The Jericho
kimberlites are part of a small Jurassic kimberlite cluster in
the northern Slave craton, Canada. A variety
of dating techniques were applied
to constrain the nature and age of two Jericho
kimberlites, JD-1 (170·2 ± 4·3 Ma
Rb-Sr phlogopite megacrysts, 172·8 ± 0·7 Ma U-Pb eclogite rutile, 178 ± 5 Ma
U-Pb eclogite zircon lower intercept) and JD-3 (173 ± 2 Ma Rb-Sr phlogopite
megacryst; 176·6 ± 3·2 Ma U-Pb perovskite), and all yielded identical results
within analytical uncertainty. As there is no discernible difference in the
radiometric ages obtained for these two pipes, the composite Rb-Sr phlogopite
megacryst date of 173·1 ± 1·3 Ma is interpreted as the best estimate for the
emplacement age of both Jericho pipes. The initial Sr isotope composition of
0·7053 ± 0·0003 derived from phlogopite megacrysts overlaps the range
(0·7043-0·7084) previously reported for Jericho whole-rocks. These strontium
isotope data, combined with the radiogenic initial 206 Pb/ 204
Pb ratio of 18·99 ± 0·33 obtained in this study, indicate that the Jericho
kimberlites are isotopically similar to Group 1 kimberlites as defined in
southern Africa. The Jericho kimberlites are an important new source of mantle
xenoliths that hold clues to the nature of the Slave craton subcontinental
mantle. A high proportion (30%) of the Jericho mantle xenolith population
consists of various eclogite types including a small number (2-3%) of apatite-,
diamond-, kyanite- and zircon-bearing eclogites. The most striking aspect of the
Jericho zircon-bearing eclogite xenoliths is their peculiar geochemistry.
Reconstructed whole-rock compositions indicate that they were derived from
protoliths with high FeO, Al 2 O 3 and Na 2 O
contents, reflected in the high-FeO (22·6-27·5 wt %) nature of garnet and the
high-Na 2 O (8·47-9·44 wt %) and high-Al 2 O 3
(13·12-14·33 wt %) character of the clinopyroxene. These eclogite
whole-rock compositions are highly enriched in high field strength elements
(HFSE) such as Nb (133-1134 ppm), Ta (5-28 ppm), Zr (1779-4934 ppm) and Hf
(23-64 ppm). This HFSE enrichment is linked to growth of large (up to 2 mm)
zircon and niobian rutile crystals (up to 3 modal %) near the time of eclogite
metamorphism. The diamond-bearing eclogites on the other hand are characterized
by high-MgO (19·6-21·3 wt %) garnet and ultralow-Na 2 O (0·44-1·50 wt
%) clinopyroxene. Paleotemperature estimates indicate that both the zircon- and
diamond-bearing eclogites have similar equilibration temperatures of 950-1020°C
and 990-1030°C, respectively, corresponding to mantle depths of 150-180 km.
Integration of petrographic, whole-rock and mineral geochemistry, geochronology
and isotope tracer techniques indicates that the Jericho zircon-bearing eclogite
xenoliths have had a complex history involving Paleoproterozoic metamorphism,
thermal perturbations, and two or more episodes of Precambrian mantle
metasomatism. The oldest metasomatic event (Type 1) occurred near the time of
Paleoproterozoic metamorphism (∼1·8 Ga) and is responsible for the extreme HFSE
enrichment and growth of zircon and high-niobian rutile. A second thermal
perturbation and concomitant carbonatite metasomatism (Type 2) is responsible
for significant apatite growth in some xenoliths and profound light rare earth
element enrichment. Type 2 metasomatism occurred in the period 1·0-1·3 Ga and is
recorded by relatively consistent whole-rock eclogite model Nd ages and
secondary U-Pb zircon upper intercept dates. These eclogite xenoliths were
derived from a variety of protoliths, some of which could represent
metasomatized pieces of oceanic crust, possibly linked to east-dipping
subduction beneath the Slave craton during construction of the 1·88-1·84 Ga
Great Bear continental arc. Others, including the diamond-bearing eclogites,
could be cumulates from mafic or ultramafic sill complexes that intruded the
Slave lithospheric mantle at depths of about 150-180
km.