Geochemistry of the highly depleted peridotites drilled at ODP Sites 1272

and 1274 (Fifteen-Twenty Fracture Zone, Mid-Atlantic Ridge): Implications

for mantle dynamics beneath a slow spreading ridge

M. Godard ⁎, Y. Lagabrielle, O. Alard, J. Harvey

Géosciences Montpellier, UMR 5243 CNRS-UM2, Université Montpellier 2, cc60, Place Eugène Bataillon, 34095 Montpellier cedex 5, France

Received 16 July 2007; received in revised form 28 November 2007; accepted 28 November 2007

Editor: Dr. R.W. Carlson

Available online 15 December 2007

Abstract

During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge (MAR) was drilled in the vicinity of the Fifteen-Twenty Fracture Zone

(FZ) that offsets one of the slowest portions of the spreading ridge. We present here the results of a bulk rock multi-elemental study of 27

peridotites drilled at Sites 1272 and 1274 (to the south and the north of the FZ, respectively).

The peridotites comprise mainly of harzburgites with minor dunites. Clinopyroxene (Cpx), which is interstitial and interpreted as secondary, is

observed in Site 1274 peridotites. Sites 1272 and 1274 peridotites have low Al2O3 contents (≤1 anhydrous wt.%), high Mg# (N91.5), and bulk

rock trace elements compositions mostly below 0.1×primitive mantle (PM). These peridotites, and in particular Site 1272 peridotites, represent

the most depleted peridotites yet sampled at a slow spreading ridge. Their compositions indicate high degrees of partial melting and melt

extraction. A single open-system melting event (melting plus percolation of melts produced within upwelling mantle) can explain their highly

depleted yet linear chondrite-normalized REE patterns, characterized by a steady depletion from HREE to LREE. Late melt-rock reactions and

precipitation of Cpx explains the slightly less depleted compositions of Site 1274 peridotites. Hence, the differences in composition between Sites

1272 and 1274 peridotites do not provide evidence for regional variations in the degrees of partial melting from the south to the north of the FZ.

The occurrence of highly refractory peridotites in the Fifteen-Twenty area suggests we sampled a more actively convecting mantle than generally

supposed below slow spreading centers.

© 2007 Elsevier B.V. All rights reserved.

Keywords: abyssal peridotites; Mid-Atlantic Ridge; trace element geochemistry; mantle dynamics

1. Introduction

Mid ocean-ridge magmatism is one of the most obvious

expressions of plate tectonics. Melting of the uprising mantle

produces the igneous basement of Earth's ocean floor (i.e., Mid-

Ocean Ridge Basalts (MORB), sheeted dykes and gabbros).

Abyssal peridotites are commonly considered as the comple-

mentary product of MORB (Dick and Bullen, 1984; Johnson

et al., 1990; Snow et al., 1994; Hellebrand et al., 2001; Alard

et al., 2005). Yet, recent studies have suggested some degree of

chemical disequilibrium betweenMORB and abyssal peridotites

in terms of isotopic (Salters and Dick, 2002), trace element (Niu,

2004) and modal (Kelemen et al., 1997) compositions, and

possibly old (N1 Gyr) ages for abyssal peridotites (Brandon

et al., 2000; Harvey et al., 2006), thus bringing a renewed

interest on the origin of the mantle fragments exposed on the

seafloor and its relation to ridge magmatism.

Regional variations in MORB composition indicate possible

mineralogical heterogeneities in the MORB mantle source

(Hirschmann and Stolper, 1996; Salters and Dick, 2002) as well

Available online at www.sciencedirect.com

Earth and Planetary Science Letters 267 (2008) 410–425www.elsevier.com/locate/epsl

⁎ Corresponding author. Tel.: +33 4 67 14 39 37; fax: +33 4 67 14 36 03.

E-mail address: Marguerite.Godard@univ-montp2.fr (M. Godard).

0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2007.11.058

as changes in melt production and extraction processes in the

mantle during upwelling and along ridges (Klein and Langmuir,

1987; Asimow et al., 1997). Concurrently, studies of ophiolitic

sections show that the composition of the residual mantle is

controlled by the paleo-ridge structure (Godard et al., 2000; Le

Mée et al., 2004). In particular, they indicate late reequilibration

with MORB in dunites cross-cutting harzburgites and at the

mantle–crust transition beneath ridge axis (Kelemen et al.,

1997; Dijkstra et al., 2001). These observations suggest a

control of melt transport processes on the geochemistry of

peridotites and of their igneous products by favoring and/or

limiting spatially melt/rock interactions during mantle upwel-

ling (Jull et al., 2002; Dijkstra et al., 2003). Trace elements are a

useful tool to characterize these magmatic processes because of

their large range of incompatibilities between the peridotite

main constituting minerals – i.e. clinopyroxene, orthopyroxene

and olivine – and silicate melts; this allows to constrain chem-

ical interactions between melts and mantle rocks during melting

and melt transport, and the associated changes in modes and

melt/rock ratios (Navon and Stolper, 1987; Godard et al., 1995;

Vernières et al., 1997; Shaw, 2000). Yet, trace element data in

abyssal peridotites are scarce. First, mantle exposures are

extremely rare; they are mostly limited to fracture zone walls

and to tectonic windows along the axis of slow and ultra-slow

spreading ridges (Lagabrielle et al., 1998; Sauter et al., 2004).

Second, to avoid the effects of alteration, geochemical data on

abyssal peridotites have been obtained mainly by in situ SIMS

analyses on clinopyroxenes, which have (partially) survived

alteration (Johnson and Dick, 1992; Dick and Natland, 1996;

Hellebrand et al., 2001). As a result, our geochemical trace

element dataset concern a limited range of trace elements

(mostly Rare Earth Elements (REE)), and it is obviously biased

toward abyssal peridotites having preserved clinopyroxene.

Progress in Inductively Coupled Plasma – Mass Spectrometry

(ICP-MS) techniques now allows measuring a large spectrum of

trace elements at very low concentration levels such as those

typically found in abyssal peridotites (Bodinier and Godard,

2003). Recently, Niu (2004) and Paulick et al. (2006) showed

that the bulk rock geochemistry of abyssal peridotites preserve

its primary signature for many trace elements during serpenti-

nisation and seafloor weathering and thus can be used to get

insight into mantle processes beneath ridges.

During ODP Leg 209 (May–July 2003 (Kelemen et al.,

2004)), a systematic sampling of a portion of the slow spreading

Mid-Atlantic Ridge was carried out to the north and south of the

Fifteen-Twenty Fracture Zone (FZ). This area is characterized by

extensive outcrops of peridotites and of gabbroic rocks exposed

on both flanks of the spreading axis (see review in Lagabrielle et

al., 1998). We present here the results of the bulk rock major and

trace element study of 27 peridotites selected amongst the least

altered samples drilled at ODP Sites 1272 and 1274 (respectively,

south and north of the Fifteen-Twenty FZ). This study allows us to

characterize the variation of the mantle composition at a regional

scale as well as at the scale of the borehole. We then discuss these

variations in terms of magmatic processes and their relationship

with mantle dynamics beneath the Fifteen-Twenty FZ area.

Fig. 1. The Fifteen-Twenty Fracture Zone (FZ) on the Mid-Atlantic Ridge (location in insert). Sites where peridotites and gabbros were sampled by dredging (Bonatti

et al., 1992; Cannat et al., 1992) or by drilling (ODP Leg 209, (Kelemen et al., 2004)) are located, together with the position of dredges that sampled basalts (from

enriched to the south to N-MORB to the north (Dosso et al., 1991, 1993)). Symbols are represented in legend.

