Contrasting styles of hydrous metasomatism in the upper mantle: An ion microprobe investigation

Geochimica

et Cosmochimica

Pergamon

Acta, Vol. 60, No. 8, pp. 1367- 1385. 1996 Cotwright 0 1996 Elsevier Science Ltd P&ted in the USA. All tights reserved 0016.7037/96 $15.00 + .OO

PI1 SOO16-7037( 96)00018-X

Contrasting

styles of hydrous metasomatism in the upper mantle: An ion microprobe investigation

K. E. JOHNSON,’ A. M. DAVIS,’ and L. T. BRYNDZIA’ ‘Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148. USA *Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA ‘U.S. Environmental Protection Agency, Center Hill Facility, 5995 Center Hill Avenue, Cincinnati, OH 45224, (Received

June

7,

1994; accepted

in revised

form

January

6,

USA

1996)

Abstract-Trace element compositions of amphibole ( Lphlogopite) -bearing spine1 lherzolite xenoliths have been investigated with regard to their oxidation state and tectonic setting to evaluate the processes associated with the formation of hydrous upper mantle. The oxidation states of these xenoliths appear to be directly related to the style of metasomatism and the tectonic environment. Suites having oxygen fugacities at or above QFM (Dish Hill, California; Ichinomegata, Japan; Mont Briancon, Massif Central, France; Dreiser Weiher, Eifel, Germany) are all modally metasomatized, containing amphibole f phlogopite. These suites are either from regions overlying Cenozoic continental arcs (Dish Hill and Ichinomegata) or from Cenozoic continental rifts superimposed on ancient arc terranes (Massif Central and Eifel) . Although trace element enrichment does not appear to be directly linked to the oxidation state, three distinct cases of metasomatic activity can be defined for these oxidized, hydrous xenolith suites. Both Case 1 and Case 2 are exemplified by xenoliths from Dish Hill and Ichinomegata. Case 1 is a less intense form of metasomatism resulting in the formation of amphibole by the simple hydration of the mantle. The original depleted trace-element composition of the mantle is preserved. Amphiboles produced in Case 2 metasomatism are enriched in incompatible elements, but coexist with depleted clinopyroxene. The formation of enriched amphibole is consistent with crystallization from a volatile-bearing melt phase. The Eifel and Mont Briancon trace element compositions that appear to represent equilibrium crystallization of incompatible element-enriched amphibole and clinopyroxene from a migrating melt phase (Case 3). Chemical evidence suggests that phlogopite in xenoliths from Dreiser Weiher, Mont Briancon, and Ichinomegata has a secondary origin. Mica formation, therefore, represents a distinct metasomatic episode. 1. INTRODUCTION

tion of many arc magmas (e.g., Gill, 1981, and references therein; Peacock, 1990, 1993; Ionov and Hofmann, 1995). This addition of water and other volatiles via subduction processes is also the probable source of a large proportion of mantle volatiles (Ito et al., 1983; Peacock, 1990, 1993). The oxidized nature of amphibole-bearing xenoliths (Wood et al., 1990; Johnson et al., 1990), may therefore, be attributed to subduction zone metasomatism caused by the high fluid fluxes in this environment (Ito et al., 1983; Peacock, 1990, 1993). In other tectonic settings, such as continental rifts, hydrous metasomatism has been attributed to interactions of peridotite with carbonated melts (e.g., Menzies and Wass, 1983; Menzies et al., 1985; O’Reilly and Griffin, 1988; Meen et al., 1989a; Witt and Seek, 1989). Carbonated melts also have the potential to affect both the redox conditions and trace element composition of the mantle that they traverse. It has been demonstrated that carbonate melt/mantle wallrock interactions can produce a metasomatic increase in the LILE content of mantle clinopyroxenes (Roden and Shimizu, 1993; Brenan and Watson, 1991). Exsolved CO, formed in association with such a melt is a viable agent for metasomatic oxidation (Johnson, 1990). Carbonated melts, therefore, provide an alternative metasomatic agent (Dupuy et al., 1992; Brenan and Watson, 1991; Meen et al., 1989b). Some investigators have suggested that the presence of secondary phlogopite is a strong indicator that carbonated melts have

In recent years, numerous workers have shown that the upper mantle is quite heterogeneous with respect to trace element and isotopic compositions (e.g., Sen et al., 1993; Roden and Shimizu, 1993; Chen et al., 1991; O’Reilly and Griffin, 1988; Roden et al., 1984; Downes and Dupuy, 1987; Frey and Prinz, 1978; Frey and Green, 1974), and to some extent, major element compositions (e.g., Elthon, 1992; Bonatti and Michael, 1989; Boyd, 1987; Dawson et al., 1981). From a geochemical standpoint, it is of great interest to characterize the nature of these heterogeneities and to perform measurements that enable the processes involved in their generation to be understood and quantified. Currently, several kinds of mantle processes including partial melting, modal metasomatism, and cryptic metasomatism can be identified and in some measure understood. Modal metasomatism resulting in the formation of amphibole in the upper mantle has been attributed to the reaction of mantle peridotite with low-density fluid phases such as H20 and CO* (Mcguire et al., 1991; Bodinier et al., 1990; Witt and Seek, 1989; Menzies and Wass, 1983; Dawson and Smith, 1982). Metasomatism in the sub-arc mantle occurs as a consequence of the dehydration of the subducted slab and is a process observed in many sub-arc related peridotites (Bielski-Zyskind et al., 1984; Ozawa, 1988; Neal and Taylor, 1989; Peacock, 1993). The hydration of the sub-arc mantle is presumed to be an important process in the genera1367

1368

K. E. Johnson,

A. M. Davis,

been a metasomatizing agent (e.g., O’Reilly and Griffin, 1988; Meen et al., 1989a). However, phlogopite and highK amphibole can be stabilized in the subduction environment at depths > 110 km (Sudo and Tatsumi, 1990; Thompson, 1992; Davies, 1994). Therefore, in areas of ancient (e.g., Paleozoic) convergence, a subduction-derived fluid component could be preserved in these phases and recycled to the surface by, e.g., a later episode of magmatism related to rifting. The primary aim of this study is to investigate the relationship between modal metasomatism, oxidation state, and trace element enrichment, and to consider the compositions of metasomatic fluids in the cases described for this study. Towards this purpose we have collected ion-microprobe trace element data for coexisting clinopyroxene and amphibole, and calculated the oxygen fugacities for these samples. The localities examined represent diverse tectonic environments, which may be correlated with specific oxygen fugacity characteristics (e.g., Mattioli et al., 1989; Wood et al., 1990). However, correlations between oxygen fugacity and trace element composition have not been firmly established, and will be examined further by this study. 2. SAMPLE

AND

LOCALITY

DESCRIPTIONS

In consideration of possible spatial variations in chemistry and oxidation state of spine1 lherzolites, the results of this investigation (Japan, Ichinomegata; Dish Hill, California; the Massif Central of France, Mont Briancon; and Germany, Eifel Volcanic Field, Dreiser Weiher locality) are compared to data for samples from a broad spectrum of geographic locations and tectonic settings including the southwestern United States (Puerto Necks, New Mexico), and Australia (Anakie Hills). The majority of these samples are Crdiopside-bearing, Type I spine1 lherzolites according to the classification of Frey and Prinz ( 1978). Textures are described according to Mercier and Nicolas ( 1975). Mineral chemistry data for Mont Briancon and Ichinomegata IM-series are presented in Wood and Virgo ( 1989); for the Eifel refer to Popp and Bryndzia ( 1992). Mineral chemistry is summarized in Table 1 and a complete presentation of data not referenced is available upon request from the first author. 2.1.

Anakie

Hills,

New

South

Wales,

Australia

The Anakie Hills locality is composed of Quatemary Scotia cones associated with the Cenozoic alkaline intraplate volcanism of eastern Australia ( Wass and Hollis, 1983; Hollis, 1985). In particular, the east Anakie Hill contains abundant granuhtic and eclogitic xenoliths, that have been the subject of crustal evolution studies (e.g., Wass and Hollis, 1983; Hollis, 1985; Sutherland, 1985). Spine1 lherzolite xenoliths also occur, but have not been extensively studied. The spine1 lherzolite xenoliths in this study are predominantly of coarsegrained, protogranular and equigranular mosaic textures. These textures represent deformation-induced recrystallization. All samples contain either melt in patches or along grain boundaries and mineral fractures. Olivine and orthopyroxene are commonly 4-7 mm; clinopyroxene 2-3 mm; spinels are commonly I mm or less and associated with melt. Larger (up to 5 mm) anastamosing spinels are generally not associated with melt. These samples are representative of southeastern Australia anhydrous spine1 lherzolite xenoliths as summarized by Hollis ( 1985). 2.2.

