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Anhydrous partial melting of MORB pyrolite and other peridotite compositions at 10 kbar: Implications for the origin of primitive MORB glasses (1987)

Falloon, T. J. ; Green, D. H.

Springer

Mineralogy and petrology 37 (1987), S. 181-219

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ISSN: 1438-1168

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Description / Table of Contents: Zusammenfassung Vier Peridotit-Zusammensetzungen wurden bei 10 kbar unter wasserfreien Bedingungen partiell aufgeschmolzen, und die Ergebnisse mit möglichen primitiven Schmelzen Mittel-Ozeanischer Rücken verglichen. Die folgenden perioditischen Zusammensetzungen wurden untersucht: „MORB pyrolite” [mögliche Ausgangszusammensetzung für primitive (Mg# 〉 0.68) MORB-Glaszusammensetzungen bei 10 kbar], „Hawaiian pyrolite” (representativ für ’angereicherten’ Oberen Mantel); „Tinaquillo lherzolite” (representativ für verarmten' Oberen Mantel) und spinel lherzolite, KLB-1 (im Gleichgewicht mit primitiver MORB-Glaszusammensetzung). Die Schmelzen im Gleichgewicht mit diesen Ausgangszusammensetzungen wurden mittels „Sandwich-Experimenten” ermittelt. Die primitive MORB-Glaszusammensetzung DSDP 3-18-7-1 wurde mit MORB pyrolite und KLB-1 equilibriert, während eine Modell-Zusammensetzung vonJaques and Green (1980) in Verbindung mit „Hawaiian pyrolite” und „Tinaquillo lherzolite” vermischt wurde. Die Resultate der Experimente werden mit einem 10 kbar Aufschmelzungsmodell zur Entstehung primitiver MORB-Gläser verglichen. Die Schmelzen im Gleichgewicht mit den vier Peridotit-Ausgangszusammensetzungen unterscheiden sich wesentlich von primitiven MORB-Gläsern, sowohl hinsichtlich ihrer Hauptelemente als auch ihrer Plot-Parameter im Basalttetraeder. Primitive MORB-Glaszusammensetzungen stellen keine primären Schmelzen dar, sondern sind durch Olivinfraktionierung von primitiven Magmen abzuleiten. Die Resultate dieser Untersuchungen werden mit früheren 10 kbar Experimenten verglichen.

Notes: Summary Anhydrous partial melting experiments on four peridotite compositions have been conducted at 10 kbar providing a relatively internally consistent set of data on the character of primary melts expected from the oceanic upper mantle in the mid-ocean ridge setting. The four peridotite compositions are: “MORB pyrolite” (considered to be suitable for the production of primitive (Mg#⩾0.68) MORB glasses at 10 kbar), “Hawaiian pyrolite” (representative of “enriched” upper mantle), Tinaquillo lherzolite (representative of more “depleted” upper mantle), and the spinel lherzolite KLB-1 which is a suitable composition for the production of primitive MORB glasses. The equilibrium liquids were determined by “sandwich” experiments. The primitive MORB glass DSDP 3-18-7-1 was used in experiments using MORB pyrolite and KLB-1, while a calculated 10 kbar liquid composition fromJaques andGreen (1980) was used in experiments with Hawaiian pyrolite and Tinaquillo lherzolite. The results of the experiments are used to test a 10 kbar melt model for the generation of primitive MORB glasses, which are parental magmas to typical MORB compositions. The melt compositions from the four peridotites studied are significantly different from primitive MORB glasses in major element chemistry and plot away from the field of primitive MORB glasses in the CIPW molecular normative “Basalt tetrahedron”. The results indicate that primitive MORB glasses are derivative compositions lying on olivine fractionation lines from picritic parents, which themselves are primary magmas at pressures greater than 10 kbar. The results of this study are integrated with previous 10 kbar experimental studies.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01161817

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Petrology and geochemistry of back-arc basin basalts from Lau Basin spreading ridges at 15°, 18° and 19°S (1992)

Falloon, T. J. ; Malahoff, A. ; Zonenshaina, L. P. ; [et al.]

