Mantle Wedge Peridotite Research Articles

Re–Os isotopic systematics of primitive lavas from the Lassen region of the Cascade arc, California

Rhenium–osmium isotopic systematics of primitive calc-alkaline lavas from the Lassen region appear to be controlled by mantle wedge processes. Lavas with a large proportion of slab component have relatively low Re and Os abundances, and have radiogenic Os and mid ocean ridge basalt-like Sr and Pb isotopic compositions. Lavas with a small proportion of slab component have higher Re and Os elemental abundances and display mantle-like Os, Sr, Nd, and Pb isotopic compositions. Assimilation with fractional crystallization can only generate the Re–Os systematics of the Lassen lavas from a common parent if the distribution coefficient for Re in sulfide is ∼40–1100 times higher than most published estimates and if most incompatible element abundances decrease during differentiation. High Re/Os ratios in mid ocean ridge basalts makes subducted oceanic crust a potential source of radiogenic Os in volcanic arcs. The slab beneath the southernmost Cascades is estimated to have 187Os/ 188Os ratios as high as 1.4. Mixing between a slab component and mantle wedge peridotite can generate the Os isotopic systematics of the Lassen lavas provided the slab component has a Sr/Os ratio of ∼7.5×10 5 and Os abundances that are 100–600 times higher than mid ocean ridge basalts. For this model to be correct, Os must be readily mobilized and concentrated in the slab component, perhaps as a result of high water and HCl fugacities in this subduction environment. Another possible mechanism to account for the correlation between the magnitude of the subduction geochemical signature and Os isotopic composition involves increasing the stability of an Os-bearing phase in mantle wedge peridotites as a result of fluxing with the slab component. Melting of such a source could yield low Os magmas that are more susceptible to crustal contamination, and hence have more radiogenic Os isotopic compositions, than magmas derived from sources with a smaller contribution from the slab. Thus, the addition of the slab component to the mantle wedge appears to result in either the direct or indirect addition of radiogenic Os to arc magmas.

P– T evolution of ‘crustal’ garnet peridotites and included pyroxenites from Nonsberg area (upper Austroalpine), NE Italy: from the wedge to the slab

Up-to-date thermobarometers have been applied to garnet-bearing peridotites and pyroxenites from Nonsberg area (upper Austroalpine, NE Italy), a remnant of the lower crust of the Central European Variscan belt. The Nonsberg ultramafic rocks record an unusual metamorphic history, from high-temperature, moderate-pressure, spinel-facies conditions to low-temperature, high-pressure, garnet-facies conditions. This peculiar P– T evolution is interpreted as a result of subduction processes, which involved mantle-wedge peridotites overlying a subducting continental slab. The following scenario is proposed based on our P– T estimates and available physical models for subduction zones. Relatively high-temperature (∼1200°C, 1.31–1.58 GPa) mantle-wedge peridotites (spinel lherzolites) were intruded by hot (>1400°C) hydrous melts rising from deeper, inner portions of the wedge. Coarse-grained, garnet-free pyroxenites were segregated from these melts and formed veins in the host peridotites. Convection within the wedge induced by the downgoing slab caused the peridotites to flow towards the slab while cooling at essentially constant pressure. The peridotites were then dragged to greater depths at slightly decreasing, or nearly constant, temperature by the downward mantle flow near the wedge corner and near the wedge-slab interface and eventually emplaced into the underlying slab. Entrainment in the continental crust, either as tectonic slices or as sinking mantle blobs, was probably favored by density contrast between overlying mantle and underlying crust. Mantle flow and entrainment in the cold continental crust caused the peridotites to cool down to ∼850°C before, or while, being subducted together with the slab to depths of about 90 km. During this T-decrease, P-increase path, garnet formed at the expense of spinel and pyroxenes, giving rise to garnet ± spinel-bearing lherzolites and pyroxenites.