Dish

Hill,

California

USA

Dish Hill is a Tertiary alkali basalt cone located in the southwestem Basin and Range Province (Wilshire et al., 1988). Xenoliths from this locality include spine1 lherzolites, websterites, granulites, and rare garnet lherzolites, of which Type I spine1 lherzolites com-

and L. T. Bryndzia prising -88% of the suite (Wilshire et al., 1988). The amphibolebearing Type I xenoliths, which comprise 13% of the total suite population, are the subject of this study. The dominant textural type is equigranular. The amphiboles are Ti-pargasites (Popp and Bryndzia, 1992), frequently associated with intergranular melt. This amphibole-melt occurrence represents the decompressional breakdown of amphibole as described by Takahashi (1980). The texture of DH102 is transitional from porphyroclastic (opx porphyroclasts up to 7 mm) to equigranular. Orthopyroxene also contains inclusions of spinel, clinopyroxene, and melt. Grain sizes are otherwise as follows: olivine and orthopyroxene 2-4 mm, clinopyroxene 2-3 mm, spine1 generally
Eifel

(Dreiser

Weiher),

Germany

Dreiser Weiher is a Quaternary maar volcano of the West Eifel, characterized by alkali basalt-hosted mantle xenoliths (Stosch and Lugmair, 1986). This region is a part of the Rhenish Shield that has been undergoing uplift since the mid-Tertiary (Stosch and Lugmair, 1986). Xenoliths types occurring at Dreiser Weiher are dominated by Type Ib (anhydrous ) and Type Ia (hydrous) spine1 lherzolites, with each type comprising about one third of the total xenolith population (Stosch and Seek, 1980). Samples in this study are Type la, composed of olivine, orthopyroxene. clinopyroxene, spinel, amphibole, and glass patches of similar origin as described for Dish Hill. In addition to the mineralogy of Stosch and Seek (1980). samples in this study also contain phlogopite. and carbonate not previously noted for xenoliths from this locality. Textures are porphyroclastic with relic orthopyroxene up to 8 x 2 mm in an equigranular matrix of the other phases, that are -I mm in diameter. 2.4. Ichinomegata,

Japan

Ichinomegata Crater is an andesitic cone located on the Oga Peninsula of Northern Honshu Island (Kuno, 1967; Takahashi, 1980). It represents a rare occurrence of mantle xenoliths in calcalkaline volcanics (Kuno, 1967). Spine1 lherzolites are the predominant xenolith type, containing the assemblage olivine, orthopyroxene, clinopyroxene, and spine1 + pargasitic amphibole ? Ca-plagioclase (Takahashi, 1980). The partial fusion of amphibole produces brown glass along grain boundaries and in patches directly associated with amphibole. Samples in this study are representative of the plagioclase-free amphibole-bearing suite and have equigranular mosaic textures. Olivine, orthopyroxene, and clinopyroxene range in size from 3-5 mm. Spinels occur along grain boundaries and are generally
Massif

Central

(Mont

Briancon),

France

Mont Briancon represents the Quaternary phase of alkaline volcanism associated with the late-Tertiary uplift of metamorphic and granitic basement (Dowries and Dupuy, 1987). Four predominant textures have been identified for Massif Central xenoliths: protogranular, porphyroclastic, equigranular, and secondary recrystallized (Downes and Dupuy, 1987). This progression of textures is thought to be related by increasing degrees of deformation (Mercier and Nicolas, 1975). Textures have regional association, with the protogranular (oldest) texture comprising -50% of the xenohths at Mont Briancon (Downes and Dupuy, 1987). Grain sizes are generally greater than 5 mm. Spine1 lherzolite is the predominant lithology. Hydrous phases (amphibole and phlogopite) are
Puerto

Necks

(Cerro

Negro),

New

Mexico,

USA

The Puerto Necks (also referred to as the Puerto Plugs) are a cluster of several volcanic cones in northwestern New Mexico (Kudo

Metasomatism Table 1. Summary

in the upper

1369

mantle

of Mineral Chemistr/

Samples

CN87-1

CN87-2

ANWllP

ANWt14

ANWllS

DHIOP

DH142

DHI 54

FF (moss.) FF (probe) aht) Cr/(Cr+Al)

0.16 0.001 0.14

0.13 0.10 0.007 0.12

0.15 0.13 0.002 0.10

0.14 0.15 0.002 0.10

0.27 0.011 0.16

0.30 0.27 0.023 0.11

0.28

0.28 0.009 0.19

0.014 0.13

WO En Fs Cr/(Cr+Al)

44 51 5 0.1

35 59 6 0.11

48 46 4 0.09

46 48 6 0.06

45 48 7 0.06

48 46 6 0.11

36 54 8 0.09

46 49 5 0.09

90

81

89

87

68

90

90

88

(Fo)

90

89

90

87

87

90

90

89

T Wells QFM N &W

937 -0.7

1057 -2.8

889 -0.5

959 -1.9

943 -1.6

893 0.2

936 -0.1

692 -0.4

Spine1

Orthopyroxene

Olivine

(En)

et al.. 1972: Wilshire et al.. 1988). Cerro Negro is a composite of three cones and was the site sampled for this study. These Tertiary alkaline basaltic cones are part of the Rio Grande Rift system. and are host to abundant crustal (sedimentary) and peridotite xenoliths (Wilshire et al., 1988). Type I anhydrous spine1 Iherzolites and harzburgites are the most common peridotite types, with minor websterites, dunites, and Al-augite pyroxenites (Wilshire et al., 1988). Samples in this study are Type I porphyroclastic harzburgites. All

Table 1. Mineral

Chemistry

Samples

samples contain intergranular melt patches frequently associated with disseminated fine-grained (< 1 mm), red-brown spinels and clinopyroxene. Parallel spine1 lineations, as in CN87-1, indicate modification of the protogranular texture by plastic deformation. These “veinlet” spinels may be up to 2 mm in diameter and are not associated with melt. Both CN87-1 and CN87-2 contain orthopyroxene porphyroclasts (up to 7 mm) Olivine and orthopyroxene are commonly 2-4 mm; clinopyroxene l-2 mm.

Data (continued).

MBR8309

lM8705

185-l

DW55-23

DW55-41

DW55-49

DW56-19

DW56-47

0.29

0.26 0.26 0.01 0.21

0.28 0.27 0.011 0.31

0.31 0.28 0.028 0.44

0.32 0.30 0.016 0.60

0.27

0.34

0.011 0.14

0.31 0.33 0.023 0.17

46 48 6 0.06

47 48 5 0.12

47 49 4 0.11

44 50 6

50 48 2

90

69

90

89

90

89

91

89

Spinel FF (moss.) FF (probe) a(mt) Cr/(Cr+Al)

0.074 0.45

0.35

46 48 4

44 50 6

46 47 5

86

93

90

66

88

93

90

68

Ctinopyroxene wo 6-l I3 Cr/(Cr+Al) Orthopyroxene

Olivine

(Fo)

(En)

T Wells 952 958 1079 1042 967 945 1030 876 QFM N &W 0.8 0.9 0.6 0.6 0.6 0.3 0.1 1.2 Data summarized lrom Johnson (1990). CN = Cerro Negro Cone, Puerto Necks Locality; ANW = Anakie Hills; DH = Dish Hill; MBR = Mont Bdancon , Massif Central; IM and I85 = Ichinomegata, DW = Dreiser W&her, the Eifel. QFM (N & W) = f02 calculated according to WOOD et al. (1990) and presented relative in log units to QFM. FF(moss) = ferrous/ferric ratios determined by Mossbauer spectroscopy; FF(probe) calculated from probe data following Wood and Virgo (1989). a(mt) calculated according to Nell (1969). TW = temperature estimates using the opx-cpx thermometer of Wells (1977). Dish Hill (DH) analyses lrom S. Esperanca (unpubtished data).

