A Discussion on volcanism and the structure of the Earth - Aspects of magmatic evolution of Réunion Island

1972 ◽  
Vol 271 (1213) ◽  
pp. 105-130 ◽  
Keyword(s):  
Partial Melting ◽  
Olivine Basalt ◽  
Low Velocity ◽  
Rock Types ◽  
Degree Of Coherence ◽  
Primitive Magma ◽  
Rapid Ascent ◽  
Basalt Composition ◽  
High Level ◽  
Velocity Layer

As with the Hawaiian islands, the volcanic construction of Reunion can be related to two main phases of activity—a shield-form ing stage of predominantly olivine-basalt composition, and a declining stage comprising more varied products (basalt-trachyte). The older of the two Reunion volcanoes (Piton des Neiges) appears to have completed both these stages, but the younger volcano (Piton de la Fournaise) is still in the shield-forming stage, and is lagging approximately 1.5 Ma* behind Piton des Neiges in its evolution. The chemical data indicate a considerable degree of coherence between the various rock types produced during the different stages of development, and it is concluded that they all stem from essentially the same hypersthene-normative, picritic, primitive magma, generated by partial melting of peridotite in the low-velocity layer of the upper mantle. The shield-forming lavas are believed to represent relatively rapid ascent of this magma, only modified by olivine fractionation, but the declining stage seems to require intermediate-depth fractionation (olivine + pyroxene) to account for its initially nepheline-normative character, followed by high level fractionation (olivine+plagioclase + pyroxene + magnetite, etc.) to produce the hawaiite—mugearite—benmoreite—trachyte sequence. Volume considerations appear to favour an open system of basalt extraction, involving a relatively modest (7 to 15 %) degree of partial melting and continuous replenishment of the mantle source beneath Reunion, rather than a closed system with its restricted basalt potential even if as much as 50 % partial melting is postulated.

Mineralogy and petrology of Quaternary lavas from the Snake River Plain, Idaho

1977 ◽  
Vol 14 (9) ◽  
pp. 2140-2156 ◽  
Author(s):  
Mavis Z. Stout ◽  
J. Nicholls
Keyword(s):  
Partial Melting ◽  
Olivine Basalt ◽  
Parental Magma ◽  
Snake River Plain ◽  
Snake River ◽  
Spinel Lherzolite ◽  
Low Velocity ◽  
Basaltic Rocks ◽  
Alkali Olivine Basalt ◽  
River Plain

Quaternary basaltic and iron-rich flows from the Snake River Plain, Idaho, have chemical characteristics of both tholeiitic and alkali olivine basalt suites. The minerals present are olivine (Fo80–Fo44), Ca-rich augite (CaO 18 wt.%), plagioclase (An71–An40), and oxides. The spinel phases include a Mg–Al rich variety as inclusions in olivine phenocrysts. Both magnetite and ilmenite are present and temperatures estimated from the compositions of coexisting groundmass Fe–Ti oxides range from 915 to 1095 °C. Temperatures estimated from the compositions of coexisting groundmass olivine and pyroxene range from 928 to 1030 °C. Oxygen fugacities are in the expected basalt range (log [Formula: see text] values from −9.6 to −12.8). Silica activities calculated for these rocks fall in the alkali olivine basalt field but are above the Ab–Ne silica buffer.The aphyric basaltic rocks could be representative of liquids generated by partial melting (15–20%) of pyrolite. The generation of these rocks would leave a residuum of lherzolite composition. Equilibrium of the basalts with the residual composition recalculated to spinel lherzolite could occur between 15 and 25 kbar (15 × 105 and 25 × 105 kPa) at approximately 1300 °C. This is in agreement with the seismically determined depth of the low velocity zone (of partial melting) at approximately 60 km.A crystal fractionation model suggests that the porphyritic basalts are products of minor fractionation and accumulation from parental magma represented by the aphyric basalts. The iron-rich lavas (total FeO 17–18 wt.%) appear to be later stage fractionation products of the porphyritic basalts. However, basalts with approximately 14 wt.% FeO can be generated from partial melting of a model mantle.

scholarly journals Pervasive low-velocity layer atop the 410-km discontinuity beneath the northwest Pacific subduction zone: Implications for rheology and geodynamics

2021 ◽  
Vol 554 ◽  
pp. 116642
Author(s):  
Guangjie Han ◽  
Juan Li ◽  
Guangrui Guo ◽  
Walter D. Mooney ◽  
Shun-ichiro Karato ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Northwest Pacific ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
Velocity Layer

Phase evolution in silver-doped BiPbSrCaCuO(2223)/Ag superconducting composites

1993 ◽  
Vol 8 (9) ◽  
pp. 2187-2190 ◽  
Author(s):  
Y.C. Guo ◽  
H.K. Liu ◽  
S.X. Dou
Keyword(s):  
Partial Melting ◽  
Phase Evolution ◽  
Avrami Equation ◽  
Phase Assemblage ◽  
Final Phase ◽  
Silver Doping ◽  
Superconducting Tape ◽  
Diffusion Controlled ◽  
High Level ◽  
Very High

