Shervais, J. W., Jean, M. M.; Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone. Geochim. Cosmochim. Acta 95 (2012) 270-285, doi: 10.1016/j.gca.2012.07.006

Abstract. Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This “subduction factory” is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones.

The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source:

C_wr,add=[C_cpx-obs/[[D_cpx/(D_bulk-PF)][1-(PF/D_bulk)]^(1/P)]]-[C_0,wr]
where C_wr,add = concentration of FME added to mantle wedge during a given melt increment, C_cpx-obs = concentration of observed pyroxene, D_cpx and D_bulk = mineral and bulk partition coefficients, P = melt proportion, and F = melt fraction required to model the observed MREE–HREE concentrations. Application of this algorithm to CRO peridotites shows that fluid influx must be continuous with open system melting, which allows us to calculate FME concentrations for small melt increments. Addition of the calculated FME concentrations to depleted MORB mantle (DMM) asthenosphere or refractory arc mantle (RAM) results in pooled magmas that match primitive arc tholeiites and boninites.

Fig. 5. Model volcanic arc melt compositions derived from FME-enriched sources as calculated in text (C_0,wr + C_wr,add) compared to primitive arc volcanics and boninites taken from the Georoc database. Squares = 10% non-modal batch melt of DMM + fluid for each locale; circles = 10% non-modal batch melt of refractory arc mantle (RAM) formed by early garnet phase melting. In all cases, fluid composition added is for 0.5% fractional melt. The correspondence of the model arc volcanic melts with primitive arc volcanics shows that our algorithm for calculating FME addition is consistent with our inferred process of continuous FME enrichment during melting. Note that melts derived from the DMM source have flat REE patterns, whereas melts derived from the RAM sources show light REE depletion, similar to boninites; however, all sources are FME enriched. en/image018.jpg" style="height:566px;width:1005px;"/>

Humphreys, E. R., Niu, Y.; On the composition of ocean island basalts (OIB): The effects of lithospheric thickness variation and mantle metasomatism Lithos 112 (2009) 118-136, doi: 10.1016/j.lithos.2009.04.038

Abstract. We have examined island-averaged geochemical data for 115 volcanic islands with known eruption ages and ages of the underlain lithosphere from the Pacific, Atlantic and Indian Oceans. These age data allow calculation of the lithosphere thickness at the time of volcanism. After correcting the basalts (including alkalic types) (< 53% SiO₂) for fractionation effect to Mg^# = 0.72, we found that the island-averaged Si₇₂ and Al₇₂ decrease whereas Fe₇₂, Mg₇₂, Ti₇₂ and P₇₂ increase with increasing lithosphere thickness. The island-averaged [La/Sm]_CN and [Sm/Yb]_CN ratios also increase with increasing lithosphere thickness. These statistically significant trends are most consistent with the interpretation that the mean extent of melting decreases whereas the mean pressure of melting increases with increasing lithosphere thickness. This is physically consistent with the active role the lithosphere plays in limiting the final depth of intra-oceanic mantle melting. That is, beneath a thin lithosphere, a parcel of mantle rises to a shallow level, and thus melts more by decompression with the aggregated melt having the property of high extent and low pressure of melting. By contrast, a parcel of mantle beneath a thick lithosphere has restricted amount of upwelling, and thus melts less by decompression with the aggregated melt having the property of low extent and high pressure of melting. This demonstrates that oceanic lithosphere thickness variation exerts the first-order control on the geochemistry of ocean island basalts (OIB). Variation in initial depth of melting as a result of fertile mantle compositional variation and mantle potential temperature variation can influence OIB compositions, but these two variables must have secondary effects because they do not overshadow the effect of lithosphere thickness variation that is prominent on a global scale. The mantle potential temperature variation beneath ocean islands cannot be constrained with the existing data. Fertile mantle source heterogeneity is required to explain the large OIB compositional variation on a given island, between islands and between island groups. The OIB mantle source heterogeneity must have multiple origins, but an incipient melt in the seismic low-velocity zone and its metasomatic lithologies in the lithosphere are best candidates that contribute to the incompatible element enriched OIB geochemistry on two different time scales: (1) melt–lithosphere interaction during OIB magmatism, and (2) recycled metasomatized lithosphere in the OIB source regions.