0.65 0.78

0.48 0.51

1.03 0.424 11.7 0.977 0.816 0.117(0.0047) 8.35

27.2 0.39(0.26) 1.95 0.024(0.021)

0.0073(0.0018) 0.0390(0.0044)

22.6 0.0218(0.0051) 0.298 0.247(0.035) 1.89 1.50 0.551 2.45 0.539 3.68 0.839 2.34 0.362 2.06 0.286

DH102

0.838 0.290 6.90 0.72(0.12) 0.583(0.090) 0.144(0.050) 5.00 0.4 0.45

0.78 0.64

41.3 3.72(0.57) 19.8 0.098(0.048)

0.281 0.149(0.017)

17.3 0.699 0.950 0.239(0.050) 1.26(0.19) 1.08 0.422 2.07 0.518 2.83 0.532(0.060) 1.93 0.346 I.72 0.264(0.033)

DH154

0.54 0.3 7.4 0.381(0.087) 0.279(0.069) 0.045(0.038) 0.12

39.9 3.12(0.76) 33.6 0.135(0.057)

0.291(0.052) 0.602(0.097)

8.4 0.247 I .31 0.257 (0.052) 1.35(0.19) 0.674 0.322 1.15(0.14) 0.208(0.033) 1.45 0.299(0.046) 0.861 0.078(0.017) 0.589(0.087) 0.098(0.022)

DH142

0.23 0.39

0.726 0.444 68.1 0.64(0.14) I .41(0.17) 0.083(0.081) 7.71

18.5 co.90 180 co.13

5.70(0.78) 6.95(0.94)

20.3 12.0 37.6 6.29 27.2 7.14 1.83 6.22 0.82(0.11) 4.31 0.97(0.10) 2.15 0.378(0.046) 1.46(0.20) 0.254(0.049)

DW56-19

0.27 0.39

1.02 0.536 68.7 0.89(0.18) l.SS(O.18) <0.17 5.24

16.0 <1.02 178 <0.15

4.64(0.59) 6.38(0.81)

20.5 11.4 40.4 6.91 29.2 6.43 I .82 5.98 0.87(0.11) 4.25 O.QQ(O.11) 2.12 0.407(0.050) 1.70(0.21) 0.388(0.058)

DW55-23

0.38 0.48

1.37 0.585 70.7 1.07(0.20) 1.48(0.18) 0.123(0.072) 8.70

20.7 0.65(0.55) 219 <0.18

5.95(0.84) 8.0(1 .l)

16.6 12.2 42.2 6.02 25.2 5.31 1.66 5.04(0.55) 0.502(0.097) 0.590(0.090) 1.79 01 0.296(0.045) 1.42(0.20) 0.258(0.047)

DW55-41

0.39 0.30

0.961 0.536 43.5 0.352(0.050) 1.06 0.045(0.033) 7.82

18.2 0.38(0.23) 127 0.112(0.038)

3.90 3.18

18.3 8.65 18.1 1.98 7.26 2.12 0.784 3.19 0.529 2.93 0.635 2.04 0.288 1.54 0.224

MBR8309

12’

0.81 <0.03

I .03 0.302 cl.3 0.353(0.086) 0.470(0.092) co.10 3.53

12.7 0.73(0.42) 3.99(3.31) 0.358(0.09)

0.31 O(O.034) 0.192(0.023)

12.0 0.682 1 .OQ 0.170(0.044) 0.82(0.16) 0.818 0.338 1.75 0.300(0.039) 2.35 0.491(0.060) 1.69 0.281(0.03) 1.52 0.229(0.032)

ANWI

0.0067(0.0015) 0.002(0.004) 0.0007(0.0002) 0.0069(0.0015) 0.0219(0.0026) 0.0066(0.0014) 0.0113(0.0019) O.OOQO(O.OOl7) 0.0023(0.0004) O.OOlQ(O.OOO6) FI (wt.%) are enrichment factors calculated according All Data in Table 2A determined at the University of Chicago. (ANW112’ is a duplicate analysis performed on both instruments). Tiii’ and Zr/Zr’ Salters and Shimizu (1988); values al.0 indicate relative enrichment when compared to chondrite-normalized REE of comparable ionic radius., e.g. Ti’ = [EuN + TbNY2.

TilTi’ ZrlZr’

0.48 0.50

0.911 0.391 17.0 0.572(0.064) 0.759 co.074 7.73

0.993 0.450 11.4 0.52(0.14) l.OO(O.14) <0.12 4.33

CR03 (wl.%) Ti02 (wt.%) zr Nb Hf Ta Nb/Ta

13.0 0.31(0.24) 5.47 0.0789(0.0263)

11.3 Cl.1 2.38 0.185(0.089)

K RI sr Pa

0.103 0.192

0.273(0.039) 0.276(0.039)

(La/Yb)N (Ce/Yb)N

15.4 0.204 0.982 0.444 2.21 4.30 0.459 1.73 0.339 2.64 0.585 1.75 0.229 I .38 0.286

lM8705

Q Ho Er Tm Yb LU

185-l

Data for Clinopyroxenes.

22.5 0.633 1.64 0.31(0.08) 2.34(0.34) 1.33(0.15) 0.503 3.45 0.625(0.072) 3.75 0.796(0.099) 2.57 0.345(0.044) 1.60(0.19) 0.345(0.049)

Y La ce R Nd sol Eu a Tb

same

Table 2A. Ion-microprobe

to

Metasomatism Table

2B. (cont.)

sample

ton-microprobe

Data

ANWlOl

ANW104

spots

3

Y La ce R Nd sm Eu a Tb e, Ho Ef Tm Yb LU

(VAX) (wt.%)

for Clinopyroxenes.

2

ANWlll

ANW112

1

2

ANW114 3

18.4 18.0 44.0 5.90 26.5 5.45 1.72 5.25 0.90 5.32 0.78 3.26 0.38 2.32(0.29) 0.32

16.7 17.2 40.1 5.20 21.5 4.51 n.d. 3.55 0.59 3.53 0.88 2.94 0.40 2.84 0.36

10.1 0.44 0.76 0.11 0.82 0.65 0.26 1.50 0.29 1.98 0.44 1.83 0.20 1.47 0.13

16.8 14.2 25.7 3.03 13.7 2.85 1.02 3.26 0.52 3.28 0.74(0.08) 2.44(0.31) 0.28 2.15(0.46) 0.30(0.05)

4.56 4.07

5.04 4.61

3.92 3.42

0.19 0.13

4.27 2.88

0.68 0.52 37.0 1.53 0.55

0.63 0.33 45.8 0.50 0.85

0.58 0.30 3.65 n.d. 0.37

0.58 0.29 9.79 1.30 0.56

0.71 0.30 27.2 1.34(0.15) 0.56

1371

mantle

17.7 16.6 39.7 5.32 2.46 5.38 1.63 4.19 16.7 4.16 0.79 2.68 0.33(0.06) 2.36 0.29(0.04)

(La/Yb)N (Cc/Y b)N CR03 Ti02 Zr Nb Hf

in the upper

ANWl15

CN87-1

2 16.8 16.5 45.8 6.36 26.9 6.50 1.46(0.96) 4.49(1.07) 0.72 5.10 0.85(0.09) 2.16(0.43) 0.26 1.76(0.34) 0.26(0.03)

CN87-2

2

2

3.92(0.58) n.d 0.03 n.d. n.d. n.d. n.d. 0.13(0.04) 0.040(0.007) 0.44(0.07) 0.12(0.02) 0.40(0.06) 0.08 0.55(0.08) 0.08

6.95(1.82) n.d n.d n.d. 0.03 0.1 O(O.09) 0.04(0.007) 0.06 0.1 l(O.04) 0.96(0.64) 0.24(0.11) 0.74 0.14(0.08) 0.91(0.54) 0.10(0.08)

n.d. n.d.

6.06 6.29

n.d. n.d.

0.77 0.05 0.17(0.07) 0.08 n.d.

0.62 0.29 0.36(0.49) l.ll(O.17) 1.20

0.51 0.1 0.16 0.1 n.d.

TilTi’ 0.17 0.27 0.69 0.26 0.18 n.d. 0.76 ZrlZr’ 0.43 0.25 0.39 0.39 0.13 0.002 n.d. 0.14 Data in Table 28 determined at the University of Edinburgh. Concentrations are given in ppm unless otherwise noted. Uncertainties are given (in parentheses) when they exceed 10% of the am&t present. Upper limits are Z.?&ma. based upon counting statistics. n.d. indicates values below the detection limit. Tiii’ and Zr/Zr* were determined as in Table 2.A. The notation HFSE’ is defined as the average chondrite normalized values for the neighboring REE.

