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.

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.

Geothermobarometry of spinel peridotites from southern British Columbia: implications for the thermal conditions in the upper mantle

2013 ◽  
Vol 50 (10) ◽  
pp. 1019-1032 ◽  
Author(s):  
A.M.R. Greenfield ◽  
E.D. Ghent ◽  
J.K. Russell
Keyword(s):  
British Columbia ◽  
Microprobe Analysis ◽  
Thermal Conditions ◽  
Spinel Lherzolite ◽  
Mantle Lithosphere ◽  
Exchange Reactions ◽  
Alkali Basalts ◽  
South Central ◽  
Peridotite Xenoliths ◽  
Mineral Pair

Spinel lherzolite xenoliths within alkali basalts exposed at Rayfield River and Big Timothy Mountain, south-central British Columbia, represent samples of the underlying lithospheric mantle. Electron microprobe analysis shows that most xenoliths comprise compositionally homogeneous grains of olivine, orthopyroxene, clinopyroxene, and spinel. We applied the following mineral-pair geothermometers to these rocks: orthopyroxene–clinopyroxene, spinel–orthopyroxene, and spinel–olivine. Temperatures calculated using the Brey and Köhler calibration of two-pyroxene thermometry were constrained in pressure by being required to lie on a model geotherm we develop for this region of B.C. The model geotherm is constrained to produce a temperature at the Moho (33 km) of 825 ± 25 °C to match the lowest temperature peridotite xenoliths recovered in this study. Although the overall effect of pressure on the temperature calculations is negligible (∼2 °C for 0.1 GPa), the simultaneous solution of the model geotherm and the pressure-dependent Brey–Köhler two-pyroxene thermometry removes the need for adopting an arbitrary pressure. We take these temperatures to represent peak mantle lithosphere temperatures. Fourteen Rayfield River xenoliths return two-pyroxene temperatures between 841 and 962 °C corresponding to depths of 34–42 km. Orthopyroxene–spinel and olivine–spinel results are 889 ± 60 and 825 ± 88 °C, respectively. Five Big Timothy xenoliths have two-pyroxene temperatures spanning 840–1058 °C and corresponding to depths of 34–48 km. Mean orthopyroxene–spinel and olivine–spinel temperatures are 844 ± 63 and 896 ± 232 °C, respectively. We argue that the differences in ranges of temperature do not represent closure temperatures imposed during cooling either in the mantle or during transport by the magma. Rather, these differences reflect differences in the original calibrations of the geothermometers or different degrees of equilibration in exchange reactions in dry rocks. Isochemical phase diagrams (pseudosections) constrain the pressure–temperature (P–T) field in which spinel is stable. These diagrams suggest that the spinel-bearing peridotites equilibrated at pressures ranging from ∼9.6 to 14 kbar (10 kbar = 1 GPa).

Ultramafic xenoliths from the Elwin Bay kimberlite: the first Canadian paleogeotherm

1977 ◽  
Vol 14 (6) ◽  
pp. 1202-1210 ◽  
Author(s):  
Roger H. Mitchell
Keyword(s):  
Upper Limb ◽  
Upper Mantle ◽  
Spinel Lherzolite ◽  
Garnet Lherzolite ◽  
Arctic Canada ◽  
Iron Enrichment ◽  
Ultramafic Xenoliths

Ultramafic xenoliths from the Elwin Bay kimberlite provide samples of the upper mantle beneath arctic Canada. The compositions of coexisting pyroxenes have been used to estimate the temperatures and pressures of equilibration of the three texturally and mineralogically distinct types of xenolith, i.e. spinel lherzolite (840–935 °C), coarse garnet lherzolite (925–1085 °C at 39.5–49.5 kbar (3.95–4.95 × 106 kPa)) and porphyroclastic garnet lherzolite (1090–1180 °C at 47.0–51.5 kbar (4.7–5.2 × 106 kPa)). The garnet lherzolite data define an inflected paleogeotherm whose upper limb lies at shallower depths than found for the Thaba Putsoa – Mothae paleogeotherm but which is identical to the Montana paleogeotherm. No evidence is found for iron enrichment of the upper mantle in this region.