Fig. 3. Island-averaged major element data corrected for fractionation effect to Mg^# = 0.72 plotted as a function of the lithosphere thickness. Each data point represents average composition for a given volcanic island; error bars represent 2 standard deviations from the mean […].The geochemical data we used are exclusively from the GEOROC database. These include mostly bulk-rock analyses and some glass analyses for major and trace elements of over 20,000 samples ranging in composition from highly evolved andesites/basaltic andesites (minor), to tholeiitic basalts (abundant), to alkali rich basalts (relatively abundant) and to rocks highly enriched in alkalis such as basanite or rarely nephelinite (minor) from 189 ocean islands in the Pacific (108 islands), Atlantic (56 islands) and Indian (25 islands) ocean basins.

Arevalo, R., Jr., McDonough, W. F., Luong, M.; The K/U ratio of the silicate Earth: Insights into mantle composition, structure and thermal evolution. Earth Planet. Sci. Letters 278 (2009) 361-369, doi: 10.1016/j.epsl.2008.12.023

Abstract.The abundance of K in the silicate Earth provides control on the composition of the Earth's interior, the dominant mode of mantle convection, the thermal evolution of the planet, and the concentration of Pb in the core. Because K acts as a volatile species during accretion, the K content of the silicate Earth is determined as a function of the terrestrial K/U ratio. A comprehensive examination of MORB from the Atlantic, Indian and Pacific oceans, including both normal- and enriched-type samples, reveals a composite MORB source K/U ratio of 19,000 ± 2600 (2σ). In comparison, ocean island basalts and average continental crust have average K/U values of 11,900 ± 2200 and 13,000 ± 3000, respectively. The fractional contributions of these reservoirs establishes the K/U ratio of the silicate Earth to be 13,800 ± 2600 (2σ), equating to 280 ± 120 μg/g K in the silicate Earth. As a result, the planet's convective Urey ratio is verified to be ~ 0.34, which indicates a current mantle cooling rate of 70–130 K Gyr^− 1 after taking into account potential heat flux across the core–mantle boundary. Additionally, the Earth's balance of radiogenic heat flow and budget of ⁴⁰Ar necessitate a lower mantle reservoir enriched in radioactive elements. The bulk Earth Pb/U ratio, determined here to be ~ 85, suggests ~ 1200 ng/g Pb in the bulk Earth and ≥ 3300 ng/g Pb in the core.

Fig. 4. Literature data for global oceanic and continental flood basalts, ocean island volcanics and continental arc rocks compiled from the GEOROC database […].

Cole, R. B. and Stewart, B. W.; Continental margin volcanism at sites of spreading ridge subduction: Examples from southern Alaska and western California. Tectonophysics 464 (2009) 118-136, doi: 10.1016/j.tecto.2007.12.005

Abstract. Episodes of spreading ridge subduction occurred during late Oligocene to early Miocene time along western California and during late Paleocene to early Eocene time along southern Alaska. In each case, ridge subduction and subsequent slab window formation has had a profound influence on continental margin magmatism. Foremost, in each setting there was a hiatus in arc magmatism following ridge subduction, followed by the onset of volcanism in zones of local extension within arc-front, forearc, and accretionary prism settings. These near-trench volcanic rocks are distinctive from adjacent arc rocks and represent unique episodes of continental margin magmatism.

The western California volcanic rocks were erupted into several local extensional basins and form discrete volcanic centers within each basin. The southern Alaskaolcanic rocks, which form the Caribou Creek volcanic field, were erupted in a broad zone of extension that trended orthogonally to the continental margin. Basalts from the western California volcanic centers and the Caribou Creek volcanic field are tholeiitic and have depleted Nd and Sr isotope compositions with _Ndt) as high as + 9.3 and + 10.9 and ⁸⁷Sr/⁸⁶Sr(t) as low as 0.70258 and 0.70278, respectively. These basalts are unique because they are the most geochemically depleted basalts yet documented along the continental margin of the northern Cordillera. The basalts of each group also have high Ti contents (TiO₂ above 1.5%) and low ratios of fluid-mobile and other large ion lithophile elements compared to high field strength elements. For example, Ba/Ta ratios among the basalts range from about 40 to 600, with most samples below 270, while arc basalts typically have Ba/Ta ratios greater than 450. The basalts also have Th/Yb and Ta/Yb ratios in the range of mid-ocean-ridge basalts and do not exhibit the typical enrichment in Th/Yb that characterizes arc basalts. The basaltic andesites through dacites and rhyolites have more emso-nriched isotope compositions, with _Nd(t) and ⁸⁷Sr/⁸⁶Sr(t) ranging from + 6.3 to − 3.2 and 0.70390 to 0.71131, respectively, among the western California samples and from + 10.1 to + 7.3 and 0.70299 to 0.70413, respectively, in the southern Alaska Caribou Creek samples. Compared to the basalts, the intermediate to acidic samples of each group also show enrichment in the light rare earth and large ion lithophile elements (e.g., Ba, Rb, Th, K). These data indicate that assimilation of crustal rocks was important in the evolution of each volcanic suite, although in the case of the Caribou Creek volcanic field, the crustal rocks are of oceanic island arc affinity and did not impart a strong geochemical enrichment to the magmas.