411M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

Table 1Bulk rock composition of Sites 1272 and 1274 peridotites

Site-hole 1272A 1272A 1272A 1272A 1272A 1272A 1272A 1272A 1272A 1272A 1272A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A 1274A

Core-section 9R-1 13R-1 13R-2 17R-1 21R-1 21R-1 21R-2 23R-1 24R-1 24R-2 27R-1 1R-1 3R-1 3R-1 4R-2 5R-2 6R-2 7R-1 8R-1 12R-1 12R-1 13R-1 15R-1 15R-2 16R-1 17R-1 27R-1

Interval (cm) 39–46 128–133 16–21 102–107 31–36 127–132 77–81 26–29 129–134 40–45 31–36 85–88 42–46 107–112 6–10 55–59 130–135 37–42 135–137 1–5 138–143 91–95 94–99 43–48 92–98 126–130 143–146

Depth (mbsf) 44.79 61.98 62.36 80.62 99.21 100.17 101.0 108.76 114.79 115.26 127.31 0.85 17.32 17.97 22.82 28.29 32.75 36.17 41.35 59.41 60.78 65.31 74.94 75.9 84.62 89.56 147.53

Lithology Ol-Hz Hz Hz Hz Hz Hz Hz Du Hz Hz Hz Hz Hz Hz Hz Hz Hz Hz Du Hz Hz Hz Ol–Hz Hz Hz Hz Hz

Primary modal compositions (%)

Olivine N80 70 70 75 75 70 75 N98 75 70 75 70 76 75 73 74 73 73 98 75 75 75 87 83 81 74 75

Opx bb20 30 30 25 25 27 b25 25 30 25 27 23 23 25 24 24 24 – 23 23 24 12 15 18 24 23

Cpx b2 b1 b1 3 N2 N2 2 1 2 2 N1 1 1.0 b0.5 b1 2 b1 2 1

Sp b2 b1 b1 b2 b1 b2 b1 b1 b0.5 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 ~1

Major element composition in wt.% (XRF)

SiO2 34.82 37.84 37.75 37.45 36.97 37.11 37.17 33.62 36.81 36.78 37.48 38.78 37.82 38.81 39.54 37.80 40.20 40.32 34.37 39.04 35.65 37.78 35.53 37.58 37.52 38.66 37.93

TiO2 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.03 0.01 0.01

Al2O3 0.34 0.64 0.65 0.57 0.54 0.55 0.52 0.07 0.60 0.47 0.61 0.76 0.59 0.59 0.88 0.68 0.72 0.79 0.11 0.62 0.57 0.66 0.25 0.54 0.53 0.77 0.70

Fe2O3 7.40 7.03 7.04 7.15 7.20 7.21 7.03 7.63 7.10 7.43 7.09 7.48 7.49 7.84 7.58 7.38 7.59 7.62 7.90 7.73 7.13 7.38 7.82 7.39 7.53 7.22 7.47

MnO 0.10 0.10 0.10 0.09 0.09 0.10 0.10 0.10 0.10 0.09 0.10 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.10 0.09 0.10 0.10 0.09 0.12 0.10

MgO 41.08 40.24 39.99 40.30 40.48 40.14 38.56 42.04 39.64 39.93 40.11 40.62 40.45 41.88 40.72 39.99 40.71 40.28 42.24 40.88 37.64 39.04 40.89 39.99 40.69 39.93 40.62

CaO 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.06 0.05 0.20 1.03 0.50 0.54 0.71 0.46 0.86 0.84 0.13 0.65 0.52 0.31 0.18 0.51 0.24 0.74 0.39

Na2O 0.12 0.12 0.10 0.12 0.10 0.10 0.09 0.10 0.09 0.08 0.08 0.05 0.04 0.04 0.04 0.05 0.10 0.05 0.03 0.05 0.04 0.08 0.05 0.08 0.04 0.03 0.04

K2O 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

P2O5 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

LOI 16.32 14.67 14.65 14.80 15.16 15.16 16.03 16.08 15.04 14.95 14.26 11.22 13.37 10.64 10.87 13.49 10.29 10.21 14.94 11.21 13.98 15.07 15.67 13.63 13.86 11.96 12.45

Total 100.24 100.72 100.35 100.53 100.59 100.43 99.56 99.56 99.46 99.79 99.95 100.08 100.39 100.47 100.49 99.98 100.61 100.25 99.85 100.32 95.66 100.43 100.52 99.86 100.55 99.45 99.73

Mg# 91.7 91.9 91.8 91.8 91.8 91.7 91.6 91.6 91.7 91.4 91.8 91.5 91.4 91.4 91.41 91.48 91.40 91.28 91.37 91.29 91.27 91.29 91.20 91.47 91.46 91.64 91.50

Trace element composition in ppm (XRF)

Cr 1613 2295 2433 2157 2009 2103 1914 650 2638 1925 2508 3758 2147 1995 3821 2305 2349 2711 1176 2289 1816 2484 1504 3238 3172 3910 3259

Ni 2329 2019 2034 2116 2112 2099 2247 2556 2130 2087 2024 2236 2166 2192 2057 2124 2051 2057 2500 2194 2239 2197 2295 2105 2212 2137 2208

Trace element composition in ppm (ICP-MS)

Duplicate (n) (2) (2) (2) (2) (2) (2) (2) (2) (2) (2)

Li 0.20 0.37 0.33 0.65 0.42 0.28 0.27 0.43 0.51 0.78 0.74 0.83 1.37 0.87 1.33 1.31 0.21 0.96 0.58 0.66 0.30 0.94 0.98 1.02 1.05

Sc 3.9 5.7 5.5 5.6 5.8 6.2 5.8 2.8 6.1 5.6 7.0 5.9 5.5 5.3 6.9 6.8 7.5 8.1 3.4 6.6 5.9 5.5 4.2 5.7 4.1 9.9 5.4

Ti 33 47 44 18 15 10 7 6 13 15 12 38 32 30 43 43 35 33 10 30 44 46 42 74 162 54 78

Co 103 95 88 94 95 93 92 101 93 105 95 120 119 125 128 113 118 102 133 103 117 117 123 114 114 146 111

Cu 7.0 6.0 5.4 5.3 6.1 5.4 5.1 5.0 5.5 5.7 5.8 18.8 13.3 4.5 11.7 14.6 6.4 6.0 5.2 6.2 4.6 5.6 4.9 5.1 9.6 6.1 4.1

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Trace element composition in ppb (ICP-MS)

Rb 13.6 14.3 11.4 13.8 12.7 10.7 8.4 9.0 12.2 9.0 18.1 20.4 19.4 41.6 39.6 23.3 38.0 37.3 2.8 26.0 15.5 13.2 10.7 13.3 10.8 12.5 53.8

Sr 633 655 673 645 674 645 657 665 709 738 810 1070 747 316 1762 1251 600 838 868 730 1057 28001 830 890 3038 1213 760

Y 27.0 31.6 33.1 23.1 27.7 24.1 20.4 9.6 24.6 25.8 23.0 140.6 96.4 89.3 99.8 122.5 106.0 101.2 32.9 81.9 63.3 59.2 88.5 144.3 115.2 106.6 80.0

Zr 6.8 9.4 13.0 12.1 5.7 – – 17.5 13.3 12.7 20.7 24.3 14.9 22.4 21.1 9.7 8.1 11.4 32.8 15.2 32.8 25.8 18.6 60.0 91.0 – 106.2

Nb 5.8 10.3 10.4 6.7 9.5 10.0 13.6 9.9 9.9 9.5 10.8 12.2 10.5 13.8 11.4 8.8 10.6 14.6 10.1 15.8 8.5 6.9 7.8 11.2 10.4 13.8 22.9

Cs 0.6 – – – 0.2 – – – 0.8 1.0 1.2 1.6 1.6 1.3 3.1 1.5 2.4 3.2 0.5 2.4 1.2 0.9 1.5 0.4 0.8 1.0 1.4

Ba 18.7 50.4 39.5 38.1 21.4 – 47.3 95.6 12.2 15.8 24.6 78.0 92.0 185.5 307.4 209.2 140.4 308.7 122.1 134.8 77.0 112.4 83.4 92.8 69.0 136.8 106.4

La – – – – 0.54 – – 0.71 0.27 0.45 0.23 0.37 1.01 – 1.06 4.04 0.58 2.30 – 2.83 6.24 – – – 1.32 – 7.34

Ce – 1.29 1.16 – – – 1.62 – 0.94 1.35 1.62 0.88 2.79 1.39 2.31 9.25 1.01 3.61 1.34 5.21 1.40 0.59 1.75 1.24 1.64 7.11 26.76