3. METHODS 3.1. Major

and

Trace

Element

Chemistry

Major element analyses of coexisting mineral phases in the xenoliths were determined using a JEOL 733/JXA-8600 microprobe. The wavelength dispersive analyses were obtained under the following conditions: 15 kV accelerating potential, 30 nA beam current, and 2-5 p beam size. Standards used were PI40 olivine (Mg), diopside (Si,Ca), ilmenite (Ti,Mn), nickel metal (Ni) , Kakanui hornblende (Na,K), NU chlorapatite (Cl), KL83 15 spine1 ( Al,Fe), and KL83 16 spine1 (Cr). KL8315 and KL83 16 are Mossbauer-analyzed spine1 standards that were analyzed during each probe session to estimate the corrections necessary for the calculation of ferric iron in unknown spinels according to their Cr/Al ratios (see Wood and Virgo, 1989). Raw data were converted into weight percentages using ZAF processing and the reported compositions represent averages of five to fifteen spot analyses per sample (Table 1) REE data for amphibole and clinopyroxene were determined using a modified AEI IM-20 ion microprobe (SIMS) (Tables 2 and 3). Polished thin sections were bombarded with a primary beam of 20 keV IhO- ions focussed into a 5-20 pm spot. The primary beam current was 15 nA for trace element measurements. For details of ion-microprobe analytical methods and data reduction refer to Davis et al. ( 1991). Additional clinopyroxene data for localities characterized by anhydrous and reduced xenoliths, Anakie Hills and Puerto Necks, were determined using a Cameca IMS4f ion microprobe. Polished thin sections were bombarded with a primary beam of 15 keV IhO- ions focussed into a 5-30 Frn spot and a primary beam current of 8 nA. Analytical and data reduction methods follow Zinner

and Crozaz ( 1986). One of the Anakie Hills clinopyroxene samples, ANWl12, was also analyzed using the AEI IM-20. This permits a comparison between data from the two ion microprobes (Table 2).

3.2.

Determination

of Spine1

Ferric

Iron

Content

Spine1 mineral separates were obtained using a Frantz isodynamic magnetic separator followed by handpicking of grains under a binocular microscope. Spine1 concentrates (the most magnetic fraction) were first leached in cold concentrated HF for 24 h and then in room temperature concentrated HCl for 4 h. Three or four successive treatments were generally sufficient to produce up to 95% pure samples. Fifty to one hundred milligrams of cleaned concentrate spine1 then were handpicked to obtain >99% purity. Mossbauer spectroscopy provides the most precise determinations of spine1 Fe3+ content (Dyar, 1984; Wood and Virgo, 1989). However, appropriate corrections can be made to microprobe data, provided that Mossbauer-analyzed spinels have been used as standards, to yield errors on the order of 20.3 log units. The details of this correction method are described in Wood and Virgo (1989). Room temperature Mossbauer spectra of a number of spinels were obtained to check the microprobe ferric iron determinations. Fe304 activities then were calculated using the expression given by Wood et al. ( 1990) and oxygen fugacities were estimated as previously described. Spectral data were fit using a computer program obtained from the Argonne National Laboratory (Davidson, 1959) following the methods described in detail by Wood and Virgo (1989).

TilTi' ZrlZr' Concentrations the detection

F (wt.%) Cl (wt.%)

Nb Hf Ta NblTa 0.01 n.d. 1.41

0.0125 0.01 0.87 0.65 0.41 otherwise noted. Uncertainties by electron microprobe. Data

0.598(0.069) 0.930 0.130(0.041) 4.60

1.36 1.64 11.5

1.36 0.80(0.23) 6.40 0.261(0.031)

0.0078(0.0011) 0.0417

0.478

0.443 0.269(0.031) 2.34 1.83 0.701 3.67 0.735 4.87 1.19 3.52 0.471 2.86

2 32.5 0.0322(0.0045)

DH102

(Pargasite).

1.00 0.911 20.6 0.404(0.055) 0.908 0.089(0.046) 4.54

0.030(0.019)

38.7 2.33(0.27) 34.0

0.133 0.166

3.25 0.467 2.46 0.484

4.19 1.04

0.730 3.21 0.593

3.34 1.83

3 31.3 0.478 1.71 0.468

lM8705

for Amphiboles

0.60 0.96 are given in ppm unless limits. Cl was determined

0.0321(0.0032) 0.01

0.733 0.649 45.8 3.41 1.24(0.13) 0.270(0.075) 12.60

4460 14.7 34.8 96.5

K Fb sr Ba

Cr203 (wt.%) Ti02 (wt.%) zr

1.11 1.34

(LalYb)N (Ce/Yb)N

0.314(0.040)

Lu

0.860 2.97 0.540(0.061) 3.09 0.743

Eu a Tb

2.31 0.405 2.11

1.58 7.40 2.02

Pr Nd sm

oy Ho Er Tm Yb

1 23.4 3.39 10.29

spots Y La ce

analyses

I-85-1

3. Ion-microprobe

f-we

Table

1.06 are given determined

0.20

0.0349 n.d.

1.06 1.50 23.7 0.375(0.071) 0.672(0.086) 0.119(0.050) 3.15

171 4.17(0.64) 85.1 0.92(0.12)

0.168 0.349

0.296

0.423(0.054) 2.61 1.32 0.505 2.63 0.452 3.16 0.728 2.13 0.268 1.53

4 21.1 0.371 1.99

OH136

0.59 when they exceed at the University

1.62

0.40 n.d.

0.592 3.66 82.2 38.5 2.28 1.16 32.63

162

10377 14.6 419

3.70 4.64

33.9 4.96 23.1 5.93 2.27 6.16 0.921 5.42 1.06 2.86 0.392 1.97 0.278

5 27.0 10.5

OH154

1.11 0.45 10%. Upper limits of Chicago.

0.224 0.01

0.86 0.50 are 2 sigma, based

0.288 0.01

1.86 1.81 79.4 48.4 1.42 1.39 34.82

2.19 2.55 82.6 83.9 2.07 2.79 30.07

11577 15.4 430 196

6.72(0.87) 8.91(1.15)

0.178(0.040)

6.69 27.9 5.73 2.18 6.58 0.738(0.088) 3.92 0.712(0.092) 1.94 0.150(0.029) 1.28(0.17)

1 19.3 12.5 42.5

DW55-23

10195 16.0 470 327

9.05(1.40) 10.9(1.7)

0.265(0.050)

8.14 32.0 6.70 2.53 6.53 0.715(0.103) 4.28 0.762(0.083) 1.97 0.230(0.038) 1.23(0.19)

1 22.1 16.1 50.1

DW56-19

upon

counting

1.36 0.50

0.235 n.d.

1.96 2.47 82.2 57.1 1.34(0.14) 1.64 34.82

184

12436 24.7 583

11.8(2.0) 14.3(2.4)

3.50 0.586(0.069) 1.84 0.170(0.030) 0.935(0.157) 0.223(0.044)

49.6 7.20 27.9 6.00 1.74 5.19(0.53) 0.718(0.092)

1 17.3 15.9

DW55-41

statistics.

n.d.

0.20 0.52

0.332 0.03

indicates

2.24 2.96 161.9 273 .21(0.15) 7.26 37.60

10362 9.60 840 189

14.7(2.1) 19.7(2.8)

2.7 0.266(0.037) 1.39(0.20) 0.125(0.050)

5.63 0.644

3.28 9.78 1.27

56.3 10.6

1 23.8 29.6 102 14.1

DW55-49

2.64 0.72 values

0.0998 n.d.

2.74 40.6 4.95 1.57 0.859 5.76

1.34

3530 9.86 375 995

3.87 3.23

0.370

4.15 1.04 2.84 0.363 1.92

9.54 2.30 0.800 4.11 0.576

3 21.9 10.7 23.0 2.74

MBR8309

belo\

Metasomatism 3.3.

Uncertainties

1373

the oxidation state. a number of calibrations detined by the mantle assemblage olivsine-

6Fe,SiO, + O2 = 3Fe2Si?Oh + 2Fe?O,, (0 llllll,lcI <,)7x ‘pmcl may be applied (Ballhaus et al., 1991; Wood et al., 1990; Mattioli and Wood. 1988: O’Neill and Wall. 1987). Factors to be considered when choosing thermometer and barometer calibrations have been discussed in numerous studies of mantle oxidation state (e.g., Woodland et al., 1992: Amundsen and Neumann, 1992; Ballhaus et al.. 1990, 1991 ), These discussions have concluded that both the Wood et al. ( 1990) and Ballhaus et al. ( 1990, 1991) oxygen barometer calibrations yield the most accurate determinations of oxygen fugacity because their spine] activity models account for the effects of the Cr content. Spine] activity models that do not take into consideration the effects of Cr content (e.g., Mattioli and Wood, 1988; O’Neill and Wall, 1987) yield fb, determinations up to 2 log units lower. The selection of an appropriate thermometer calibration is dependant upon the oxygen barometer applied. For example, Ballhaus et al. ( 1991 ) revised the olivine-spine] thermometer to be consistent with their oxygen barometry calibration. by correcting the Cr-spine] activity model as a part of the temperature determination. In contrast, Wood et al. ( 1990) have applied the spine1 activity mode1 of Nell ( 1989 j to the oxygen barometer. thereby correcting for the spine1 Cr content effects in a step separate from the temperature determination. This correction permits the application of independent thermometers such as Wells ( 1977). Chen et al. ( 1991) have noted the limitations of the Wells thermometer, that, for example, cannot be applied to pyroxene-free systems such as dunites. However. their comparison of therrnometry methods demonstrated that, given the proper assemblage, the Wells thermometer was comparable in accuracy to more recent calibrations such as Sachtleben and Seek ( I98 I ), O’Neill and Wall ( 1987). and Ballhaus et al. (1991). In addition. thermometry errors can be mitigated by presenting h,. data relative to an oxygen buffer such as QFM (Woodland et al., i992). Pressure cannot be calculated for spine1 Iherzolites, so reasonable pressure estimates must be made based upon the spine1 stability range ( IO-20 Kbars). Oxygen fugacities have been calculated at 15 kbars, yielding an estimated fi,. error of kO.18 log units for a pressure uncertainty of up to 6 kbars (Woodland et al., 1993). 4. RESULTS

