ORIGINAL PAPER

Diamonds and eclogites of the Jericho kimberlite(Northern Canada)

Andrea De Stefano M. G. Kopylova P. Cartigny V. Afanasiev

Received: 17 June 2008 / Accepted: 21 January 2009 / Published online: 18 February 2009

Springer-Verlag 2009

Abstract We studied diamonds and barren and dia-

mondiferous eclogite xenoliths from the Jericho kimberlite

(Northern Slave craton). The majority of the diamonds are

non-resorbed octahedral crystals, with moderately aggre-

gated N (IaB \ 50%, N \ 300 ppm) and d13C = -5 to-41%. The diamonds belong to eclogitic (90% of thestudied samples), websteritic (7%) and peridotitic

(3%) assemblages. The Jericho diamonds differ from the

majority of eclogitic diamonds worldwide in magnesian

compositions of associated minerals and extremely light C

isotopic compositions (d13C = -24 to -41%). We pro-pose that metasomatism triggered by H2O fluids may have

been involved in the diamond formation. Multiple episodes

of the metasomatism and associated melt extraction of

various ages are evident in Jericho eclogite xenoliths where

primary garnet and clinopyroxene have been recrystallized

to more magnesian minerals with higher contents of some

incompatible trace elements and to hydrous secondary

phases. The model is supported by the general similarity of

mineral compositions in diamondiferous eclogites to those

in diamond inclusions and to secondary magnesian garnet

and clinopyroxene in recrystallized barren eclogites. The

ultimate products of the metasomatism could be webste-

ritic diamond assemblages sourced from magnesian

eclogites.

Keywords Mantle metasomatism Eclogite Diamond inclusion Websteritic diamonds Carbon isotopes Rare earth elements Slave craton

Introduction

Diamondiferous eclogites provide us with a valuable

insight into a parent rock for diamond and a diamond-

friendly environment of the upper mantle. Among various

occurrences of diamond-bearing eclogites worldwide,

eclogite xenoliths of the Jericho kimberlite (Northern Slave

craton) stand out, as they are unusually Mg-rich

(Cookenboo et al. 1998; Kopylova et al. 1999a) and are

not a common source for diamond (Heaman et al. 2006).

Several explanations put forward for the origin of the

Jericho eclogites (Kopylova 2003; Heaman et al. 2006)

would benefit from additional constraints that can be

derived from eclogitic diamonds in the pipe. This study

therefore aims to characterize diamonds from the Jericho

kimberlite and elucidate their formation processes based on

the morphology, nitrogen content and aggregation state,

carbon isotopic composition, and mineral inclusion chem-

istry. The study also offers a first glimpse on the parental

rocks for diamonds in the mantle below the northern part of

the Slave craton. We show that the vast majority of the

Communicated by T. L. Grove.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00410-009-0384-7) contains supplementarymaterial, which is available to authorized users.

A. De Stefano (&) M. G. KopylovaDepartment of Earth and Ocean Sciences,

University of British Columbia, 6339 Stores Road,

Vancouver, BC, Canada

e-mail: adestefano@eos.ubc.ca

P. Cartigny

Laboratoire de Geochimie des Isotopes Stables,

Universite de Paris VII, Institut de Physique du Globe et URA

CNRS 1762, 4 Place Jussieu, 75251 Paris Cedex 05, France

V. Afanasiev

Institute of Geology and Mineralogy,

3 Koptiuga St., 678170 Novosibirsk, Russia

123

Contrib Mineral Petrol (2009) 158:295315

DOI 10.1007/s00410-009-0384-7

http://dx.doi.org/10.1007/s00410-009-0384-7

diamonds are sourced from eclogites, and these uncommon

magnesian eclogites host diamonds with an uncommon

isotopic composition of carbon. Our study explores possi-

ble causes of these traits of diamonds and their parent

eclogites and a pronounced distinction with respect to other

worldwide eclogites. Detailed studies of secondary

recrystallization in Jericho eclogites and trace element

analyses of garnet and clinopyroxene led us to conclude

that metasomatism and melt extraction may have played a

role in the recrystallization of eclogites as well as in the

formation of diamonds. The ultimate products of such

metasomatism could be websteritic diamond assem-

blages sourced from magnesian eclogites.

Mantle xenoliths of the Jericho kimberlite

The Jericho kimberlite (11128.900W, 6559.190N) is sit-uated *150 km north of the Lac de Gras kimberlite fieldand was emplaced in the Archean granitoids (Contwoyto

batholith, 2.582.59 Ga, van Breemen et al. 1989) of the

Slave craton. The Jericho kimberlite is a part of a mid

Jurassic (173.1 1.3 Ma, Heaman et al. 2006) kimberlite

cluster, which includes six other kimberlite pipes and

dykes. The petrography, geochemistry and emplacement

history of the kimberlite have been characterized most

recently by Kopylova and Hayman (2008).