The depleted Nd and Sr isotope and trace element compositions, coupled with the relatively high Ti content and low Ba/Ta and Th/Yb ratios among the basalts, indicates that the basalt magmas of each group formed by a low-degree of partial melting from a depleted mantle source without a strong flux of fluid-mobile large ion lithophile elements as would occur above an actively dewatering subducted slab. This petrogenesis is consistent with a slab window model in which parental magmas were derived by decompression melting of suboceanic (sub-slab) mantle that upwelled into the opening that formed beneath each continental margin following spreading ridge subduction.

Fig. 3. Graphs showing selected major element variations with respect to silica for rocks of the Caribou Creek volcanic field of southern Alaska and near-trench volcanic rocks of western California (data compiled from [Cole and Basu, 1992], [Cole and Basu, 1995] and [Cole et al., 2006]). Major element oxides are in wt% and normalized to 100%, volatile-free. […]Also shown are the range of TiO₂ for Tertiary through modern arc rocks of western North America (dashed outline), recent analogs (Late Miocene to Quaternary) of mafic volcanic rocks that erupted above slab windows (solid outline), and late Tertiary to Quaternary back arc basin basalts and basaltic andesites. The arc data were obtained from the GEOROC database and include Quaternary age samples from the Aleutian arc on the Alaska Peninsula, Quarternary age samples from the Cascade arc, and Tertiary age samples from the Sierra Nevada arc (total n=3095) […].

Jackson, M. G., Hart, S. R., Saal, A. E., Shimizu, N., Kurz, M. D., Blusztajn, J. S., Skovgaard, A. C.: Globally elevated titanium, tantalum, and niobium (TITAN) in ocean island basalts with high ³He/⁴He. Geochemistry Geophysics Geosystems 9, doi: 10.1029/2007GC001876 (2008)

Abstract. We report evidence for a global Ti, Ta, and Nb (TITAN) enriched reservoir sampled by ocean island basalts (OIBs) with high ³He/⁴He ratios, an isotopic signature associated with the deep mantle. Excesses of Ti (and to a lesser degree Nb and Ta) correlate remarkably well with ³He/⁴He in a data set of global OIBs, demonstrating that a major element signature is associated with the high ³He/⁴He mantle. Additionally, we find that OIBs with high ³He/⁴He ratios have moderately radiogenic ¹⁸⁷Os/¹⁸⁸Os (>0.135). The TITAN enrichment and radiogenic ¹⁸⁷Os/¹⁸⁸Os in high ³He/⁴He OIBs indicate that they are melts of a mantle domain that hosts a nonprimitive (nonchondritic) component. The observation of TITAN enrichment in the high ³He/⁴He mantle may be important in balancing the Earth's budget for the TITAN elements. Understanding the origin of the TITAN enrichment is important for constraining the evolution of the enigmatic high ³He/⁴He mantle domain.

Figure 1: Relationships between TITAN anomalies, ¹⁸⁷Os/¹⁸⁸Os, and ³He/⁴He in representative hot spot lavas. …The ³He/⁴He, ¹⁸⁷Os/¹⁸⁸Os, and trace element data for representative OIB samples are from the GEOROC database (http://georoc.mpch-mainz.gwdg.de/) and from the helium database of Abedini et al. [2006].

Bianco, T. A., Ito, G., Van Hunen, J., Ballmer, M. D., J. J. Mahoney: Geochemical variation at the Hawaiian hot spot caused by upper mantle dynamics and melting of a heterogeneous plume. Geochemistry Geophysics Geosystems 9, doi: 10.1029/2008GC002111 (2008)

Abstract. Geochemical variations within the young Hawaiian Islands occur in two particularly prominent forms: differences between volcanic stages and differences between the “Loa” and “Kea” subchains. These observations have been interpreted to reveal spatial patterns of compositional variation in the mantle, such as concentric zoning about the hot spot or elongate streaks along the hot spot track. Our numerical models of a hot plume of upwelling mantle that is interacting with, and melting beneath, a moving lithospheric plate suggest some of the above interpretations should be reevaluated. The mantle plume is assumed to be uniformly isotopically heterogeneous, thus without any compositional zoning. Nonetheless, our models predict geographic zoning in lava isotope composition, an outcome that is caused by differences in melting depths of distinct source components and plume-lithosphere interaction. Isotope compositions of model volcanoes that grow as they pass over the melting zone can explain some of the gross aspects of isotope variation at Hawaii. The results illustrate that chemical zoning at the surface is not necessarily a map of zoning in the mantle, and this affects further inferences about the chemical structure of the mantle.