Element

Chemistry

Major element compositions for the constituent phases have been determined and are summarized in Table I, Spine1

70-l

I

1

ANAKIEHILLS DISHHILL DREW WdtER lCHiNO,,EGATA MONTBRIANCON FUEPCONED(S

0-l 80

mantle

in Jo, Determinations

In order to determine of the oxygen barometer orthopyroxene-spine].

4.1. Major

in the upper

05

90

OLIVINE

95

I 100

Mg#

FIG. I. Major element characteristics of spinels. High Cr contents for Dreiser Weiher suggest re-equilibration with intruded melt.

40

50

CPX

60

Mg#

FIG. 2. Major element characteristics of clinopyroxenes. Weiher xenoliths again define a distinct type of metasomatism. bols for the localities are the same as in Fig. I.

Dreiser Sym-

constitutes about 3% by volume of the lherzolites in this study, and contains on the order of 1% FeZ03 by weight. Spine1 compositions are characterized by high aluminum contents, generally in excess of 40 wt’% A&O,, with Cr/(Cr + Al) ranging from 0.10 to 0.60 (Fig. I ). Spinels from Dreiser Weiher are more enriched in Cr and Fe’+ (Table 1 and Fig. 1 ). Clinopyroxene composition5 range from Wois to WosU; however, the vast majority of samples have compositions > Wolz Clinopyroxenes are Cr-diopsides with Cr/( Cr + Al) values ranging from 0.06 to 0. I2 for the anhydrous and amphibole-bearing suites. The Dreiser Weiher diopsides have higher Cr contents (Table I ). Clinopyroxene Mg-numbers show a wider range of variation compared to the Cr content. This is especially evident for suites such as the Puerto Necks and Ichinomegata (Fig. 2). Orthopyroxene , compositions range from En,, to En,,. wtth most fallmg between En,, and E~Y,,~).Olivine compositions range from FoX7 to Fotj7 (Table I ) Amphiboles are Ti-rich pargasites with TiOz ranging from 0.82-3.66 wt% (Table 3 ). The Dreiser Weiher amphiboles are distinguished by their comparativ,ely high K20, TiOL, and Cr?O, contents. Amphiboles in 185-I and MBR8309 have intermediate K concentrations, suggesting a transition to a K-rich environment where phlogopite might occur with amphibole. High Ti ( > I .S wt% ) is another characteristic of interaction between matic magma and mantle peridotite (Witt and Seek, 1989). One Dish Hill sample (DHl02) and the Mont Briancon sample (MBR 8309) also contain amphiboles that suggest a similar metasomatic origin (Table 3 ). In contrast, the Ichinomegata and remaining Dish Hill samples contain amphiboles that are comparatively depleted in both Cr and Ti, suggesting that they resulted from hydration of peridotite by low density fluid. Amphibole-spine1 textural associations and pargasitic amphibole compositions are indicative of formation by hydration of pyroxene (e.g., Dawson and Smith, 1982). Amphibole Nb/Ta ratios < 18, as observed for 18% 1, IM8705, DH 102, DHl36, and MBR8309

1374

K. E. Johnson, 10

A. M. Davis,

1 sigma

600

700

600

900

TEMPERATURE

1000

1100

1200

(C)

FIG. 3. Oxidation state relations for hydrous and anhydrous Iherzolite xenoliths. nfo, relative to QFM is defined as the difference between the calculated oxygen fugacity of the sample and the QFM buffer at the temperature determined for each sample. Amphibolebearing lherzolites generally plot at or above the QF’M buffer. Data from this study (Table 1). Johnson ( 1990). and Wood and Virgo ( 1989). Temperatures were determined according to Wells (1977) with an uncertainty of 50°C. Oxygen fugacities were determined according to Wood et al. ( 1990) with an uncertainty of kO.18 log units when referenced to the QFM buffer. Symbols are the same as in Fig. 1.

(Table 3) support a simple hydration origin, while Nb/Ta ratios > 18 suggest crystallization from a melt (Ionov and Hofmann, 1995 ) Phlogopite is present in all of the Dreiser Weiher and one of the Ichinomegata samples (I85 1) Although not observed in MBR8309, its occurrence has been noted for other Mont Briancon samples of comparable texture (Downes and Dupuy, 1987). The phlogopites in all of the samples in this study may be classified as secondary or metasomatic in origin based upon composition. Dreiser Weiher phlogopites are generally higher in TiO:, with only one sample (DW55-49) at <3.35 wt% (Table 4). Although the DW55-49 and 185-l phlogopites are lower in TiOL, their compositions are within the accepted range for secondary phlogopites (e.g., 0.5-5.7 wt% TiOz, Hartmann and Wedepohl, 1990). Other chemical characteristics indicative of a metasomatic origin include high CrzOl (> 1.O wt%), high total Fe (Fe0 > 4.4 wt%), and low F (<0.04 wt%), as defined by earlier studies (e.g., Meyer, 1977; Dawson and Smith, 1977; Delaney et al., 1980; Field et al., 1986; Hartmann and Wedepohl, 1990). 4.2. Trace Element

Chemistry

A summary of the incompatible element data (Table 2) shows that two types of clinopyroxenes can be distinguished. Clinopyroxenes from ultramafic xenoliths may be classified as Type Ia, LREE-depleted or Type Ib, LREE-enriched (Frey and Prinz, 1978). Type Ia clinopyroxenes occur in the xenoliths from Dish Hill and Ichinomegata (Fig. 4). Clinopyroxenes from the Eifel and Mont Briancon are Type Ib, although the Eifel samples display more extreme enrichment (Fig. 4). Clinopyroxenes from amphibole-free samples

and L. T. Bryndzia

of the Ichinomegata, Dish Hill, and Mont Briancon localities have the same chemical characteristics as the amphibolebearing xenoliths (Fig. 5). The occurrence of both enriched and depleted clinopyroxenes in subduction related mantle suggests that even in this fluid-rich environment metasomatic activity may have a very localized effect, as hypothesized for other tectonic settings (e.g., Sleep, 1988; Nicolas, 1990; Mcguire et al., 1991; Nielson and Wilshire, 1993). For example, one observes a range in both trace element concentrations and oxygen fugacity in hydrous xenoliths (Fig. 6). Amphiboles, constituting l-2% of the samples by volume coexist with abundant interstitial glass and both Type Ia and Type Ib clinopyroxene. Primitive mantle normalized trace element plots and chondrite normalized REE plots of the amphibole-pyroxene pairs illustrate these classifications (Fig. 4). Examination of clinopyroxene-amphibole pairs reveal three distinct types of trace element behavior. Case 1 corresponds to similarly depleted LREE, HFSE, and LILE concentrations in both pargasite and clinopyroxene (Fig. 4ab). Case 2 corresponds to the “apparent” preferential parttioning of LREEs, Nb, Ta, and LILEs into amphibole vs. clinopyroxene (Fig. 4c-d). Finally, Case 3 corresponds to similar LREE-enriched concentrations in amphibole and clinopyroxene, with “apparent” preferential partitioning of Nb, Ta, and LILEs (Ba and K) into pargasite (Fig. 4e-h). All three types occur at higher oxidation states (i.e., at or above QFM), yet no correlation of fo2 with incompatible element concentrations are evident (Fig. 6). The processes and metasomatizing agents responsible for the development of these three distinct trace element patterns for amphiboles will be examined further. 4.3. Oxygen

