Trace Element Systematics Research Articles

Alkaline magmatism from Kutch, NW India: implications for plume–lithosphere interaction

Geochemical data are presented for primitive alkaline rocks from the Kutch region, north–northwest of Deccan Volcanic Province (DVP) of west central India, which is generally regarded as related to the Reunion Plume. The trace element systematics of these rocks are similar to those of ocean-island basalts, but there is considerable compositional variation, which is related to a strong overprint from the lithosphere on plume-derived magmas. This subcontinental lithospheric mantle (SCLM) component has geochemical characteristics that overlap those observed in spinel lherzolite xenoliths entrained in these rocks. Phlogopite and apatite in the SCLM are of metasomatic origin attributed to the infiltrating fluids and/or melts derived from rising mantle plume material. The composition of the alkaline rocks is consistent with a regional upwelling of deep mantle related to marginal rifting and with OIB-type geochemical characteristics. Thermal inhomogeneities within such plume swath resulted in small diapirs, which may have undergone melt segregation at the base of the lithosphere (∼100 km) and incorporated varying amounts of SCLM during ascent.

Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data

Owing to advances in microanalytical techniques over the last 15 years, there is a growing database on the volatile contents of subduction-related magmas as recorded in melt (glass) inclusions trapped in phenocrysts in volcanic rocks. Basaltic magmas from subduction zones show a wide range of water contents, ranging from as high as 6–8 to <0.5 wt.% H 2O. Variations are related in some places to primary factors such as proximity to the arc front or extent of subduction-related mantle enrichment inferred from trace element systematics. Some low values of H 2O in melt inclusions result from shallow degassing before crystallization and entrapment of inclusions. The dissolved CO 2 contents of melt inclusions from basaltic arc lavas range from below detection (∼25 ppm) to nearly 2500 ppm. This variability is caused by degassing of low-solubility CO 2 before inclusions are trapped. The estimated primary CO 2 content of arc basaltic magma inferred from global arc volcanic CO 2 emissions and magma flux is >3000 ppm, suggesting that no melt inclusions sample undegassed arc magmas. The Cl and S contents of arc basaltic magmas are greater than midocean ridge basalts, indicating that these volatiles are also recycled from subducted sediment and altered oceanic crust back into the mantle wedge. Comparison of the fluxes of volatiles subducted back into the mantle along subduction zones and returned from the mantle to the surface reservoir (crust, ocean, and atmosphere) via magmatism suggests that there is an approximate balance for structurally bound H 2O and Cl. In contrast, ∼50% of subducted C appears to be returned to the deep mantle by subduction, but uncertainties are relatively large. For S, the amount returned to the surface reservoir by subduction zone magmatism is only ∼15–30% of the total amount being subducted. Dacitic and rhyolitic magmas in arcs contain 1–6 wt.% H 2O, a range that overlaps considerably with the values for basaltic magmas. Either basaltic parents for these differentiated magmas are relatively H 2O-poor, or intermediate to silicic arc magmas form through open-system processes involving variable amounts of crustal melting, mixing with basalt and basaltic differentiates, and fluxing of CO 2-rich vapor from mafic magma recharged into silicic magma bodies. Consideration of H 2O–CO 2 relations and gaseous SO 2 emissions for intermediate to silicic arc magmas shows that such magmas are typically vapor-saturated during crystallization in the middle to upper crust. Gas emissions thus reflect migration and accumulation of volatiles within complex open magmatic systems.

