JOURNAL OF PETROLOGY
VOLUME 45
NUMBER 2
PAGES 439–455
2004
DOI: 10.1093/petrology/egg102
Re---Os and Lu---Hf Isotope Constraints on the
Origin and Age of Pyroxenites from the Beni
Bousera Peridotite Massif: Implications for
Mixed Peridotite---Pyroxenite Mantle Sources
D. G. PEARSON* AND G. M. NOWELL
DEPARTMENT OF GEOLOGICAL SCIENCES, DURHAM UNIVERSITY, SOUTH ROAD, DURHAM DH1 3LE, UK
RECEIVED NOVEMBER 15, 2002; ACCEPTED AUGUST 16, 2003
A suite of pyroxenites from the Beni Bousera peridotite massif,
northern Morocco, have been analysed for Re---Os and Lu---Hf
isotopic compositions. Measured sections of the massif indicate that
pyroxenite layers make up between 1 and 9% by volume of the total
outcrop. Clinopyroxenes from two Cr-diopside pyroxenites have
unradiogenic Hf isotope compositions (eHfi ÿ77 to ÿ85)
whereas those of the Al-augite suite are more radiogenic (eHfi 94
to 256). In general, the Nd---Hf isotope compositions of the
pyroxenites lie close to the mantle array. One garnet pyroxenite lies
significantly below the mantle Hf---Nd isotope array such that it
requires an ancient history characterized by high Lu/Hf and Sm/
Nd but low Lu/Hf relative to Sm/Nd. As with the Sm---Nd and
Rb---Sr systems, parent---daughter and isotopic ratios for the Lu---Hf
system have been recently decoupled by a partial melting event
associated with transfer of the massif from mantle to crust. This
created highly fractionated Sm/Nd and Lu/Hf ratios in many rocks
and the pyroxenites can be referred to as ‘residual’. The near-solidus
extraction of a siliceous melt from the pyroxenites is also a possible
explanation for the orthopyroxene-rich margins to numerous pyroxenite layers, via reaction with peridotite. Pyroxenite Os isotope
compositions are much more radiogenic than their host peridotites.
In contrast to the non-systematic Nd and Hf model ages, a large
portion of the pyroxenite Re---Os model ages cluster between 12 and
14 Ga, within error of the model ages defined by many Ronda
pyroxenites and close to the precise 143 007 Ga Lu---Hf
isochron defined by clinopyroxenes from the peridotites. The
Re---Os system thus seems to have been more robust to late-stage
melting events that decoupled Sm/Nd and Lu/Hf isotope systematics in the pyroxenites. In contrast to pyroxenites measured from
Ronda, some Beni Bousera pyroxenites have relatively radiogenic Os
isotope compositions at high Os concentrations (018 to 42 ppb),
*Corresponding author. E-mail: [email protected]
comparable with values reported for some cratonic pyroxene-rich
xenoliths. In contrast to cratonic eclogites, most pyroxenites analysed
here and those reported in the literature lie close to the mantle Nd---Hf
isotope array. The Nd---Sr---Pb---Hf isotopic compositions and stable
isotope characteristics of these pyroxenites reflect signatures from
recycled oceanic crust and sediment. Hence, mixing of such material,
if present within the convecting mantle, with peridotite, could account
for some of the heterogeneity seen in oceanic basalts. Small amounts
of pyroxenite incorporated into peridotite can also produce the radiogenic Os isotope signatures evident in the source of oceanic basalts.
However, these observations alone do not require pyroxenite to be an
integral part of the convecting upper-mantle magma source region.
The spectrum of Nd, Hf and Os isotope compositions also makes
them a suitable component to explain some of the isotopic characteristics of the source regions of ultrapotassic magmas.
KEY WORDS:
osmium isotopes; hafnium; pyroxenites
INTRODUCTION
Pyroxenite layers within orogenic peridotite massifs provide direct evidence of mantle heterogeneity and have
been used as key pieces of evidence for ‘marble-cake’
mantle models (Allegre & Turcotte, 1986; Kellog,
1992). Some recent models to explain the detailed
major, trace element and isotopic systematics of
mid-ocean ridge basalts (MORB) have advocated contributions from pyroxenites to the melting regime (e.g.
Prinzhofer et al., 1989; Langmuir et al., 1992; Lundstrum
Journal of Petrology 45(2) # Oxford University Press 2004; all rights
reserved
JOURNAL OF PETROLOGY
VOLUME 45
et al., 1995; Hirschmann & Stolper, 1996; Blichert-Toft
et al., 1999a). In addition, recent petrogenetic models
for continental ultrapotassic volcanics involve melting of
veined lithospheric mantle, in which the vein component
is pyroxene-rich (Foley, 1992; Carlson & Nowell, 2001).
Because of their potential significance in such geodynamic and petrogenetic models, it is important to
improve characterization of the elemental and isotopic
systematics of pyroxenites. This will help to constrain the
distinguishing geochemical criteria that might indicate a
contribution from pyroxenite to magma sources and in
turn will allow further testing of marble-cake mantle
models.
This study focuses on the Beni Bousera peridotite
massif, northern Morocco, as an example of an orogenic
peridotite massif with relatively abundant pyroxenites.
Previous work on this peridotite massif has revealed that
the pyroxenites probably originate from a variety of
sources and are likely to be variable in age (Loubet &
Allegre, 1982; Kornprobst et al., 1990; Pearson et al.,
1993; Kumar et al., 1996; Blichert-Toft et al., 1999a). In
general, the age of the pyroxenite layers has not been well
constrained, yet this information is important in the context of the applicability of the massif to ‘marble-cake
mantle’ or ‘plum-pudding’ mantle models. In this study,
we have analysed the Lu---Hf and Re---Os isotopic compositions of a suite of well-characterized pyroxenites
(Pearson et al., 1993) from the Beni Bousera peridotite
massif. Our objectives were to try to further constrain the
timing of formation of pyroxenite formation and to evaluate the Hf and Os isotopic characteristics of such rocks,
in terms of them being a possible component in the
sources of magmas originating from the oceanic and
continental lithospheric mantle.
GEOLOGICAL SETTING
The Beni Bousera peridotite massif is situated in the Rif
mountains of northern Morocco and is part of the
Betic---Rif orogenic belt (Fig. 1). The massif is surrounded
by migmatitic graphite---sillimanite---garnet gneisses (kinzigites), that are part of a lower-crustal assemblage
exposed with the peridotite body. The tectonic setting
and emplacement age of the Beni Bousera massif is identical to that of the Ronda massif in southern Spain (Fig. 1).
The two peridotite bodies are compositionally very similar, differing mainly in their degree of mineralogical
equilibration during emplacement. Both massifs contain
pyroxenite layers with graphitized diamonds (Pearson
et al., 1989; Davies et al., 1993) and hence ultimately
originate from the diamond stability field. It is possible,
indeed likely, that Beni Bousera and Ronda were derived
from very similar portions of lithospheric mantle underpinning the Betic---Rif region prior to lithospheric
NUMBER 2
FEBRUARY 2004
Fig. 1. Regional geological map of the western Mediterranean area
showing the location of the Beni Bousera and Ronda peridotite massifs.
delamination and extension. Further evidence to support
this idea will be presented below.
ABUNDANCE OF PYROXENITE
LAYERS AND THEIR
MINERALOGICAL ZONATION
Detailed descriptions of the field occurrence, mineralogy
and petrology of the Beni Bousera pyroxenite suite have
been given by Kornprobst et al. (1969), Pearson (1989),
Pearson et al. (1993), Kumar et al. (1996) and Pearson &
Nixon (1996). Here, we concentrate on a few salient
features that we believe are relevant to petrogenetic
models for the pyroxenites. Carefully measured sections
within the peridotite massif indicate that locally pyroxenites can constitute between 1 and 9% by thickness of the
section (Pearson & Nixon, 1996; Fig. 2). The value of 9%
is probably a minimum for the section concerned because
of the likely thinning of some layers to extents that render
them undiscernible in the field. Despite the high abundance of pyroxenites in some places, a value of 1---3% is
440
PEARSON AND NOWELL
BENI BOUSERA PYROXENITES
Fig. 2. Histogram of pyroxenite layer thicknesses in a measured 175 m
section (continually exposed) of the Oued el Jouj, SE Beni Bousera. Data
from Pearson (1989). Pyroxenites make up 9% of the thickness of the
section (defined perpendicular to the layer orientation). This section is
one of the more pyroxenite-rich regions.
the favoured estimate for the massif as a whole
(Kornprobst, 1969; Allegre & Turcotte, 1986; Pearson
et al., 1993). These lower values are similar to the abundance of pyroxenites estimated from measured sections in
the Horoman peridotite (Takazawa et al., 1999). In terms
of frequency distribution, thinner layers (520 cm) are
much more abundant than thick layers (20---260 cm
thick; Fig. 2; Allegre & Turcotte, 1986) as observed for
Horoman pyroxenites (Takazawa et al., 1999). In the
thinner layers (550 cm) the cumulative frequency distribution approximates to log-normal. For thicker layers
this relationship breaks down. As noted by Allegre &
Turcotte (1986), the number of very thin layers (51 cm)
is low, probably because of their destruction by convection/diffusive re-equilibration and also the difficulty in
identifying them in the field.