Fugacity

Relations

There are distinct differences between the results in terms of geographic and tectonic environment. Oxygen fugacity values for the hydrous xenoliths range from -0.4 to +I.2 log units relative to the QFM buffer (Table 1 and Fig. 3). Samples from Ichinomegata, Dish Hill, Mont Briancon, and the Eifel are all relatively oxidized, with most samples near or above QFM (+0.6 to +0.9; -0.4 to +0.2; +0.8; and +O. 1 to + 1.2 respectively). These regions represent mantle

Tabie 4. Maior Element Data for Phlcgopite. sample

185-l

DW56-19

DW55-23

DW55-41

DWSS-49

DW56-47

Si02 Ti02 A1203 Cr203 Fe0 z Nil cao sao Na20 K20

39.38 1.29 15.79 0.78 9.04 0.04 21.04 0.28 0.04 0.16 0.54 7.81

40.16 4.14 15.73 1.77 4.27 0.03 20.81 0.38 0.01 0.30 0.86 8.33

40.66 3.75 15.84 1.05 5.96 0.04 20.53 0.27 0.02 0.13 0.77 7.72

40.37 3.98 15.98 1.56 5.00 0.03 20.95 0.27 0.03 0.23 0.78 7.63

40.36 1.08 15.92 1.71 3.96 0.03 23.58 0.34 0.02 0.31 0.95 7.91

40.20 4.16 15.91 1.17 5.40 0.04 20.56 0.33 0.02 0.10 0.63 8.12

H2D F

3.59 0.03

2.85 0.20

2.93 0.20

2.83 0.21

3.36 0.29

2.99 0.20

Total 99.81 99.84 100.32 99.85 99.82 99.85 HZ0 and F (wt.%) determined by Ion-microprobe at the University of Chicago.

1

0.1

0.001

0.01

’

I

I

I

0 CPX e AMPH I

I

FIG, 4. (a) -(h)

I

I

I

I

I

DHlo2

I

I

I

I

f

I

I

I

I

I

r

W

Er Tm Yb Lu

0.1 Er Tm Yb Lu discussion.

Sm Eu Gd Tb Dy Ho

REE

P Nd F SmZr I-If Ti Y YbLu

It48705 Chondrite-Normalized

U NbTa K LaCePbSr

La Ce Pr Nd

CsRbf3aTh

IM8705 Primitive Mantle-Normalized

Incompatible element behavior far coexisting clinopyroxene (open dots) and pargasitic amphibole rclosed dots). See text for full

Sm ELI Gd Tb Dy Ho

REE

P Nd F Sm7-r Hf Ti Y YbLu

Chondrite-Norm&ad

U NbTa K LaCePbSr

I

DH102 Primitive Mantle-Normalized

La Ce Pr Nd

CsRbEktTh

o.ol I

0.00001

E

i! 9 ‘i3 .I E 'in =I e! .-t”

Q, 32

10

1376

K. E. Johnson,

I”““’ ’ I”““” II”“’ ’ I”“” ’ I”““’ ’ I”“” ’ I 077 7 8r s z d B d 8 d d

A. M. Davis,

and L. T. Bryndzia

I”“1 8

’ ::

’

]““I” 0 F

’ In

7

Metasomatism

in the upper

mantle

1”’

I

I

I

11”

0 In

8 r

0 r

I,

1

m

-> -F -z 2 rJl -IL -s -aL 2 -3 -3 -Y f -Z -3 -e 4 -3 1”’ 8 F

I 5:

I

‘11’~’

I 0 T-

~~!JpUOt.j3//r~J~U!lLl

I UY

I

1378

K. E. Johnson,

A. M. Davis,

and L. T. Bryndzia

,111 8

I 0 UJ

I

I

IIII 0 t-

I In

I

I 7

FIG.

1

1

MBRa306

0

I I I 11 Le Ce Pr Nd

O MEW307

MBR5305

0

Mont Briarwon

La Ce Pr Nd

REE Data

REE Data

I ’ 1 ’ I”“’ Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

INM Chonclrite-Normalized

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

INAA Chondrite-Normalized

S. incompatible element data for clinopyroxenes from anhydrous xenoliths. INAA data for the Anakie Hills and Puerto Necks (h). LNAA data from Johnson ( 1990).

.:

lchinomegata

0.01

0.01

10

’

’

xenoliths

from

Mont

I I I I La Ce Pr Nd

I

r

l

I

I

Briancon

l

l

l

Xenoliths

1

1

1

and Ichinomegata

I

l

I

(a),

and ion-microprobe

Er Tm Yb Lu

I

Xenoliths

P Nd F SmZr Hf Ti Y YbLu

REE Anhydrous

I

Anhydrous

r I 1 I I I Srn Eu Gd Tb Dy Ho

Chondrite-Normalized

I I I I I I I I I I I CsflbBaTh U NbTa K LaCePbSr

CN87-1

A ANwll5 v

_ -

-

l

o ANWlOl ANW104 q ANWlll w ANW112

data for amphibole-free

.-

t

0 za

Primitive Mantle-Normalized

2 \o

K. E. Johnson. A. M. Davis, and L. T. Bryndzia

1380

2 ;; *

megacrysts, and the occurrence of eclogites imply past subduction influences (Menzies et al., 1987; Mukhopadhyay and Manton, 1994). Isotopic, trace element, and mineral chemistry data for xenoliths from the Eifel and the metamorphic terrane of the Massif Central also support a subduction-related enrichment of the mantle (Kempton et al., 1988; Harmon et al., 1987; Downes et al., 1989). Studies of the Victoria, Australia, locality have not addressed subduction-related enrichment processes, and it seems unlikely in this region, although the effects of consequent metasomatism (Roden et al., 1984) by alkaline magmas may be the cause of the apparently more reduced oxidation states (Johnson, 1993).

10

1

Ic I

n VIM +

0

-2

I

-1

b

A fO2

relative

2

1

to QFM

FIG. 6. Chondrite-normalized La/Yb ratios versus Af,, relative to QFM for hydrous xenoliths (Dreiser Weiher, Mont Briancon, Ichinomegata, Dish Hill). The four most enriched amphiboles (La/ Yb > 5) are from Dreiser Weiher.

that has been subjected to Cenozoic subduction-hydrationoxidation processes (Ichinomegata, Dish Hill) or ancient subduction in relatively old continental lithosphere that may show a metasomatic overprint related to later continental rifting (Mont Briancon, Dreiser Weiher). The presence of amphibole is characteristic of the arc-related mantle environment. Petrographic relations (e.g., melt patches with spinel) in amphibole-free samples from both Mont Briancon and Ichinomegata suggest the removal of amphibole by decompressional melting. In contrast to these oxidized samples, anhydrous xenoliths from intraplate localities. not related to present or past subduction yield oxygen fugacities values down to 4 log units below QFM (Fig. 3 ) Peridotites from mid-ocean ridges are also reduced, with an average ,fo2 value of -0.9 log units relative to QFM (Bryndzia and Wood, 1990), consistent with values measured for quenched MORB glasses (Christie et al.. 1986). The results of these studies and new data presented here support the hypothesis that oxidation and hydration of the overlying mantle accompanies subduction (Peacock, 1990. 1993; Wood et al., 1990). 5. DISCUSSION 5.1. Preservation Subduction

of Evidence

for a History

of

It is proposed here that subduction processes have affected the lithospheric mantle even in areas where these processes have been overprinted by Cenozoic rifting. Some xenoliths erupted within rift environments may, therefore, preserve evidence of ancient subduction processes. For example, Type IB xenoliths from the southwestern USA exhibit carbon isotopic heterogeneities (Mattey et al., 1985) and He-Ar systematics (Kyser and Rison. 1982) that suggest the mixing of a recycled subduction component with depleted mantle (Menzies et al., 1987). In addition. strontium isotopic evidence. the chemical composition of amphibole and mica