Lithium, boron and lead isotope and trace element systematics of Quaternary basaltic volcanic rocks in northeastern Japan: mineralogical controls on slab-derived fluid composition

Across-arc Li–B–Pb isotope systematics, together with trace element geochemistry and phase relationships in subducting slabs are used to understand the petrogenesis of lavas from the northeastern Japan arc. Pb/Nb ratios and Pb isotope compositions of the arc lavas decrease with depth to slab whereas Li/Y, U/Nb, Th/Nb and La/Sm show an upward profile across the arc, with peaks at 180 km depth to the Wadati–Benioff zone (WBZ). The peaks in these profiles may reflect the influence of lawsonite breakdown in the slab at 180 km depth. B/Nb ratios of the arc lavas do not show clear across-arc variation and the δ 11B values show a concave upward across-arc profile with minimum values at around 180 km in depth of the WBZ. These results possibly reflect the decomposition of tourmaline at ∼180 km. Our results suggest that accessory minerals and minor phases in the subducting slab play important roles in determining the chemical compositions of fluid released by dehydration, and the resulting trace element and isotope composition of arc lavas. Li isotope compositions of the lavas are MORB-like and do not show systematic across-arc variation, contrasting with Izu arc lavas, which show maximum δ 7Li values at the volcanic front and a systematic decrease towards the back arc. The strikingly different results for the northeastern Japan arc and the Izu arc are likely to be caused by differing extents of Li isotope fractionation due to differing subduction zone physical characteristics, such as thermal structure of the subducting slab and subduction angle. Thus, not only chemical characteristics of the subducting slab but also the physical, as well as chemical characteristics of the subduction zone could affect the Li isotope compositions of subducting slabs, producing heterogeneity in the Li isotope composition of the mantle.

Origin of enriched ocean ridge basalts and implications for mantle dynamics

The Mid-Atlantic Ridge (MAR) south of the Kane Fracture Zone at ∼23°N (the MARK area) is distant from hot spots and a type area for “normal” mid-ocean ridge basalt (N-MORB) depleted in highly incompatible elements. High-density sampling reveals that a small proportion of basalt are enriched in incompatible elements (enriched mid-ocean ridge basalts, E-MORB) from the MARK area. It is apparent that enriched magma sources, not associated with hot spots, are widespread in the upper mantle and are a common occurrence on both fast- and slow-spreading ridges. Evaluation of the trace-element systematics shows that E-MORB generation requires two stages. Low-degree melts metasomatise the upper mantle to create an enriched source, which later undergoes large extents of melting. A significant time lapse between the two events is required by differences in radiogenic isotope ratios. Atlantic, Pacific, and Indian ocean ridges that are far from hot spots show “mantle isochron” ages of ∼300 Ma for the Sm–Nd, Rb–Sr, and 238U– 206Pb systems after corrections for melting, but these ages need not be indicative of a specific event. Instead, they can result from continuous processes of formation and destruction of enriched mantle sources by melting and convective mixing. A two-box model describing these processes illuminates relationships between mantle isochron ages and upper mantle dynamics. If formation–destruction of enriched mantle is at steady state, constant “mantle isochron” ages are maintained and depend on the residence time of enriched mantle sources, the half-life of the radioactive system, and the daughter element behavior during mantle melting. The common ages of the Sr, Nd, and Pb systems reflects their long half-lives and similar melting behavior. In contrast, 207Pb/ 204Pb– 206Pb/ 204Pb ages are approximately twice as old due to the short half-life of 235U relative to the age of the Earth. For the long-lived systems, the mantle isochron ages approximate the residence time of the enriched reservoir, if its mass is a few percent of the system. We propose that the first stage of melting occurs at depth in subduction zones where the mantle wedge is enriched by the addition of low-degree melts of subducted crust. The second stage of greater extents of melting occurs beneath ocean ridges. The model results suggest that the mantle is currently in quasi-steady state and that the size of the system (N-MORB plus E-MORB sources) is similar to the upper mantle. The time scale of ∼300 Ma for survival of E-MORB sources indicates rapid convective stirring and efficient reprocessing of the upper mantle by plate tectonics.