Pb isotope composition of Klyuchevskoy volcano, Kamchatka and North Pacific sediments: Implications for magma genesis and crustal recycling in the Kamchatkan arc

Pb isotope data are used to constrain the chemical contribution of the subducted components in the recycling beneath Klyuchevskoy volcano, the most active volcano in the Kamchatkan arc. The Pb isotope ratios of Klyuchevskoy basalts ( 206Pb/ 204Pb= 18.26–18.30, 207/Pb 204Pb= 15.45–15.48, 208/Pb 204Pb= 37.83–37.91) define a narrow range that falls within the Pacific mid-ocean ridge basalt (MORB) field and are among the least radiogenic island arc basalts measured to date. These data are similar to data from three other Quaternary Kamchatkan volcanoes: Tolbachik, Kumroch-Shish, and Maly Semiachik. In contrast, North Pacific sediments (primarily siliceous oozes) collected parallel to the Kamchatkan trench during Ocean Drilling Program Leg 145, have Pb isotope ratios ( 206Pb/ 204Pb= 18.51–18.78, 207Pb/ 204Pb= 15.56–15.64, 208Pb/ 204Pb= 38.49–38.75) that are more radiogenic than either the Klyuchevskoy basalts or Pacific MORB. Incorporation of even a small amount of sediment in the source of the Klyuchevskoy magmas would shift the Pb isotope ratios of the erupted basalts from the MORB field to more radiogenic values. The absence of 10Be and elevated Pb isotope ratios in the Kamchatkan volcanic lavas, despite the presence of distinctively radiogenic Pb in the North Pacific sediments makes it unlikely that sediments or sediment-derived fluids are involved in the source magmas beneath Kamchatka. The Kamchatkan arc thus represents an “end-member” whereby little or no sediment is involved in terms of elemental recycling and arc magma genesis. The major and trace elements, Pb, Sr and Nd isotope data of the Kamchatkan basalts are most consistently explained if derived from a fluid-fluxed, peridotitic mantle wedge source, wherein the fluid composition is dominantly controlled by dehydration of altered oceanic crust, imparting a radiogenic 87Sr/ 86Sr, and MORB-like Pb isotope signature to the mantle source. The erupted Klyuchevskoy lavas preserve a slab signature derived from incompatible elements that are strongly partitioned into the fluid. The 30 km of arc crust through which the Klyuchevskoy magmas traverse prior to eruption is not composed of older crust, but must be juvenile, similar in isotopic composition to MORB.

Garnet Growth during Amphibolite Anatexis: Implications of a Garnetiferous Restite

Formation of a garnet clinopyroxenite restite may have important physicochemical consequences for deep crustal and mantle processes. We have experimentally simulated the dehydration-melting of amphibolite, and report the modal and compositional evoltuion of the mafic granulitic mineral assemblage (the restite), with an emphasis on garnet. A natural, low-K, calcic amphibolite was powdered and run in sealed, gold capsules in a piston-cylinder apparatus at 10 kbar, 750-1000°C, 1 to 9-day runs, and $$fO_{2}$$ ~ Ni-NiO. Garnet first appears in 8-day runs at $$825 \pm 25^{\circ} C$$. Above 850°C, clinopyroxene and poikilitic garnet comprise most of the restite. Garnet peaks at 950°C (at ~32 vol.% of the material within the capsule, or 41 vol.% of the restite assemblage) and disappears between 975-1000°C. There is no cross-over of Fe-Mg partitioning between coexisting liquids and garnets. Liquids remain Mg-poorer than garnets throughout the melting interval but do increase in Mg# with increasing temperature; thus the hotter liquids may be less likely to react with mantle wedge peridotite. The modes are consistent with REE patterns of Archean tonalitic-trondhjemitic rocks generated by the partial melting of garnet-bearing mafic crust. A garnet clinopyroxenite with 41 vol.% garnet has a density of $$3.5 g/cm^{3}$$, which may induce lower crustal delamination in hot, nonthickened, amphibolitic crust. This crust is seismically indistinguishable from lithosphere or mantle. Because garnet growth traps water-bearing hornblende inclusions, the restite may contain up to 0.3 wt.% water. Thus, delivery of poikilitic garnet to the mantle, possibly by delamination or by subduction, may be an important mechanism for adding water to the mantle.

Origin and differentiation of picritic arc magmas, Ambae (Aoba), Vanuatu

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg ∼80 to 93.4) and clinopyroxene (mg ∼80 to 92), and include accessory Cr-rich spinel (cr ∼50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to ∼20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with ∼10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo2 ∼FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has ∼15 wt% MgO and an mg Fe2 value of ∼81. It requires deep (∼3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T p ∼1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge, Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ɛNd (+8.0–+8.4) and low 87Sr/86Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (ɛNd=+7.5−+8.4; 87Sr/86Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ɛNd (+4.6–+4.9) and (87Sr/86Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.