5.2. Hydrous Mantle

Metasomatic

Processes

in the Upper

Mantle metasomatism has been invoked to explain the decoupled behavior of major and trace elements observed in some mantle xenoliths. In the previous discussion, modal metasomatism has been proposed to occur at high oxidation states (i.e., >QFM) and in tectonic environments that may have a direct or indirect relationship to past or present subduction. Previous investigations (e.g., Mattioli et al., 1989; Wood et al., 1990; Woodland et al., 1992) have shown that cryptic metasomatism can occur across the entire spectrum of oxidation states, and bears no relationship to a specific tectonic setting. Modal metasomatism, unlike cryptic metasomatism, does appear to have a tectonic correlation, generally being associated with oxidized mantle that has a history of subduction (Menzies et al., 1987; Wood et al., 1990; Johnson, 1990). However, modal metasomatism need not be accompanied by incompatible element enrichment. An examination of mineral-melt partition coefficients (Fig. 7) can aid in interpreting the three types of pairedmineral compositions discussed below. The equilibrium case should produce similar REE patterns with amphibole slightly enriched relative to clinopyroxene in all of the REEs. Niobium and Ta patterns mimic the REE concentrations. LILEs such as Cs, Rb, Ba, and Sr should also display significant enrichment in amphibole vs. clinopyroxene. Case 3 corresponds to equilibrium partitioning of trace elements between clinopyroxene and amphibole, resulting in similar REE patterns, but with amphibole relatively enriched in Rb, K, Nb, Ta, and Sr (Fig. 4e-h) Dreiser Weiher and Mont Briancon are examples of Case 3 metasomatism. These characteristics should be expected for equilibrium partitioning between coexisting clinopyroxenes and amphiboles in the presence of a melt (Fig. 7). The Dreiser Weiher mineral pairs show the strongest evidence for a melt-metasomatic origin for coexisting enriched clinopyroxene and amphibole based upon the highest Cr contents and high NblTa ratios. The obvious variation in absolute trace element abundances suggests differing metasomatic histories and/or melt compositions for the Eifel and Massif Central. Case 2 trace element behavior is exemplified by xenoliths from both Dish Hill and Ichinomegata (Fig. 4c-d). In this instance the amphibole has crystallized from a LREE-enriched melt to produce an “apparent” partitioning of LREEs into amphibole vs. clinopyroxene. In reality the “depleted”

1381

Metasomatism in the upper mantle

0.00011 CsRbE!aTh

0.1

’

,Y, La Ce

” NbTa

Pr

I Nd

K LaCePbSr

I Sm

1 Eu

P NdF

/ Gd Tb

Cy

Sm.2

Ho

Hf i3

I Er

I Tm

Y Yi,Lu

I yb

0

CPX

.

AMPH

I Lu

FIG. 7. Mineral-melt distribution coefficients for amphibole and clinopyroxene suggest that under equilibrium conditions REE compositions are not significantly fractionated between these two phases. Data from Irving ( 1978, 1980), Irving and Frey ( 1984), Adam et al. (1993). and Ionov and Hofmann (1995).

clinopyroxene represents a premetasomatic phase in equilibrium with the other lherzolite phases (olivine and orthopyroxene) The secondary, metasomatic amphiboles share many of the chemical characteristics of Case 3 amphibole (Fig. 4c-d). Conversely, the clinopyroxenes may be expected to have greater chemical affinity with Case 1 clinopyroxenes. Case 1 appears to result from the simple hydration of primary clinopyroxene to form pargasite with no disturbance to the original trace element composition of the parental clinopyroxene (IM8705 and DH 102; Fig. 4a, b) This type of metasomatism preserves depleted MORB-type mantle characteristics that have resulted from a prior episode of partial melting. Case 1 metasomatism may be interpreted as less intense than Case 2, resulting from interaction of mantle peridotite with an exsolved volatile fluid phase rather than a melt. This correlates with the “volatile front” hypothesized in numerous studies to advance through the system ahead of the migrating melt (e.g., Menzies et al., 1985; Nielson and Wilshire. 1993; lonov and Hofmann, 1995). A correlation in Cr contents between coexisting amphibole and clinopyroxene should be expected (Fig. S), though at much lower CrlAl ratios than for Case 3. NblTa ratios are also consistent with the volatile front model. These three cases can be interpreted to represent increasingly severe metasomatic effects, beginning with Case 1 where both amphibole and clinopyroxene are depleted Type Ia, progressing to an intermediate state exemplified by Case

2, and culminating with Case 3 metasomatism, where both clinopyroxene and amphibole are LREE enriched. Case I results from the migration of a volatile fluid phase which is capable of hydrating the system and mobilizing LILEs (i.e., Rb, Ba, Sr, K) (e.g., Ito et al., 1983; Tatsumi et al., 1986; Peacock, 1990, 1993 ), but cannot produce LREE or HFSE enrichment, as experimental studies have shown (e.g., Schneider and Eggler, 1986; Michael, 1988; Meen et al., 1989b). The disequilibrium in chemical characteristics of amphibole and clinopyroxene in Case 2 metasomatism requires that the amphibole has formed from a volatile-enriched melt, rather than by simple hydration of the system. Case 3 represents the crystallization of both secondary amphibole and clinopyroxene from a volatile-enriched melt as secondary phases. The difference between Case 2 and Case 3 may represent differences in the volume of melt and/or in the composition of the metasomatizing melt. Cases 1 and 2 both have examples in the Dish Hill and Ichinomegata localities, and overly mantle that has experienced subduction during the Cenozoic. These xenoliths represent subductionrelated hydrous mantle metasomatism. Case 3 xenoliths in this study have an obvious Cenozoic rift association, and their clinopyroxene compositions are of the type generated by alkaline metasomatism (e.g., Frey and Prinz, 1978; Sen et al.. 1993). The ultimate source for volatiles in the system may have an ancient subduction origin (e.g., Ito et al., 1983; Menzies et al., 1987). Secondary metasomatic phlogopite observed in Dreiser Weiher and Ichinomegata (185- 1) xenoliths, and inferred for Mont Briancon, implies the occurrence of a distinct micametasomatic event. Amphibole + mica modal metasomatism represents a more complex tectonic and magmatic history. Numerous studies have concluded that amphibole and phlogopite represent distinct episodes of metasomatism (e.g., O’Reilly and Griffin 1988; Menzies et al., 1987; Voshage et al., 1988; Hartmann and Wedepohl, 1990). The metasomatizing agent that produces mica is presumed to be a highK melt (e.g., O’Reilly and Griffin, 1988). These multiple episodes of metasomatism may represent distinct tectonic

= a + g

6

Cl

0.3 -

l

n

0.2-

0

0.1 -

0.01 0.00

0.02

0.04

Cr/(Cr FIG.

liths.

l 0.06

+

Al)

e

I 0.06

0.10

0.12

AMPH

8. Cr/(Cr + Al) for spine1 and amphibole for hydrous

xeno-

K. E. Johnson, A. M Davis, and L. T. Bryndzia

1382

events, such as the preservation of ancient subduction characteristics that have been overprinted by continental rifting as at Dreiser Weiher and Mont Briancon (e.g., Menzies et al., 1987), or the inverse relationship possibly relating to Ichinomegata (Voshage et al., 1988). 5.3. Composition-Volume

Relationships

The mass transfer of trace elements during the metasomatic process can be quantified in terms of compositionvolume calculations. Vander Auwera and Andre (1991) have demonstrated that such calculations can be used to estimate the amount of a specific component (n) added to the system, and to limit the possible number of reactions necessary to describe the system. Following their method we have determined composition-volume relations for the hydrous xenoliths in this study using the Gresens ( 1967) equation. The first assumption required is that the reaction of pyroxene has produced the metasomatic amphibole phase with little or no change in the absolute volume of the system. Minor and trace elements used in the calculation were selected to represent the important elemental groups that participate in metasomatism: the transition trace elements (Cr, Ti, Nb), the LILEs elements (Rb, K, Ba, Sr), and the LREEs (La, Ce). The generalized form of Gresens’ composition-volume equation is

100 Lfv (Ml@)

Cf: - C!l = X,

(2)

where A is the reacting phase and B is the metasomatic product; fv is the net volume change factor for the reaction, that is, the amount by which the volume of component A (V,) is multiplied to produce the volume occupied by the metasomatic phase B (V,); gBlgA is the ratio of specific gravities for phase A and B ; C is the concentration of component (n) in each phase; and X, is the composition-volume factor which indicates a net gain of component (n) when positive and a net loss when negative. For the simple hydration reaction assumed here a constant volume relationship

Table Sample

5. Composition-Volume I85

1

Relationships

is assumed (fv = 1) because the new metasomatic phase is embedded in a matrix and at conditions that do not allow for a large volume change. As long asfv > 0.95, the assumption of constant volume is valid (Vander Auwera and Andre, 1991). When the elemental concentrations are converted to weight percent, then X,, represents the actual weight percent of the component added to the system (Gresens, 1967). The results of the calculation have been presented in Table 5. It is evident that even the depleted Case 1 amphiboles (DH102 and IM8705) show a slight net gain in most of the components under consideration. Case 2 amphiboles are in general more enriched in most components than Case 1; however, it is surprising that 185-l and DH154 show a net loss in Cr despite the presumed magmatic influence. The Case 3 xenoliths (Dreiser Weiher and Mont Briancon) display the overall enrichment expected for these environments, consistent with more extensive and probably repeated episodes of metasomatism. 6. CONCLUSIONS