Pr 0.32 – – 0.23 0.35 – – – – – – – – – – 1.69 – – – – – – 0.39 – – 0.88 4.56

Nd 1.65 – – – 1.49 – – – 1.18 1.68 – – 1.58 1.38 1.27 6.63 2.30 3.29 1.50 3.39 1.52 0.60 1.33 1.32 2.00 2.91 17.49

Sm – – – – – – – – – – – 3.55 3.40 3.60 3.47 – – – 4.06 – – 2.13 3.13 3.54 5.87 3.86 6.54

Eu – 0.54 – – – – – – – – – 0.68 – – – – 0.92 – 0.78 – – 3.32 1.03 1.35 2.76 1.17 1.15

Gd 1.41 1.62 – – 1.54 – – – 1.35 2.00 2.02 3.58 2.36 3.02 2.43 5.77 3.89 3.63 2.42 4.00 1.49 1.29 4.47 7.30 12.84 4.85 8.37

Tb 0.41 0.42 0.40 0.25 0.34 – – – 0.23 0.27 0.32 1.29 0.80 0.77 0.70 1.34 1.01 0.72 0.41 0.83 0.47 0.40 1.31 2.25 2.74 1.16 1.83

Dy 4.61 – – 3.09 3.45 – – – – – – 19.32 11.90 10.55 11.75 15.77 13.98 11.86 6.27 11.23 8.67 6.80 14.98 24.29 23.32 14.39 15.58

Ho 1.38 1.59 1.64 1.19 1.46 1.40 1.27 0.91 1.57 1.62 1.69 6.11 4.06 3.89 4.18 5.30 4.56 4.41 1.59 3.80 2.98 2.49 4.19 6.62 5.51 4.61 3.73

Er 5.30 4.99 6.04 5.78 6.52 5.74 4.43 2.75 6.66 6.51 6.27 25.44 17.28 16.50 17.48 21.96 19.99 19.46 6.42 16.53 13.72 11.89 14.79 22.25 16.65 19.81 12.84

Tm 1.14 1.19 1.15 1.39 1.72 1.59 1.29 0.73 1.66 1.55 1.55 5.28 3.69 3.53 3.95 4.83 4.38 4.25 1.58 3.53 3.27 2.68 2.72 4.14 2.95 4.38 2.49

Yb 10.3 12.7 12.0 14.3 17.5 16.6 14.3 7.8 16.0 16.0 15.9 44.6 33.0 32.6 35.8 39.8 37.2 37.5 15.0 30.4 31.2 24.0 22.1 30.9 21.5 39.5 19.9

Lu 2.42 2.75 2.95 3.64 4.38 4.05 3.45 2.04 4.09 3.67 4.09 9.62 7.48 7.10 8.18 8.85 8.36 8.44 3.83 6.98 6.95 5.16 4.32 5.86 4.29 8.32 4.66

Hf 1.4 1.2 1.4 1.7 1.5 – – – 1.0 1.5 1.7 3.6 1.3 1.8 1.4 3.2 3.2 3.4 1.8 3.3 1.3 0.7 1.5 2.7 8.7 – 8.9

Ta 0.6 0.6 1.2 0.7 0.9 0.8 0.9 1.1 – – – 3.7 2.2 2.9 3.8 1.9 3.2 5.7 1.7 6.9 1.5 0.6 0.8 1.5 1.7 3.4 2.2

Pb – – – – – – – – – 15.3 – 11.4 – 9.2 9.4 4.8 – 15.6 19.2 22.9 10.8 23.7 19.5 – 21.7 19.3 –

Th 13.2 – – 1.27 1.36 0.12 – 0.25 0.37 0.64 0.64 0.47 0.72 0.54 – 3.92 2.45 1.53 1.26 3.02 0.47 0.31 – 0.39 1.74 1.36 4.85

U 1.56 0.93 0.83 – 1.14 0.85 – 1.79 1.55 1.20 1.62 – – – – 3.25 – 2.77 – 1.41 1.15 429 1.04 0.76 3.35 0.90 1.80

The primary modal composition of Site 1272 peridotites was estimated from hand sample (in italic) and thin section observations. The mode of Site 1274 peridotites was calculated by a mass-balance equation relating

bulk rock and mineral compositions in the CFMAS system. Mineral compositions (unpublished) were obtained by electron microprobe at Service Commun “Microsonde-Sud”, Université Montpellier 2 (France). The

precision of modal estimates is, on average, 2%. Major elements, Ni and Cr were analyzed by XRF, at the Open University (UK) and at Geo Labs (Canada) for Sample 1272A-21R-2, 77-81 cm. Li, Ti, Sc, Co, Cu and

trace elements (REE, Rb, Sr, Zr, Nb, Cs, Ba, Hf, Ta, Pb Th and U) were analyzed by ICP-MS at Géosciences Montpellier. Results for which the analytical error is greater than 50% are reported in italics (see Appendix).

Duplicate analyses were performed for several samples (numbers of duplicates are indicated in brackets). These analyses were reproducible within the analytical error. Abbreviations: Hz: harzburgite; Ol-Hz: olivine-rich

orthopyroxene-poor harzburgite; Du: dunite; Opx=orthopyroxene; Cpx=clinopyroxene; Spl=spinel; LOI=loss on ignition; Mg#=cationic ratio: 100×cationic Mg/ (Mg+Fetotal).

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2. The Fifteen-Twenty Fracture Zone on the

Mid-Atlantic Ridge

2.1. Geological setting

The Fifteen-Twenty FZ offsets by ~175 km one of the slowest

portions of the Mid-Atlantic Ridge (MAR, Fig. 1), with a mean

full spreading rate of 25 km/m.y. (Fujiwara et al., 2003). This

region is characterized by extensive exposures of peridotites and

gabbroic rocks and corresponds spatially with gravity highs; it is

interpreted as a magma-starved area. It is bordered by large

concentric negative Bouguer gravity anomalies, centered at

~14° and 16°N; these gravity lows are interpreted as centers of

magmatic segments where there is accretion of thick igneous

crust (Escartin and Cannat, 1999; Fujiwara et al., 2003). The

negative gravity anomaly of the spreading segment to the south

of the FZ is about twice as large as the one of the segment to the

north of the FZ. Geochemical variations are observed from the

south to the north of the FZ (Fig. 1): (i) basalts sampled along the

southern segment are enriched MORB while, to the north of the

FZ, their composition changes progressively from enriched to

depleted MORB in the northernmost part of the segment (Dosso

et al., 1991, 1993), (ii) peridotites sampled to the south of the FZ

have highly refractory compositions, characterized by Cr-rich

spinels (Cr# (cationic Cr/(Cr+Al))N40), uncommon in MAR

peridotites (Bonatti et al., 1992; Cannat et al., 1992)). These

characteristics have been tentatively ascribed to the presence of

an H2O-rich and/or hot mantle beneath the southern segment

(Bonatti et al., 1992).

During ODP Leg 209, we drilled 8 sites along the MAR axial

valley and on the immediate valley walls from 14°43′ to 15°44′N

(Kelemen et al., 2004). We recovered peridotites, generally

associated with intrusive gabbroic rocks, at six sites (Fig. 1).

At most sites, the primary composition of peridotites had

been almost completely obliterated by late melt impregnation

(Sites 1270, 1271 (Paulick et al., 2006) and 1275 (Kelemen

et al., 2004)) or alteration processes (Site 1268 (Bach et al.,

2004; Kelemen et al., 2004; Paulick et al., 2006)). For this

study, which aimed at characterizing primary mantle pro-

cesses, we focused our sampling on the two remaining sites

where the mantle peridotites were the least altered and where

hand samples did not show textural evidence of melt impreg-

nation: Site 1272 and Site 1274.

2.2. Sampling and petrography of peridotites at ODP Sites

1272 and 1274

The various lithologies recovered at Site 1272 and Site 1274

are documented in Kelemen et al. (2004). Bach et al. (2004)

provide a detailed description of the alteration of the peridotites.

A systematic characterization of the primary and secondary

mineral compositions was carried out by Moll et al. (2007) at

Sites 1272 and 1274. The detailed lithostratigraphic column of

the boreholes drilled at Sites 1272 and 1274 is in the EPSL-

online background dataset. The principal observations relevant

to this paper and the main petrographic characteristics of the

studied peridotites are summarized below.