A notable feature of the Beni Bousera pyroxenites is
the mineralogically zoned character of some layers
compared with a more homogeneous mineralogy in
others (Kornprobst, 1969; Pearson et al., 1993; Kumar
et al., 1996). This mineralogical zonation has been previously ascribed to combinations of high-pressure crystal
fractionation within magmatic veins and melt-rock reaction with the surrounding peridotite. Recently, experiments that melt eclogite/peridotite ‘sandwiches’ have
been carried out that shed new light on the likely origin
of the mineralogical variation within these veins (Yaxley
& Green, 1998).
Near-solidus partial melts of pyroxenite and eclogite
are siliceous and hence highly reactive towards the host
peridotite (Yaxley & Green, 1998). Migration of siliceous
melts from pyroxenites into peridotite will increase the
modal proportion of orthopyroxene in the wall rock and
may result in mantling of the pyroxenitic residue by
orthopyroxenite (Yaxley & Green, 1998). Symmetrical,
thick orthopyroxenite margins tend to be present on
garnet pyroxenite layers that are moderately light rare
earth element (LREE) depleted, suggesting low fractions
of melt loss (Pearson, 1989; Pearson et al., 1993). The
highly LREE-depleted garnet pyroxenites indicate
greater extents of partial melting and tend not to have
orthopyroxene-rich margins in many cases. This is consistent with larger degrees of melting resulting in less
SiO2-rich melts. The occurrence of orthopyroxenite margins to websterite and garnet clinopyroxenite layers is
widespread at Beni Bousera (Kornprobst, 1969; Pearson
et al., 1993) and could probably originate in this way
rather than as a result of high-pressure crystal fractionation as proposed previously by Pearson et al. (1993). None
the less, crystal fractionation at upper-mantle depths is
likely to have operated during the intrusion and crystallization of the layers in the peridotite host.
SAMPLES AND ANALYTICAL
TECHNIQUES
Samples
Samples were selected from the extensive pyroxenite suite
analysed by Pearson et al. (1993) and the reader is referred
to this work for more petrological detail; a summary is
provided in Table 1. Briefly, three websterites belonging
to the Cr-diopside pyroxenite suite were studied together
with four garnet pyroxenites and one websterite from the
Al-augite pyroxenite suite. Major and trace element geochemistry of the whole rocks and minerals have been
presented by Pearson et al. (1993) and Pearson & Nixon
(1996). All layer thicknesses exceeded 10 cm and two
garnet pyroxenites were in excess of 2 m thick (Table 1).
All samples analysed here were taken from the centre of
layers. The letter M denotes a sample from the layer
centre, taken as part of a sample set collected across a
given layer. GP147 M contains graphite pseudomorphs
after diamond, up to 15% by volume. This observation
defines the ultimate derivation of the massif to be from
within the diamond stability field (Pearson et al., 1989;
Pearson & Nixon, 1996).
All clinopyroxenes within the pyroxenites contain
abundant exsolution lamellae of orthopyroxene. In addition, clinopyroxenes within the garnet pyroxenites commonly contain exsolved blebs of garnet. Discrete garnet
crystals frequently contain 10 mm needles of rutile; however, this phase was not observed as an interstitial phase
in the samples analysed. Bulk isotopic compositions for
garnet pyroxenites are calculated using a mode of 60%
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NUMBER 2
FEBRUARY 2004
Table 1: Summary of pyroxenite petrographical characteristics
Sample
Layer
Suite
Lithology
Mineralogy
Features
0.30
0.50
Cr-diopside
Olivine websterite
ol (10%), cpx (50%), opx (40%)
Simple layer
Al-augite
Garnet clinopyroxenite
cpx (60%), gt (40%)
Simple layer
0.18
0.10
Al-augite
Garnet clinopyroxenite
cpx (60%), gt (40%)
Zoned, layer, margins gt-poor
Cr-diopside
Websterite
cpx (40%), opx (60%)
Zoned, patches of orthopyroxenite
2.50
2.60
Al-augite
Garnet clinopyroxenite
cpx (60%), gt (40%)
Zoned layer, narrow orthopyroxenite margins
Al-augite
Garnet clinopyroxenite
cpx (60%), gt (40%)
Zoned layer, narrow upper orthopyroxenite---
thickness (m)
GP30
GP37
GP87
GP101
GP139
GP147
websterite margin; contains graphite pseudomorphs after diamond
0.15
0.14
Cr-diopside
Websterite
cpx (40%), opx (60%)
Simple layer
GP194
Al-augite
Garnet clinopyroxenite
cpx (60%), gt (40%)
Zoned, layer margins websteritic
GP236
0.40
Al-augite
Websterite
cpx (40%), opx (60%)
Simple layer
GP188
Table 2: Lu---Hf and Sm---Nd isotope data for Beni Bousera pyroxenites
Sample no.
Lu
Hf
176
Lu/177 Hf
GP101CPX
0.080
0.577
0.0197
GP101WR
GP188CPX
0.085
0.689
0.0175
176
Hf/177 Hf
eHfI
Sm
Nd
147
0.282527
0.282560
22
ÿ8.5
1.26
0.743
4.25
2.50
0.179
0.184
0.512188
0.512175
0.282548
50
ÿ7.7
1.06
11.8
3.31
0.313
2.62
0.200
0.107
0.910
0.207
0.209
0.512215
0.512242
0.512727
22
9.6
0.403
0.374
0.994
0.411
0.245
0.550
0.512765
0.513078
21
2.3
7.6
44
8.2
7.9
0.238
0.050
2.91
0.513395
14
7.0
ÿ3.9
ÿ4.5
0.299
0.320
0.288
0.266
0.627
0.726
0.513080
0.513112
18
7.4
7.7
ÿ2.4
ÿ4.3
1.58
0.827
2.54
0.220
0.377
2.25
0.513982
0.514537
19
25.7
31.2
ÿ11.9
ÿ20.9
0.865
1.28
1.28
1.61
0.410
0.480
0.513925
0.514041
18
24.5
27.4
ÿ12.4
ÿ13.7
1.49
0.53
1.12
0.07
0.804
4.58
0.513243
0.513725
15
10.1
8.6
ÿ4.8
8.6
0.941
1.11
0.615
0.700
0.925
0.438
0.513178
0.513205
14
8.4
10.4
ÿ4.7
ÿ3.2
16
GP188WR
GP236MCPX
0.050
0.574
0.0124
GP236MWR
GP147MCPX
GP147MGT
0.081
2.02
0.709
0.202
0.0162
1.42
GP147MGT(R)
147 WR
GP37 CPX
0.858
0.016
0.506
0.684
0.241
0.0033
GP37GT
1.82
0.623
0.415
GP139CPX
0.738
0.098
0.660
1.62
0.159
0.0086
GP139GT
2.77
0.283
1.39
147 BULK
GP37WR
GP37 BULK
GP139GT(R)
GP139WR
GP139 BULK
1.167
1.09
0.152
0.283098
0.283144
0.283037
0.283590
0.283581
14
14
43
0.283133
0.283125
15
0.283501
0.283631
50
9.4
26.2
14
24.7
0.283484
0.283550
18
0.283103
0.283677
0.283695
10
0.283143
0.283164
8
16
16
25.6
12.1
11.8
12.4
12.1
Sm/144 Nd
143
Nd/144 Nd
eNdI
9
ÿ8.7
ÿ9.0
0.18
ÿ8.3
ÿ7.8
1.7
ÿ0.13
20
16
35
12
14
10
38
DeHf
6.3
7.0
WR, whole rock; GT, garnet, CPX, clinopyroxene, BULK, bulk composition calculated from a mode of 60% cpx, 40% garnet.
