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

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).

1987 ◽  
Vol 24 (8) ◽  
pp. 1679-1687 ◽  
Author(s):  
Dante Canil ◽  
Mark Brearley ◽  
Christopher M. Scarfe

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.


Geosphere ◽  
2011 ◽  
Vol 7 (3) ◽  
pp. 710-723 ◽  
Author(s):  
Urmidola Raye ◽  
Elizabeth Y. Anthony ◽  
Robert J. Stern ◽  
Jun-Ichi Kimura ◽  
Minghua Ren ◽  
...  

2004 ◽  
Vol 118 (2) ◽  
pp. 164
Author(s):  
George W. Douglas

In Canada, Lemmon's Holly Fern, Polystichum lemmonii, is restricted to the Baldy Mountain area on the eastern side of the Okanagan River valley in south-central British Columbia. This population represents the northern limits of the species which ranges south through northern Idaho, Washington and Oregon to northern California. In British Columbia, P. lemmonii is associated with ultramafic rocky ridges within a montane forest at an elevation of 1900 m. The population in the Baldy Mountain area is relatively small, unprotected and potentially imperilled by mining exploration, forest road construction or wildfires.


2006 ◽  
Vol 120 (2) ◽  
pp. 163
Author(s):  
George W. Douglas ◽  
Jenifer L. Penny ◽  
Ksenia Barton

In Canada, Dwarf Woolly-heads, Psilocarphus brevissimus var. brevissimus, is restricted to the Similkameen River valley, south of Princeton in southwestern British Columbia and the extreme southeast and southwest corners of Alberta and Saskatchewan, respectively. This paper deals with the three British Columbia populations which represent the northwestern limit of the species which ranges from south-central British Columbia, southward in the western United States to Montana, Idaho, Washington, Oregon, Nevada, Utah, Wyoming, California and Baja California, Mexico. In British Columbia, P. brevissimus is associated with calcareous vernal pools and ephemeral pond edges in large forest openings. This habitat is rare in the area the few existing populations could easily be extirpated or degraded through slight changes in groundwater levels, coalbed methane gas drilling, housing development or recreational vehicles.


2010 ◽  
Vol 142 (2) ◽  
pp. 135-142 ◽  
Author(s):  
Peter J. Landolt ◽  
D. Thomas Lowery ◽  
Lawrence C. Wright ◽  
Constance Smithhisler ◽  
Christelle Gúedot ◽  
...  

AbstractLarvae of Abagrotis orbis (Grote) (Lepidoptera: Noctuidae) are climbing cutworms and can damage grapevines, Vitis vinifera L. (Vitaceae), in early spring by consuming expanding buds. A sex attractant would be useful for monitoring this insect in commercial vineyards. (Z)-7-Tetradecenyl acetate and (Z)-11-hexadecenyl acetate were found in extracts of female abdominal tips. In multiple field experiments, male A. orbis were captured in traps baited with a combination of these two chemicals but not in traps baited with either chemical alone. Males were trapped from mid-September to early October in south-central Washington and south-central British Columbia. Other noctuid moths (Mamestra configurata Walker, Xestia c-nigrum (L.), and Feltia jaculifera (Guenée)) were also captured in traps baited with the A. orbis pheromone and may complicate the use of this lure to monitor A. orbis. Abagrotis discoidalis (Grote) was captured in traps baited with (Z)-7-tetradecenyl acetate but not in traps baited with the two chemicals together.


2012 ◽  
Vol 2012 ◽  
pp. 1-12
Author(s):  
Teresa A. Newsome ◽  
Jean L. Heineman ◽  
Amanda F. Linnell Nemec

Critical height ratios for predicting competition between trembling aspen and lodgepole pine were identified in six juvenile stands in three south-central British Columbia ecosystems. We used a series of regression analyses predicting pine stem diameter from the density of neighbouring aspen in successively shorter relative height classes to identify the aspen-pine height ratio that maximizedR2. Critical height ratios varied widely among sites when stands were 8–12 years old but, by age 14–19, had converged at 1.25–1.5. MaximumR2values at age 14–19 ranged from 13.4% to 69.8%, demonstrating that the importance of aspen competition varied widely across a relatively small geographic range. Logistic regression also indicated that the risk of poor pine vigour in the presence of aspen varied between sites. Generally, the degree of competition, risk to pine vigour, and size of individual aspen contributing to the models declined along a gradient of decreasing ecosystem productivity.


1978 ◽  
Vol 15 (10) ◽  
pp. 1669-1672 ◽  
Author(s):  
B.N. Church

New localities of shackanite and related analcite-bearing lavas have been discovered in a broad field of early Tertiary phonolite and mafic phonolite in south-central British Columbia. The development of primary and secondary analcite in these rocks is probably the result of cooling lava during and shortly after extrusion.The possibility of leucite to analcite transformation in Daly's shackanite is unlikely because of lack of petrographic evidence and a preponderance of Na2O over K2O in bulk rock composition. It is also unlikely that analcite, and particularly groundmass analcite, crystallized at great depth and was transported to surface during eruption.


1977 ◽  
Vol 14 (11) ◽  
pp. 2578-2592 ◽  
Author(s):  
J. W. Hillhouse

Paleomagnetic evidence indicates that the extensive early Mesozoic basalt field near McCarthy, south-central Alaska, originated far south of its present position relative to North America. Results obtained from the Middle and (or) Upper Triassic Nikolai Greenstone suggest that those basalts originated within 15° of the paleoequator. This position is at least 27° (3000 km) south of the Upper Triassic latitude predicted for McCarthy on the basis of paleomagnetic data from continental North America. The Nikolai pole, as determined from 50 flows sampled at 5 sites, is at 2.2° N, 146.1° E (α95 = 4.8°). The polarity of the pole is ambiguous, because the corresponding magnetic direction has a low inclination and a westerly declination. Therefore, the Nikolai may have originated near 15° N latitude or, alternatively, as far south as 15° S latitude. In addition to being displaced northward, the Nikolai block has been rotated roughly 90° about the vertical axis. A measure of the reliability of this pole is provided by favorable results from the following tests: (1) Within one stratigraphic section, normal and reversed directions from consecutive flows are antipolar. (2) Consistent directions were obtained from sites 30 km apart. (3) Application of the fold test indicated the magnetization was acquired before the rocks were folded. (4) The magnetizations of several pilot specimens are thermally stable up to 550 °C. The stable component is probably carried by magnetite with lamellar texture, a primary feature commonly acquired by a basalt at high temperature during initial cooling of the magma. Geologic and paleomagnetic evidence indicates that the Nikolai is allochthonous to Alaska and that, together with associated formations in southern Alaska and British Columbia, it is part of a now disrupted equatorial terrane.


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