Vendian Volcanism, Weathering, and Phosphorus Cycle Variation in the East European Platform

Two geochemical types of volcanic ash are recognized in the Redkino Horizon of the East European Platform based on the systematics of trace elements. The first type developed in the northeastern area of the platform is characterized by Ni/Cr and Ce/Yb ratios typical of the silicic tephra of recent volcanic eruptions in the Kuril–Kamchatka and Aleutian island-arc systems. The respective ratios in ash of the second type are typical of volcanics from the lower Vendian Berestovets Formation. Depletion of the upper Vendian pelites of the East European Platform in Ca, Sr and P (relative to the PAAS) indicates the continental chemical weathering of provenance.

Fluid-controlled quartz recovery in granulite as revealed by cathodoluminescence and trace element analysis (Bamble sector, Norway)

Dispersed quartz veinlets in Proterozoic enderbitic charnockite from Tromoy, Bamble sector (southern Norway), with Ti-concentrations of mostly 80–180 ppm, produce bright blue cathodoluminescence (CL), against contrasting micro-textures of secondary pure quartz with low CL intensity. The textures comprise grain boundary alteration, healed micro-fractures, patches of secondary quartz, and euhedral quartz nuclei. The secondary quartz locally makes up 50% of the total quartz. Trace element analysis has been done by electron-probe micro-analysis (EPMA), laser-induced coupled plasma mass spectrometry (LA-ICPMS), and additional electron-paramagnetic resonance (EPR) spectroscopy. These studies testify systematic trace element reduction and extensive quartz recovery during retrograde metamorphism. We assume healing of a part of the aqueous fluid inclusions during a late thermal event, possible correlated with Paleozoic magmatic activity in the Oslo-Rift. During final cooling, aqueous fluids were largely retrapped in patches of secondary quartz associated with healed fractures, whereas carbonic inclusions (without secondary quartz) may have survived retrograde metamorphism. The variable but generally high salinity of fluid inclusions is assumed to be in part the result of quartz recovery by which ion concentrations were increased in the percentage range.

Upper mantle processes beneath the 2.7 Ga Abitibi belt, Canada: a trace element perspective

Abstract Mantle processes associated with volcanism in the Abitibi belt remain poorly understood despite numerous detailed studies of its volcano-sedimentary successions. This paper employs log plots of Nb/Y and Zr/Y to distinguish the sources of well-defined Abitibi volcanic suites and to clarify processes in the upper mantle during greenstone belt development. Mafic rocks of the Southern Volcanic Zone in the Abitibi belt define two main trends on log plots of Nb/Y versus Zr/Y. Trends for the Kidd–Munro Assemblage and Malartic Block correspond to mixtures of strongly deleted plume asthenosphere and adakitic melts. The shared trace element systematics of these volcanic successions confirm they are parts of a single authochthonous supracrustal assemblage that extended across the southern Abitibi belt. The younger Kamiskotia Assemblage and Blake River Group correspond to crustal extension episodes within an Abitibi island arc. Trace element plots establish that they were derived from mantle plume-related asthenosphere and not the late Archean MORB-source upper mantle associated with island arc assemblages elsewhere in the southern Superior Province. The data suggests widespread lateral flow of mantle plume asthenosphere occurred in the vicinity of the Abitibi arc. Mantle mixing with adakitic melts continued between ∼2716 and ∼2698 Ma. The data are not consistent with wet komatiite models that generate ultramafic volcanic rocks from typical upper mantle following hydration of a mantle wedge. Chemical mixing trends for mafic magma sources indicate melt hybridization of the Abitibi mantle wedge. The trends require a slab melt origin for adakitic magmas rather than internal differentiation of underplated mafic crust. Comparisons with other 2.7 Ga greenstone belts suggest that mantle plume interaction with Archean subduction zones, as proposed for the Abitibi belt, can be distinguished in the NbZrY systematics of associated mafic volcanic rocks.