Sm---Nd data for CPX separates taken from Pearson et al. (1993). R, repeat run of same solution. M, middle of layer as part
of a sample traverse, all samples from the centre of layers. Initial ratios corrected to 22.5 Ma. CHUR values: 176 Lu/177 Hf ¼
0.0334, 176 Hf/177 Hf ¼ 0.282772; DM values: 176 Lu/177 Hf ¼ 0.0384, 176 Hf/177 Hf ¼ 0.28325. All Hf data normalized to 176 Hf/
177
Hf ¼ 0.282160 (Blichert-Toft et al., 1997; Nowell et al., 1998). 176 Lu decay constant from Scherer et al. (2001).
clinopyroxene and 40% garnet (Table 2). This ratio is
typical of modes estimated visually and calculated from
mineral and bulk major element compositions. The
coarse grain size and modal heterogeneity within layers
makes it very difficult to define a mode for any given layer
and the 60:40 ratio is the best estimate for any given
garnet clinopyroxenite. Not all samples analysed for
Re---Os isotopes in this study have corresponding
442
PEARSON AND NOWELL
BENI BOUSERA PYROXENITES
Lu---Hf isotope analyses. This simply reflects the lack of
remaining sample for adequate separation of pure
mineral separates.
Analytical techniques
Techniques used in mineral separation and picking have
been described by Pearson et al. (1993). In contrast to the
leaching procedure employed by Pearson et al. (1993), we
used only a 40 C 6 M HCl ultrasonic leach to clean
minerals for Hf analysis because of concern that the dilute
HF---HCl leaching procedure used for Sr---Nd isotope
analysis (Pearson et al., 1993) might leach Hf from the
separates.
Splits (10 mg) of the leached mineral separates were
taken for trace element analysis by inductively coupled
plasma mass spectrometry (ICP-MS) following established procedures (Ottley et al., 2003), except that we
used ultra-pure acids and ultra-clean work environments
throughout. The final solutions were diluted to 50 ml of
35% HNO3 and run directly on the ICP-MS system.
Repeat analyses of standards show that Lu/Hf is reproducible to 15% (1 relative standard deviation) using
this procedure. Given the current disagreement on the
half-life of 176 Lu, this level of parent---daughter isotope
ratio precision is more than adequate for our purposes.
The bulk (95%) of the leached mineral separate was
used for Lu---Hf isotope analysis. We used a simple twocolumn pre-concentration procedure that employs a 5 ml
cation separation as the first step, using 1N HF---1N HCl
to elute Hf and 6N HCl to elute Nd, followed by a mixed
sulphuric acid---H2O2 anion column for final purification
of the Hf. The procedure provides a rapid, low blank
method for the analysis of Sr, Nd and Hf isotopes in
geological samples in two column steps (Dowall et al.,
2003). Hf blanks were 60 pg and are insignificant for the
levels of Hf analysed here. Measurements were made on
a ThermoFinnigan Neptune plasma ionization multi collector mass spectrometer. Whole rocks and clinopyroxenes were analysed using an ESI PFA-50
nebulizer with quartz, cyclonic Scott-type double pass
spray chamber. Twelve analyses of the JMC-475 standard during this session gave a 176 Hf/177 Hf value of
0282150 7 (2 S.D.; i.e. 265 ppm external reproducibility). Garnets were analysed using a Cetac Aridus desolvating nebulizer and a high-sensitivity skimmer cone
that produced a sensitivity of 470 V/ppm Hf at an uptake
rate of 80 ml/min, for the analytical session in question.
Nine analyses of the JMC-475 standard during this session gave a 176 Hf/177 Hf value of 0282148 3 (2 S.D.;
i.e. 11 ppm external precision). 176 Hf/177 Hf values were
corrected to an accepted value of 0282160 (Blichert-Toft
et al., 1997; Nowell et al., 1998). The long-term average
for the JMC 475 standard on the Durham Neptune
system is 0282155 9 (n ¼ 195; Nowell et al., 2003a)
and is within 17 ppm of the accepted value. Full details of
mass spectrometry procedures, sensitivity and instrumental performance have been given by Nowell et al. (2003a).
Whole-rock Re---Os chemical procedures followed the
methods of Pearson & Woodland (2000). Samples were
analysed by negative thermal ionization mass spectrometry (N-TIMS) on a ThermoFinnigan Triton mass
spectrometer. All analyses were carried out on the secondary electron multiplier, via ion-counting, in peakhopping mode using a Ba(OH)2 activator solution.
Using this procedure, our long-term mean 187 Os/188 Os
value for 161 runs of the University of Maryland College
Park standard at signal sizes equivalent to those of the
samples was 011383 32 (2 S.D.; 28 per mil) and is
within error of the value of 0113791 3 produced from
static Faraday runs of large loads by Walker et al. (1997).
The mean 189 Os/188 Os over this period is 121976 192
(2 S.D.; equates to 16 per mil).
Lu---Hf AND Sm---Nd ISOTOPE
SYSTEMATICS
Element partitioning and isotope
systematics
Measured whole-rock vs calculated bulk isotopic
compositions
For comparative purposes, bulk compositions have been
calculated for the garnet clinopyroxenites. This allows the
bulk Nd---Hf isotope compositions to be easily compared
with the websterites and with basaltic rocks. We use
calculated bulk compositions rather than the measured
whole-rock compositions presented in Table 2 for various
reasons. First, there is evidence for late-stage, grainboundary, LREE---HFSE (high field strength element)
enriched metasomatic phases in orogenic peridotites
(e.g. Reisberg et al., 1989; Bodinier et al., 1996). This
can result in measured whole rocks having less radiogenic
Nd isotope compositions than their clinopyroxene, in
assemblages that contain no primary LREE-enriched
phases (Pearson, 1989). Second, whole rocks show considerable evidence of serpentinization and other types of
alteration. This can also be a source of LREE-enriched
crustal material. Crustal input to the whole-rock powders
is clearly shown to be the case for Sr isotopes by detailed
leaching studies (Zindler et al., 1983; Pearson et al., 1993).
Hence, it is likely that the measured whole-rock pyroxenites will not faithfully record the Nd or Hf isotope
composition of the pyroxenite and we prefer to use calculated bulk isotopic compositions for the garnet pyroxenites. In practice, the measured whole-rock
compositions for the rocks studied here are either within
error, or close to within error (05 epsilon units) of the
calculated bulk compositions (Table 2). In some cases, the
measured whole rocks are further off the mantle Nd---Hf
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JOURNAL OF PETROLOGY
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isotope array than the calculated bulk rocks. The small
differences between measured and calculated bulk
Nd---Hf isotope characteristics in no way alter the conclusions arrived at below. The following discussion will
refer to the calculated bulk values when discussing
pyroxenite bulk isotopic compositions. Because of the
likely open-system behaviour of the measured whole
rocks, they will not be included in any isochron
regressions.
Mineral equilibria and isotopic compositions
Lu/Hf and Sm/Nd are partitioned between garnet and
clinopyroxene in the manner expected, i.e. garnets have
considerably greater Lu/Hf and Sm/Nd than their
coexisting pyroxenes. Garnet 176 Lu/177 Hf ratios are predictably high (041---142; Table 2). Clinopyroxenes equilibrated with garnet have correspondingly low 176 Lu/
177
Hf (00032---0016) compared with clinopyroxenes
from garnet-free assemblages (0012---002). 147 Sm/144 Nd
values of some clinopyroxenes are among the highest ever
measured in mantle clinopyroxenes and are testament to
the extreme LREE depletion of some of the pyroxenites,
as noted by Loubet & Allegre (1982) and Pearson et al.
(1993). This feature led those workers to propose a latestage partial melting event to explain the LREE-depleted
nature of many pyroxenites. The presence of orthopyroxenite reaction rims in many of the Beni Bousera
pyroxenite layers supports this idea.
Garnets from garnet pyroxenites have more radiogenic
measured Hf than their coexisting clinopyroxenes, as
expected. There is evidence of some minor, late-stage
disturbance of both Lu---Hf and Sm---Nd systems from
the small to moderate differences in initial isotopic compositions at the preferred emplacement age of 225 Ma
(Table 2) that is reflected in some very anomalous
isochron ages (see below). Clinopyroxenes from the two
Cr-diopside pyroxenites have unradiogenic Hf isotope
compositions (eHfi ÿ76 to ÿ84; Table 2) whereas
those of the Al-augite suite are more radiogenic (eHfi þ96
to þ263). This range extends up eHfi ¼ þ42 when the
data of Blichert-Toft et al. (1999a) are considered. These
values are much more restricted than the values obtained
for Archaean eclogite xenoliths (cpx eHfi up to 166; Jacob
et al., 2002) and garnet---spinel (alkremite) mantle xenoliths from kimberlites (garnet eHfi up to þ24 000; Nowell
et al., 2003a).
