The geochemistry of carbon in mantle peridotites

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The geochemistry of carbon in mantle peridotites E. A. MATHEZ', V. J. DIETRICH’ and A. J. IRVING’ ‘Department of Geological Sciences, University of Washington, Seattle, WA 98195, U.S.A. ‘Institut fur Kristaiiographie und Petro8raphie. Eidgenossische Technische Hochschuie, CH-8092 Zurich, Switzerland (Received December 6, 1983; accepted in revised/arm June 2 1, 1984) A~~ct~ar~n abundances have been determined in mantle xenoiiths from alkalic basaits and kimberlites and interpreted in terms of the nature and ~st~bution of the C-rich phases. Anhydrous Crdiopside Group I spine1 ihenoiites from basalts typically contain 15-50 ppm C, and amphi~i~~~ng ones have only marginally higher concentrations (40-100 ppm). Carbon abundances in Al-au&e Group 11 pyroxenites are not significantly different from those of the Group I rocks. Although most LREEdepleted iherzoiite xenoiiths contain less C than enriched samples, there is no clear relationship between abundances of C and the incompatible trace elements. In the suite of deformed cumulate peridotite and dunite xenoiiths of the 1801 Kaupulehu flow of the Huaialai volcano, Hawaii, C abundances are clearly related to texture, modal composition, and style of deformation. The most C-rich rocks are wehrlites in which the ciinopyroxenes deformed more brittly and thus possess higher fluid inclusion and crack densities than the surrounding oiivines. Regardless of their iithology, ail xenoliths from kimberlites (including both peridotites and eclogites) are C-rich compared to those from basal& Most of the C in these xenoiiths exists as calcite or carbonaceous matter associated with serpentine veins and was thus probably contributed by the kimberiite host. Primary carbonates are extremely rare in all xenoiiths, although ~asionahy they have been observed as daughter products in fluid inclusions. Although most C exists as inclusions of COrrich vapor, condensed carbonaceous matter also appears to occur in ail rocks as discrete piaty grains and as a film on natural surfaces such as grain boundaries and cracks. INTRODUCI’ION

THE ROLESof the major volatile elements in petrogenesis and the ways in which they affect the bulk physical properties of the mantle are mainly inferred from experimental studies of natural or synthetic systems. For example, experiment has established that H and C exert a profound influence on the temperatures at which model mantle assemblages begin melting and on the compositions of these melts (e.g., WYLLIE and HUANG, 1976; EGGLER, 1978). Hydrogen and C may be particularly important in magma generation if fluids or volatile-rich melts can migrate to and concentrate in appropriate source regions. In fact, the existence of such volatile-rich matter has been hypothesized from geochemical studies of mantle peridotites (e.g., FREY and GREEN, 1974; MENZIES and MURTHY, 1980; FREY, 1983). Physical properties

of the mantle,

including

seismic

anomalously high electrical conductivities, which is proximate with the LVZ in many areas, can be

accounted for by the presence of small quantities of nraahite as an interconnectine _.Dhase on grain bound. . aries. Indeed, it appears that volatile-rich phases are invoked to account for a wide variety of observations. But because their specific roles in the real Earth BOWEN’s ( 1928)

CARBON ABUNDANCES Sample preparation and analytical procedure

velocities and electrical conductivities, may also be influenced by volatile-rich phases. The low velocity zone (LVZ), for example, is generally attributed to the presence of small concentrations of presumably volatile-rich interstitial melt, and recently DUBA and SHANKLAND(1982) have argued that ;he zone of

remain largely matters of speculation,

of volatiles as Maxwell’s demons of petrology remains appropriate today. This investigation concerns the C-rich phases in mantle peridotites brought to the surface as xenoiiths in alkalic basaits and kimberlites. Data are presented on the abundances of C in xenoliths and interpreted in terms of the origin and distribution of C-rich phases. These phases include ill-defined carbonaceous matter (MATHEZ and DELANEY,1981) and carbonates in addition to the ubiquitous inclusions of C-rich fluids (ROEDDER,1965; GREEN and RADCLIWE, 1975; MURCK et al., 1978; BERGMANand DUBESSY, 1984). Petrographic observations permit identification of some of the processes and geochemicai characteristics to which C abundances are related.

characterization

Preparation procedures were designed to minimize the

loss of CO2by the opening of high pressure vapor inclusions in the constituent minerals of the peridotites, to minimize the adsorption of atmospheric components on active surfaces of freshly crushed material, and to avoid contaminants originating from the volcanic or meteoric environments or acquired during the preparation procedure itself. To the extent that it was possible, the analyzed material was taken from the most interior portions of the xenoliths. Samples wereC~SM in agateor St&l to -c1A mm maximum grain size. The mean grain size was maximized by periodic removal of the cl.4 mm fraction as crushing progressed. The crushed material was split into two portions, one of which was washed in distilled water and the other in cold 1 N HCI for 3 minutes to allow distinction between calcite and other forms of C.