Fig. 3: Composition versus thickness […] Colored shapes are data for Mauna Loa (blue squares), Loihi (cyan squares), Koolau (cyan crosses), West Molokai (green triangles), West Molokai postshield (green stars), Mauna Kea (red circles), Kilauea (black diamonds), Kohala (black circles), and Kohala postshield (black stars) (see GEOROC database http://georoc.mpch-mainz.gwdg.de and references therein […]

Brandenburg, J. P., Hauri, E. H., Van Keken, P. E., Ballentine, C. J.; A multiple-system study of the geochemical evolution of the mantle with force-balanced plates and thermochemical effects. Earth and Planetary Science Letters 276 (2008) 1-13, doi: 10.1016/j.epsl.2008.08.027

Abstract. Here, multiple isotope systems are tracked simultaneously in models of mantle convection and it is show that this can provide powerful constraints on the role of oceanic crust recycling in the development of isotopic end-member compositions. The dynamical models are based on high-resolution cylindrical calculations with force-balanced plates and variable chemical density. The dynamic results span a parameter space of variable realistic excess crustal density compared to experimental estimates and convective vigor measured by plate velocities and surface heat flow. Isotope geochemistry is then modeled for the U–Th–Pb, Sm–Nd, Rb–Sr, and Re–Os isotope systems. The role of a dense crustal layer in development of a HIMU-isotope signature is confirmed. The extraction of continental crust is found to be essential for the formation of all isotope compositional end-members, including HIMU. This extraction is implemented as an ad-hoc process secondary to partial melting at mid-ocean ridges and constrained by estimated isotopic abundances in the present-day crust. Whereas previous studies generated mantle isotopic arrays that spanned DMM–HIMU, the additional isotope systems in this analysis indicate that enrichment purely from ancient oceanic crust may also generate an EM-I component without invoking the subduction of sediment. In this case, the EM-I signature may be indicative of mantle enriched by oceanic crust produced before 2.25 Byr, while the HIMU signature indicates enrichment by oceanic crust extracted more recently. However, it is found to be difficult to maintain a true DMM isotopic end member in Sr–Nd isotope space when significantly enriched end-members are present. This may highlight the sensitivity of the Rb–Sr system to mass exchange between the upper and lower mantle.

Fig. 1.  Compilation of oceanic basalt isotope data. The data in A–E was gathered from the PetDB (http://www.petdb.org/petdbWeb/index.jsp) and GEOROC (http://georoc.mpch-mainz.gwdg.de) databases respectively, and filtered to remove measurements with possible contamination or analytical errors […].

Pirrung, M., Illner, P., Matthiessen, J.; Biogenic barium in surface sediments of the European Nordic Seas., Marine Geology 250 (2008) 89-103,
doi: 10.1016/j.margeo.2008.01.001

Abstract.Barium in marine terrigenous surface sediments of the European Nordic Seas is analysed to evaluate its potential as palaeoproductivity proxy. Biogenic Ba is calculated from Ba and Al data using a conventional approach. For the determination of appropriate detrital Ba/Al ratios a compilation of Ba and Al analyses in rocks and soils of the catchments surrounding the Nordic Seas is presented. The resulting average detrital Ba/Al ratio of 0.0070 is similar to global crustal average values. In the southern Nordic Seas the high input of basaltic material with a low Ba/Al ratio is evident from high values of magnetic susceptibility and low Al/Ti ratios. Most of the Ba in the marine surface sediments is of terrigenous and not of biogenic origin. Variability in the lithogenic composition has been considered by the application of regionally varying Ba/Al ratios. The biogenic Ba values are comparable with those observed in the central Arctic Ocean, they are lower than in other oceanic regions. Biogenic Ba values are correlated with other productivity proxies and with oceanographic data for a validation of the applicability in paleoceanography. In the Iceland Sea and partly in the marginal sea–ice zone of the Greenland Sea elevated values of biogenic Ba indicate seasonal phytoplankton blooms. In both areas paleoproductivities may be reconstructed based on Ba and Al data of sediment cores.