Beni Bousera pyroxenites show much greater Sr---Nd
isotopic heterogeneity than their host peridotites (Pearson
et al., 1993). In contrast, the range of calculated bulk
pyroxenite Hf isotope compositions is considerably more
restricted than the large range shown by clinopyroxenes
from the peridotites (eHfi 14---209; Pearson & Nowell,
2003). This is because the peridotites have surprisingly
NUMBER 2
FEBRUARY 2004
radiogenic eHfi values that are much more variable than
their eNdi values (þ32 to þ 149; Pearson et al., 1993).
Measured eHfi values for the websterite clinopyroxenes
and calculated bulk eHfi values for the garnet clinopyroxenites, when combined with their Nd isotopic compositions, can be compared with the composition of oceanic
basalts that scatter about the so-called Hf---Nd ‘mantle
array’ (Fig. 3). Deviation from the mantle array can be
denoted using the DeHfi notation of Beard & Johnson
(1993). Beni Bousera pyroxenites scatter around the mantle Hf---Nd isotope array (Fig. 3). The garnet pyroxenites
analysed by Blichert-Toft et al. (1999a) plot close to the
mantle array, with the DeHfi values close to zero. Three
garnet pyroxenites (this study) plot at varying distances off
the mantle array. GP139 and GP147 have DeHfi values of
ÿ32 to ÿ43, and plot between the fields of MORB and
ocean island basalt (OIB). More extreme is GP37, which
plots well to the right of the mantle Nd---Hf array with a
DeHfi value of ÿ137. This is one of the largest deviations
below the mantle array so far observed for a mantle
sample. The radiogenic eHfi value indicates that Lu/Hf
was supra-chondritic but the displacement below the
mantle array indicates lower levels of Lu/Hf fractionation compared with Sm/Nd fractionation, relative to the
mantle array. In contrast to the garnet-bearing pyroxenites, the garnet-free Al-augite websterite GP236 plots
above the mantle array with a DeHfi value of þ63,
lying at the outer edge of the OIB field.
Although the two Cr-diopside websterites have
unradiogenic Nd and Hf isotope compositions, relative
to Bulk Earth, they plot on an extension of the mantle
array, in the field occupied by upper-crustal rocks
(Vervoort et al., 1999). These are the only pyroxenite
compositions occupying the ‘enriched’ part of the
Nd---Hf isotope diagram.
Inter-mineral Sm---Nd and Lu---Hf
isochrons
The essentially bi-mineralic nature of the garnet pyroxenites combined with their high-temperature evolution
provide the opportunity to obtain two-point intermineral isochrons for the Beni Bousera pyroxenites
(Polve, 1983; Kumar et al., 1996). Recently, BlichertToft et al. (1999a) produced six Lu---Hf isochrons for the
Beni Bousera pyroxenites. In this study, we have produced an additional three Lu---Hf and Sm---Nd
clinopyroxene---garnet isochrons. The combined isochron regressions for all datasets are presented in
Table 3. All uncertainties on ages, including means, are
quoted at the 95% confidence limit and the Lu---Hf ages
of Blichert-Toft et al. (1999a) have been recalculated to a
l value of 1865 10 ÿ11 (Scherer et al., 2001). Six
Sm---Nd garnet---clinopyroxene isochrons (excluding
GP37) give a mean of 209 40 Ma (2 S.D.), within
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Fig. 3. eHfi vs eNdi plot of the Beni Bousera pyroxenites compared with oceanic basalts, Beni Bousera peridotites and peridotite xenoliths.
Calculated bulk compositions are plotted for the garnet pyroxenites by combining clinopyroxene and garnet in a 60:40 ratio. Nd isotope data
from Pearson et al. (1993) and Table 1. Only clinopyroxenes are plotted for the websterites. Data sources for oceanic basalt fields (MORB and OIB)
have been given by Blichert-Toft (2001). Black squares, garnet pyroxenites analysed in this study; white squares, calculated bulk compositions of
garnet pyroxenites analysed by Blichert-Toft et al. (1999a); grey squares, clinopyroxenes from websterites analysed in this study. Upper panel shows
the extreme Hf isotope heterogeneity of clinopyroxenes from the Beni Bousera peridotites (Pearson & Nowell, 2003), compared with lithospheric
mantle peridotite samples as represented by xenolith suites (cratonic and non-cratonic) and the MORB---OIB fields [data from Vervoort et al. (1999)].
Data sources for xenoliths have been given by Pearson et al. (2004b).
error of the mean of eight Lu---Hf isochrons (excluding
M5-15) of 241 86 (2 S.D.; Table 3; Fig. 4). The
variability of the two-point isochron ages, in particular
the large disagreement between the Sm---Nd and Lu---Hf
isochron ages for the most aberrant samples, e.g. GP37
(Table 3), indicates that additional, open-system processes were affecting Sm---Nd and Lu---Hf equilibrium in
these rocks during or after cooling. The best agreement
between Sm---Nd and Lu---Hf isochrons is shown by samples GP147 (Lu---Hf isochron ¼ 208 25 Ma; Sm---Nd
isochron ¼ 191 11 Ma) and M5-101 (Lu---Hf
isochron ¼ 253 12 Ma; Sm---Nd isochron ¼ 240 43 Ma). Unfortunately, the isochron ages for these two
samples are significantly discrepant and it does not seem
sensible to use the level of agreement between Lu---Hf and
Sm---Nd systems as an indication of accuracy. The most
precise isochron is provided by the Lu---Hf isochron for
GP139, where the extreme Lu/Hf fractionation between
garnet and clinopyroxene provides an age of 225 11 Ma. Blichert-Toft et al. (1999a) produced a similarly
precise, but older garnet---clinopyroxene isochron age of
253 12 Ma for sample M5-101 (Table 3). However,
the garnet and clinopyroxene in this layer did not show
the extreme Lu/Hf fractionation of GP139. Because the
GP139 mineral pair show the most extreme Lu/Hf fractionation, we argue that they will be the least readily
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Table 3: Compilation of garnet---clinopyroxene
mineral isochrons for garnet clinopyroxenites using
Lu---Hf and Sm---Nd isotope systems
Sample
Lu---Hf age (Ma)
(2s)
Sm---Nd age (Ma)
(2s)
GP374
16.8
22.5
6.6
1.1
40.5
19.9
2.3
1.7
20.8
2.5
19.1
23.0
1.1
7.1
20.1
19.5
6.8
5.9
24.0
4.3
GP139
4
GP1474
2
Ga
Ii2
E22
1
M5-1013
M6-214
3
M5-153
M5-99
3
M5-1063
M5-367
3
Average
25.3
31.5
1.2
3.5
68.3
24.7
1.5
3.4
26.4
24.7
2.1
6.4
24.1
8.6
20.9
4.0
Garnet---clinopyroxene Sm---Nd and Lu---Hf isochrons for
Beni Bousera pyroxenites. Errors are at the 95% confidence
limit. Compiled from data given in the following sources:
1
Polvé (1983); 2Kumar et al. (1996); 3Blichert-Toft et al.
(1999a); 4this study. For data obtained in this study the
estimated reproducibility of 176 Lu/177 Hf ratios was 3% 2
S.D. and 0.6% for 147 Sm/144 Nd. In-run errors were used on
the isotopic ratios except where long-term standard reproducibility was greater. Where two analyses of garnet are available, we take the mean value and quadratically add the in-run
uncertainties. Uncertainties for 176 Lu/177 Hf ratio for BlichertToft et al. (1999a) study are stated as 51% and ages presented here are calculated using 1%. All Lu---Hf isochron
ages calculated using l176 Lu ¼ 1.865 10 ÿ11 (Scherer et
al., 2001). All Sm/Nd ages calculated used uncertainty on
147
Sm/144 Nd of 0.6%. Age for sample Ga from Kumar et al.
(1996) calculated using cpx (1) and garnet (1).