Springer

Mineralogy and petrology 47 (1992), S. 1-35

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ISSN: 1438-1168

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Description / Table of Contents: Zusammenfassung Dieser Artikel berichtet über erste Ergebnisse einer Forschungsexpedition in das Lau Basin, ausgeführt mit dem Forschungsschiff ‘Akademiker Mstislav Keldysh’ und dem Mini-U-Boot ‘Mir’. Drei Tauchfahrten führten zur zentralen Achse der ‘King's triple junction’ (KTJ) im nordöstlichen Teil des Lau Basins östlich der Insel Niuafo'ou (15°S). Die dort beprobten Laven werden auf der Basis ihrer chemischen Zusammensetzung eingeteilt in die Typen I und II, entsprechend der Klassifikation vonSinton andFryer (1987) in ‘N-MORB’ und ‘BABB’. Eine vierte Tauchfahrt zum zentralen Lau Spreading Center (CLSC, 18°N) erbrachte Fragmente von Sulphidschloten eines aktiven ‘Black Smoker’ innerhalb einer kollabierten Calderastructur. Eine fünfte Tauchfahrt zum östlichen Lau Spreading Center (ELSC, 19°N) erbrachte Laven vom Typus I (entsprechend N-MORB). Unter Berücksichtigung früherer Arbeiten gestatten die Ergebnisse der ‘Keldysh 90’ Expedition eine geochemische Charakterisierung des Lau Basins, sowie eine Bewertung bestehender Zonierungsmodelle fur die Entstehung und Evolution von Backarc Basins. Entlang der gesamten Achse im Lau Basin, aber abseits des eigentlichen aktiven Spreading Centers, werden Boninite und Laven boninitischer Affinität beobachtet. Diese Boninite können als integraler Teil des Frühstadiums eines Backarc Spreading Centers gedeutet werden, und entstehen möglicherweise dort, wo Spreading Centers in die Kruste eines Inselbogens vordringen oder dann, wenn es zu einer abrupten Positionsänderung der Achse kommt. Die Hf-Th-Ta Konzentrationen und Verhältnisse in Lau Basin Laven gestatten die Identifikation mehrerer Mantel-Endglieder (entsprechend D-MORB, N-MORB, E-MORB, und OIB), sowie einer an H2O und LILE-LREE angereicherten Komponente, die mit Subduktionstätigkeit und anschließender Mantelmetasomatose in Zusammenhang gebracht wird. Die angereicherte Komponente war vermutlich inhomogen aufgrund variabler Anteile subduzierter Sedimente. Die Laven des Lau Basins bestätigen die vonSinton andFryer (1987) vorgeschlagene geochemische Sonderstellung von BABB Magmentypen.

Notes: Summary Fresh back-arc basin lavas were recovered during five dives of the submersible ‘Mir’ during the 1990 cruise of the research vessel ‘Akademik Mstislav Keldysh’ to the Lau Basin. Three dives were conducted on the central spreading center of the ‘King's Triple Junction’ (KTJ) in the northeastern part of the Lau Basin east of Niuafo'ou Island at approximately 15°S. The lavas from the KTJ can be divided into types I and II based on their similarities to N-MORB and the BABB magma type ofSinton andFryer (1987) respectively. One dive each was made on the Central Lau Spreading Center (CLSC) at 18°S and the Eastern Lau Spreading Center (ELSC) at 19°S. Lavas sampled on the CLSC were associated with active hydrothermal sulphide chimneys occurring at the base of a collapsed caldera structure on the central volcanic axial high. Sampled lavas from both the CLSC and ELSC are all of type I geochemistry. The results of the ‘Keldysh’ 90 cruise are integrated with previous work to evaluate the geochemical characteristics of the Lau Basin crust as a whole and geochemical zonation models for back-arc basin development. An important part of this review of Lau Basin basalt geochemistry is the recognition of boninites and rocks of boninitic affinity which occur at off-ridge locations throughout the Lau Basin. These are suggested to erupt during the initial stages of development of new spreading ridges associated with episodes of ridge jump or the propagation of back-arc spreading ridges into arc crust. Hf-Th-Ta systematics of the Lau Basin lavas are used to identify unmodified mantle source compositions and possible subduction-related enriched components. Unmodified mantle source compositions range from D-, N-, and E-MORB to OIB. All Lau Basin lavas show some evidence of enrichment by a H2O ± LILE ± LREE-enriched component, a slab derived hydrous fluid. The slab-derived fluid is not homogeneous in composition perhaps reflecting the presence or absence of a subducted sediment component. The sampled lavas from the KTJ confirm the uniqueness of the BABB magma type ofSinton andFryer (1987).

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01165295

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The origin of island arc high-alumina basalts (1987)

Crawford, A. J. ; Falloon, T. J. ; Eggins, S.