2.2.1. Site 1274 peridotites

Site 1274 is located on the western flank of the MAR valley at

~15°39′N to the north of the Fifteen-Twenty FZ. The borehole

penetrated to a depth of 156 m below sea-floor (mbsf) with a total

recovery of ~35 m of core (22%). The igneous and residual mantle

protoliths of recovered corewere 71%harzburgite, 19%dunite, and

3% gabbro. Harzburgites represent the main lithology down to

~40 mbsf, where dunites make their first appearance as a few

centimeter-thick intervals. Downhole, dunites become as thick as

70 cm. From ~75 to 85 mbsf, the lithology is dominated by

sequences of alternating orthopyroxene(Opx)-poor harzburgites

and dunites. From ~95 to ~140mbsf, a prominent gouge zone (7%

of recovered samples) comprising a mixture of crushed peridotites

and the remnants of gabbroic intrusions is observed. Below this

zone, recovered peridotites comprise mainly harzburgites. Site

1274 peridotites are the freshest mantle rocks drilled during Leg

209, up to 40% of the primary minerals being preserved in some

harzburgites (Bach et al., 2004; Paulick et al., 2006).

We selected 14 harzburgites, one dunite and one Opx-poor

harzburgite with the 75–85 mbsf interval (Sample 1274A-15R1,

94–99 cm), thereafter grouped with the dunite. Site 1274

harzburgites have coarse granular textures and contain a mineral

assemblage consisting largely of olivine (Ol-73%–83%), lesser

Opx (15%–27%) with variable amounts of clinopyroxene (Cpx

~2%−b5%) and spinel (Spl≤1%). Cpx is mostly found as

anhedral grains with highly curvilinear margins, which oc-

cupy interstitial spaces associated with Ol and/or Opx in the

serpentinized matrix. It is commonly associated with Spl and

Cu-rich sulfides as symplectites. These textures suggest that

most Cpx at Site 1274 are of secondary origin. The symplectitic

association of Cpx to Spl without Opx precludes an origin by a

garnet breakdown reaction. Cpx±sulfide±Spl were probably

formed by reaction with or crystallization of percolating melts

(Kelemen et al., 2004; Seyler et al., 2007). Symplectitic textures

suggest rapid cooling of these melts.

All primary minerals have highly refractory compositions

(Moll et al., 2007): olivine have high Mg# (100×cationic Mg/

(Mg+Fe)) of 91–92 and spinels are Cr-rich (Cr#N43). Opx are

enstatites and Cpx are Cr-rich diopsides (Cr2O3~1–1.3 wt.%).

Cpx sampled above the gouge zone have low TiO2 (b0.06 wt.%)

andNaO2 (0.06–0.15wt.%) contents, except in samples collected

between 75 and 85 mbsf, which have slightly higher values for

these elements (TiO2~0.1 wt.% and NaO2 up to 0.33 wt.%).

These variations are correlated with higher Ti contents in Spl

(TiO2 up to 0.13 wt.% compared to valuesb0.05 wt.% in

harzburgites sampled in the upper part of the borehole). Below the

gouge zone, Cpx have TiO2 ranging from 0.04 to 0.09 wt.% and

high Na content (NaO2N0.5 wt.%).

2.2.2. Site 1272 peridotites

Site 1272 is located on the western flank of the Mid-Atlantic

rift valley, near the summit of the inside corner high just south of

the Fifteen-Twenty FZ. The borehole penetrated to a depth of

131 mbsf with a total recovery of 37.50 m of core (28.6%). The

upper part is composed of diabases and gabbros with

subordinate amounts of peridotites. The section below 56 mbsf

consists of 93% harzburgites with minor dunites (~3.5%)

414 M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

occurring as bands or ovoid patches, cross-cut by fine-grained

mafic rocks (~3.5%). Site 1272 peridotites are very highly to

completely altered to (dark) green serpentinite.

We selected 10 harzburgites and one dunite in the lower-

most part of borehole, within the least altered areas. Site 1272

peridotites have coarse-grained granular textures. Site 1272

peridotites are composed of Ol and Opx with minor Spl before

serpentinization. Primary Opx abundance varies greatly along the

studied section (from 30% to b20% within harzburgitic areas, to

b5% for harzburgite–dunite transitions) and these modal changes

are found over relatively short distances (tens of cm or less).

Minor Cpx (≤1%) can be observed as tiny grains associated to

Opx, but only in the less serpentinized harzburgite samples and its

abundance may have been underestimated in several samples.

Relicts of primary minerals are extremely rare. Ol, Opx and Spl

have compositions similar to that of the Site 1274 peridotites,

except for the extremely low TiO2 (~0.01 wt.%) of both Opx and

Spl. Mineral compositions do not show significant downhole

variations (Moll et al., 2007).

3. Bulk rock chemistry of Site 1272 and Site 1274 peridotites

3.1. Analytical methods

Bulk rock trace element compositions were determined on a

quadrupole VG-PQ2 ICP-MS at Montpellier University

(France) following the procedure described by Ionov et al.

(1992) and adapted by Godard et al. (2000) for the analysis of

ultra-depleted peridotites. Ti, Li, Co, Cu, Sc, REE, U, Th, Sr, Zr,

Hf, Y, Cs, Rb and Ba concentrations were determined by ex-

ternal calibration. To avoid memory effects due to the intro-

duction of concentrated Nb–Ta solutions in the instrument, Nb

and Ta concentrations were determined by using, respectively,

Zr and Hf as internal standards. This technique is an adaptation

to ICP-MS analysis of the method described by Jochum et al.

(1990) for the determination of Nb by spark-source mass spec-

trometry. The precision and accuracy of the ICP-MS analyses,

including results obtained for rock standards PCC-1 and UBN,

are given in the EPSL-online background dataset.

Results are given in Table 1.

3.2. Major and minor elements

The peridotites from Sites 1272 and 1274 plot near the most

depleted end of the mantle fractionation array on theMgO/SiO2 vs.

Fig. 2. Bulk rock compositions of Sites 1272 and 1274 peridotites illustrated on

(a) MgO/SiO2 vs. Al2O3/SiO2 and (b) FeO (wt.%) vs. MgO (wt.%) diagrams.

Compositions are recalculated on a volatile free basis. The compositions of Sites 1272

and 1274 peridotites obtained during this study are represented with large symbols

compared to shipboard data (Kelemen et al., 2004) and the variously altered samples

analyzed by Paulick et al. (2006). FeO stands for total Fe content. The thick grey line

on Fig. 2a represents the bulk silicate earth evolution (“terrestrial array” after Jagoutz

et al. (1979) and Hart and Zindler (1986)). On Fig. 2b, dashed grey lines are iso-Mg#

lines for Mg# (=100×Mg/(Mg+Fe) cationic ratio) ranging from 86 to 94 while the

thick grey line represent the stoichiometric variations of olivine Fe–Mg composition

(FeO+MgO=66.67 in mole%). Estimated compositions of the bulk silicate earth or

Primitive Mantle (PM (Jagoutz et al., 1979; McDonough and Sun, 1995)) and of the

Depleted MORB Mantle (DMM (Workman and Hart, 2005)) are represented for

comparison, togetherwith a compilation of abyssal peridotite and ophiolitic peridotite

compositions: Abyssal peridotites (Niu, 2004 and compilation of Bodinier and

Godard, 2003): SR: Spreading Ridges; TOC: Transition Ocean Continent (Iberia

Abyssal Plain); IBMa: peridotites drilled at the Izu–Bonin–Mariana fore-arc;

“Impregnated” peridotites (Leg 209): ODP Sites 1270 & 1271 (Paulick et al., 2006);

Ophiolitic peridotites (compilation of Bodinier and Godard, 2003): Semail ophiolite

(Oman); Semail ophiolite Mantle–crust Transition Zone (MTZ); Other ophiolites:

Indonesia ophiolites, Western Alps ophiolites (Lanzo, Internal Ligurides). Symbols

are represented in insert.