Constraints on magma genesis behind the Neogene Cascade Arc: Evidence from major and trace element variation of high‐alumina and tholeiitic volcanics of the Bear Creek Area

Lava flows and dikes from the Bear Creek area within the Neogene Blue Mountains Province (BMP) were erupted in an extensional tectonic environment. Major and trace element variations in the Bear Creek suite constrain models for the petrogenesis of these rocks. Bear Creek basalts, basaltic andesites, and andesites fall into two compositional groupings, high alumina and tholeiitic. The most primitive flow is high alumina in composition with an Mg # = 0.69. Various phase equilibria tests suggest that magma of this composition may be a hybrid of an eclogite‐derived melt and a peridotite contaminant, or it could be a melt derived wholly from a peridotite source. Large ion lithophile element enrichment and high field strength element depletion (especially Nb) in this sample are consistent with its derivation from a fluid metasomatized peridotite source. An alternative source is required for three Bear Creek tholeiites and high‐alumina basalts (HABs) that differ from the other Bear Creek samples primarily in having larger Nb contents relative to other incompatible trace elements. This source can be modeled as partial melts of a peridotite which has been contaminated by a pelagic sediment, presumably from the subducted slab. Least squares mixing calculations with high‐pressure mineral phases from peridotites indicate that high‐pressure fractionation may have generated the more primitive high‐alumina types in the Bear Creek suite. However, using the computer program SILMIN (Ghiorso, 1985) to semiquantitatively model shallow magmatic processes demonstrates that assimilation of greywacke and rhyolite (as a partial melt of the greywacke) and fractional crystallization (AFC) at 1 kbar pressure better explain the major element variation in the high‐alumina types. The tholeiites appear to be derived from high‐alumina parents via fractional crystallization at shallower levels in the crust. Semiquantitative modeling using trace element distributions supports a model in which the HAB suite was derived from peridotite and underwent AFC in the upper crust and the tholeiites were derived via Rayleigh fractionation from HAB parents. The trace element modeling also indicates that the range of trace element abundances cannot be explained by mixing of melts derived from eclogite and peridotite. Therefore a model in which dehydration of the Farallon plate metasomatizes mantle wedge peridotite and where pelagic sediment is incorporated into the peridotite, which subsequently melt to yield magmas that are modified by contamination and fractionation in the upper crust, explains all of the geochemical variation in the Bear Creek area rocks. This model may in part be applicable to the origin of the Picture Gorge and Slide Creek volcanic sequences in the BMP.

Migration of fluid phases and genesis of basalt magmas in subduction zones

Dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of H2O in the production of magmas at convergent plate boundaries. The subduction of oceanic lithosphere, occurring with increasing pressures and rising temperatures, causes liberation of H2O from the slab. Amphibole, which can be stable to the highest PT conditions among hydrous phases in the slab, decomposes at around 90 km depth. It follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab‐derived H2O is not a direct trigger for the production of arc magmas. Instead, the H2O released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals (serpentine, talc, amphibole, chlorite, and phlogopite). This hydrated peridotite is dragged downward on the slab toward higher PT regions and releases H2O to shallower potential magma source regions in the mantle wedge. Combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge, it is concluded that those minerals decompose beneath the forearc region. On the other hand, high PT experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front. Phlogopite in the peridotite decomposes to release H2O at a deeper level (about 200 km). H2O liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites. Along with the migration of H2O through the above processes, subduction components, especially large ion lithophile elements, can be overprinted on the magma source region, which governs the geochemical characteristics of arc magmas.