*Average Lu---Hf isochron age does not include value for M515; average Sm---Nd isochron age does not include value for
GP37.
disturbed by secondary processes and hence might provide the best estimate of when the Beni Bousera massif
passed through the Lu/Hf blocking temperature for the
garnet---clinopyroxene assemblage. We acknowledge that
the GP139 isochron age is almost within error of the age
produced by Blichert-Toft et al. (1999a) and so, in reality,
either age could be valid.
Re---Os ISOTOPE SYSTEMATICS
Although Os isotope studies of orogenic lherzolite massifs
have been carried out previously (Reisberg et al., 1991;
Kumar et al., 1996; Roy-Barman et al., 1996; Becker et al.,
2001), only two individual pyroxenite layers from Beni
Bousera have been analysed for both Re and Os. We
present an additional seven analyses, including two
Fig. 4. Lu---Hf isochrons for garnet---clinopyroxene mineral pairs in
garnet pyroxenites. Errors used are 3% (2s) for 176 Lu/177 Hf and the
in-run error for 176 Hf/177 Hf. Where garnets were run twice (Table 1),
the average isotopic composition is taken and the within-run errors are
quadratically summed. The 176 Lu decay constant used is 1865 10 ÿ11
(Blichert-Toft, 2001).
Fig. 5. Re vs common (non-radiogenic) Os concentrations in wholerock Beni Bousera pyroxenites. Also plotted are data for Ronda pyroxenites (Reisberg et al., 1991).
Cr-diopside pyroxenites. Kumar et al. (1996) made three
analyses of a single, complex zoned layer from the northern area of Beni Bousera. These analyses are included in
our plots.
Os contents of the Beni Bousera pyroxenites are mostly
in the range from 003 to 06 ppb (Fig. 5). One Crdiopside websterite (GP30) has 22 ppb Os. This total
range is similar to that found by Roy-Barman et al. (1996)
but those workers did not note which petrogenetic groups
their pyroxenites belonged to. Typical common Os
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BENI BOUSERA PYROXENITES
concentrations measured in Beni Bousera pyroxenites are
significantly higher than those found for Ronda (Reisberg
et al., 1991). Several Beni Bousera pyroxenites have relatively radiogenic Os isotope compositions at high Os
concentrations (018 to 42 ppb), comparable with values
reported for some cratonic pyroxene-rich xenoliths
(Carlson & Irving, 1994). For the Beni Bousera
pyroxenites, the high Os contents suggest crystallization
in a sulphur-saturated environment. This is confirmed by
the presence of abundant sulphide inclusions within
many of the clinopyroxenes.
Re contents and hence Re/Os ratios in the pyroxenites
are highly variable (Fig. 5). Re values as low as 0035 ppb
(GP37) are lower than those reported from basaltic
magmas (Shirey & Walker, 1998), whereas the value of
273 ppb for GP147 is considerably higher than for
typical magmatic rocks. Elevated Re contents have been
reported for pyroxenites from other massifs. A wholerock Re content of 35 ppb has been reported for a
pyroxenite from the Horoman peridotite by Saal et al.
(2001), whereas Roy-Barman et al. (1996) reported a Re
concentration of 126 ppb for a garnet separate from a
Lherz pyroxenite layer. This could provide support for
the idea that garnet has a very high partition coefficient
for Re (Righter & Hauri, 1999) and that the pyroxenites
may have formed by accumulation of garnet. However,
the Re content of some garnetiferous pyroxenites is very
low (e.g. GP37) and Re abundance does not directly
correlate with either whole-rock Yb or garnet content.
These observations argue against a strong garnet control
on Re. A possible explanation for the lack of strong
garnet control in the Beni Bousera garnet pyroxenites is
that garnet may be exsolved from a higher-T aluminous
pyroxene in some pyroxenites, whereas it is a liquidus
phase in others. An additional complicating factor is that
Beni Bousera pyroxenites contain abundant sulphide
which may also account for, or contribute towards the
high Re contents of the whole rocks. Micro-sulphide
inclusions in the garnets may also be present.
As observed previously for orogenic peridotites
(Reisberg et al., 1991; Roy-Barman et al., 1996; Saal
et al., 2001) the Os isotopic compositions of the pyroxenites are generally significantly more radiogenic than
those reported for the host peridotites (Table 3; Fig. 6).
Although the most radiogenic 187 Os/188 Os ratios for
pyroxenites are those reported from the Ronda massif
(Reisberg et al., 1991), the values for Beni Bousera and
Ronda pyroxenites overlap (Fig. 6). Additional sampling
and analysis would probably reveal very similar isotopic
ranges given the similarity of Nd---Sr isotope systematics
and emplacement ages of the two massifs. High 187 Os/
188
Os in the Beni Bousera pyroxenites is supported by
high 187 Re/188 Os such that a positive correlation is
defined on a Re---Os isochron diagram with a slope
equating to an age of 980 330 Ma. This relationship
Fig. 6. Comparison of the range in Os isotope compositions,
expressed as gOs, of Beni Bousera pyroxenites compared with their
host peridotites and pyroxenites from the Ronda massif (Reisberg et
al., 1991) and non-contaminated OIBs (see text for data sources). Beni
Bousera peridotite range taken from Pearson et al. (2004b). Range of
values for eclogite xenoliths is taken from Pearson et al. (1995a). Range
of values for continental crust is taken from Ravizza & Turekian
(1992). gOs ¼ [187 Os/188 Os(sampleT ) --- 187 Os/188 Os(ChondriteT )/187 Os/
188
Os(ChondriteT )] 100, where T is the time of eruption or massif
emplacement.
and its significance will be addressed below in more
detail. The high 187 Os/188 Os values of the pyroxenites
are significantly more radiogenic than modern-day,
uncontaminated oceanic basalts (e.g. Hauri & Hart,
1993; Marcantonio et al., 1993; Widom & Shirey, 1996;
Widom et al., 1999; Fig. 6).
DISCUSSION
The age and evolution of the Beni Bousera
pyroxenites
Lu---Hf and Sm---Nd isochrons
An 40 Ar/39 Ar plateau age of 215 17 Ma was obtained
from a plagioclase separate from a sillimanite---garnet
gneiss surrounding the Beni Bousera massif (Pearson
et al., 1993) and is taken to indicate the age of cooling
through the 300---400 C K---Ar blocking temperature,
corresponding to the final stages of crustal emplacement
of the peridotite body into the crust. The high equilibration temperatures recorded by the pyroxenite assemblages (4900 C for rim compositions; Pearson, 1989;
Pearson & Nixon, 1996) imply rapid cooling of the massif.
Hence, it is likely that any garnet---clinopyroxene isochron
for the Sm---Nd or Lu---Hf systems will ideally record the
timing of removal of the peridotite body from the mantle
into the crust, followed by rapid exhumation. As such, the
ages are likely to approximate to the massif emplacement
age. The variation of the Lu---Hf and Sm---Nd isochron
systematics was discussed above. In general, the Lu---Hf
isochrons provide the most precise ages (Table 3).
The two-point 225 11 Ma Lu---Hf isochron for
GP139, or the older 253 12 Ma isochron for
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JOURNAL OF PETROLOGY
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M5-101 (Table 3; Blichert-Toft et al., 1999a) are consistent with the higher blocking temperature for the Lu---Hf
system, whereas the younger Lu---Hf isochron ages for
GP147 and GP37 are not. The similarity of the GP139
isochron to the precise Sm---Nd isochron obtained for the
Ronda massif (Zindler et al., 1983) leads us to take the
relatively precise Lu---Hf isochron age of 225 Ma for
GP139 as the ‘emplacement age’ of the Beni Bousera
massif. The small difference between the 40 Ar/39 Ar plateau age and the Lu---Hf isochron age implies cooling
rates of the order of 400 C/Myr if the blocking temperature for Lu---Hf in the garnet---clinopyroxene system is of
the order of 800 C. If the older age of 253 Ma is viewed
as more reliable, this decreases cooling rates by almost a
factor of two. The errors involved do not allow much
certainty to be attached to these estimates. The 225 11 Ma Lu---Hf age for GP139 is in closer agreement with
the Sm---Nd age of 215 18 Ma determined for a
Ronda garnet pyroxenite (Zindler et al., 1983). The similarity of these ages indicates the approximate synchroneity of emplacement of the two massifs into the crust.
Re---Os whole-rock isochrons and model age systematics
Pearson et al. (1993) noted the complex Sm---Nd isotope
systematics of the Beni Bousera pyroxenites. Nd model
ages (relative to depleted mantle or CHUR) are extremely variable and suggest that the pyroxenite suite as a
whole could not have been derived simultaneously from
any isotopically homogeneous source with their present
Sm/Nd ratios. The same observation can be made for
the Lu---Hf system because model ages for calculated bulk
pyroxenite compositions (from garnet---clinopyroxene
pairs), or from clinopyroxene in websterites, are extremely variable.