415M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

Al2O3/SiO2 diagram (Fig. 2a), with low Al2O3/SiO2 ratios (b0.02)

and highMgO/SiO2 (N1). In spite of their high loss on ignition (up

to 16 wt.%), they do not display the major-element variations

typically associated with oceanic alteration, e.g. MgO depletion

due to seafloor weathering and/or SiO2 enrichments associated

with talc crystallization (Snow and Dick, 1995; Bach et al., 2004;

Paulick et al., 2006). The composition of harzburgites is consistent

with the evolutionary trend shown by more fertile ophiolitic and

abyssal peridotites. Dunites and Ol-rich harzburgites plot system-

atically above the mantle array. These high MgO/SiO2 values

reflect the increase of Ol/Opx from harzburgites to dunites. Sites

1272 and 1274 harzburgites have a narrow range of MgO and FeO

contents (Fig. 2b). Mg# (100×cationic Mg/(Mg+Fetotal)) of Site

1272 and 1274 peridotites is high (91.7±0.3 for Site 1272

peridotites and 91.3±0.3 for Site 1274 peridotites), and no

significant variation from harzburgites to dunites is observed.

Site 1272 and 1274 peridotites are depleted in Al2O3 (0.1–1

an. wt.%; an.: anhydrous) and CaO (b1.2 an. wt.%). These

compositions reflect their low pyroxene content and, for CaO,

their low Cpx content. In spite of some scattering in CaO (0.2–

1.2 an. wt.%), Site 1274 harzburgites have CaO/Al2O3 (0.5–

1.4) close to primitive mantle and Depleted MORB Mantle

values (0.7–0.9 (Jagoutz et al., 1979; Hart and Zindler, 1986;

Workman and Hart, 2005)). Site 1272 harzburgites have Al2O3

contents similar to Site 1274 peridotites but they are highly

depleted in CaO (b0.05–0.2 an. wt.%) and low CaO/Al2O3

(b0.4). This may indicate a stronger Cpx-depletion in these

samples and/or preferential CaO loss during alteration.

Peridotites at Site 1272 and above the gouge zone at Site 1274

have low Ti (b50 ppm) contents typical of highly refractory

peridotites (Bodinier and Godard, 2003). Peridotites sampled

within the 75–85 mbsf interval and at the bottom of the borehole

are distinguished by higher Ti contents (74–162 ppm).

3.3. Trace elements

Site 1272 and 1274 peridotites are strongly depleted in

lithophile trace elements (mostlyb0.1×primitive mantle (PM)),

with concentrations well below the average composition of

abyssal peridotites (Figs. 3 and 4). Site 1272 harzburgites have

more depleted and homogeneous compositions (Yb=10–

17.5 ppb) compared to Site 1274 harzburgites (Yb=16–

44.5 ppb). They display linear to slightly concave-upward

REE patterns (Fig. 3a), characterized by a steady decrease of

normalized REE abundances from heavy REE (HREE) to

middle REE (MREE) (Dy/Yb~0.2×chondrites), which be-

comes less marked from MREE to light REE (LREE) (Ce/

Gd~0.25×chondrites). Site 1274 harzburgites sampled above

Fig. 3. REE composition of (a) Site 1272 and (b) Site 1274 peridotites normalized to CI chondrites and extended trace element patterns normalized to primitive mantle

values of (c) Site 1272 and (d) Site 1274 peridotites. Normalizing values were taken from Sun and McDonough (1989). Symbols are indicated in insert. The average

composition of abyssal peridotites (thick line—determined after a compilation of Niu, 2004 and Bodinier and Godard, 2003 datasets) and the composition of the

harzburgites sampled in the Semail (Oman) ophiolite (Oman harzburgites (Godard et al., 2000)) are shown for comparison.

416 M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

the gouge zone have similar patterns (Dy/Yb~0.23×chon-

drites; Ce/Gd~0.3×chondrites) (Fig. 3b), except for three

peridotites sampled within the 75–85 mbsf interval (Sections

1274A-15R-1 to 1274A-16R-1). The latter are characterized by

a flatter MREE-HREE segment on chondrite-normalized

patterns (Dy/Yb=0.45–0.75×chondrites) and lower LREE/

MREE ratios (Ce/Gd~0.1×chondrites). The harzburgite sam-

pled at the bottom of the borehole (Core 1274A-27R) is dis-

tinguished by an almost flat LREE chondrite-normalized pattern

(Dy/Yb~0.5×chondrites; Ce/Gd~1×chondrites), and a slight

negative Eu anomaly (Eu/Eu⁎=0.5). At both sites, dunites have

REE patterns similar to neighboring harzburgites; they are

characterized by slightly lower REE contents (Fig. 3).

Site 1272 and 1274 peridotites have U-shaped normalized trace

element patterns, with no significant variations in trends from one

rock type to the other (Fig. 3c and d). These patterns reflect a strong

depletion of the most incompatible REE as well as of Zr and Hf

relative to HREE. More particularly, Zr shows variable, but

systematic, depletion relative toMREE andHf. Both Site 1272 and

1274 peridotite trace element patterns are characterized by selective

enrichments in highly incompatible elements (Cs to Ta) relative to

LREE (e.g., averageTh/Ce~12×PM-). They showalso prominent

spikes in Sr and Pb (on average, Sr/Ce ~250×PM and Pb/Ce

~100×PM) and selective enrichments in U relative to the

neighboring elements Th and LREE (U/Th=1.5–29×PM, except

sample 1274A-13R-1, 91–99 cmwith U/ThN5000×PM- Fig. 4c).

Finally, Sites 1272 and 1274 peridotites are enriched in Nb and Ta

relative to LREE (Nb/Ce=2–35×PM) and, to a lesser extent,

relative to Th (Nb/Th up to 9×PM). Nb/Ta ratio is on average

below PM estimates in Site 1272 peridotites (Nb/Ta=9–17 com-

pared with PM=17.6 (Sun and McDonough, 1989)); Site 1274

peridotites are distinguished by even lower Nb/Ta values (2–11).

The Zr–Hf ratio is, on average, lower than PM estimates (Zr/

Hf=11±8; PM=36.25 (Sun and McDonough, 1989)). Nb/Ta is

correlated to Zr/Hf variations in Site 1274 peridotites while Nb/Ta

is constant in Site 1272 peridotites (Fig. 4d).

4. Discussion

The peridotites drilled along theMARduringODPLeg 209, to

the south (Site 1272) and to the north (Site 1274) of the Fifteen-

Twenty FZ, are the most depleted abyssal peridotites yet sampled

at a slow spreading center. As illustrated on Figs. 2–4, they have

compositions similar to those of the most refractory ophiolitic and

abyssal peridotites sampled so far (Parkinson and Pearce, 1998;

Godard et al., 2000). The occurrence of such a highly depleted

mantle at one of the slowest segments of theMid-Atlantic Ridge is

in contradiction with the generally accepted idea that there is a

Fig. 4. Trace element compositions (in ppm) of harzburgites and dunites drilled at Sites 1272 and 1274 on (a) Ce versus Yb, (b) Th and (c) U versus Ce, and (d) Nb/Ta

ratio versus Zr/Hf ratio diagrams, compared to the compositions of abyssal peridotites (including Izu–Bonin Mariana peridotites) and of peridotites from ophiolitic

massifs (including the Semail (Oman) ophiolite) compiled by Bodinier and Godard, 2003). Estimated compositions of the Depleted MORBMantle (DMM, (Workman

and Hart, 2005)), the bulk silicate earth (PM) and the average compositions of normal and enriched MORB (N-MORB and E-MORB) and Ocean Island Basalts (OIB)

are represented for comparison (PM, MORB and OIB values after (McDonough and Sun, 1995)). The symbols are the same as in Fig. 2.

417M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

correlation between ridge spreading rates and the fertility of the

exposed mantle, the most refractory peridotites being associated

with fast spreading centers, where high volumes of melts are

extracted (e.g., Niu and Hekinian 1997).

In the following, we discuss the petrogenesis of Sites 1272 and

1274 peridotites on the basis of geochemical data; in particular,

we examine the roles of melting, melt extraction and melt-rock

interaction in their formation. We also discuss the geodynamic

significance of the occurrence of these highly depleted peridotites

in the slow spreading Fifteen-Twenty FZ area. However, in order

to assess the chemical variations associated with mantle pro-

cesses, the influence of late hydrothermal processes on the com-

position of these peridotites needs to be considered first.