Springer

Contributions to mineralogy and petrology 97 (1987), S. 417-430

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al2O3 either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas. Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na2O, MgO and Al2O3. These mismatches are easily explained if the HAB are plagioclase-accumulative. Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass). Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo94, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with 〉10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with 〈7% MgO, but plagioclase nucleation is delayed by their low but significant (〈1%?) H2O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al2O3 contents (〈17.5%). The majority of arc basalts, however, have Al2O3 contents in excess of 18%, reflecting plagioclase accumulation. We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (〈10%) partial melting of peridotite at P〈18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00372004

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Experimental Study of the Influence of Water on Melting and Phase Assemblages in the Upper Mantle (2014)

Green, D. H., Hibberson, W. O., Rosenthal, A., Kovacs, I., Yaxley, G. M., Falloon, T. J., Brink, F.

Oxford University Press

In: Journal of Petrology

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Publication Date: 2014-09-26

Description: The role of water in the uppermost mantle has been explored to 6 GPa (~200 km) by a novel experimental approach in which the silicate melting solidus, the stability of hydrous phases and the H 2 O contents in nominally anhydrous minerals (NAMs) were determined. The composition studied is a fertile lherzolite modelled as a source for mid-ocean ridge basalts (MORB). The use of crushed olivine as traps for melt or fluid inclusions allows a distinction to be made between quenched hydrous silicate melt and quench material from water-rich vapour phase. The vapor-saturated solidus (water-rich vapor) of fertile lherzolite increases in temperature ( T ) from a minimum of 970°C at 1·5 GPa (~50 km) to 1375°C at 6 GPa. The Ca-rich amphibole pargasite is stable to the vapour-saturated solidus to 3 GPa (~100 km). Based on normative components, at 2·5 GPa the near-solidus melt (1–2%) in mantle with very low H 2 O content is transitional between sodic–dolomitic carbonatite and olivine melilitite. With higher melt fraction (~5%) at higher T or higher H 2 O content it is olivine-rich basanite. Both immediately below and above the solidus, the H 2 O content in residual lherzolite is ~200 ppm retained in NAMs at 2·5 and 4 GPa. The experimentally determined vapour-saturated solidus corrects recent numerical models of melting of lherzolite + H 2 O based on inferred high solubilities of H 2 O in NAMs and accounts for a discrepant experimental determination of the vapour-saturated solidus in which very high water/rock ratios were used. At 2·5 ± 0·1 GPa, the water content of experimental charges was varied from 0·05 to 14·5 wt %. Below the solidus and with increasing water content from 0·05 to 2·9 wt %, pargasite decreases in K 2 O and Na 2 O content and is absent in experiments with 7·25 and 14·5 wt % H 2 O. Also with increasing water content from 0·05 to 14·5 wt % H 2 O, the Na 2 O content of clinopyroxene decreases from 1·6 wt % to below the limit of detection (0·2 wt %). The destabilization of pargasite and change of clinopyroxene composition at 2·5 GPa and 1000°C are attributed to the leaching role (Na 2 O and K 2 O particularly) of the water-rich vapour at high water/rock ratios. The hydrous mineral pargasite is the major site of H 2 O storage in fertile uppermost mantle lherzolite but pargasite is unstable at pressures ( P ) 〉3 GPa (~100 km depth), causing a sharp drop in the water storage capacity of the upper mantle from 〉2000 to ~200 ppm. For small H 2 O contents (〈2000 ppm approximately), the temperature of the vapour-undersaturated solidus of fertile upper mantle lherzolite decreases sharply with increasing P at ~90 km depth. The negative d T /d P for the vapour-undersaturated solidus has important rheological and geodynamic consequences. In oceanic intraplate settings, the geotherm passes from subsolidus pargasite-bearing lherzolite to garnet lherzolite with incipient melting, creating the rheological boundary at ~90 km depth, between lithosphere and asthenosphere. The asthenosphere becomes geochemically zoned with the ‘enriched’ intraplate basalt source (〉500 ppm H 2 O) overlying the ‘depleted’ MORB source (~200 ppm H 2 O) in the deeper asthenosphere. Water also plays a significant role at convergent margins, where hydrous silicate melting in the mantle wedge is initiated at the vapour-saturated solidus. Melting of lherzolite at or near the vapour-saturated solidus does not fully dehydrate residual lherzolite or harzburgite. Residual lithosphere returned to the upper mantle may carry ~100–200 ppm H 2 O. At 6 GPa the low K/Na model mantle composition (MORB-source mantle) with 〉200 ppm H 2 O has normal rather than supercritical melting behaviour with the solidus at 1375°C, which is ~350°C below the C + H-free solidus.

Print ISSN: 0022-3530

Electronic ISSN: 1460-2415

Topics: Geosciences