Loubet & Allegre (1982) and Pearson et al. (1993) suggested that a recent partial melting event may have disrupted the parent---daughter ratios. The recent nature of
this event means that insufficient time has elapsed to
allow its expression isotopically, thereby decoupling
parent---daughter and isotopic ratios. This results in considerable variation in model age and whole-rock isochron
systematics. The evidence for this event is the extreme
LREE depletion shown by some pyroxenite layers,
together with the orthopyroxene-rich margins of numerous layers, which may document the extraction of a
dacitic near-solidus melt, leaving a residual pyroxenite.
This partial melting event, particularly if of a nonequilibrium nature, affecting pyroxenes that had probably partially exsolved orthopyroxene, may contribute to
the extreme inter-mineral Lu/Hf---Sm/Nd fractionations
observed between garnet and clinopyroxene that differ
significantly from experimental values (Blichert-Toft et al.,
1999a).
NUMBER 2
FEBRUARY 2004
Regression of the Beni Bousera pyroxenite whole-rock
Re---Os data does not produce a line that has a high
probability of fit. A model 3 regression (assuming scatter
as a result of assigned errors and variation in initial Os
isotope ratio) of all the Beni Bousera pyroxenite data,
including those of Kumar et al. (1996), gives an age of
980 330 Ma (2s). Initial ratio variation is likely to be
highly correlated with Re/Os, such that artificial trends
can be generated in samples that are unrelated to each
other. These trends are a particular danger when evaluating low-probability-of-fit regressions such as those that
can be made with the pyroxenite data. More useful
information can be obtained by examining the Re---Os
model age systematics.
Although variable, five of the nine whole-rock pyroxenites [including the layer studied by Kumar et al. (1996)
as one sample] have Re---Os model ages ranging between
1 and 14 Ga (Table 4). The melts from which the pyroxenites originally crystallized may not have had Os
isotopic ratios that fell exactly on the mantle evolution
curve. This is likely given their complex Nd---Hf isotope
systematics and highly varied oxygen isotope compositions. This will have little effect on the model ages calculated for extremely radiogenic pyroxenites, but could be
important for samples with relatively unradiogenic compositions such as GP30 and GP194 M. Interestingly, four
out of seven pyroxenites from the Ronda massif (Reisberg
et al., 1991) have whole-rock Re---Os model ages in this
range. The Re---Os model age systematics are much more
coherent than Sm---Nd and Lu---Hf model ages in the
pyroxenites. This observation suggests that the Re---Os
isotopic system might be more robust to disturbance from
a late-stage partial melting event than the Lu---Hf and
Sm---Nd systems. Re and Os may be relatively unfractionated by the extraction of a low-degree, S-undersaturated melt from the pyroxenites. Any small fractionation
produced by low degrees of partial melting of the pyroxenites may be insufficient to disturb Re/Os significantly
and hence results in only minor alteration of the calculated model ages. In contrast, the presence of residual
garnet during this partial melting event will have a
greater effect on the fractionation of Sm/Nd and
Lu/Hf. The much steeper intersection of the pyroxenite
Os isotope evolution curves with the Primitive Mantle
evolution curve, compared with the shallow-angle intersections of the Hf and Nd isotope evolution curves,
means that minor variations in Re/Os will not greatly
affect the Re---Os model age.
Whether the Re---Os model ages reflect the timing of
pyroxenite formation or some later, major Re---Os fractionation event is debatable. Kumar et al. (1996) suggested, on the basis of three portions of the same
pyroxenite layer yielding model ages in the 12---13 Ga
range, that this represented the formation age of some of
the pyroxenite layers. Our more extensive dataset
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BENI BOUSERA PYROXENITES
PEARSON AND NOWELL
Table 4: Whole rock Re---Os isotope compositions of pyroxenites
Sample
Re (ppb)
Os (ppb)
Os
GP30
0.135
0.051
0.078
2.28
0.399
0.244
2.28
0.398
0.241
0.712
2.73
0.337
0.182
0.329
0.166
0.835
0.183
0.551
0.487
0.540
0.484
0.651
0.634
0.545
0.522
0.537
0.515
GP37
GP87 M
GP101
GP147 M
GP188
GP194 M
GP236 M
GP236MR
187
Re/188 Os
187
0.182
0.613
1.56
Os/188 Os
0.12640
0.13370
0.20892
10.4
78.8
gOsi
TMA (Ga)
ÿ1.0
4.7
0.67
1.4
0.41
0.33
63
4.0
1.1
0.39
0.11
0.54
1.2
0.08
0.12
0.10
0.10
0.10
0.31311
0.83374
143
7.45
1.82
0.27621
0.18361
115
5.83
5.94
0.23933
0.23649
86
2.3
1.2
84
1.2
531
44
d18 O
4.9
7.3
8.7
7.5
9.3
7.1
5.6
Errors on 187 Os/188 Os all better than the standard reproducibility of 2.5 per mil (2s). Error on 187 Re/188 Os is 3% (2s). Oxygen
isotopic compositions taken from Pearson et al. (1993). Parameters used for TMA model age calculation: 187 Re/188 Os ¼
0.4243, 187 Os/188 Os ¼ 0.1287. Error on TMA model ages includes the uncertainty in the Bulk Earth evolution curve and errors
in parent---daughter ratio and isotopic ratio measurements. Os , common (non-radiogenic) Os values calculated assuming an
atomic fraction of 187 Os ¼ 0.0146. gOsi values calculated to an emplacement age of 22.5 Ma and a chondritic 187 Os/188 Os of
0.12757 corrected to 22.5 Ma. R, repeat dissolution and analysis. Oxygen isotope values are for clinopyroxenes [data from
Pearson et al. (1991)].
supports the significance of the 12---13 Ga age in the
evolution of the Beni Bousera massif, especially when
considered with the 14 07 Ga Lu---Hf isochron age
of the peridotite clinopyroxenes (Pearson & Nowell,
2003), the 12 Ga Re---Os model age of a very high-Os,
low-Re peridotite (Pearson et al., 2004b) and the
14---16 Ga Sm---Nd model ages of the surrounding kinzigite crustal units (Polve, 1983). The coincidence of these
ages suggests that a major melting event took place in the
peridotites, approximately coincident with differentiation
in the overlying crust. This melting event was probably
responsible for the removal of the peridotite body from
the convecting mantle, into the depleted lithospheric
mantle, where the peridotites evolved highly radiogenic
Hf isotopic compositions, characteristic of ancient lithospheric mantle (Fig. 3; e.g. Pearson & Nowell, 2004a;
Pearson et al., 2003). The pyroxenites may have intruded
the peridotites during this initial melting event. Although
the Os isotopic compositions of the pyroxenites could
have evolved to their present-day values if the peridotites
had been their source, the extreme oxygen isotopic compositions of many of the pyroxenites (Table 4; Pearson
et al., 1991) rule out any genetic relationship. Hence,
pyroxenite formation, as magmatic veins, may have
merely been triggered by the large-scale crust---mantle
differentiation but the source for many pyroxenites
seems to have been from crustal precursors based on
oxygen isotope systematics. The significance of this differentiation event regionally is indicated by the similarity
in the Ronda pyroxenite Re---Os model ages and by the
c. 13 Ga melting age indicated by the Ronda peridotite
Re---Os and Sm---Nd isotope data (Reisberg et al., 1989;
Reisberg & Lorand, 1995). Furthermore, the age systematics of minerals and whole rocks in the different
systems applied, together with the petrological similarities
between the massifs, strongly indicates that the Beni
Bousera and Ronda peridotite bodies were originally
derived from a contiguous portion of the asthenosphere,
differentiated into the lithospheric mantle c. 14---13 Gyr
ago and then emplaced and exhumed as two separate
portions into the crust at 22 Ma.