4.1. Alteration of Sites 1272 and 1274 peridotites

In Sites 1272 and 1274 peridotites, alteration is revealed by (i)

the occurrence of hydrous minerals in the secondary assemblage,

such as brucite and iowaite (Bach et al., 2004), and (ii) high losses

on ignition (up to ~15% and ~20% in, respectively, Sites 1274

and 1272 peridotites (Kelemen et al., 2004)). On the basis of a

detailed geochemical study, Paulick et al. (2006) show that,

except for the addition of water (and volatiles), alteration did not

produce significant changes in the bulk chemistry of the

peridotites at these two sites: (i) variations in major elements

and in themore compatible trace elements (e.g., Sc, Ti, HREE) are

consistent with the primary modal characteristics of the studied

peridotites and with the composition of their primary minerals,

and (ii) the composition of more incompatible REE is correlated

to that of elements immobile during alteration such as Th and

High Field Strength Elements (Zr, Hf, Nb and Ta).

In detail, however, some elements have probably been

remobilized by hydrothermal fluids, e.g. CaO in some of Site

1272 peridotites or Cs, Rb, Ba, Sr and U at both sites. None-

theless, we note that, in several samples, the latter are broadly

correlated with elements having the same degree of compat-

ibilities but considered as immobile during alteration such as Th,

Nb, Ta or LREE (Paulick et al., 2006). This suggests that Sites

1272 and 1274 peridotites may have preserved their primary

mantle signature even for some mobile elements; however, a

detail study of primary (relict) minerals and of their associated

alteration products is required to determine and quantify the

effects of alteration on the abundances of these elements. There-

fore, they will not be considered in the following discussion.

4.2. Composition of the mantle exposed at the Fifteen-Twenty

FZ: insights into its magmatic history

Themantle sampled at Sites 1272 and 1274 is highly refractory:

(i) the peridotites are mainly harzburgites with low Cpx contents

and high Cr-spinels (Cr#N43), (ii) their bulk rock composition is

characterized by lowAl2O3 contents (≤1 an. wt.%) and highMg#

(~91.5) and (iii) they have ultra-depleted trace element composi-

tions. The peridotites sampled at Site 1274 are distinguished from

those sampled at Site 1272 by (i) the occurrence of higher amounts

of Cpx, (ii) their higher Al2O3 and HREE bulk rock contents (Yb,

calculated on a volatile free basis, ranges from 0.07 to

0.12×chondrites in Site 1272 peridotites and from 0.13 to 0.30

in Site 1274 peridotites), and (iii) significant downhole variations

in minor (Ti, Na) and trace element (REE) compositions in

minerals (Cpx and Sp (Suhr et al., 2004; Moll et al., 2007; Seyler

et al., 2007)) and in bulk rock (this study).

Sites 1272 and 1274 peridotites are significantly more

depleted than the peridotites sampled in the vicinity of Site

1272, at ODP Sites 1270 and 1271 (Fig. 1). Sites 1270 and 1271

peridotites are distinguished by high Al, Fe and trace element

contents (Figs. 2 and 4). They result from extensive melt/rock

interaction and late precipitation of abundant plagioclase and

clinopyroxene (melt “impregnation”) within a mantle protolith,

that had the composition of Sites 1272 and 1274 peridotites

(Kelemen et al., 2004; Paulick et al., 2006). Melt “impregna-

tion” is typical of peridotites sampled in the shallowest part of

the mantle, in areas associated with the ridge magma feeding

system (e.g., the mantle crust transition zone in ophiolites). Sites

1272 and 1274 peridotites preserve the geochemical signature

of the mantle far from these melt accumulation areas.

4.2.1. Sites 1272 and 1274 peridotites: geochemical signatures

of melting and melt extraction processes

The highly refractory compositions of Site 1272 and Site

1274 peridotites indicate that they have undergone high degrees

of partial melting. The progressive depletion in both HREE and

Al2O3 contents (Figs. 2–4) from peridotites sampled at Site

1274 to those sampled at Site 1272 suggests an increasing

degree of melt extraction from the north to the south of the

Fifteen-Twenty FZ. However, Sites 1272 and 1274 peridotites

do not show significant variations of geochemical indicators of

partial melting, such as bulk rock Mg#, spinel Cr# and Ti in

pyroxenes. In addition, although they are highly depleted, they

have relatively linear REE patterns and spiked trace element

patterns. These features do not seem consistent with a simple

“melting only” history (Bodinier and Godard, 2003).

Trace element variations associated to partial melting are

commonly modeled using a closed-system approach. It is based

upon the assumption that melt can only be removed from the

residue during melting (see review in Shaw, 2000): melt is

(continuously) extracted, although a fraction of the melt may

remain in the residue, but no melt migrating from elsewhere in

the melting region can pass through the melting peridotites

(Fig. 5a). This approach has been used to reproduce Cpx REE

patterns in numerous abyssal peridotites (e.g., Johnson et al.,

1990). On the other hand, several studies suggest that melting

and melt extraction represent open-system processes (Ozawa

and Shimizu, 1995; Vernières et al., 1997): a fraction of the

newly formed melts could percolate upward and, thus, interact

chemically with the melting mantle column (Fig. 5b). To

characterize the behavior of REE within the residue during

closed-system and open-system melting processes, we carried

out a series of numerical experiments using the model of

Vernières et al. (1997). For simplicity, we assumed a homo-

geneous DMM source, and reactions in the garnet-stability field

during the onset of melting were neglected. Melting experiments

were stopped when the Cpx-out curve was reached: Cpx-out is

generally considered the limit at which melt productivity

418 M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

decreases strongly beneath ridges (Asimow et al., 1997). The

results, best fitting the composition of Sites 1272 and 1274

peridotites, are illustrated on Fig. 5.

Closed-system modeling allows reproducing the HREE

composition of peridotites sampled at Sites 1274 and 1272 for

degrees of melt extraction of, respectively, ~14% (Ol 70) and

~18% (Ol 76) in the illustrated example (Fig. 5c). These high

melting estimates are consistent with those inferred of the basis of

petrographic indicators: more than 15% partial melting (Walter

et al., 1995; Niu, 1997) is needed to produce the modal

compositions of the studied peridotites (OlN70-Cpx/Opxb0.10)

and high Spl Cr# values suggest 15–18% partial melting

(Hellebrand et al., 2001). However, closed-system modeling fails

to reproduce the relatively flat REE patterns that characterize

peridotites sampled at both sites. Furthermore, latemelt entrapment

will not “flatten” the REE patterns because the in situ melt in local

equilibrium with the peridotites is also highly LREE depleted.

During the first stages of partial melting, the open-system

model also produces a decrease in LREE relative to less in-

compatible REE, yet to a lesser extent than the closed-system

model. At higher degrees of partial melting, REE patterns

become linear in the partially molten peridotites and in the

associated melts: LREE tend to be “buffered” by percolating

melts while HREE are governed by local changes in the matrix

composition and melt/rock ratio and therefore decrease steadily

during melting (Godard et al., 1995; Vernières et al., 1997). This

model allows reproducing the composition of Site 1272

harzburgites for degrees of melt extraction slightly lower than

closed-system melting (~15%), thus suggesting that HREE are

leached out of the residue more efficiently during open-system

Fig. 5. Evolution of the trace element composition of peridotites in an upwelling melting mantle column. Model of (a) closed-system and (b) open-system partial

melting associated with mantle upwelling. REE evolution predicted using the “Plate model” of Vernières et al. (1997) for (c) closed-system melting and (d) open-

system melting. In both models, we assumed that the melt is incompletely extracted. The maximum fraction of melt remaining in the matrix after each melting

increment is 2%. Numbers on the chondrite-normalized REE patterns (thick lines) indicate olivine content (in percent) in residual peridotites before melting (thick line

noted 57) and after the melting simulations. The composition of the “in situ” melt in equilibrium with the matrix and of the cumulated melt at the top of the melting

column (extracted melt) at the end of the melting processes are indicated by thick grey lines and thick black lines respectively. For both models, we used the estimated

composition of DMM (Workman and Hart, 2005)) as the initial composition of the upwelling peridotites, and the mineral/melt partition coefficients compiled by

Bedini and Bodinier (1999) for Cpx and Ol, and by Niu et al. (1996) for Opx. The initial modal composition was fixed as follows (spinel neglected): 57% Ol, 28% Opx

and 15% Cpx. The melting reaction was taken from Walter et al. (1995) (1.7 GPa reaction at the bottom of the column, and 1.1 GPa reaction at the top). The

composition of Sites 1272 and 1274 peridotites are shown for comparison (same symbols as on Fig. 3).