Generation of Archean tonalites

Research Article| April 01, 1989 Generation of Archean tonalites J. Arkani-Hamed; J. Arkani-Hamed 1Department of Geological Sciences, Brock University, St. Catharines, Ontario L2S 3A1, Canada Search for other works by this author on: GSW Google Scholar Wayne T. Jolly Wayne T. Jolly 1Department of Geological Sciences, Brock University, St. Catharines, Ontario L2S 3A1, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information J. Arkani-Hamed 1Department of Geological Sciences, Brock University, St. Catharines, Ontario L2S 3A1, Canada Wayne T. Jolly 1Department of Geological Sciences, Brock University, St. Catharines, Ontario L2S 3A1, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1989) 17 (4): 307–310. https://doi.org/10.1130/0091-7613(1989)017<0307:GOAT>2.3.CO;2 Article history 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 J. Arkani-Hamed, Wayne T. Jolly; Generation of Archean tonalites. Geology 1989;; 17 (4): 307–310. doi: https://doi.org/10.1130/0091-7613(1989)017<0307:GOAT>2.3.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 Thermal evolution models of young subducting oceanic plates under Archean conditions suggest that tholeiitic basalts in the subducting wet crust melted at shallow depths (50 to 80 km) and produced Archean tonalites. The latent heat of fusion consumed by the crust kept the overlying peridotitic mantle wedge below the melting temperature of peridotite. This prevented both fusion in the wedge and production of cale-alkaline island-are volcanic rocks similar to those observed in modern subduction zones. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Secular variation of magma source compositions beneath the Northeast Japan arc

Sr and Nd isotopic compositions were determined for Tertiary volcanic rocks with ages of 22, 16 and 8 Ma along the volcanic front in the NE Japan arc in order to examine the secular variation of magma source compositions in the mantle wedge. These rocks show no through-time variation in both Sr and Nd isotope ratios (0.70412–0.704759 and 2.09–4.12 ∈-units, respectively). It follows that the subduction components do not increasingly pollute the magma source region during the continuous subduction of oceanic lithosphere. The degree of pollution by the subduction components is controlled by the amount of clinopyroxene and aluminous phases in mantle wedge peridotites, because H 2O which causes partial melting of the mantle wedge beneath a volcanic front is supplied by the breakdown of both amphibole and chlorite in the dragged hydrated peridotite formed by the addition of slab-derived fluid beneath the fore-arc region.

Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: Evidence from high-pressure experiments and natural rocks

Dehydration experiments on synthetic serpentine spiked with 11 trace elements (Cs, Rb, K, Ba, Sr, La, Sm, Tb, Y, Yb and Nb) have been carried out at 12 kbar and 850°C to examine the nature of element migration processes in the upper mantle beneath volcanic arcs. The results indicate that an element with a larger ionic radius is more readily transported by the aqueous fluid phase through the dehydration process. The geochemistry of arc magnesian andesite and basalt, which are part of the compositional spread included in the range of arc primary magmas, has shown that source mantle materials of subduction zone magmas are enriched in incompatible elements with larger ionic radii compared to the source mantle of oceanic magmas; the enrichment should be caused by the transfer of elements by the fluid phase from the downgoing lithosphere. The downgoing slab is anhydrous beneath volcanic arcs, and the relatively shallow breakdown of hydrous phases means that the mantle wedge peridotite is infiltrated by the slab-derived fluid phase beneath the fore-arc regions. Drag of the metasomatized peridotite wedge downwards with the subducted lithosphere transports a larger-ion-element-enriched source to the regions of arc-magma generation.

Major-element chemistry of plutonic rock suites from compressional and extensional plate boundaries

Chemical criteria have been developed to distinguish plutonic rock suites from compressional and extensional plate margins by comparing type examples of Mesozoic and Cenozoic plutonic suites generated at these plate boundaries. Compressional plutonic rock suites are characterized by intermediate unimodal frequency distributions of differentiation index and normative plagioclase, calc/alkali indexes in the range of 60–64, and distinctive patterns on AFM diagrams. Extensional plutonic rock suites are characterized by bimodal distribution of differentiation index and normative plagioclase, calc/alkali indexes in the range of 50–56 and the presence of peralkaline rocks. These criteria are useful in determining tectonic settings of plutonic rock suites of unknown environment. No single criterion should be used to distinguish tectonic setting. The distinguishing chemical features of compressional plutonic rock suites may directly be related to melting and dehydration of the subducted ocean crust. Other processes that may be important are: lowering of the solidus of the overlying peridotitic mantle wedge resulting in partial melting; fractionation of both primary melts; extensive reaction of these melts with the continental crust. The characteristics of extensional plutonic rock suites may directly be related to melting of anhydrous peridotitic mantle; small amounts of melting of continental crust and a lack of mixing of the two magmas. Basic rocks from extensional suites may be generated by smaller amounts of melting at greater depths than those from compressional suites resulting in some with alkaline affinities.