1849

1850

E. A. Mathez. V. J. Dietrich and A. J. Irving

Whereas the purpose of the acid-washing procedure was simply to eliminate obvious carbonate contaminants. to some extent it represents a compromise. On the one hand. C con~ntmtions in acid-washed @its of samples known to contain abundant calcite, such as most xenoiiths from kimberiites. are generally higher than those in acid-washed splits of samples known to contain little or no calcite. This. along with analyses of splits for which grain size and strengths of the acid wash were s~ati~iy varied. suggests that only 80-90’S of the &cite component is typically removed. On the other hand, we are cautioned by the fact that the effects of acid-washing on xenoiiths are not completely understood. In fact, studies of such effects have demonstrated #at even weak acids are capable of removing elements such as the alkalis and significantiy modifying Pb and Sr isotonic ratios (ea.. ZINDLERetal., 1983). The effects of acid-washing are d&ssed further below. The adsorption of significant quantities of atmospheric CO2 and CO on active surfaces produced by crushing has been documented by BARKER and TORKELSON(1975) and GRAHAMef al. (1979). The importance of adsorption processes is emphasized by the fact that our I98 1 reanalyses of powders initially produced and analyzed in 1977 (BASALTIC V~L~AN~SMS~~DYPR~JE~T,~9Si)in~~te~n~n~tions that are typically twice the original values, whereas analyses of coame-gtained samples of the same rocks prepared in the manner described here are significantly below concentrations repotted for the original powders (compare UM samples of Table I, this work, and Table 1.2.I I .6, BASALTWVOLCANISM STUDY PROJECT,1981). The adsorption of C-species from the atmosphere is probably time dependent and may occur as a continuous displacement of adsorbed Nz or other atmospheric components. Carbon anaiyses were obtained with a Couiomat C anaiyzer at ETH, Ziirich. Approximately IO mg of sample along with Li-tetraborate flux were fused at i25O’C in oxygen. liberating ail C and CO*. The CO2 content of the oxygen carrier gas was determined by a couiometric-aikaiimetric titration tecitnique, as described by SIXTA (1977). The titration was continuous untiT no further COs was liberated from the charge (usually about 3 minutes). Samples were always observed to be completely molten upon their removal from the furnace. In order to confirm that ail COz was released during the initial fusion, several samples were subjected to multiple fusions. Additional CO2 was not encountered in these subsequent fusions. Errors and sample heterogeneities

At the low concentrations encountered in this study, both anaiyticai precision and the accuracies of analyses are difficult to estimate. Although the total machine and procedure errors, as established empirically from repeated analyses of various materials and Mat&s, is probably 4% relative to absolute concentrations, sample heterogeneities impose much larger practical uncertainties that are both difficult to estimate and to separate from the pure analytical errors. This problem is illustrated in Fig. IA, which shows the distribution of individual analyses around their respective mean values for splits for which muitipie analyses exist. It can be seen that a number of analyses fall beyond the ranges anticipated from simple analytical errors. Furthermore, the distribution itself is not the Gaussian one that normally characterizes such errors, Several samples of material taken from different parts of the same xen&th and prepared independently were akso anaivxed. Thse data are listed in Table I and plotted in Fig. _IB. The C contents of some pairs of sampks are essentially identical. but in other pairs they are significantly different. Note further that in two xenohths (H-353 and P-I) C concentrations in the acid- and water-washed splits of one sample from each xenoiith differ significantly from the respective splits of samples taken from different parts of

20

m-

%Deviation

from mean

FIG. 1.(a) Variation of replicate C analyses of sm@e splits of rock samples and (b) variation of duplicate C analyses of samples taken from different parts of the same xenoiith. Inhomogeneity in C contents exists on the scale of the individual sample size f IO-100 gm) or greater. Two samples for which C contents he outside the scale of this figure are also shown.

the xenofiths (Tabie I). These analyses serve to demonstmte that inhomogeneities exist in C contents on the scale of the sample size (10-100 pm) or @eater. Heterogeneity on this scale and of the order indicated in Fig. I8 has been documented for diamond and graphite in eciogite xenoiiths from various South African kimberiites (ROBINSC~N, 1979). it is clear from petrographic examination that heterogeneity such as that in H-30-3 and P-i cannot generally be accounted for by variations in fluid inclusion abundance. (This is not the case for the Hualaiai rocks. which are discussed below.) Rather, the variations in C contents across megwopic portions of xenoliths probably reflect differences in the amounts of carbonaceous matter on grain boundaries.

Spine/ from

Iber~~Iite

and pyroxenirr

xen~~ith.~

basalts

Abundances of C in water- and acid-washed splits of sampies of petidotite xenoliths from basaits are listed in Table 1, and the data for acid-washed samples are also summarized in Fig. 2, It is evident that the acid-washing procedure reduces C abundances in most samples. Furthermore, the va~abiIity in C contents exhibited by sampies washed in water IS considerably reduced by acid-washing. This is consistent with petrographic observations which suggest that the acid-soluble carbonate component tn most samples is a contaminant originating from the meteoric or volcanic environment and is not “rndigenous” to the xenolith, as discussed below

C in peridbtite TABLE

SAHPLC

Carbon

1.

