Lherzolite nodules from a Pleistocene cinder cone in central Yukon

1978 ◽  
Vol 15 (2) ◽  
pp. 220-226 ◽  
Author(s):  
P. D. Sinclair ◽  
D. J. Tempelman-Kluit ◽  
L. G. Medaris Jr.
Keyword(s):  
Upper Mantle ◽  
Cinder Cone ◽  
Mineral Chemistry ◽  
Spinel Lherzolite ◽  
Mantle Origin

Fresh spinel lherzolite nodules occur in basaltic tuff on the flank of a Pleistocene cinder cone built on Selkirk Lavas in central Yukon. The nodules are mineralogically and chemically similar to others from diverse localities. The texture and mineral chemistry are consistent with an upper mantle origin for the Selkirk nodules. Equilibration temperatures for the nodules have been determined to be about 1100 °C.

Upper mantle origin for Harding County well gases

Nature ◽  
1987 ◽  
Vol 325 (6105) ◽  
pp. 605-607 ◽  
Author(s):  
Thomas Staudacher
Keyword(s):  
Upper Mantle ◽  
Mantle Origin

Petrology of ultramafic xenoliths from Rayfield River, south-central British Columbia

1987 ◽  
Vol 24 (8) ◽  
pp. 1679-1687 ◽  
Author(s):  
Dante Canil ◽  
Mark Brearley ◽  
Christopher M. Scarfe
Keyword(s):  
British Columbia ◽  
Phase Equilibrium ◽  
Partial Melting ◽  
Upper Mantle ◽  
Mantle Xenoliths ◽  
Spinel Lherzolite ◽  
Equilibrium Constraints ◽  
Host Magma ◽  
South Central ◽  
Ultramafic Xenoliths

One hundred mantle xenoliths were collected from a hawaiite flow of Miocene–Pliocene age near Rayfield River, south-central British Columbia. The massive host hawaiite contains subrounded xenoliths that range in size from 1 to 10 cm and show protogranular textures. Both Cr-diopside-bearing and Al-augite-bearing xenoliths are represented. The Cr-diopside-bearing xenolith suite consists of spinel lherzolite (64%), dunite (12%), websterite (12%), harzburgite (9%), and olivine websterite (3%). Banding and veining on a centimetre scale are present in four xenoliths. Partial melting at the grain boundaries of clinopyroxene is common and may be due to natural partial melting in the upper mantle, heating by the host magma during transport, or decompression during ascent.Microprobe analyses of the constituent minerals show that most of the xenoliths are well equilibrated. Olivine is Fo89 to Fo92, orthopyroxene is En90, and Cr diopside is Wo47En48Fs5. More Fe-rich pyroxene compositions are present in some of the websterite xenoliths. The Mg/(Mg + Fe2+) and Cr/(Cr + Al + Fe3+) ratios in spinel are uniform in individual xenoliths, but they vary from xenolith to xenolith. Equilibration temperatures for the xenoliths are 860–980 °C using the Wells geothermometer. The depth of equilibration estimated for the xenoliths using geophysical and phase equilibrium constraints is 30–40 km.

Age and radiogenic isotopic systematics of the Borden carbonatite complex, Ontario, Canada

1987 ◽  
Vol 24 (1) ◽  
pp. 24-30 ◽  
Author(s):  
Keith Bell ◽  
John Blenkinsop ◽  
S. T. Kwon ◽  
G. R. Tilton ◽  
R. P. Sage
Keyword(s):  
Upper Mantle ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Growth Curves ◽  
Carbonatite Complex ◽  
Northern Ontario ◽  
Mantle Origin ◽  
Mid Ocean Ridge ◽  
Ocean Ridge ◽  
Isotope Correlation

Rb–Sr and U–Pb data from the Borden complex of northern Ontario, a carbonatite associated with the Kapuskasing Structural Zone, indicate a mid-Proterozoic age. A 207Pb/206Pb age of 1872 ± 13 Ma is interpreted as the emplacement age of this body, grouping it with other ca. 1900 Ma complexes that are the oldest known carbonatites associated with the Kapuskasing structure. A 206Pb–238U age of 1894 ± 29 Ma agrees with the Pb–Pb age but has a high mean square of weighted deviates (MSWD) of 42. A Rb–Sr apatite–carbonate–mica whole-rock isochron date of 1807 ± 13 Ma probably indicates later resetting of the Rb–Sr system.An εSr(T) value of −6.2 ± 0.5 (87Sr/86Sr = 0.70184 ± 0.00003) and an εNd(T) value of +2.8 ± 0.4 for Borden indicate derivation of the Sr and Nd from a source with a time-integrated depletion in the large-ion lithophile (LIL) elements. These closely resemble the ε values for Sr and Nd from the Cargill and Spanish River complexes, two other 1900 Ma plutons. The estimated initial 207Pb/204Pb and 206Pb/204Pb ratios from Borden calcites plot significantly below growth curves for average continental crust in isotope correlation diagrams, a pattern similar to those found in mid-ocean ridge basalts (MORB) and most ocean-island volcanic rocks, again suggesting a source depleted in LIL elements. The combined Nd and Sr, and probably Pb, data strongly favour a mantle origin for the Borden complex with little or no crustal contamination and support the model of Bell et al. that many carbonatites intruded into the Canadian Shield were derived from an ancient, LIL-depleted subcontinental upper mantle.