A granite?gabbro complex from Madagascar: constraints on melting of the lower crust

The Ranomandry Complex is a Neoproterozoic, nested intrusion from central Madagascar composed of a gabbroic core within a coeval peraluminous granite ring intruding pelitic metasediments. Although xenocryst entrainment and magma mixing have both contributed to marginal phases of the granite, the primary melt is characterised by steep LREE/HREE ratios and negligible, or slightly positive, Eu anomalies. Both isotopic and trace element systematics preclude an origin from either partial melting of the metapelitic country rock or from fractional crystallisation of the gabbroic magma. However, trace-element modelling suggests an origin from the dehydration melting of a plagioclase-poor, garnet-bearing metagreywacke at temperatures of 850–900 °C and at lower crustal pressures (>10 kbar). Melting of an enriched subcontinental mantle generated gabbroic magmas that caused advective heating and anatexis at the base of thickened continental crust during their ascent. Transport of the resulting granite magma was facilitated by the pre-existing plumbing system that overcame thermal and mechanical problems associated with both diapirism and self-propagating dykes as mechanisms for long-distance transport of granite magmas. The nested geometry of the intrusions is an indication of a structurally homogeneous lower crust that contains no pre-existing shear zones or fault systems.

Magmagenesis at the Eocene Electric Peak–Sepulcher Mountain complex, Absaroka Volcanic Province, USA

Igneous rocks exposed on Electric Peak and Sepulcher Mountain represent the preserved intrusive and extrusive components of an early Eocene, calc-alkaline eruptive center in the Absaroka Volcanic Province (AVP) of Montana and Wyoming, USA. The Electric Peak stock has an outcrop area of ∼1 km 2 and consists of six discrete phases ranging in composition from quartz diorite to granite, representing multiple small intrusions of compositionally distinct magmas. Lava flows and dikes on Sepulcher Mountain are basaltic–andesitic to dacitic in composition with silicic rocks generally peripheral to mafic rocks. Important geochemical characteristics of the rocks from both localities include high Ni and Cr concentrations in andesitic and dacitic rocks, lower rare earth element (REE) concentrations in evolved rocks relative to more mafic rocks, and variable Sr and Nd isotopic ratios. Although petrographic evidence permissive of magma mixing is limited, compositional data suggest that andesitic rocks are hybrids formed by mixing variably fractionated and contaminated mantle-derived mafic magmas with diverse composition silicic crustal melts derived from partial melting of amphibolitic lower- to mid-crustal rocks. Isotopic and trace element systematics of mafic rocks, such as high ratios of large ion lithophile to high field strength elements, also suggest that parental magmas were derived from melting of subduction-modified lithospheric mantle. The significance of these results is that the calc-alkaline nature of the center was inherited from crustal processes, whereas the arc-like trace element signature reflects generation of the magmas in a tectonic environment related to geometrically complex subduction of the Farallon plate under the North American continent.

Mantle Source Characteristics of Alkali Basalts and Basanites in an Extensional Intracontinental Plate Setting, Western Anatolia, Turkey: Implications for Multi-stage Melting

The alkaline volcanic province of Western Anatolia is characterized by intra-continental plate alkali olivine basalts and basanites extruded along localized extensional basins during Late Miocene (<11 Ma) to Quaternary (>0.13 Ma) time. The rocks are characterized by low 87Sr/86Sr (0.70311-0.70325) and high 143Nd/144Nd (0.51293-0.51298) ratios; they have OIB-like trace-element patterns characterized by enrichment in LILE, HFSE, and L-MREE, and a slight depletion in HREE relative to N-MORB. Trace-element modeling indicates that the mafic magmas formed by variable degrees (˜2% -10%) of partial melting of an isotopically homogenous garnet-bearing mantle source. The degree of melting decreased progressively from early-formed alkali olivine basalts to later basanites. Major- and trace-element systematics reveal two distinct depth ranges of melt segregation from the source mantle: (1) garnet and spinel + garnet stability zone for the Late Miocene rocks; and (2) spinel and spinel + garnet stability zone for the Quaternary rocks. Source concentrations, calculated using an inverse numerical method, show that the mantle from which the alkaline magmas were generated was enriched in all incompatible elements (e.g., LILE, HFSE, and LMREE) relative to depleted MORB mantle (DMM) and primitive mantle (PM) compositions. The isotopically depleted nature of the alkaline rocks relative to bulk silicate earth (BSE) further indicates that this enrichment is a recent event related to small-degree, multi-stage melting processes that involve local metasomatism of the mantle.