Geochemistry of ultramafic xenoliths and their host alkali basalts from Tallante, southern Spain

1986 ◽  
Vol 50 (356) ◽  
pp. 231-239 ◽  
Author(s):  
C. Dupuy ◽  
J. Dostal ◽  
P. A. Boivin
Keyword(s):  
Upper Mantle ◽  
Mantle Metasomatism ◽  
Southern Spain ◽  
Stability Field ◽  
Spinel Lherzolite ◽  
Alkali Basalts ◽  
Spinel Lherzolites ◽  
Ultramafic Xenoliths ◽  
Liquid Segregation ◽  
Garnet Stability Field

AbstractUltramafic xenoliths enclosed in Plio-Quaternary alkali basalts from Tallante near Cartagne (southern Spain) are composed mainly of spinel lherzolites which are probably upper mantle residues. In many xenoliths, the spinel lherzolite is cut by pyroxenite or gabbroic anorthosite veinlets generally 0.2–3 cm thick. The clinopyroxenite veinlets were formed by high-pressure crystal-liquid segregation from alkali basalt magmas formed earlier than the host basalts, whereas mantle metasomatism played a role in the genesis of gabbroic anorthosites. Close to the contact with the veinlets, the spinel lherzolites are enriched in Ca, Fe, and some incompatible elements including light REE due to the migration of a fluid from the veinlets into the surrounding lherzolites. The host alkali basalts were derived from a heterogeneous, incompatible element-enriched upper-mantle source probably similar in composition and nature to the composite xenoliths, but were formed in a garnet stability field.

Post-entrainment mineral–melt reactions in spinel peridotite xenoliths from Inver, Donegal, Ireland

1997 ◽  
Vol 134 (6) ◽  
pp. 771-779 ◽  
Author(s):  
CLIFF S. J. SHAW ◽  
ALAN D. EDGAR
Keyword(s):  
Partial Melting ◽  
Alkali Basalt ◽  
Melt Inclusions ◽  
Alkali Feldspar ◽  
Spinel Peridotite ◽  
Spinel Solid Solution ◽  
Spinel Lherzolite ◽  
Host Magma ◽  
Chemical Signatures ◽  
Na Depletion

Spinel lherzolite and harzburgite xenoliths hosted in an alkali basalt dyke near Inver, Donegal, Ireland show abundant evidence of interaction between xenolith minerals and the host melt. Of particular interest are primary Cr-diopside and spinel with sieve-textured coronas. Coronas on primary Cr-diopside are up to 3 mm wide and are associated with veinlets of devitrified glass. The coronas comprise secondary Cr-diopside with vermicular, interstitial alkali feldspar and chlorite grains up to 100 µm in size. The inclusion-free Cr-diopside cores are Al- and Na-rich whereas the coronas are Al- and Na-depleted and Ti-enriched. Sieve-textured spinels have similar texture to the clinopyroxene grains and are also associated with veinlets of infiltrated glass. However, the interstitial inclusions in the sieve-textured region are chlorite and nepheline. Inclusion-free spinel is part of a chromite–spinel solid solution and is Ti-poor. Spinel in the coronas has a greater chromite and ulvospinel component and falls close to a mixing line with spinel in the host alkali basalt. In addition to the sieve-textured grains, primary olivine in contact with infiltrated glass has Fe-rich rims, and orthopyroxene has broken down to form rims of olivine, clinopyroxene and a K-rich phase similar in composition to alkali-feldspar. Comparison of the compositions of the inclusion-free cores and sieve-textured rims shows that the rims have chemical signatures consistent with partial melting, that is, Al and Na depletion for clinopyroxene and Cr-enrichment for spinel. The textures of the coronas, particularly those around spinel and the reaction margins on orthopyroxene are identical to those produced during dissolution experiments.We suggest that silicate liquid from the host magma infiltrated the xenoliths during their ascent and since it was not in equilibrium with the xenolith minerals caused reaction. The occurrence of K-bearing interstitial minerals in the sieve-textured grains and reacted orthopyroxenes indicate that the coronas did not form by simple melting since none of the minerals that underwent breakdown are K-bearing. We suggest that the sieve-textured grains formed initially by partial melting and reaction associated with decompression and infiltration of liquid from the host magma. The melts included in the reacted phases were enriched in K by diffusion from the Si-poor infiltrated melt into the more Si-rich melt inclusions in the coronas.