Mantle xenoliths found within the Jericho kimberlite are

up to 30 cm in diameter and include eclogite, coarse and

deformed peridotite, megacrystalline websterite and

ilmenitegarnet wehrlite (Kopylova et al. 1999b). The

xenoliths constrain the mantle lithological column at

depths between 40 and 220 km. The upper (40160 km)

lithospheric part of the column consists of coarse, textur-

ally equilibrated peridotites and eclogites. The lower part

of the column ([160 km) has been altered by astheno-spheric melts and contains websterites and deformed

peridotites with young, unequilibrated textures (Kopylova

et al. 1999b).

Eclogites comprise 25% of the mantle xenolith popu-

lation of the Jericho kimberlite. They are composed of

primary pyrope and omphacite, with occasional olivine,

orthopyroxene, and kyanite. Petrographic observations

proved the existence of two groups of eclogites, massive

and foliated, with distinct mineral chemistry and fabric

(Kopylova et al. 1999a). Foliated texture is partly con-

trolled by preferential replacement of garnet and

clinopyroxene by secondary volatile-rich phases along

specific planes. Clinopyroxene in foliated eclogite has

higher contents of jadeite and hedenbergite; garnet has

higher grossular content (Kopylova et al. 1999a). By

mineral chemistry, the massive eclogites match eclogites of

Groups A and B of Coleman et al. (1965), whereas foliated

eclogite belong to Groups B and C (Kopylova et al.

1999a). Only the massive eclogite with Mg-rich garnet and

jadeite-poor clinopyroxene is known to contain diamond

(Cookenboo et al. 1998; Kopylova et al. 1999a; Heaman

et al. 2006). Worldwide, most other diamondiferous

eclogites are less magnesian than the corresponding Jericho

rocks (Heaman et al. 2006).

Complex history of Jericho eclogites is recorded in their

mantle metasomatic mineral assemblages and in multiple

formation ages. The metasomatic assemblage includes

amphibole, phlogopite, rutile, zircon, apatite and ilmenite

(Kopylova et al. 2004). These minerals developed mostly

in foliated eclogites. Zircon-bearing eclogites characterized

by high Na and low Mg contents and similar in mineral

chemistry to foliated eclogites, have been dated (Heaman

et al. 2006) and yielded a range of ages reflecting different

metamorphic and metasomatic events. Hf model ages of

foliated zircon eclogites of subduction origin record a

generation of an oceanic crust between 2.0 and 2.1 Ga

(Schmidberger et al. 2007). The oldest event of mantle

metasomatism occurred simultaneously with metamor-

phism at *1.8 Ga and formed zircon and rutile. Finally,the second episode of metasomatism occurred in the period

1.01.3 Ga and caused apatite growth and light rare earth

element enrichment (Heaman et al. 2006). Zircon-bearing

foliated eclogites were interpreted as metasomatised pieces

of the oceanic crust possibly linked to easterly subduction

off the western margin of the Slave craton and formation of

the Great Bear magmatic arc 1.881.84 Ga ago (Heaman

et al. 2006). Ages of the diamondiferous eclogites are not

known and may well be different from the above ages of

foliated zircon eclogites.

Analytical techniques

Morphology, surface features, and body colour are docu-

mented for 208 small (\3 mm) diamonds ranging inweight from 0.154 to 31.172 mg (0.00080.1559 carats)

(Electronic Table 1). The cathodoluminescence (CL)

characteristics of ten polished diamond plates were exam-

ined using a Philips XL 30 scanning electron microscope

with a CL attachment consisting of a Hamamatsu R376

photomultiplier tube [Department of Earth and Ocean Sci-

ences (EOS), University of British Columbia (UBC),

Vancouver]. The accelerating voltage was 20 keV and the

electron beam current was 100 lA. The spectral range ofthe system was 300800 nm. Infrared (FTIR) spectra were

obtained for 25 diamonds using a Nicolet 710 FTIR spec-

trometer with a Nic-Plan IR microscope attachment and

liquid nitrogen reservoir. Absorption spectra were measured

in transmission mode in the range of 4,000650 cm-1 at a

resolution of 8 cm-1 by averaging the signals of 256 scans.

296 Contrib Mineral Petrol (2009) 158:295315

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The rough stones were mo