of magma/fluid interaction. At Hekla, Li concentrations and isotope compositions do not indicate any extensive fluid exsolution. Hence, the heavy Fe isotope composition of the dacites and rhyolites can be predominately attributed to fractional crystallisation. Iron isotope analyses on single samples from other Icelandic volcanoes (Torfajökull, Vestmannaeyjar) confirm heavy Fe isotope enrichment in evolving magmas. Our results suggest that the iron isotope composition of highly evolved crust can be slightly modified by magmatic processes.

Using amphibole phenocrysts to track vapor transfer during magma crystallization and transport: An example from Mount St. Helens, Washington

In order to evaluate and further constrain models for volatile movement and vapor enrichment of magma stored at shallow levels, amphibole phenocrysts from 2004–2005 Mount St. Helens dacite were analyzed for major and selected trace elements (Li, Cu, Zn, Mn, and REE) and Li isotopes. Several recent studies have examined fluid-mobile trace element abundances in phencryst phases and melt inclusions as a means of tracking volatile movement within subvolcanic magmatic systems, and high Li contents in plagioclase phenocrysts from 1980 and 2004 Mount St. Helens dacites have been interpreted as evidence that shallow magma was fluxed by a Li-bearing vapor phase prior to eruption. In amphibole phenocrysts, Zn and Mn behave compatibly, correlating to FeO ⁎ and Al 2O 3, and show no systematic change with time. In contrast, Li and Cu abundances in amphibole vary by up to 3 orders of magnitude (7.6–1140 μg/g and 1.7 to 94 μg/g, respectively), and do not generally correlate with either major or trace elements. However, they do correlate moderately well ( R 2 = 0.54, >> 95% confidence) with each other and show systematic temporal variations that are opposite to those observed for plagioclase, precluding a simple 1-step diffusion model for Li enrichment. We propose a Diffusion-Crystallization Multi-Stage (DCMS) model to explain the temporal variations and co-variations of Li and Cu. In early erupted dacite (October–December 2004) profiles of Li isotopes in conjunction with measured 7Li intensities and core-to-rim increases in Li concentration are characteristic of Li diffusion into the amphiboles, consistent with prior models of plagioclase enrichment. In amphiboles from 2005 dacite, average Li and Cu concentrations are high (∼ 260–660 μg/g and ∼ 29–45 μg/g, respectively) and in contrast to amphiboles from earlier-erupted dacite, correlate weakly with Al 2O 3 wt.%. Amphibole Al 2O 3 concentrations are an indicator of pressure, with high-Al amphiboles crystallizing at higher pressures, and we suggest that Li and Cu are partitioned into a fluid phase during ascent and crystallization of the magma so that amphiboles crystallizing at lower pressure have correspondingly lower Li and Cu concentrations. However, low Li and Cu in amphiboles from the dacite at the start of the eruption also require crystallization from a low Li–Cu bearing melt or residence times long enough for amphiboles to re-equilibrate with a Li–Cu depleted melt. Estimated residence times suggest that amphiboles in early dacite could have been present since the end of the 1980–1986 eruptive episode at Mount St. Helens.

Volatile (S, Cl and F) and fluid mobile trace element compositions in melt inclusions: implications for variable fluid sources across the Kamchatka arc

Volatile element, major and trace element compositions were measured in glass inclusions in olivine from samples across the Kamchatka arc. Glasses were analyzed in reheated melt inclusions by electron microprobe for major elements, S and Cl, trace elements and F were determined by SIMS. Volatile element–trace element ratios correlated with fluid-mobile elements (B, Li) suggesting successive changes and three distinct fluid compositions with increasing slab depth. The Eastern Volcanic arc Front (EVF) was dominated by fluid highly enriched in B, Cl and chalcophile elements and also LILE (U, Th, Ba, Pb), F, S and LREE (La, Ce). This arc-front fluid contributed less to magmas from the central volcanic zone and was not involved in back arc magmatism. The Central Kamchatka Depression (CKD) was dominated by a second fluid enriched in S and U, showing the highest S/K2O and U/Th ratios. Additionally this fluid was unusually enriched in 87Sr and 18O. In the back arc Sredinny Ridge (SR) a third fluid was observed, highly enriched in F, Li, and Be as well as LILE and LREE. We argue from the decoupling of B and Li that dehydration of different water-rich minerals at different depths explains the presence of different fluids across the Kamchatka arc. In the arc front, fluids were derived from amphibole and serpentine dehydration and probably were water-rich, low in silica and high in B, LILE, sulfur and chlorine. Large amounts of water produced high degrees of melting below the EVF and CKD. Fluids below the CKD were released at a depth between 100 and 200 km due to dehydration of lawsonite and phengite and probably were poorer in water and richer in silica. Fluids released at high pressure conditions below the back arc (SR) probably were much denser and dissolved significant amounts of silicate minerals, and potentially carried high amounts of LILE and HFSE.