Chemical variation among French ultramafic xenoliths—evidence for a heterogeneous upper mantle

1975 ◽  
Vol 40 (310) ◽  
pp. 153-170 ◽  
Author(s):  
R. Hutchison ◽  
A. L. Chambers ◽  
D. K. Paul ◽  
P. G. Harris
Keyword(s):  
Partial Melting ◽  
Upper Mantle ◽  
Bulk Composition ◽  
Kimberlite Pipe ◽  
Chemical Variation ◽  
Peridotite Xenoliths ◽  
Ultramafic Xenoliths ◽  
Host Rocks

SummarySome 200 ultramafic xenoliths and their basaltic hosts from five French localities were studied. New analyses are presented, which show the five host-rocks to be nepheline- and olivine-normative. Seven bulk analyses of xenoliths from four localities, together with analyses of their constituent diopsides and, for six, of their orthopyroxenes, are also presented. Xenoliths from four occurrences appear to have equilibrated at pressures between about 8 to 18 kb at sub-basaltic solidus temperatures. Suites of xenoliths are chemically different. Histograms were used to determine compositions of depleted and ‘undepleted’ upper mantle. A suite of peridotite xenoliths from the Bult-fontein kimberlite pipe is no less depleted in fusible oxides than xenoliths from two French localities. ‘Undepleted’ upper mantle is very similar to ‘pyrolite’ in composition, except that the latter has much higher TiO2, Na2O, and K2O contents. No xenolith encountered in this work has a bulk composition that could yield more than 12% oceanic tholeiite on partial melting.

Magma generation in the upper mantle, field evidence from ophiolite suites, and application to the generation of oceanic lithosphere

1978 ◽  
Vol 288 (1355) ◽  
pp. 527-546 ◽  
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Upper Mantle ◽  
Oceanic Lithosphere ◽  
Single Parent ◽  
Spinel Lherzolite ◽  
Mantle Material ◽  
Magma Generation ◽  
Field Evidence ◽  
Lower Series

The Bay of Islands ophiolite may be divided into a lower series of ultramafic tectonites representing mantle material and a higher series of cumulate and extrusive rocks and sediments which may be correlated with oceanic crust. The tectonite series consists of a lower spinel—lherzolite member overlain by harzburgites. Both are cut by numerous olivine-pyroxene veins which represent early crystallization products from a picritic tholeiite magma derived at 18-22 kbar by approximately 23 % partial melting of the spinel lherzolite. The remainder of this magma crystallized as differentiated cumulate and extrusive rocks under crustal conditions. The petrogenetic model derived implies a rising diapiric body beneath an accreting centre and allows for the production of tholeiitic, transitional and mildly alkaline basalts from a single parent. The nature of the basalt erupted depends upon the rate of upwelling and crystallization in the diapir. The model suggests that crystallization takes place in the diapir at depths of less than 60 km.

Mantle xenoliths from Tertiary lavas and dykes on Ubekendt Ejland, West Greenland

1998 ◽  
pp. 152-154
Author(s):  
Stefan Bernstein ◽  
C. Kent Brooks
Keyword(s):  
Field Work ◽  
Mantle Xenoliths ◽  
Alkaline Basalt ◽  
Basalt Flow ◽  
Volcanic Province ◽  
East Greenland ◽  
Original Series ◽  
West Greenland ◽  
Ultramafic Xenoliths ◽  
Extensive Collection

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Bernstein, S., & Brooks, C. K. (1998). Mantle xenoliths from Tertiary lavas and dykes on Ubekendt Ejland, West Greenland. Geology of Greenland Survey Bulletin, 180, 152-154. https://doi.org/10.34194/ggub.v180.5099 _______________ Mantle xenoliths were found in Tertiary alkaline (basanitic) lavas on Ubekendt Ejland in West Greenland in the mid 1970s by J.G. Larsen. Microprobe analyses of olivine, pyroxene and spinel in two mantle xenoliths, suggested that the xenoliths on Ubekendt Ejland are highly depleted and have high modal olivine contents, and low modal orthopyroxene and clinopyroxene (Larsen 1982). In this respect the mantle xenoliths from Ubekendt Ejland are very similar to the spinel harzburgites from Wiedemann Fjord, in the Tertiary volcanic province of East Greenland (Brooks & Rucklidge 1973; Bernstein et al. 1998). Larsen (1981) also reported dykes containing mantle nodules and a varied suite of cumulates and megacrysts, one of which has subsequently been dated to 34.1 ± 0.2 Ma (Storey et al. 1998) The basalt flow that carries the xenoliths is from what is defined as the Erqua Formation which occurs at the top of the lava succession in western Ubekendt Ejland (Fig. 1; Drever & Game 1948; Larsen 1977a, b). The basalts have not been dated, but are younger than 52.5 Ma, which is the date obtained for the underlying formation (Storey et al. 1998). During July 1997, we spent three weeks collecting xenoliths and prospecting for xenolith-bearing dykes in the Uummannaq district of central West Greenland. The field work resulted in an extensive collection of xenoliths from an alkaline basalt flow described by Larsen (1977a, b), as well as the discovery of a dyke carrying a large number of ultramafic xenoliths of various origins. 

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