Re---Os and Lu---Hf isotope constraints
on the origin of the pyroxenites
Previous models for the genesis of the Beni Bousera
pyroxenites have suggested multiple origins for the different pyroxenite layers (Kornprobst, 1969; Polve & Allegre,
1980; Allegre & Turcotte, 1986; Kornprobst et al., 1990;
Pearson et al., 1993; Kumar et al., 1996). The complex
and diverse Re---Os and Lu---Hf isotope systematics found
in this study support this notion. If the pyroxenite layers
are viewed as oceanic crust thinned by mantle convection
and diffusion (Allegre & Turcotte, 1986) then there
should be a simple relationship between increasing pyroxenite age and decreasing thickness. Our dataset, combined with that of Kumar et al. (1996), does not show any
such relationship and thus we discount the notion that the
layers simply represent thinned oceanic crust in favour of
models that involve high-pressure crystal---liquid equilibria. This latter origin probably involved derivation of
some pyroxenites from recycled oceanic crustal protoliths, as suggested by available oxygen and sulphur isotopic data (Pearson et al., 1991, 1993).
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JOURNAL OF PETROLOGY
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Fig. 7. Modelling of the eHf---eNd isotopic evolution of subducted E-MORB (enriched MORB) and N-MORB generated at time t from a depleted
mantle (DM) source. The isotopic composition of DM at time t (open circles) is calculated assuming a present-day average 176 Hf/177 Hf---143 Nd/
144
Nd of the depleted MORB source mantle (DMM) to be 0283200 and 0513150, respectively, and formation of the DM reservoir from Bulk
Silicate Earth (BSE) at 4 Ga. The isotopic evolution and present-day isotopic compositions of E- and N-MORB generated from DM at time t are
represented by continuous lines with filled circles and are calculated assuming present-day average Lu/Hf and Sm/Nd ratios for E- and N-MORB
(Chauvel & Blichert-Toft, 2001). Field of pelagic sediments and vector for terrigenous sediments taken from Vervoort et al. (1999) and references
cited by Blichert-Toft (2001).
The Hf---Nd isotope systematics of the pyroxenites can
be evaluated in terms of recycling models. The relatively
low Lu/Hf of MORB compared with their Sm/Nd ratios
means that, if no fractionation occurs during subduction,
recycled ancient MORB will generate Hf---Nd isotope
characteristics that will evolve below the mantle array
with time (Fig. 7). Three of the garnet pyroxenites plot
below the mantle Hf---Nd array (Figs 3 and 7). GP139 and
GP147 plot close to, or within the field occupied by 1---2
Gyr old subducted normal MORB (N-MORB). As such,
the initial Nd---Hf isotopic compositions of these two
garnet pyroxenites could have originated via evolution
from subducted basic---ultrabasic crustal protoliths or
high-pressure cumulates. Initial Nd---Hf isotopic compositions for GP37 are significantly outside the field for
isotopically evolved subducted MORB. The isotopic
composition of this sample cannot easily be generated
by any melt, even allowing for 1---3 Gyr of isotopic evolution. However, compilation of measured Lu/Hf and
Sm/Nd systematics in mantle minerals (Pearson et al.,
2004b) shows that clinopyroxene can have Lu/Hf significantly below chondritic values while retaining moderately high Sm/Nd, such that if GP37 originally
crystallized as a high-T pyroxenite and subsequently
exsolved garnet on cooling, its long-term isotopic
evolution could evolve to the bulk composition observed
at 225 Myr ago. This indicates the likelihood that crystal
fractionation occurring at upper-mantle depths was an
important process in forming the pyroxenites.
GP188 and GP101 have enriched Hf---Nd isotope signatures, outside the OIB field, but plot on the mantle
array. The most likely explanation for these characteristics is that the pyroxenites have incorporated a significant
amount (2---5%) of subducted sediment into their source,
as proposed by Pearson et al. (1993) on the basis of high
D7/4 Pb isotope systematics combined with unradiogenic
Nd and radiogenic Sr isotopic compositions. In the case
of Nd---Hf isotopes, the sediment can be constrained to be
of turbiditic, or possibly pelagic turbiditic character,
rather than true pelagic sediment or red clay (e.g.
Vervoort et al., 1999). This is because the high Lu/Hf of
pelagic or red clays generates distinctively high eHf isotopic compositions at a given eNd (Fig. 7).
The anomalous oxygen and sulphur isotopic compositions of the pyroxenites indicate a role for recycled oceanic crustal protoliths (Pearson et al., 1993). The relatively
high Os contents of some of the pyroxenites combined
with their very variable Re contents clearly indicate that
the pyroxenites cannot be metamorphosed MORB (e.g.
Roy-Barman et al., 1996) because of the low Os contents
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PEARSON AND NOWELL
BENI BOUSERA PYROXENITES
of most MORB. It is possible that some layers containing
corundum might represent metamorphosed aluminous
oceanic crustal cumulates (Kornprobst et al., 1990) but
many have the petrological and geochemical characteristics of high-pressure crystal fractionation products
superimposed upon their recycled crustal isotopic signatures. The high Os abundances reported here suggest
that their parental melts must have been sulphur
saturated.
Implications for mixed peridotite---pyroxenite
source regions
The attraction of involving pyroxenite in the sources of
mantle-derived magmas is two-fold: (1) it increases the
amount of melt at a given P---T condition as a result of
the lower solidi of most pyroxenites [see summary by
Hirschmann & Stolper (1996)]; (2) because garnet is
stable on the pyroxenite solidus well into the spinelperidotite stability field (e.g. Irving, 1974), a ‘residual
garnet’ elemental signature can be generated by shallow
melting. Here we will concentrate on the isotopic character of likely pyroxenite components in mantle source
regions, with reference to the Beni Bousera pyroxenites.
The numerous studies conducted on the Beni Bousera
and Ronda massifs have not reached agreement on
whether the massif was isolated in the lithospheric mantle
for 13 Gyr, or remained as a fragment of ancient
depleted mantle, ‘foundered’ in the asthenosphere. We
show above that long-term residence of the massif in the
lithospheric mantle is most likely. However, this conclusion does not affect our purpose here. We aim to characterize the isotopic signatures of ancient pyroxenitic
mantle material that could have remained as discrete
heterogeneities within the convecting or lithospheric
mantle to act as potential components in mantle-derived
magma sources. Because we have no convincing direct
samples of recycled material from within the convecting
mantle, the Beni Bousera pyroxenites are probably our
best analogues. As such their geochemical characteristics
can be used to constrain models relating to the petrogenesis of oceanic basalts (e.g. Hauri, 1996) and of potassic
igneous rocks thought to originate from partial melting of
veined continental lithospheric mantle (e.g. Carlson et al.,
1996; Carlson & Nowell, 2001). Isotopic signatures that
may be indicative of pyroxenite contributions to either
oceanic or continental magma sources, based on observations from Beni Bousera pyroxenites and other orogenic
massif pyroxenites, are as follows.
(1) Radiogenic Os isotope compositions. Pyroxenites
have 187 Os/188 Os ratios that are almost exclusively more
radiogenic than their host peridotites or the range shown
by uncontaminated oceanic basalts (Fig. 6).
(2) Variable oxygen isotopic compositions. Pearson
et al. (1993) reported variable d18 O values that were
lighter (49%) and heavier (94%) than the typical
mantle value of 52%. These values have been
subsequently confirmed by laser-fluorination methods
(D. P. Mattey & D. G. Pearson, unpublished data, 1994).
Some pyroxenites have oxygen isotopic compositions
that are indistinguishable from typical mantle.
(3) Nd and Sr isotopic compositions are very variable
and can be similar to peridotite values, more depleted
(GP37), or considerably more enriched.
(4) Hf isotope compositions are also variable and
range from within the MORB---OIB field to considerably
more radiogenic values.
(5) Combined Nd---Hf isotope systematics can be
distinctive, even if the eNd and eHf values are within
the respective ranges of oceanic basalts. One Beni
Bousera pyroxenite plots well below the mantle Nd---Hf
isotope array, with low DeHf. This type of signature is
rare in the Beni Bousera samples analysed so far.
Ancient garnet-bearing xenoliths from cratonic areas
have particularly extreme Hf---Nd isotope signatures that
scatter both well above and well below the mantle array
(Jacob et al., 2002; Nowell et al., 2003b).
Any one of these signatures in isolation is not particularly distinctive, but a combination of several features
provides a strong indication of the possible presence of
pyroxenitic material in the source regions (e.g. Carlson
et al., 1996; Carlson & Nowell, 2001). The prominence of
any of these chemical signatures in a mantle-derived
magma obviously depends on the extent and nature of
mixing between pyroxenite- and peridotite-derived melts
and the relative concentrations and abundance of pyroxenite in the mantle.