Mobile element analysis by secondary ion mass spectrometry (SIMS) of impactite matrix samples from the Yaxcopoil-1 drill core in the Chicxulub impact structure

The concentrations of the fluid mobile trace elements lithium, beryllium, boron, and barium were measured in samples of the altered matrix of several impactite breccias of the Yaxcopoil-1 drill core using secondary ion mass spectrometry (SIMS) to determine the extent of transport due to aqueous or hydrothermal processes. Three of the elements, Li, Be, and B, have higher concentrations in the upper suevite impact breccias than in the lower impact melt deposits by factors of 3.5, 2.2, and 1.5, respectively. Lithium and B are the most enriched elements up section, and appear to have had the greatest mobility. The similar fractionation of Li and B is consistent with fluid transport and alteration under low-temperature conditions of less than 150 °C based on published experimental studies. In contrast to Li, Be, and B, the concentration of Ba in the altered matrix materials decreases upward in the section, and the concentration of Ba in the matrix is an order of magnitude less than the bulk concentrations, likely due to the presence of barite. The origin of the elemental variations with depth may be related to different protolith compositions in the upper versus the lower impactite units. A different protolith in the altered matrix is suggested by the Mg-rich composition of the lower units versus the Al-rich composition of the upper units, which largely correlates with the mobile element variations. The possibility that vertical transport of mobile elements is due to a postimpact hydrothermal system is supported by published data showing that the sediments immediately overlying the impactites are enriched in mobile elements derived from a hydrothermal system. However, the mobile elements in the sediments do not have to originate from the underlying impactites. In conclusion, our data suggests that the impactites at this location did not experience extensive high-temperature hydrothermal processing, and that only limited transport of some elements, including Li, Be, and B, occurred.

Adakites—the key to understanding LILE depletion in granulites

For more than 20 years, it has been argued that granulites of the lower continental crust are depleted in fluid-mobile elements such as Cs, Rb, U and Th, either because they were removed by melting of the lower crust or through dehydration. We argue that there is little evidence for a simple relationship between granulite-facies metamorphism and element depletion, and propose that, for felsic orthogneisses, the depletion may be a primary feature of crust generation processes. Modern adakites show a range of fluid-mobile element ratios and adakites from the Austral Volcanic Zone (AVZ), Chile show fluid-mobile element ratios very similar to those found in lower crustal felsic granulites. This suite of adakites has a strong slab-derived component. We propose that modern adakites of the type found in the Austral Volcanic Zone are analogous, in their genesis, to lower crustal felsic granulites and that their mafic precursors were already depleted in fluid-mobile trace elements, prior to melting. An incremental melting model, in which fluid-mobile elements are released from an eclogitic slab prior to partial melting, can explain the fluid-mobile element behaviour of both modern adakites and lower crustal felsic granulites.

Petrogenesis of Group I Kimberlites from Kimberley, South Africa: Evidence from Bulk-rock Geochemistry

Fresh samples of hypabyssal kimberlite from the five major kimberlite pipes in the Kimberley area of South Africa have been analysed for their bulk-rock major and trace element geochemistry. The geochemical data allow identification of the influence of crustal contamination in certain samples, best illustrated in terms of elevated SiO2 ,A l 2 O 3, Pb and heavy rare earth element (HREE) contents. Samples devoid of such crustal contamination show coherent major and fluid-immobile trace element variations, whereas fluid-mobile trace elements are scattered. Kimberlites rich in macrocrysts are shown to reflect substantial (up to 35%) entrainment of mantle peridotite, with Ni---SiO2 and Sc---SiO2 variations defining mixing trajectories towards garnet lherzolite. The likely primary magma(s) parental to the Kimberley kimberlites is suggested to have a composition of 26---27 wt % MgO, 26---27 wt % SiO2, 2 2w t % Al2O3 and Mg number 086. Subtle differences in chondritenormalized REE abundance patterns can be explained by small variations in the degree of partial melting within the range 04---15%, leaving residual garnet. The data are satisfied by melting a source enriched relative to chondrites by a factor of 10 in light REE (LREE), with chondritic or lower HREE abundances. Extended normalized trace element diagrams exhibit significant negative K, Rb, Sr and Ti anomalies that are interpreted to be primary magma characteristics, despite evidence for secondary mobility of K, Sr and Rb. A model is proposed in which fluid or melt from a sub-lithospheric source region precipitates phlogopite en route to metasomatizing the overlying subcontinental mantle lithosphere, imprinting its geochemical signature on a source region previously depleted in HREE relative to primitive mantle. Subsequent 1% melting of the metasomatized source produces a kimberlite with compatible element characteristics strongly influenced by depleted lithospheric peridotite (high Mg number, high Ni, low HREE), but with incompatible elements (and their isotope ratios) characteristic of the deeper source. The similarity of incompatible element ratios (Nb/U, Nb/Th, Ce/Pb) in the kimberlite magmas to those of ocean island basalts from the South Atlantic suggests an ultimate origin in an upwelling mantle plume.