The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot

We present comprehensive radiogenic isotope (Os, Pb, Hf, Nd, Sr) and trace element data on basaltic lavas from Pitcairn Island and the Pitcairn seamounts and examine the origin of the enriched mantle isotopic signature (EM-1) found in these lavas. The 187Os/ 188Os ratios of the lavas range from 0.131 to 0.254, while those of the high-Os concentration samples (>50 pg/g) lie between 0.131 and 0.148. All 187Os/ 188Os ratios are higher than the bulk silicate Earth reference value of 0.127. Since ancient subcontinental lithospheric mantle (SCLM) is expected to have a 187Os/ 188Os ratio less than 0.127, it appears that recycled SCLM plays no role in the Pitcairn source. Variations in 187Os/ 188Os ratios appear to be unconnected with those of 206Pb/ 204Pb ratios in Pitcairn lavas, suggesting that Pb and Os isotopic variations are controlled by different factors. Modeling shows that variations in Pb isotopic compositions may mainly reflect the proportion of recycled sediment in the source, while those of 187Os/ 188Os ratios may reflect the proportion of peridotite mantle versus recycled oceanic crust. The occurrence of negative Nb anomalies in some of the lavas, a correlation between Nb anomaly and 87Sr/ 86Sr ratios (0.7036–0.7051), and extremely unradiogenic and strongly correlated Nd and Hf isotopic compositions ( ϵ Nd of −5.9 to +1.1 and ϵ Hf of −5.3 to +2.2) together suggest that the Pitcairn mantle source contains a recycled continental crustal component. The slope of the ϵ Hf vs. ϵ Nd correlation is shallower for Pitcairn Island than for the Pitcairn seamounts or the global OIB array, and may be due to a variable ratio of recycled mud to sand in the Pitcairn source. A trace element mixing model also indicates the presence of small amounts of recycled pelagic and terrigenous sediment and permits variable amounts of depleted components such as recycled MORB, gabbro and depleted mantle. The 206Pb/ 204Pb ratios of the Pitcairn lavas vary between 17.47 and 18.10 and are very unradiogenic compared to those of other ocean island basalts. By contrast, 208Pb/ 204Pb ratios are high and relatively homogeneous at values of ∼39.0. This observation along with the measured Th/U ratios of the lavas, which range up to 14.1, indicate a long-term history of U loss in the Pitcairn source. In 207Pb/ 204Pb– 206Pb/ 204Pb space, the data form a linear array that can be interpreted in terms of mixing between a minor recycled sediment end member and more depleted material. Lead isotopic compositions suitable for the recycled end member were investigated using a three-stage evolution model by Monte Carlo methods and suggest ages between 0.7 and 1.9 Ga for the recycled sediment. The relationships between measured Th/U and radiogenic 208Pb*/ 206Pb* ratios suggest that the isotopic arrays displayed by the lavas were produced by mixing, probably occurring during magma genesis.

Boron isotope and trace element systematics of the three volcanic zones in the Kamchatka arc