419M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

melting. The highly homogeneous composition of Site 1272

harzburgites suggests that any pre-existing chemical hetero-

geneities (suggested by downhole variations in Opx/Ol) has

been wiped out during partial melting and melt extraction. In

detail, however, we observe differences between modeled and

measured compositions. Measured LREE are higher than

predicted compositions: late entrapment of small fractions of

“in situ” melts (up to 0.5%) can explain these differences. Also,

our simplified model does not take into account initial melting in

the garnet-stability field. This process strongly decreases MREE

content relative to HREE during the onset of melting (e.g.,

Hellebrand et al. 2002): it would explain the MREE depleted

compositions of Site 1272 harzburgites compared to our models.

The accumulated melts produced during closed-system and

open-system melting have the same MORB-type composition.

They are however significantlymore depleted than averageMORB

(Yb~0.25×N-MORB (Sun and McDonough, 1989)). Indeed, the

degrees of partial melting inferred for the residual peridotites are

higher (≥15%) than those generally inferred for the formation of

MORB (~5% (Workman and Hart, 2005)). Peridotites exposed at

ridges represent the upper part of the melting “pyramid”, which has

undergone the highest degree of partialmelting. In contrast,MORB

represent the accumulation of melts formed by variable degrees of

partial melting at different depths, thus giving an average (and thus

lower) estimate of the degree of partial melting of the upwelling

mantle source (Langmuir et al., 1992).

Finally, although open-system melting of a homogenous

source can account for the composition of Site 1272 peridotites,

it does not allow to reproduce the composition of Site 1274

peridotites, even for small extents of melting (Fig. 5). Also,

melting and melt extraction cannot be solely invoked to explain

the selective enrichments in highly incompatible elements ob-

served at both sites.

4.2.2. Melt flow, dunitization and refertilization at Site 1274

The variations in the modal and chemical compositions of

peridotites observed downhole at Site 1274 occur at scales too

small (a few meters or less) to be attributed to differences in the

degree of melting, as it would imply unrealistic small-scale

thermal gradients. According to recent experiments (Bulatov

et al., 2002), melting of pyroxenite layers within a peridotite

matrix produce olivine-rich peridotites; hence, the sequences of

alternating harzburgites, Opx-poor harzburgites and dunites ob-

served between ~75 and 85 mbsf could reveal an ancient

pyroxenitic layering. However, at Site 1274, olivine-rich

peridotites are enriched in fertile components (e.g., Na and Ti

or trace elements) compared to surrounding harzburgites,

whereas the melting residues of both peridotite and pyroxenite

are similarly depleted during experiments. Selective enrich-

ments in olivine-rich peridotites suggest a major role of melt-

rock reactions in the formation of these zones (Kelemen et al.,

1997). In this scheme, the sequence of alternating Opx-poor

harzburgites and dunites between 75–85 mbsf can be interpreted

as a melt focusing zone, draining reactive (Opx-undersaturated)

melts that produced locally olivine-rich peridotites (dunites

representing the end products of the reaction); in return, this

favored focused transport of melts in chemical disequilibrium

with host peridotites, because of the higher permeability of

pyroxene poor peridotites with respect to melts (Godard et al.,

1995; Kelemen et al., 1997). The reactivity of the percolating

melts toward Opx would imply that they were not formed locally

but at larger depths (Kelemen et al., 1997). Dunites cross-

cutting harzburgites in the upper part of the borehole may have

been formed by the same process (Suhr et al., 2004). The

“dunitization” scenario is consistent with the observation of (i)

constant bulk rock Mg# and constant spinel Cr# in Site 1274

peridotites showing that modal changes were not produced by

various degrees of melting, (ii) selective enrichments in bulk

rock incompatible element contents and high Ti in spinels

indicating chemical interaction with a basaltic melt (Bodinier

et al., 1990; Kelemen et al., 1997).

Site 1274 peridotites are characterized also by the ubiquitous

occurrence of secondary Cpx, having highly variable composi-

tions (Suhr et al., 2004; Seyler et al., 2007). Their chemistry

together with their interstitial texture suggests that Cpx pre-

cipitated after melts in disequilibrium with the residue of partial

melting. Cpx-Spl symplectitic textures suggest a late “quench-

ing” process, probably as a near-solidus melt-freezing reaction,

Fig. 6. Bulk rock REE content (on a volatile free basis) of peridotites sampled at

the top (a) and at the bottom (b) of the borehole at Site 1274, compared to the

calculated composition of peridotites formed after addition (mixing) of 2% to

5% Cpx within a depleted matrix. The depleted matrix has the average

composition of Site 1272 peridotites (thick grey line). Cpx compositions are

represented in inserts. In Fig. 6a, they have the average compositions of Cpx

sampled at the top of the borehole (Gréau, unpublished data). Cpx compositions

on Fig. 6b are reported in Seyler et al. (2007). Compositions produced by the

addition of 2% Cpx are represented by thick dotted black lines and those

produced by the addition of 5% Cpx are represented by thick black lines. These

values were chosen on the basis of thin section observation and modal

reconstitutions (Table 1).

420 M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

in the spinel stability field, at the boundary of the partially

molten domain, similarly to the late fertilization processes

described in the Ronda orogenic massif (Lenoir et al., 2001) and

the Oman ophiolite (Godard et al., 2000; Takazawa et al., 2003).

The late addition of Cpx account for the slightly enriched and

more variable bulk rock trace element content of Site 1274

peridotites compared to Site 1272, and more particularly for the

differences in composition between the top and the bottom of the

borehole (Fig. 6).

4.2.3. Selective enrichments in highly incompatible elements:

an effect of mantle metasomatism?

Sites 1272 and 1274 peridotites have highly depleted com-

positions, yet they show selective enrichments in highly

incompatible elements considered as immobile during altera-

tion (Th, Nb and Ta) relative to elements with similar degree of

incompatibility (e.g., LREE-Figs. 3 and 4). Their U-shaped

spiked-patterns resemble that of the most refractory peridotites

sampled in orogenic and ophiolitic massifs and by basalt-borne

xenoliths; such selective enrichments have been commonly

ascribed to chemical interaction with volatile-rich mantle

fluids and/or the late precipitation of microphases (e.g., rutile

or baddeleyite) unevenly distributed on mineral boundaries

(Bodinier and Godard, 2003; Pearson et al., 2003). Similar

processes may occur in the mantle sampled by abyssal

peridotites: beneath ridges, volatile-rich fluids may form at

depthsN100 km during the onset of melting (Presnall et al.,

2002; Dasgupta and Hirschmann, 2006) or by melt-freezing

reactions at the asthenosphere–lithosphere boundary, a process

already suggested to explain pervasive metasomatism in orog-

enic massifs (Lenoir et al., 2001). We note also that Nb–Ta and

Zr–Hf are strongly fractionated and that their evolution is

correlated, but only in Site 1274 peridotites (Fig. 4d). Niu

(2004) tentatively ascribed these variations, commonly

observed in abyssal peridotites, to post melting interaction

with basaltic melts. This hypothesis is consistent with the

evidence of late melt-rock interaction in Site 1274 peridotites.

4.3. Formation of highly depleted peridotites at the Fifteen-

Twenty FZ

4.3.1. The Fifteen-Twenty FZ peridotites: an ancient mantle or

not?

Exposures of highly depleted mantle sections are not

uncommon in low to very low magma budget extensional

settings, at the foot of continental margins (the Newfoundland

margin (Müntener and Manatschal, 2006)) or in oceanic

environments (South West Indian Ridge (Seyler et al., 2003)).