Fluid-mobile trace element constraints on the role of slab melting and implications for Archaean crustal growth models

We use published and new trace element data to identify element ratios which discriminate between arc magmas from the supra-subduction zone mantle wedge and those formed by direct melting of subducted crust (i.e. adakites). The clearest distinction is obtained with those element ratios which are strongly fractionated during refertilisation of the depleted mantle wedge, ultimately reflecting slab dehydration. Hence, adakites have significantly lower Pb/Nd and B/Be but higher Nb/Ta than typical arc magmas and continental crust as a whole. Although Li and Be are also overenriched in continental crust, behaviour of Li/Yb and Be/Nd is more complex and these ratios do not provide unique signatures of slab melting. Archaean tonalite-trondhjemite-granodiorites (TTGs) strongly resemble ordinary mantle wedge-derived arc magmas in terms of fluid-mobile trace element content, implying that they-did not form by slab melting but that they originated from mantle which was hydrated and enriched in elements lost from slabs during prograde dehydration. We suggest that Archaean TTGs formed by extensive fractional crystallisation from a mafic precursor. It is widely claimed that the time between the creation and subduction of oceanic lithosphere was significantly shorter in the Archaean (i.e. 20 Ma) than it is today. This difference was seen as an attractive explanation for the presumed preponderance of adakitic magmas during the first half of Earth's history. However, when we consider the effects of a higher potential mantle temperature on the thickness of oceanic crust, it follows that the mean age of oceanic lithosphere has remained virtually constant. Formation of adakites has therefore always depended on local plate geometry and not on potential mantle temperature.

Lithium isotope evidence for light element decoupling in the Panama subarc mantle

Research Article| June 01, 2000 Lithium isotope evidence for light element decoupling in the Panama subarc mantle Paul B. Tomascak; Paul B. Tomascak 1Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, USA Search for other works by this author on: GSW Google Scholar Jeffrey G. Ryan; Jeffrey G. Ryan 2Department of Geology, University of South Florida, Tampa, Florida 33620, USA Search for other works by this author on: GSW Google Scholar Marc J. Defant Marc J. Defant 2Department of Geology, University of South Florida, Tampa, Florida 33620, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Paul B. Tomascak 1Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, USA Jeffrey G. Ryan 2Department of Geology, University of South Florida, Tampa, Florida 33620, USA Marc J. Defant 2Department of Geology, University of South Florida, Tampa, Florida 33620, USA Publisher: Geological Society of America Received: 18 Oct 1999 Revision Received: 03 Mar 2000 Accepted: 14 Mar 2000 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2000) 28 (6): 507–510. https://doi.org/10.1130/0091-7613(2000)28<507:LIEFLE>2.0.CO;2 Article history Received: 18 Oct 1999 Revision Received: 03 Mar 2000 Accepted: 14 Mar 2000 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paul B. Tomascak, Jeffrey G. Ryan, Marc J. Defant; Lithium isotope evidence for light element decoupling in the Panama subarc mantle. Geology 2000;; 28 (6): 507–510. doi: https://doi.org/10.1130/0091-7613(2000)28<507:LIEFLE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The systematics of fluid-mobile trace elements in arc lavas from Panama, relative to their Li isotopic compositions, provide unique evidence for the fertilization and subsequent differential extraction of mobile species from the subarc mantle. Calc-alkaline lavas that crystallized between 20 and 5 Ma (Old Group) that possess δ7Li as high as +11.2 have low B/Be. Otherwise identical (and similarly old) calc-alkaline lavas with high B/Be (to 23), have mid-ocean ridge basalt (MORB) like δ7Li (+4.7 to +5.6). Adakite lavas (<3 Ma; Young Group) possess δ7Li from +1.4 to +4.2 and have consistently lower B/Be than Old Group lavas, consistent with derivation from melting of a devolatilized MORB slab. If Li and B had comparable fluid mobility in the subarc mantle, then slab fluids would carry both high B concentrations and elevated δ7Li signatures into arc sources, and samples with the highest δ7Li would also have the highest B/Be. Our data suggest that although both Li and B are initially derived from the slab, older δ7Li signatures may be preserved in the mantle beneath arcs. As a result, regions of the lithospheric mantle will develop Li isotope signatures that are heavier than typical MORB mantle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Timescales of destructive plate margin magmatism: new insights from Santorini, Aegean volcanic arc