Trace element concentrations and B isotope compositions were determined for lavas from the Eastern Volcanic Front (EVF), the Central Kamchatka Depression (CKD), and the Sredinny Range (SR) in the Kamchatka arc. Trace element ratios and δ11B values of the EVF lavas show across-arc variations that are consistent with those observed in the adjacent Kurile arc: B/Nb, Pb/Nb, and δ11B are highest at the volcanic front (14 to 22, 2.0 to 2.4, and +4.9 to +5.6‰, respectively) and systematically decrease with increasing slab depth (to ∼3, ∼0.9, and −2‰, respectively) whereas Rb/Nb, Ba/Nb, K/Nb, Be/Nb, and Li/Zr do not vary significantly across the arc. The values for the SR (B/Nb = 1.7 to 2.0, Pb/Nb = 0.9 to 1.2, and δ11B = −3.7 to −1.1‰) located far behind the EVF are consistent with these across-arc trends. However, the CKD lavas exhibit elevated B/Nb (6 to 13) and δ11B values (−0.6 to +3.6‰) although the slab depths are greater than at the EVF. Despite variations in B/Nb and δ11B, the values for all these three volcanic zones of Kamchatka can be explained in terms of simple mixing between a depleted mantle wedge and slab-derived aqueous fluids with a relatively homogeneous δ11B (+6.2 ± 0.5‰). This mixing trend resembles those observed in Kurile, Izu, and Mariana, suggesting that fluids derived from altered oceanic crust and/or serpentinized peridotite in the subducting slab predominate in the slab-mantle interaction beneath these western Pacific arcs. Given this mixing relationship, the high B/Nb and δ11B values observed in the CKD indicate particularly large fluid influx beneath this region, which may partly result from subduction of the altered crust of the Emperor seamount chain in addition to the altered Mid-Ocean Ridge Basalt (MORB) crust. The compositions of the SR lavas in this study are consistent with melting of a weakly fluid-metasomatized mantle. However, the origin of magma in this volcanic zone is complicated, judging from the variable chemical characteristics reported for the SR lavas.

‘Olmec Blue’ and Formative jade sources: new discoveries in Guatemala

The discovery of jadeite rocks (jadeitites) in Guatemala in 1954 led to the recognition of the Motagua River Valley as a Maya jade source. While abundant and accessible, what is produced today for the tourist trade, mainly between Estancia de La Virgen and Teculutan (Harlow, 1994), is mostly opaque and dull-colored. It scarcely resembles the translucent and colorful bluegreen jade found in Formative horizons in Mexico. This qualitative disparity has led to a search for Olmec jade sources elsewhere in Mesoamerica. The Sierra de las Minas and the highlands south of the Motagua River have recently yielded some remarkablytranslucent, fine-grained jadeitites that closely resemble those worked in Formative times. Geological reconnaissance has expanded the known jade-bearing area at least six-fold. The first deposits examined are at an elevation of ~1,750 meters, 13 kilometers north of the limits of detailed geological mapping, on Cerro Bandera Perdida (see FIGURE 1). An ancient, and possibly trade-route related, dry-stone trackway parallels the 1,700 meter contour for at least four kilometers and leads to the mine area at Los Cedros. Alluvial jade was also found 8 kilometers further northwest at Finca Santa Clara on the Rio Blanco near the intersection of Alta Verapaz and El Progreso. Another jade deposit was visited west of the town of Rio Hondo. These sources extend the described jade-bearing zone 10 km north and 18 km east. Some 10 km south of the Motagua, alluvial jadeitite found in the Rio El Tambor is significant for its origin across a major fault, long considered the southern geological limit for jadeitite occurrence. In 1998 the floodwaters of Hurricane Mitch exposed much new jade which has been traced upriver into the department of Jalapa, at elevations from 600 to 1,400 meters. In 2001 more jadeitites and related rocks were found in the quebradas of the Rio Jalapa and at an outcrop near La Ceiba, all lying in east-west running bands of serpentinite. This southern band of the Guatemalan jade zone extends 18 km south of the northern zone and has the same eastwest extent Matching jade artifacts with jadeitite sources has been a problematic endeavor. Chemical composition has been used for sourcing many lithic materials, but jadeitites are inherently inhomogeneous which precludes matching by whole-rock chemical analysis (see Lange, 1993). Multivariate trace-element systematics shows promise but a statistically useful array of Olmec artifacts or potential source materials have yet to be measured. Minor mineral constituents are better than whole-rock composition for sourcing, because they do not get lost in the noise of whole-rock analysis. Minor paragonite or barium-rich muscovite mica and titanite are this fingerprint for most Guatemalan jadeitites and Mesoamerican jades. However, for a jadeitite source material to be considered the counterpart of one exploited in the Formative period, it must also share the visual characteristics of Olmec jade. These visible internal features, such as globular clusters of whitish secondary minerals are further identifying fingerprints that reflect the unique geological history of each deposit. Some of the recently recovered jadeitites have the combination of composition, minor minerals, and visual characteristics (details to be reported elsewhere; Harlow et al., in prep.) that point to these newly discovered (or rediscovered) deposits as excellent candidates for an Olmec-blue jade source. FIGURE 2 shows polished surfaces of jadeitites, retrieved from southern highland rivers, and a similar Olmec jade from the Mesoamerican Collection of the AMNH. Polished specimens from the Rio Jalapa and Carrizal Grande (FIGURE 3) illustrate that Guatemala’s new (or newly rediscovered) mines are again producing jades of the quality and color that figured in long-range Formative trade some 3,000 years ago.

