Source: https://www.scribd.com/document/8480559/A-Study-of-Peridotitic-Garnet-Xenocryst-Compositions-from-Selected-Ultramafic-Bodies-in-the-Northern-Alberta-Kimberlite-Province-Implications-for-Ma
Timestamp: 2019-04-23 20:58:11+00:00

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Electron microprobe (EMPA) major-element analyses of peridotitic garnet xenocrysts from the northern Alberta kimberlite province typically have well-defined lherzolitic paragenesis with geochemical affinities that are uncharacteristic of garnet in diamondiferous kimberlite. Yet approximately 67% of the Buffalo Head Hills kimberlite field bodies contain diamonds with at least three kimberlite occurrences having estimated diamond contents of between 13 and 55 carats per hundred tonnes. This conundrum is important because the major element composition of periodotitic garnet has been used extensively to establish criteria for target evaluation in diamond exploration. A comprehensive set of garnet xenocrysts from the three separate ultramafic rock fields in northern Alberta were analyzed by LA-ICP-MS. These trace element data provide information additional to EMPA data that quantify parameters indicative of diamond potential and provide new information on the chemical nature of the lower crust-subcontinental lithosphere beneath northern Alberta. This report shows that distinct compositional changes in garnet xenocryst Ti, REE, Y and Zr provide a means of separating garnets into distinct geochemical groups that disclose evidence for varying degrees of depletion or re-enrichment of the protolith. Based on garnet compositions—and using TNi as a proxy of depth—at least five lithological transitions are inferred for the lower crustal-sublithospheric mantle underlying northern Alberta. From low to high-T, these regions include the following: fertile lherzolite, chromite–clinopyroxene–garnet equilibrium trend garnet and wehrlite (1130ºC). These compositional groups can serve as a proxy for future evaluation of garnet compositions in Alberta because they also distinguish inter- and intra-field mantle variations. In terms of diamond prospects in northern Alberta, one transitional mantle layer associated with the Buffalo Head Hills field includes a predominance of 1000º to 1130ºC, low-Ti, Y and Zr-depleted lherzolite that implies a diamond window in the mantle underlying the Buffalo Head Hills of between 160 km and 180 km. In contrast, both the Mountain Lake and Birch Mountain areas seem to be characterized by either a hot, less depleted asthenospheric-type mantle, or by mantle regions characterized by relatively cool geotherms. These findings have significant implication for the documentation and evaluation of known occurrences of kimberlite and in the evaluation of surficial kimberlite-indictor mineral surveys critical to target selection in Alberta and other areas of the western North America.
by the ERCB/AGS of any manufacturer’s product.
probe mount preparation and electron microprobe analytical work.
Finally, Steven Creighton of the University of Alberta and Melissa Kirkley of Diamondex Resources Ltd.
are thanked for peer-review comments that improved the overall manuscript.
asthenospheric-type mantle, or by mantle regions characterized by relatively cool geotherms.
in Alberta and other areas of the western North America.
inaccessible geologic environments, such as the lithospheric upper mantle and lower crust.
xenocrysts, however, are common in all northern Alberta ultramafic bodies.
the Mountain Lake, Buffalo Head Hills and Birch Mountains areas, respectively.
Figure 1. Ultramafic rock occurrences in the northern Alberta kimberlite province on the inferred basement domain map of Ross et al. (1994).
Hills and of Ross et al. (1994).
diamond location markers depict diamondiferous and barren kimberlites, respectively. Mountain Lake is reportedly subeconomic.
1991; Thériault and Ross, 1991; Ross et al., 1994).
ultrabasic, olivine alkali basalt/basanite; Eccles, 2004; Eccles et al., 2004).
the depth of the lithosphere-asthenosphere boundary (LAB) to approximately 180 km depth.
Lake cluster: south body, Buffalo Head Hills field: K2, K6, K11 and K14 bodies, and Birch Mountains: Kendu, Legend and Xena bodies (Table 1).
and Eccles et al. (2008).
valid throughout the probing session.
elements varying between 0.01 ppm and 0.05 ppm.
equilibrium with mantle olivine having an average Ni concentration of 2900 ppm (Ryan et al., 1996).
an uncertainty in experimental calibration of ±70ºC (2σ; Canil, 1999).
line showing that TMn is increasingly offset to higher temperatures with increasing T.
garnet temperatures are calculated using the methods of Canil (1999). Mn-in-garnet temperatures are calculated using the methods of Grütter et al.
(1999). Trend lines are represented by solid lines. The TNi versus TMn 1:1 line is represented by a dashed line.
TNi versus TMn 1:1 line is represented by a dashed line.
olivine-associated younger mantle could provide unrealistic (higher) garnet TNi temperature estimates.
significant impact on the major conclusions of this study.
Head Hills has two thermal distributions, one at 825º–850ºC and a broad TNi range from 1025º to 1275ºC.