Modally metasomatized xenoliths are, in general, more oxidized than both cryptically metasomatized and unmetasomatized xenoliths, and frequently have arc associations. Dish Hill, Mont Briancon, Dreiser Weiher, and Ichinomegata are all related to present or past continental arcs, all are modally metasomatized, and have oxidation states above QFM. The subduction-related mantle, as exemplified by these localities, is expected to show evidence for interaction with slab derived fluids and melts with the overlying depleted lithosphere, as demonstrated by these hydrated and oxidized suites. The importance of melt infiltration, rather than low density fluid migration, to metasomatism has been demonstrated by investigations in cryptic as well as modal metasomatism. In the case of modal metasomatism, the trace element compositions of the resultant amphibole permit the discrimination of volatile-induced vs. melt-induced origins. The occurrence

for Hydrous

Xenoliths.

lM8705

DH102

DH142

DH154

DW56-19

DW55-23

DW55-41

MBR8309

0.330

0.520

-0.246

0.999

0.899

1.229

0.379

XCr

-0.200

0.089

X Ti

0.199

0.520

1.176

1.200

3.370

1.934

3.777

2.014

2.204

XNb

0.004

0.004

-0.011

0.000

0.043

0.094

0.045

0.053

0.006

XK

0.250

-0.130

-0.310

1.580

0.750

1.280

1.510

1.520

0.210

XRb

0.150

0.020

0.004

0.011

0.120

0.270

0.160

0.170

0.100

xsr

0.038

0.034

0.006

0.061

0.477

0.347

0.359

0.431

0.290

XBa

1.050

-0.001

-0.003

0.009

1.810

2.040

2.190

3.650

1.111

XLa

0.032

0.003

0.000

0.001

0.120

0.090

0.050

0.090

0.020

XC9 0.100 0.009 0.002 0.008 0.390 0.370 0.290 0.380 0.060 Xn values are composition-volume enrichment factors calculated according to Gresens (1967). Positive values indicate overall enrichment; negative values indicate overall depletion of metasomatic amphibole relative to the precursor depleted clinopyroxene. Calculated using data from Tables 2 and 3. Depleted values for Dreiser Weiher are estimated from Mont Briancon value.

Metasomatism in the upper mantle of phlogopite indicates a distinct metasomatic episode also produced by melt infiltration. Recent studies in cryptic metasomatism have also demonstrated the significance of melt infiltration (e.g., Sen et al., 1993). These studies further demonstrate that the formation of enriched clinopyroxene in the Dreiser Weiher samples could have occurred as an event distinct from amphibole, although the compositions of the parental melts would have to be similar in order to produce the apparent equilibrium trace element concentrations observed. Thus, it is evident that melt infiltration is the key to all types of metasomatism and not just the migration of a volatile fluid phase (Sen et al., 1993; Roden and Shimizu, 1993; Brenan and Watson, 1991). The importance of melt infiltration has also been demonstrated for metasomatic oxidation (Dyar et al., 1992; Woodland et al., 1992; Mcguire et al., 1991; Wood et al., 1990; Wood and Virgo, 1989). The proposed metasomatizing agent for Case 1 is a free volatile fluid phase ( Hz0 or CO*), for Case 2 a small volume of volatile enriched melt, and for Case 3 a larger volume of volatile-bearing melt, or repeated episodes of melt infiltration (e.g., Davies 1994). Once all volatiles have been consumed by the formation of amphibole, trace element-enriched clinopyroxene crystallizes. Composition-volume calculations place quantitative constraints on the types of metasomatic activity described. These results indicate that there is not as great a difference between Case 1 and Case 2 amphiboles as might be expected. This suggests that Case 2 amphibole formation is still largely a hydration process, and not strongly influenced by melt migration. The presence of carbonate in the Dreiser Weiher samples raises the possibility of carbonated melts as the metasomatizing agent for this locality. For example, the following Dreiser Weiher characteristics, enrichment in Zr and Ti relative to Hf in amphibole and clinopyroxene, and clinopyroxenes that are high in MgO (Table 1 ), are commonly associated with metasomatism by carbonated alkaline basalts (Dupuy et al., 1992; Brenan and Watson, 1991; Jones, 1989; Meen et al., 1989b; Green and Wallace, 1988). The low halogen content of amphiboles and phlogopites (Tables 3 and 4) rule out any significant contribution of Cl or F complexing. In conclusion, the type of metasomatism observed is a function of the different fluid/rock regimes, volatile contents, and metasomatizing melt compositions that occur in these environments. However, in general it may be stated that modal metasomatism requires a higher melt volatile content, most probably in the form of dissolved HZ0 or carbonate. The data presented here clearly show that oxidation is associated with modal metasomatism, although oxidation may not correlate with incompatible element enrichment. Cryptic metasomatism appears to be decoupled from oxidation state, probably because of the ease with which fo, can be perturbed in rocks lacking sufficient amounts of a phase (spine1 or amphibole) capable of buffering the fo,. Acknowledgments-This research was supported by the National Aeronautics and Space Administration through grants NAG 9-5 1 (to R. N. Clayton) and NAG 9-111 (to A. M. Davis) and by grants to B. J. Wood from the Gas Research Institute (Contract #5087-2601569) and the National Science Foundation (EAR-8804063). Research at the University of Edinburgh was supported by the Natural Environmental Research Council (U.K.). C. Graham is thanked for

1383

access to the Edinburgh instrument. J. Craven and R. Hinton are gratefully acknowledged by K. E. Johnson for their technical assistance. We also thank the following persons for making samples and/ or analyses available to this study: R. J. Arculus (Ichinomegata), S. Sorenson (Eifel and Hawaii), B. J. Wood (Anakie Hills, Massif Central), S. Esperanca (Puerto Necks), and C.-Y. Chen (INAA data). R. N. Clayton, C. I. Chalokwu, and G. McClellan are acknowledged for preliminary discussions and comments. A. D. Brandon, M. F. Roden, and B. J. Wood are thanked for their critical reviews which enhanced the presentation of our story. Editorial handling: J. D. Morris REFERENCES Adam J., Green T. H., and Sie S. H. (1993) Proton microprobe determined partitioning of Rb, Sr, Y, Zr, Nb and Ta between experimentally produced amphiboles and silicate melts with variable F contents. Chem. Geol. 109,29-49. Amundsen H. E. F. and Neumann E.-R. ( 1992) Redox control during mantle/melt interaction. Geochim. Cosmochim. Acta 56, 24052415. Ballhaus C., Berry R. F., and Green D. H. (1990) Oxygen fugacity controls in the Earth’s upper mantle. Nature 349, 437-440. Ballhaus C., Berry R. F., and Green D. H. (1991) High pressure experimental calibration of the olivine-orthopyroxene-spine1 oxygen geobarometer: implications for the oxidation of the upper mantle. Contrib. Mined Petrol. 107, 27-40. Bielski-Zyskind M., Wasserburg G. J., and Nixon P. H. (1984) SmNd and Rb-Sr systematics in volcanic and ultramafic xenoliths from Malaita, Solomon Islands, and the nature of the Ontong Java Plateau. J. Geophys. Rex 89, 2415-2424. Bodinier J.-L., Vasseur G., Verniers J., Dupuy C., and Fabries J. ( 1990) Mechanisms of mantle metasomatism: geochemical evidence in the Lherz erogenic massif. J. Petrol. 31, 597-628. Bonatti E. and Michael P. J. ( 1989) Mantle peridotites from continental rifts to ocean basins to subduction zones. Earth Planet. Sci. Lat. 91, 297-311. Boyd F. R. (1987) High- and low-temperature garnet peridotite xenoliths and their possible relationship to the lithosphere-asthenosphere boundary beneath southern Africa. In Mantle Xenoliths (ed P. H. Nixon), pp. 403-412. Wiley. Brenan J. M. and Watson E. B. ( 199 1) Partitioning of trace elements between carbonate melt and clinopyroxene and olivine at mantle P-T conditions. Geochim. Cosmochim. Acta 55,2203-2214. Bryndzia L. T. and Wood B. J. ( 1990) Oxygen thermobarometry of abyssal spine1 peridotites: The redox state and C-O-H volatile composition of the Earth’s sub-oceanic upper mantle. Amer. J. Sci. 290, 1093-1116. Chen Y. D., Pearson N. J., O’Reilly S. Y., and Griffin W. L. ( 1991) Applications of olivine-orthopyroxene-spine1 oxygen geobarometers ro the redox state of the upper mantle. In J. Petrol. Spec. Lherzolites Issue, pp. 291-306. Oxford Univ. Press. Christie D. M., Carmichael I. S. E., and Langmuir C. H. (1986) Oxidation states of Midocean ridge basalt glasses. Earth Planet. sci. L&t.

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