It is unlikely that partial melts of pyroxenite layers can
be easily extracted from a peridotite matrix because such
melts are siliceous and will react with the surrounding
peridotite (Yaxley & Green, 1998). At high pressures,
the pyrope---omphacite---orthopyroxene thermal divide
prevents the mixing of siliceous liquid produced from
eclogite/pyroxenite melting with the nephelinenormative picritic liquids produced from metasomatized
lherzolite. The siliceous liquids react with and metasomatize the surrounding lherzolites. Eventually, residual
phase compositions in the eclogite/pyroxenite and metasomatized lherzolite converge but a modally heterogeneous, refertilized peridotite results (Yaxley & Green,
1998). This refertilized mantle can then produce nepheline-normative melts at the solidus that retain the isotopic
memory of the heterogeneous source mixture. Veining
within the asthenosphere may be on a much finer scale
than the decimeter scale most evident in the lithosphere
as sampled by massifs and xenoliths. It is possible that
pyroxenite veins may become intimately mixed into the
peridotite, making a fertile peridotite composition. This
would simplify the problem of extracting melts from a
mineralogically zoned source. The intimate physical and
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JOURNAL OF PETROLOGY
VOLUME 45
diffusional mixing of garnet pyroxenites with spinel-facies
peridotites at Beni Bousera and Ronda produces Fe-rich,
‘extra-fertile’ garnet peridotite with olivine Mg-numbers
as low as 84---86 (Pearson et al., 1995b). Melts of this
material will be strongly influenced by the garnetpyroxenite ingredient in the mixture and could give
Fe-rich melts with radiogenic Os. Such a source would
appear simply as Fe-rich, rather than pyroxenitic and
may be suitable for the origin of Fe-rich picrites (e.g.
Gibson et al., 2000).
Although potentially complex, the processes involved
in melting heterogeneous mantle ultimately involve net
transfer and mixing of pyroxenite components into peridotite. This ‘mixed’, refertilized source can then remelt.
Because we do not have samples of the near-solidus
pyroxenite melt that would mix with a peridotite, and
because there is abundant evidence in the Beni Bousera
and other massifs for physical mixing of pyroxenite into
peridotite, we chose to crudely model the interaction by
simple mixing of the two end-members. Given that derivative small-degree melts from the pyroxenites are likely
to be higher in incompatible elements than their source,
the estimates so derived are likely to be overestimates of
the required pyroxenite-derived mass flux for Sr, Pb and
Nd but underestimates for Os if mixing occurs via melt
interacting with peridotite. An additional reason for modelling simple solid---solid mixing is that the measured
sections documenting pyroxenite abundance in the Beni
Bousera massif (Fig. 2) allow some quantitative bounds to
be placed on the mass-balance effects of pyroxenite--peridotite mixtures in a lithospheric mantle environment
such as that from which the massif was derived.
In terms of element balance for radiogenic isotope
systems, input of 10% ‘typical’ pyroxenite into a fertile
peridotite produces an increase in Sr and Nd abundance
of 10%, increases Pb abundances by 30%, Hf abundances by 50% and decreases Os abundances by 10%
(Fig. 8). Whatever mixing scenario is favoured, it is clear
that significant elemental flux enters the ‘mixed’ peridotite from the pyroxenite and this will affect isotopic
systematics (Fig. 8). This, in turn, constrains some of
the likely isotopic variations to be expected when
ancient recycled materials contribute to magma source
regions.
Becker (2000) has noted that if the Re/Os and U/Pb
isotopic characteristics of subducted eclogites and blueschists are used in mixing calculations to simulate mixing
between oceanic crustal material and mantle, excessive
amounts (70---90%) of 05---2 Gyr old recycled material
are required in the source. One reason for this is the very
low Os abundances of subducted metabasalts (typically
5 ppt). Although the Os isotopic compositions of the Beni
Bousera pyroxenites are not as elevated as values predicted for ancient MORB, they have considerably higher
Os concentrations than MORB/metabasalts (by a factor
NUMBER 2
FEBRUARY 2004
Fig. 8. Simple mixing model of pyroxenite with peridotite. (a) Variation in elemental abundance of mixtures of typical pyroxenite compositions into fertile peridotite. Shaded field illustrates the 1---10% range of
pyroxenite thickness extrapolated to mass fraction assuming equal densities. Elemental abundances used are: pyroxenite-----Sr 17 ppm; Pb
02 ppm; Nd 12 ppm, Hf 06 ppm; Os 02 ppb; peridotite-----Sr
95 ppm; Pb 005 ppm; Nd 063 ppm, Hf 01 ppm; Os 33 ppb.
(b) Variation in Os isotopic composition (as per cent difference from
starting peridotite) of mixtures of pyroxenite compositions (from
Tables 2) into fertile peridotite (187 Os/188 Os ¼ 012623; Os 358 ppb).
Shaded vertical box represents range of pyroxenite abundances in the
Beni Bousera peridotite massif. Shaded horizontal box represents the
approximate range in Os isotopic compositions observed in ocean island
basalts (OIB; see Widom et al., 1999).
of 10---100), and so have a much more dramatic effect on
mixing relationships with peridotite (Fig. 8). This greatly
alleviates some of the mass-balance problems identified
by Becker (2000). Os isotopic variation remains highly
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BENI BOUSERA PYROXENITES
dependent on which pyroxenite composition is used
(Fig. 8). Very large amounts (470%) of a relatively unradiogenic, moderately low-Os pyroxenite such as GP37
are required to elevate the Os isotopic composition above
1% of the original peridotite value. In contrast, high-Os
pyroxenites such as GP147, with a radiogenic Os isotope
composition, can create 41% variation in Os isotopic
composition (i.e. within the range of OIB magmas) by
mixing in 5---10% by mass. This mass fraction is within
the range observed in the Beni Bousera massif, although
this is not necessarily a constraint on the mixing relations
in a particular petrogenetic scheme.
Although the largest changes in elemental concentrations are observed for the Hf mass balance over the
1---10% pyroxenite mixing range modelled in Fig. 8, the
precise effects on the Hf isotopic composition of the mix
are difficult to predict. This is because of the extreme Hf
isotopic and elemental abundance variability of the pyroxenites and peridotites. Peridotite---pyroxenite mixtures
could lie anywhere within a polygon defined by the
extremities of the pyroxenite---peridotite fields of Fig. 3
if the peridotite end-member was a Beni Bousera peridotite. Extreme Nd---Hf isotope compositions (eHf and eNd
commonly being 4 þ50 and lying well above and below
the mantle array) have been observed for ancient eclogites and alkremites sampled from the lithospheric mantle
( Jacob et al., 2002; Nowell et al., 2003b) and indicate the
potential variation available for veined melting models
within the ancient lithospheric mantle.
In contrast to cratonic eclogites, most pyroxenites analysed here and by Blichert-Toft et al. (1999a) lie close to
the mantle Nd---Hf isotope array. Hence, mixing of such
material with convecting mantle peridotite, less variable
in its Hf isotopic composition than the Beni Bousera
peridotites, could account for the more coherent (with
respect to the mantle Nd---Hf isotope array) heterogeneity
seen in oceanic basalts (MORB and OIB). The coherency
of the mantle Nd---Hf isotope array suggests a minimal
role for ancient recycled materials with the extreme,
diverse isotopic characteristics of ancient (3 Ga) eclogites.
Moreover, the coherency of the Nd---Hf isotopic systematics in oceanic basalts suggests that ancient subducted MORB alone is unlikely to be the sole recycled
ingredient in their source regions, except for HIMU
basalts (Fig. 7), and indicates the likely addition of
continental or continent-derived material (Fig. 7; e.g.
Blichert-Toft et al., 1999a). The Beni Bousera pyroxenites
have a spectrum of radiogenic and stable isotopic characteristics that include combinations of recycled oceanic
crustal and sedimentary signatures (Pearson et al., 1993;
Tables 2 and 4). This suggests, whether they evolved in
the lithospheric mantle or not, that these pyroxenites
provide perhaps the closest analogy that we have for
any proposed pyroxenitic component in oceanic mantle
magma source regions.
ACKNOWLEDGEMENTS
We thank Chris Ottley for assistance with ICP-MS measurements, and Gareth Davies and Peter Nixon for field
assistance and collaboration on other aspects of Beni
Bousera. The instrumentation used in this study was
funded by HEFCE/NERC grant DUPEEQ to D.G.P.
Helpful and detailed reviews by L. Reisberg, J. BlichertToft, F. Frey and R. Carlson, and editorial comments by
M. Wilson considerably improved the quality and focus
of this paper.
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