Table 1.The data sources and methods for several parameters applied in this study are listed below

Iwamori, H., Albarčde, F.; Decoupled isotopic record of ridge and subduction zone processes in oceanic basalts by independent component analysis. Geochemistry Geophysics Geosystems 9, doi: 10.1029/2007GC001753 (2008)

Abstract. Isotopic variability in oceanic basalts indicates possible interactions among multiple mantle components or geochemical end-members. Beyond the standard principal component analysis, which has been used so far to identify mantle components, the relatively new independent component analysis is well suited for extracting independent features in multivariate compositional space. Radiogenic isotopic compositions of oceanic basalts from the Atlantic and South Indian oceans, including both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), show that two independent compositional vectors (referred to as independent components or ICs) account for most of the observed variations with three isotopic ratios of Pb (856 MORB and 781 OIB) or five isotopic ratios of Pb, Sr, and Nd (672 MORB and 597 OIB). In both cases, the first IC distinguishes OIB from MORB, while another maps the geographical distribution of a mantle component and in particular the DUPAL anomaly. This property shows that the two ICs indeed distinguish independent information and reflect two distinctive geodynamic processes, a feature which is not present in the conventional analysis of mantle isotopic variability. The first IC that distinguishes OIB from MORB is similar to the isotopic trend reproduced in the MORB-recycling model of Christensen and Hofmann (1994). The second IC that discriminates geographical distribution is characterized by simultaneous enrichment/depletion of Pb, Rb, and Nd relative to U-Th, Sr, and Sm, respectively, which can be explained by elemental fractionation associated with aqueous fluid-mineral reactions. These geochemical characteristics, together with the fact that most of the observed multidimensional isotopic space is spanned by the joint distribution of the two ICs, indicate independent but overlapping differentiation processes which mostly take place within the depleted mantle domain. They are likely to reflect ridge versus subduction zone processes, or melting versus interaction with aqueous fluid. We use the regional distribution of the second, “enriched” IC to redefine the DUPAL anomalous mantle and show that in addition to its Southern Ocean type locality, it also distributes itself broadly in the Northern Hemisphere.

Figure 4. Independent components (IC1 and IC2) in the ²⁰⁴Pb/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb-²⁰⁸Pb/²⁰⁶Pb-⁸⁷Sr/⁸⁶Sr-¹⁴³Nd/¹⁴⁴Nd system, for 1269 data sets (672 MORB and 597 OIB data) […]The isotopic data are compiled from literature (the PetDB database, Agranier et al. [2005], and Meyzen et al. [2005, 2007]) for MORB and the GEOROC database for OIB. From the GEOROC database, basaltic rocks with SiO₂ contents between 53 and 35 wt % were selected.

Zhang, B.-H., Liu, Y.-S., Gao, S.; Petrogenetic significance of high Fe/Mn ratios of the Cenozoic basalts from eastern China. Science in China Series D: Earth Sciences 51 (2008) 229-239, doi: 10.1007/s11430-007-0139-0

Abstract. The Cenozoic basalts from eastern China show commonly high Fe/Mn ratios (average = 68.6 ± 11.5) coupled with OIB-type trace element signature. The Cenozoic basalts form the northern margin and the southern margin of the North China Craton are studied in detail. Model calculations point out that the coupling feature of high Fe/Mn ratio with OIB-type trace element signature of these basalts cannot be produced by neither pyroxene/olivine crystallization nor remelting of previously melted mantle, but require partial melting of a garnet pyroxenite-rich mantle source. Combining these features of the Cenozoic basalts with the Phanerozoic lithospheric evolution of the eastern China, we suggest that the Cenozoic basalts were derived from a garnet pyroxenite-rich mantle source associated with continental crust delamination or oceanic crust subduction.

Putirka, K.; Excess temperatures at ocean islands: Implications for mantle layering and convection. Geology 36 (2008) 283-286, doi: 10.1130/G24615A.1

Abstract. To test for the prevalence of mantle plumes and the existence of mantle layering, temperatures (T) are estimated for 28 oceanic hotspots, using olivine-liquid equilibria (T^ol-liq). There are 27 localities that have T^ol-liq hotter than mid-ocean ridges(MOR), by 99–233 °C (average = 146 ± 26 °C),which translates to mantle potential temperatures that exceed those of MOR by 114–290 °C (average = 173 ±38 °C). Thermally driven mantle plumes are thus common, not rare. Moreover, mantle temperatures at ocean islands are positively correlated with buoyancy flux and ³He/⁴He. The correlation with buoyancy affirms that oceanic swells are thermal in origin.The positive correlation with ³He/⁴He is inconsistent with the notion that high ³He/⁴He and depleted MOR mantle derive from the same layer, but instead shows that high ³He/⁴He is tied to a lower thermal boundary layer, and thus that the mantle is compositionally layered. Mantle temperatures are negatively correlated with Pb isotope ratios, supporting a model by C.Class and S.L. Goldstein that this deep, high ³He/⁴He layer may be depleted.