Tracing the evolving flux from the subducting plate in the Tonga-Kermadec arc system using boron in volcanic glass

The influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was investigated using ion microprobe measurements of B/Be and boron isotope ratios (11B/10B) to document the source and relative volumes of the fluids released from the subducting oceanic plate. We analyzed young lavas from eight different islands along the Tonga-Kermadec Arc, as well as glass shards in volcanic sediments from Ocean Drilling Program (ODP) Site 840, which record the variations in the chemistry of Tonga magmatism since 7 Ma. B/Be is variable (5.8–122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from ∼3 to 4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for arc lavas (18–607). These values are too high to be related simply to a sediment influence on petrogenesis. Together with very high δ11B values (−11.6 to +37.5) for the same shards and lavas these data indicate that most of the B is derived from fluid escaped from the subducting altered Pacific oceanic crust, rather than from sediment. High δ11B values also reflect large degrees of isotopic fractionation in this cold fast subduction zone. Lower δ11B values noted in the Kermadec Arc (17 to −4.4) are related to the influence of sediment eroded from New Zealand and slower convergence. High fluid flux (B/Be) is synchronous in Tonga and the Marianas at 3 to 4 Ma and may be related to acceleration of the Pacific Plate just prior to this time.The timing of maximum B/Be at 3 to 4 Ma correlates with maximum light rare earth (LREE) and high field strength element depletion. This suggests maximum degrees of partial melting at this time. Although thinning of the arc lithosphere during rifting to form the Lau Basin is expected to influence the arc geochemistry, variable aqueous fluid flux from the subducting plate alone appears capable of explaining boron and other trace element systematics in the Tonga-Kermadec Arc with no indication of slab melting.

The significance of phenocryst diversity in tephra from recent eruptions at Popocatepetl volcano (central Mexico)

Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO2=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic plagioclase (An33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#melt ≈ 72–76) and an evolved component magma (mg#melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#melt=50–72. The mafic end member (mg#melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H2O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg# clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing. The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano. The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that the Popo mafic end member is a magnesian andesite with variable, but high SiO2 contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites, such as the low CaO, Al2O3, FeO*, high Na2O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central to quantifying the subcrustal mass fluxes beneath Popo.

Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines)

Central Mindanao was the locus of a Pliocene (4–5 Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3 Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts are among the magmatic products. All the lavas are Na-rich (up to 4.88%), with Na 2O/K 2O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere.