The effects of solid-solid phase equilibria on the oxygen fugacity of the upper mantle

2020 ◽  
Vol 105 (10) ◽  
pp. 1445-1471
Author(s):  
Edward M. Stolper ◽  
Oliver Shorttle ◽  
Paula M. Antoshechkina ◽  
Paul D. Asimow
Keyword(s):  
Phase Equilibria ◽  
Oxygen Fugacity ◽  
Upper Mantle ◽  
Phase Changes ◽  
Primitive Mantle ◽  
Spinel Lherzolite ◽  
Garnet Lherzolite ◽  
Constant Composition ◽  
Al Content ◽  
Mantle Sources

Abstract Decades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle. Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇄ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations. Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

Kaersutite from San Carlos, Arizona, with comments on the paragenesis of this mineral

1968 ◽  
Vol 36 (283) ◽  
pp. 997-1002 ◽  
Author(s):  
Brian Mason
Keyword(s):  
Upper Mantle ◽  
Volcanic Rocks ◽  
Mantle Origin ◽  
Basic Volcanic ◽  
San Carlos

SummaryA 230 g kaersutite xenocryst from basalt at San Carlos, Arizona, has the composition SiO2 40·10 %, TiO2 4·44, Al2O3 14·53, Fe2O3 3·24, FeO 10·05, MnO 0·14, MgO 11·00, CaO 11·06, Na2O 2·99, K2O 1·62, H2O + 0·73, H2O − 0·10; D = 3·28; α 1·680, β 1·700, γ 1·715; a 9·858 Å, b 18·063 Å, c 5·315 Å, β 105° 14′. Kaersutite occurring as xenocrysts in basic volcanic rocks and tuffs, and as a phase in garnet-pyroxene xenoliths from such rocks, is probably of upper mantle origin, and may be the important potassium-bearing phase in this region.

The occurrence and significance of xenocrysts of apatite, ilmenite, and Na-Fe-Ti oxide in ultrapotassic lavas from the Leucite Hills, Wyoming

1987 ◽  
Vol 51 (360) ◽  
pp. 265-270
Author(s):  
Michael Barton
Keyword(s):  
Upper Mantle ◽  
High Field ◽  
Mineral Chemistry ◽  
Component Model ◽  
High Field Strength ◽  
Host Magma ◽  
Source Regions ◽  
Two Component ◽  
Alkaline Magmas ◽  
Low K

AbstractThe occurrence and mineral chemistry of apatite, magnesian ilmenite, and an Na-Fe-Ti oxide in lavas from the Leucite Hills are reported. Magnesian ilmenite and apatite occur as xenocrysts and as crystals in amphibole-mica-pyroxenite xenoliths. Na-Fe-Ti oxide and also rutile occur as inclusions in ilmenite. The latter mineral contains up to 7.2% MgO and shows evidence of oxidation by, and reaction with, the host magma. The apatite differs from that which occurs as phenocrysts and microphenocrysts inasmuch as REE were not detected. The occurrence of these minerals, which are important repositories for REE and High Field Strength elements, together with phlogopite in the upper mantle source regions of ultrapotassic lavas, is important and may explain some unusual aspects of the geochemistry of such lavas (low K/Rb, P2O5/Ce, Ti/Zr, high Ti/V, Zr/Nb). The source regions must be grossly heterogenous and a two-component model is suggested for the source. This model is similar to that suggested for the source regions of other alkaline magmas and is capable of explaining the unusual Nd-Sr isotopic characteristics of the Leucite Hills lavas.

Amphibole in a spinel lherzolite xenolith: evidence for volatiles and partial melting in the upper mantle beneath southern British Columbia

1984 ◽  
Vol 21 (9) ◽  
pp. 1067-1072 ◽  
Author(s):  
Mark Brearley ◽  
Christopher M. Scarfe
Keyword(s):  
British Columbia ◽  
Partial Melting ◽  
Upper Mantle ◽  
Stability Field ◽  
Spinel Lherzolite ◽  
Ultramafic Xenolith ◽  
Metasomatic Fluid ◽  
Small Grains ◽  
First Time

Pargasitic amphibole has been observed for the first time in an ultramafic xenolith from British Columbia. The xenolith is a chrome diopside-bearing spinel lherzolite trapped within an alkali basaltic lava flow at Lightning Peak, near Vernon, British Columbia. Amphibole (<5%) occurs within the xenolith as small grains, interstitial between other xenolith mineral phases, and always shows evidence of melting. Microprobe analyses of the amphibole reveal that it is a pargasite rich in MgO (MgO = 17.1–17.7 wt.%; Mg/(Mg + Fe2+) = 0.89) and CaO (10.4–10.7 wt.%). Textural and chemical evidence suggests that the pargasite is in equilibrium with the other phases in spinel lherzolite. The pargasite probably crystallized within the spinel stability field of the upper mantle from a volatile-rich metasomatic fluid that was produced by dehydration of subducted material. Melting in the amphibole may have been caused by one of three processes: superheating by the host alkali basalt, decompression as the magma ascended, or by in situ partial melting within the upper mantle. The partial melting of amphibole-bearing spinel lherzolite provides a possible mechanism for the generation of late Cenozoic alkalic magmas of the Intermontane Belt of British Columbia.

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