Figure 1. Fe²⁺O total (FeO_t) versus MgO. OIB—ocean island basalts (glass and whole rocks from GEOROC [Geochemistry of Rocks of the Oceans and Continents], http://georoc.mpch-mainz.gwdg.de/georoc/; n = 7082). Galapagos data are from PETDB (Petrological Database of the Ocean Floor) (http://www.petdb.org/petdbWeb/index.jsp) and Geist et al. (2002) for Volcan Ecuador (VE). Siqueiros data are from Perfit et al. (1996). […}

Yamagishi, Y., K. Suzuki, H. Tamura, H. Yanaka, and S. Tsuboi (2011), Visualization of geochemical data for rocks and sediments in Google Earth: Development of a data converter application for geochemical and isotopic data sets in database systems, Geochem. Geophys. Geosyst., 12, Q03016,
doi: 10.1029/2010GC003490

Abstract. Abstract [1] We developed a Keyhole Markup Language (KML) generator for converting geochemical and isotopic data sets for rocks and sediments stored in database systems into KML. Our program allows users to visualize geochemical or isotopic data easily in Google Earth. The generator accepts data files produced by the database systems PetDB, SedDB, GEOROC, and GANSEKI. The data are plotted three-dimensionally as a bar graph on the surface of the virtual Earth at the sampling site. This type of visual presentation, including information on sample localities, directly shows the distribution of isotopic or compositional anomalies of specific samples on the Earth's surface. We provide a Web application for the generator, so anyone can set the parameters for visualization over the Internet. With other KML generators we developed earlier, geochemical data can be overlain on a seismic tomographic model. This overlay image can provide information on the origin of samples in the tomographic model.

Figure 4. (a) Major element compositions of rock sampled from Karoo Province. Original geochemical data were obtained from GEOROC as a precompiled data file (KAROO PROVINCE.csv) including various volcanic rocks, e.g., basalt, andesite, and rhyolite. Major element compositions are shown as a stacked bar graph. Graphs are constructed even if the composition data in the data file are incomplete. The remainder of the total abundance of major element compositions making up 100% is expressed as “residual” and appears as a black bar in the graph. (b) Stacked bar graph showing the legend.

Shervais, J. W., Jean, M. M., Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone, Geochim. Cosmochim. Acta 95 (2012) 270-285, doi: 10.1016/j.gca.2012.07.006

Abstract. Abstract Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This “subduction factory” is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones.

The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source:

C_wr,add=[C_cpx-obs/[[D_cpx/(D_bulk-PF)][1-(PF/D_bulk)]^(1/P)]]-[C_0,wr]
where C_wr,add = concentration of FME added to mantle wedge during a given melt increment, C_cpx-obs = concentration of observed pyroxene, D_cpx and D_bulk = mineral and bulk partition coefficients, P = melt proportion, and F = melt fraction required to model the observed MREE–HREE concentrations. Application of this algorithm to CRO peridotites shows that fluid influx must be continuous with open system melting, which allows us to calculate FME concentrations for small melt increments. Addition of the calculated FME concentrations to depleted MORB mantle (DMM) asthenosphere or refractory arc mantle (RAM) results in pooled magmas that match primitive arc tholeiites and boninites.

Fig. 5. Model volcanic arc melt compositions derived from FME-enriched sources as calculated in text (C_0,wr + C_wr,add) compared to primitive arc volcanics and boninites taken from the Georoc database. Squares = 10% non-modal batch melt of DMM + fluid for each locale; circles = 10% non-modal batch melt of refractory arc mantle (RAM) formed by early garnet phase melting. In all cases, fluid composition added is for 0.5% fractional melt. The correspondence of the model arc volcanic melts with primitive arc volcanics shows that our algorithm for calculating FME addition is consistent with our inferred process of continuous FME enrichment during melting. Note that melts derived from the DMM source have flat REE patterns, whereas melts derived from the RAM sources show light REE depletion, similar to boninites; however, all sources are FME enriched.