Silver doping into (Bi, Pb)2Sr2Ca2Cu3O10 superconducting composite tapes was found to accelerate the formation process of high-Tc (2223) phase owing to lowering the partial melting point of the samples. The differential thermal analysis (DTA) results showed that the partial melting temperature of the sample was lowered by about 10 °C from 850 °C to 840 °C by silver doping. However, with sufficient sintering both the silver-doped and undoped samples can reach a very high level of high-Tc phase fraction, suggesting that the silver doping only speeds up the rate of high-Tc phase formation, but does not change the final phase assemblage of the materials. The reaction kinetics was analyzed by using the Avrami equation, and the results indicated that the conversion process of low-Tc (2212) phase to high-Tc (2223) phase was a diffusion-controlled, two-dimensional reaction. The correlation of the phase evolution with electrical property inside the superconducting tape during the process of heat treatment was also discussed.

Partial melting and the low-velocity zone

1970 ◽  
Vol 4 (1) ◽  
pp. 62-64 ◽  
Author(s):  
Don L. Anderson ◽  
Hartmut Spetzler
Keyword(s):  
Partial Melting ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Velocity Zone

Focal mechanisms of earthquakes related to the 29 November 1975 Kalapana, Hawaii, earthquake: The effect of structure models

1981 ◽  
Vol 71 (3) ◽  
pp. 713-729 ◽  
Author(s):  
R. S. Crosson ◽  
E. T. Endo
Keyword(s):  
Main Shock ◽  
Horizontal Layer ◽  
Focal Mechanisms ◽  
Local Network ◽  
The South ◽  
Low Velocity ◽  
Tectonic Model ◽  
Low Velocity Layer ◽  
Using Data ◽  
Velocity Layer

abstract Initial focal mechanism determinations for the 29 November 1975 Kalapana, Hawaii, earthquake indicated discrepancy between the mechanism determined from teleseismic data by Ando and the mechanism determined using data from the local U.S. Geological Survey network surrounding the epicenter region. The resolution of this difference is crucial to correctly understand this earthquake, as well as to understand the tectonics of the south flank of Kilauea volcano. When a model with a low-velocity layer at the base of the crust is used for projection back to the focal sphere for the local network mechanisms, the discrepancy vanishes. To further investigate this result, focal mechanisms were determined using several contrasting models for a set of well-recorded earthquakes. A large number of these earthquakes have mechanisms identical to the main shock when the low-velocity layer model is used. Dispersion of P and T axes is also minimized by use of this model. A low-angle slip direction, favored for the main shock and typical of most other solutions, exhibits remarkable stability normal to the east rift zone of Kilauea. Our results suggest a tectonic model, similar in nature to that proposed by Ando, in which the south flank of Kilauea consists of a mobile block of crust which is relatively free to move laterally on a low-strength zone at about 10 km depth. Forceful injection of magma along the rift zones provides the loading stress which is released by catastrophic failure in the weak, horizontal layer in a cycle of perhaps 100 yr.

scholarly journals Evidence of mingling between contrasting magmas in a deep plutonic environment: the example of Várzea Alegre, in the Ribeira Mobile Belt, Espírito Santo, Brazil

2001 ◽  
Vol 73 (1) ◽  
pp. 99-119 ◽  
Author(s):  
SILVIA R. MEDEIROS ◽  
CRISTINA M. WIEDEMANN-LEONARDOS ◽  
SIMON VRIEND
Keyword(s):  
Partial Melting ◽  
Granitic Magma ◽  
Geochemical Data ◽  
Mobile Belt ◽  
Tholeiitic Magma ◽  
Rock Types ◽  
Upper Proterozoic ◽  
Incompatible Elements ◽  
Wide Range ◽  
Intermediate Rock

At the end of the geotectonic cycle that shaped the northern segment of the Ribeira Mobile Belt (Upper Proterozoic to Paleozoic age), a late to post-collisional set of plutonic complexes, consisting of a wide range of lithotypes, intruded all metamorphic units. The Várzea Alegre Intrusive Complex is a post-collisional complex. The younger intrusion consists of an inversely zoned multistage structure envolved by a large early emplaced ring of megaporphyritic charnoenderbitic rocks. The combination of field, petrographic and geochemical data reveals the presence of at least two different series of igneous rocks. The first originated from the partial melting of the mantle. This was previously enriched in incompatible elements, low and intermediate REE and some HFS-elements. A second enrichment in LREE and incompatible elements in this series was due to the mingling with a crustal granitic magma. This mingling process changed the composition of the original tholeiitic magma towards a medium-K calc-alkalic magma to produce a suite of basic to intermediate rock types. The granitic magma from the second high-K, calc-alkalic suite originated from the partial melting of the continental crust, but with strong influence of mantle-derived melts.

Sign in / Sign up

Export Citation Format

Share Document