The timescales of element transfer at volcanic arcs provide important insights into magma generation, movement and storage beneath destructive margin volcanoes. Here we present major, trace and Sr-, Nd- and U-series isotope data on <200 ka old samples from Santorini in the Aegean volcanic arc as a case study of a hazardous destructive margin volcano. Samples range from low-K calc-alkaline basalts to high-K rhyolites, are enriched in light rare earth elements and fluid mobile trace elements, and have negative Ta, Nb and Ti anomalies characteristic of arc magmatism. 87Sr/ 86Sr ratios range from 0.7035 to 0.7054 and are negatively correlated with 143Nd/ 144Nd ratios, ranging from 0.51285 to 0.51263. Lavas from the second eruptive cycle (∼180 ka–∼3.6 ka) are in U–Th radioactive equilibrium, and a whole-rock, plagioclase, magnetite U–Th mineral isochron from a 67±9 ka dacite yields an age of 85 (+22/−19) ka (1σ). Samples from the Kameni islands (46 AD–1950) show small but significant uranium excesses, with ( 238U/ 232Th)=0.91–1.04 and ( 230Th/ 232Th)=0.91–0.97, suggesting that the time since U–Th differentiation is ≤147 (+27/−21) ky (1σ). A U–Th mineral isochron from the 1940 Kameni dacite yields an age indistinguishable from its eruption age. The Kameni samples show variable Ra depletion, with ( 226Ra/ 230Th)=0.92–0.99. The range in Sr and Nd isotope data reflects progressive crustal assimilation. The combined trace element and isotope data are consistent with a three-component source model involving mantle wedge, sediment melts and slab-derived fluids. While initial 238U-excesses in the second eruptive cycle have decayed, implying slab-derived fluid addition >180 ky prior to eruption, for the Kameni lavas fluid addition to the wedge is ≤150 ky. U–Th mineral isochrons indicate that crystal residence times in the subvolcanic plumbing system are short. Fractional crystallisation less than ∼1 ky prior to eruption of the Kameni dacites is suggested by ( 226Ra/ 230Th)<1. The combined data imply that timescales of magmatic differentiation in crustal reservoirs are short, that transfer time of the slab-fluid signature through the mantle wedge varies through time.

U-Th-Ra Isotope Evidence for Rates of Fluid Transport and Magmatic Processes Beneath Santorini, Aegean Volcanic Arc, Greece

The island of Santorini is situated in the Aegean and is part of the Hellenic Volcanic Arc, formed by the northward subduction of the African Plate under the Eurasian Plate. Santorini has been volcanically active since the middle Pleistocene. The two youngest (<200 ka) volcanic sequences are the second explosive cycle that terminated with the caldera forming Minoan eruption of ~1600 BC, and the Kameni island dacites, which comprise twelve historic lava eruptions from 46 AD to 1950. Selected rocks from these two sequences have been analysed for major and trace elements, for Sr and Nd isotopes, and for the short lived isotopes of U-Th-Ra by TIMS. Second eruptive cycle The compositions of the second eruptive cycle range from basaltic andesite to rhyolite, and samples are enriched in LREE, LILE, U and Th. Sr, Nd and Th isotopic compositions are also enriched relative to MORB, and they preserve good isotope-isotope correlations with S7Sr/S6Sr = 0.7035-0.7052 and 143Nd/144Nd = 0.51285-0.51267 (Fig. 1), and (23~ = 0.97-0.90. The rocks tend to have similar (23~ and (238U/232Th) activity ratios, and they are therefore in 23~ equilibrium. The within-suite trace element and Sr-Nd isotope trends can be modelled by AFC processes, indicating that variable assimilation of up to ~3% upper crustal material was responsible for the range in Sr, Nd and Th isotopes in the second eruptive cycle. The composition of the inferred pre-contamination magma reflects contributions from both sedimentderived melts (-0.3%) and hydrous fluids from the subducted slab. Relative to N-MORB, the enrichment in LREE, the high Th/Ta ratios and the low initial 143Nd/144Nd [~ 0.51285] and Th activity ratios [(23~ ~ 0.6] reflect modification of the mantle wedge by small degree melts of the subducted sediment. In contrast, the concentrations of fluid mobile trace elements [Rb, Sr, Ba, K, U and Pb] are significantly increased by introduction of slabderived fluids. The data are consistent with a fluid Sr isotope ratio of ~0.7035, as inferred from studies of other island arcs (e, g. Turner et al., 1997), and mass balance considerations indicate that fluid addition accounts for ~40% of the U budget. The

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

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 (r 2 = 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.