Bachu and Burwash, 1994) most likely reflects a higher geothermal gradient.
the discussion section of this report.
moderate-TNi garnet from the K11 and K14 bodies, and a single garnet from K6 (Figures 6, 7 and 8).
kimberlite province. TNi based on the garnet Ni thermometer of Canil (1999).
calculated using the garnet Ni thermometer of Canil (1999).
McDonough Chondrite values from McDonough and Sun (1995).
from K2, Buffalo Head Hills. Chondrite values from McDonough and Sun (1995).
garnet xenocryst cores from the K2 body. Chondrite values from McDonough and Sun (1995).
garnet xenocryst cores from the K6 body. Chondrite values from McDonough and Sun (1995).
from K11, Buffalo Head Hills. Chondrite values from McDonough and Sun (1995). McDonough and Sun (1995).
from McDonough and Sun (1995).
garnet xenocryst cores from the Kendu body. Chondrite values from McDonough and Sun (1995).
and Sun from McDonough and Sun (1995).
garnet xenocryst cores from the Xena body. Chondrite values from McDonough and Sun (1995).
exception of single higher-TNi garnet from K6 (1184ºC).
•	low TNi of between 836º and 924ºC (median 864ºC) relative to the other groups.
These high-TNi (1132º to 1186ºC) garnets are associated only with the Mountain Lake bodies (Figure 4).
•	high-Cr2O3 (6.5–9.1 wt. %) in comparison to most garnets from the other groups.
•	the lowest median TNi in this dataset of 815ºC.
•	moderate to high TNi of between 1016º and 1269ºC (median 1167ºC) relative to the other groups.
with distinctly enriched LaN-CeN in comparison to all other garnet LREEN.
history of the SLM and lower crust underlying northern Alberta. Discrimination diagrams Ti vs. TNi, Y vs.
TNi, Zr vs. TNi, Y vs. Zr and Nd/YN vs. Sc/YN are presented in Figures 12 to 16, respectively.
compositions of each individual field.
Garnet xenocryst cores from Mountain Lake are restricted to high-TNi >1130ºC domains (Figures 12–14).
metasomatism and low-Sc/YN suggestive of re-enriched garnet.
shallow lithosphere that has been impregnated by melts (Sen and Leeman, 1991; Pearson et al., 2003).
ultramafic fields; and C) individual ultramafic bodies.
garnet ‘groups’ as depicted in this study; B) ultramafic fields; and C) individual ultramafic bodies.
this study; B) ultramafic fields; and C) individual ultramafic bodies.
Fields et al. (1999e). trends from Griffin et al. (1999e).
based on the location of depleted Group A lherzolite.
spinel at Mountain Lake implies that garnet Cr content can be correlated with depth (Grütter et al., 1999).
caused by dehydration and/or melt release from subducted slabs.
the Buffalo Head Terrane in north-central Alberta from low- TNi to high- TNi as follows.
14). In addition, these garnets have high Y/Zr ratios and low-Nd/YN and –Sc/YN (Figures 15 and 16).
thus, we expect some combination of these explanations has influenced Group C garnet.
mW/m2 geotherm (Davies et al., 2004).
indicative of similar mantle within these two prairie kimberlite provinces remains to be proven.
Group F garnet also dominates the TNi >1130ºC SLM layer sampled by the K2, K11 and K14 bodies.
is at a depth of approximately 180 km. This depth matches the LAB depth prediction of Aulbach et al.
steady state chemical mantle zonation between asthenosphere and lithosphere (Moore and Lock, 2001).
Garnets of the Group G low-TNi (900º to 1000ºC) SLM layer were only observed in the Kendu body.
referred to as melt metasomatized wehrlite.
well with Ti and Zr, suggesting an enrichment process involving metasomatism.
depleted lherzolite from the Buffalo Head Hills (Figure 16).
lherzolite compositional fields in Cr2O3-CaO space.
thresholds based on the aforementioned classical mantle mineral chemistry modeling in Cr2O3-CaO space.
remaining diamond-inclusion garnets include lherzolitic (3 grains) or wehrlitic (1 grain) paragenesis.
garnet harzburgite xenolith—a chemical signature that is virtually exclusive to Archean SLM.
from Pearson encompasses diamond inclusion garnet from Pearson et al.
factor in Cr2O3 with favourable garnet having higher Cr2O3 concentrations (e.g., >6 wt. %).
indictor mineral studies critical to evaluation of future targets.
features indicative of multiple metasomatic events and extreme low- or high-T of last equilibration.
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in weight per cent. LA-ICP-MC trace element Ti values are included for comparison and validity of data.
Appendix 2. LA-ICP-MC trace element data from selected garnet xenocrysts from the northern Alberta kimberlite province. Data values in parts per million. TNi calculated using the calibration of Canil (1999).

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