Petrogenetic evolution of late Cenozoic, post-collision volcanism in western Anatolia, Turkey

Following an Eocene continent-arc collision, the Western Anatolia region experienced a complete cycle of thickening and orogenic collapse. The early stage of collision-related volcanism, which was most evident during the Early Miocene (<21Ma), produced a considerable volume of lavas and pyroclastic deposits of basaltic andesite to rhyolite composition. The volcanic activity continued into the Middle Miocene with a gradual change in eruptive style and magma composition. The Middle Miocene activity formed in relation to localised extensional basins and was dominated by lava flows and dykes of basalt to andesite composition. Both the Early and Middle Miocene rocks exhibit calc-alkaline and shoshonitic character. The Late Miocene volcanism (<11Ma) was marked by alkali basalts and basanites erupted along the zones of localised extension.The Early–Middle Miocene volcanic rocks exhibit enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE) and have high 87Sr/86Sr (0.70757–0.70868) and low 143Nd/144Nd (0.51232–0.51246) ratios. Modelling of these characteristics indicates a mantle lithospheric source region carrying a subduction component inherited from a pre-collision subduction event. Perturbation of this subduction-metasomatised lithosphere by either delamination of the thermal boundary layer or slab detachment is the likely mechanism for the initiation of the post-collision magmatism.Petrographic characteristics and trace element systematics (e.g. phenocryst assemblages and relative depletion in MREE and heavy rare earth elements (HREE)) suggest that the Early–Middle Miocene magmas underwent hydrous crystallisation (dominated by plagioclase+pyroxene+pargasitic amphibole) in deep crustal magma chambers. Subsequent crystallisation in shallower magma chambers follows two different trends: (1) anhydrous (pyroxene+plagioclase-dominated); and (2) hydrous (edenitic amphibole+plagioclase+pyroxene dominated).AFC modelling shows that the Early–Middle Miocene magmas evolved through assimilation combined with fractional crystallisation, and that the effects of assimilation decreased gradually from the Early Miocene into the Middle Miocene. This may indicate a progressive crustal thinning related to the extensional tectonics that prevailed from the latest Early Miocene onwards.In contrast, the Late Miocene alkaline rocks are characterised by low 87Sr/86Sr (0.70311–0.70325) and high 143Nd/144Nd (0.51293–0.51298) ratios and have OIB-type like trace element patterns characterised by enrichment in LILE, HFSE, LREE and MREE, and a slight depletion in HREE, relative to average N-MORB. REE modelling indicates that these rocks formed by partial melting of a garnet-bearing lherzolite source. Trace element and isotope systematics are consistent with an origin by decompression melting of an enriched asthenospheric mantle source.

B19 カムチャッカ弧の 3 つの火山帯の火山岩のホウ素同位体・微量元素システマティックス

B19 カムチャッカ弧の 3 つの火山帯の火山岩のホウ素同位体・微量元素システマティックス

Evidence from oceanic gabbros for porous melt migration within a crystal mush beneath the Mid‐Atlantic Ridge

The evolution of a gabbroic crystal mush beneath the Mid‐Atlantic Ridge has been investigated using evidence from gabbros recovered from Ocean Drilling Program (ODP) Hole 923A (Leg 153). Lithological variations occur on a vertical scale of meters and correlate with mineral compositions. This defines a detailed chemical stratigraphy in which variations in olivine, plagioclase, and clinopyroxene solid‐solution component compositions correlate with each other. The volumetrically dominant lithology (plagioclase + clinopyroxene ± olivine ± orthopyroxene gabbros) has variable grain size, grain shape, and mineral compositions. These variations correlate, such that coarser grained samples have more granular textures and lower mafic phase Mg/Fe ratios than adjacent finer grained samples. Clinopyroxene trace element systematics, determined by ion probe, cannot be explained by growth from a melt that evolved along either an equilibrium or a fractional crystallization trend. Clinopyroxene crystals are strongly zoned and enriched in Zr with respect to rare earth elements (more to less incompatible elements). These textural and geochemical characteristics are not expected from simple crystal accumulation processes or the crystallization of trapped melt. Instead, melt migration within a crystal mush is suggested as the most likely process to explain them. The meter‐scale mineral compositional variations, which correlate between phases (e.g., olivine forsterite content and plagioclase anorthite content), suggest that the porous melt flux after the formation of this layering was insufficient to destroy this correlation.

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.