They have been interpreted as evidence for the preservation of

old mantle fragments, a process favored in amagmatic settings

because (i) amagmatic rifting allows exhumation of fragments

of old lithospheric mantle, previously isolated from the

convective mantle (“raft-model” at the ocean-continent transi-

tion (Müntener and Manatschal, 2006)), or (ii) very low degrees

of melting beneath rifting areas allow preservation of ancient

compositional heterogeneities in the convective mantle (Seyler

et al., 2003). These scenarios are consistent with Os isotope data

indicating Proterozoic ages for abyssal peridotites (Brandon

et al., 2000; Alard et al., 2005; Harvey et al., 2006). However,

they imply that abyssal peridotites do not acquire their geo-

chemical characteristics during ridge accretion. Furthermore,

Os-signatures are mostly contained in sulfides armored in

minerals and they may not be related to the latest melting events

(Harvey et al., 2006). In the Fifteen-Twenty area, a set of

observations leads us to favor the alternative hypothesis that the

petrogenesis of the Fifteen-Twenty peridotites was associated to

oceanic spreading:

(i) The composition of the mantle exposed at the Fifteen-

Twenty FZ and its variations, from the downhole meter-

scale to the scale of the mantle exposure, are similar to

that of the oceanic mantle exposed in ophiolites, from

depleted mantle peridotites (Sites 1272 and 1274) to

highly reacted and impregnated peridotites (Sites 1270

and 1271). In the same way, its composition is explained

by extensive melt extraction then, locally, by the pro-

gressive overprinting of the mantle signature by MORB-

type melts (Fig. 4 a–c). Melt infiltration and late melt

impregnation implies that the mantle was partially

molten. The most probable hypothesis is that melt per-

colation occurred before or during cooling of the up-

welling mantle, right below the spreading axis, as in

ophiolites. Assuming that the highly refractory mantle at

Fifteen-Twenty is ancient would imply that melt infilt-

ration follows the remelting of this Cpx-depleted mantle,

a process of which we have no direct evidence.

(ii) Drilling during Leg 209 allowed to sample abundant

gabbros (N25% of the collected samples at most drilled

sites (Kelemen et al., 2004)) in addition to the “impreg-

nated” peridotites sampled at Sites 1270 and 1271. These

gabbros, which are intrusive into the depleted mantle,

suggest a high magmatic activity in the Fifteen-Twenty

area in spite of the lack of basaltic crust today.

(iii) The mechanisms proposed for the preservation of old

mantle fragments do not seem pertinent in the Fifteen-

Twenty area. In contrast to the major FZ (N500 km long

offset) located more to the south in the Equatorial Atlantic,

which involve mantle regions that might have interacted

with structures of the African-American transformmargin,

the Fifteen-Twenty area is associated with the accretion

of the Central Atlantic Ocean. The preservation of a

~200 km×~120 km “raft” (Fig. 1) of undelaminated

mantle seems improbable over the large time scales that

characterize the opening on this portion of the MAR

(~160 Ma (Stampfli and Borel, 2002)).

4.3.2. Geodynamical models

Three scenarios are posited to explain the depleted composi-

tions of the peridotites exposed at the Fifteen-Twenty FZ.

The first scenario is based on seismic and gravity data and

morphological observations indicating variations of the ridge

feeding system with time and along axis (e.g., MacDonald,

1998). Ridges cyclic functioning alternates periods of intense

magmatic activity and amagmatic periods during which

421M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

spreading is dominated by tectonic processes. In this scenario

(Model 1 on Fig. 7), the high degrees of melting leading to the

formation of the highly refractory mantle exposed in the Fifteen-

Twenty FZ would have occurred during a magmatic stage,

building a thick basaltic crust. The magmatic stage is followed

by a period of waning magmatic activity, during which the thick

crust migrates away from the axis. Mantle upwelling continues,

but at a slower rate, and flows away laterally and possibly along

axis, thus favoring the exposure of the depleted mantle at the

tip of segments. Melt percolation, draining melt from variable

depths and possibly variable sources, may occur in the still

partially molten peridotites and produce fertilization (Site 1274)

followed bymelt accumulation processes (Sites 1270 and 1271).

In that sense, melts trapped in the mantle would represent the

“normal” melts that were not expelled at the axis during the so-

called amagmatic period.

The second possible scenario is based upon small-scale

convection models (Model 2 on Fig. 7): the occurrence of small,

secondary convection cells with high upwelling rates, flowing

symmetrically with respect to the ridge axis, may be controlled

by the presence of fracture zones (Rouzo et al., 1995). Fracture

zones are tectonic features occurring regularly along spreading

centre and several intersections of the MAR axis with fracture

zones expose highly refractory peridotites (Lagabrielle et al.,

1998). These small-scale convection cells favor high degrees

of melting at shallower depth than predicted on the basis

of spreading rates (Langmuir et al., 1992) leading to higher

degrees of melting in the mantle. Melt produced is emplaced off

axis and/or forms gabbroic bodies in the upper part of the

upwelling mantle, and impregnated peridotites atop the shallow

melting zone.

The third possible scenario is based on the fact that fossil FZ

and pseudo-faults are present to the west of the studied area

(Fig. 1). Fossil FZs, which end abruptly without connections to

the current axis, indicate ridge jumps. Oblique pseudo-faults

indicate propagation of segment tips along the axis. Both

features reveal crustal instabilities probably linked to complex

dynamic processes in the mantle, involving the displacement of

convective cells. These instabilities will favor the sampling of

melts of different sources in the percolated areas, in particular at

Site 1274, located at the tip of an ancient FZ (Royal Through-

Fig. 1). The ridge jump will favor the exposure of the most

depleted mantle during amagmatic extension. This model does

not exclude a role of magmatic cycles (model 1) or small-scale

convection (model 2) in the formation of this highly depleted

mantle (Fig. 7).

Fig. 7. Three scenarios for the formation of highly refractory peridotites during accretion in the Fifteen-Twenty area.

422 M. Godard et al. / Earth and Planetary Science Letters 267 (2008) 410–425

5. Conclusion

Site 1272 and 1274 peridotites represent the most depleted

peridotites sampled so far at a slow spreading ridge. The com-

position of Site 1272 peridotites can be explained by open-

system partial melting and incomplete melt extraction. In ad-

dition to melting, we observed evidence for localized melt/rock

interaction with melts of variable mantle sources, followed by a

melt-freezing reaction characterized by the precipitation of

interstitial Cpx at Site 1274. The variability of composition of

the peridotites studied in this area does not relate to variable

degrees of partial melting on one side and the other of the FZ but

to variable degrees of melt-rock interaction.

The formation of such highly refractory peridotites suggest

that (i) mantle melting and melt extraction is more efficient at

slow spreading ridges than previously expected and (ii) large

amount of melts is produced and emplaced even in “amagmatic”

segments. We propose different scenarios to explain our ob-

servations: (i) melting during a period of high magmatic activity

followed by present day exhumation during amagmatic rifting or

(ii) formation of shallowmicro-convection cells close to FZ-ridge

intersections. Also, remains of older fracture zones preceding the

Fifteen-Twenty FZ appear to the west of the studied area, which

suggests a displacement of the crustal spreading centre relative to

the mantle with time. This has probably favored the sampling

of different regions of the partially molten mantle at depth and

exposure of parts of the upwelling mantle rarely exposed in

oceanic tectonic windows.

Acknowledgements

We thank the captain and crew of the JOIDES Resolution, the

Ocean Drilling Program (ODP) staff and the members of ODP

Leg 209 Shipboard Scientific Party for the tremendouswork done

during the cruise, S. Pourtales and O. Bruguier for technical

assistance on ICP-MS and C. Nevado and D. Delmas for the thin

sections. This research used data and samples supplied by ODP.

ODP is sponsored by the U.S. National Science Foundation

(NSF) and participating countries under management of Joint

Oceanographic Institutions (JOI), Inc. This work benefited from

discussions with H. Paulick, B. Ildefonse, F. Boudier, and J.-L.

Bodinier. We thank E. Hellebrand and A. Dijkstra for helpful

reviews and R. Carlson for editorial handling.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.epsl.2007.11.058.

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