RDCK

abundances

iDIAL

TYPE

H70

in

xenollths

C (wt. Wash

pprn)

HTI

Hale,

Group

spine1

from

KW77-2

lDt4-2).

Group

KH77-5

tlJ4-4)

KH77-6

rlzno-

and

Alkali?

Island,

Alaska

78

t nnflz

Group

22

1 CwlY

Group

1 orlfl7

Group

lherzolite

Group

I

splnel

lherzolite

20

Group

I

spine1

lherzolite

16

KH77-11

(UH-7)

Croup

I

spine1

lhrrzolite

KH77-15

(IJM-8)

Group

I

sainel

lherzolite

83

24

KH77-16

(UM-9)

lherzolite

15

12

Group

1 soinel

KH?7-26P

Grow

I1

KHBZ-IL

Group

I

KHBZ-IP

Group

11

wine1

22

rlino-

KHEZ-2L

lherzolite

soinel

clino-

(dike

Group

1 spine1

Group

It

in

tno50

66

20

10051

343

50

Group iuwl

f

rlino-


in

c;oup

1 spine1

Group

Group II pyroxenite

Dreiser

Weiher,

West

I

Iherzolite

spine1

32 32

31

18 19

37

23

1 spine1 tabular)

lherzolite

61

40

114854/4

Group

I

lherzolite

53

53

spine1

(coarse

Mt. Quinran, Q76-5 (UM-15) 076-8

Quint

in,

Mt.

I

spine1

Vulran’s

Throne,

Shadnll,

5 wM-IO)

SC7?-5

I

spine1

lherzolite

30

I I I

spine1 spine1 spine1

Iherzollte lherzolite lherzolite

19 67

Deleqste, D69-1

40

21

69-27

amphibole-bearinq

66

56

Salt Lake 114989/T

Arizona I

90-47

Crater, (UN-12)

Red Hill, RH7?-1

If

epxnel

websterite

Group

lherzolite

Arizdna Group

I

equant

lherzolite

splnel spine1

I

29 37 20

139

22 37

19 41

lherzolite

31

26

Australia 56

Mew South

Crater,

(UM-tf)

clssts

of Group I spine1 lherzolite) Grwp 11 spinel-olivlne clinopyroxenite

136

L7

Group

103

95

88

84 61

11

websterite

Wales, Australia Garnet clinowroxenitr

720

webste;ite

Hawaii Garnet

rlinopyroxenit

Croup

I

spine1

5%

e

lherzoltte

I

spine1

24

lherzolite

284

26

203 43

Hawail

HL 78-3

Wehrlite

Hl_78-4

otnite

HL7B-5

Olivinr Durite

websterite

160 172 44 70

Olivine Wehrlite

websterite

97

uttrs 5 11435B/l 118358/5

14

52

clinopytoxenite Group II spinel-olivine

30

32

114370117

olivine clino(rontaininq

120”

29

46

II

86

604

37 20

spinel-olivine

114386/8

35

52

fran

50 68 83

II3 123 79

Websterite

48

Hsrzburqi Dunite

46 84 29

47 74

34

te

114369/15

Dwite

114743/7 1 ?A97611

Wehrlite-dwite

compwite

Dwite-olivine

mhsterite

composite Xenoliths

>64**

Websterite

10%

qlee3)

Mine,

50**

108

Australia

Group I spine1 clinopyroxenite (rontsinino

Hualalai,

spine1

Group II pyroxenite

Victor

478

47

websterite

Roberts 110589

483

47

lherzollte lherzolite

1 spine1

Virtorla,

11435e/1o 114370/12 Group

AH77-3

95 470”

37

rlino-

pyroxen\te Croup I spine1

Arizona

AH77-2

145 497

Australia

Garnet

P-l

harzburaite

Group 11 splnel-olivine rlinopyroxenite

Cochise

ic

spinel

I I

Group

5072-4 Group

Group

x73-1 x77-1

36 72

tabular)

Iherzol,tp

Arizona

x73-12

30 122

tabular) spine1

(coarse

Owensland.

so72-3

Group Group Group

Group

farlos,

(coarse

I

Group Group

25

25

tabular)

spinet San

lherzolite

Mexico

VT73-1

amphibole

spinet

Group II spinet-olivlne clinopyroxenite

Lake Farharn. LE76-4

orsnular)

(mediun San

I

Queensland,

Germany Group

BSQ-9 BSQ-34 RSQ-35 SQ-7 wt-14)

lherzollte

amphibole

1 soinel

Group

so7z-1P West

spine1

Gerneny

Group (fine

114072/57

27

Arabia

R76-9

114854/3

Meerenfelder,

34

1

I

(qrsnuloblast Saudi

41

lherzolite

::

82

clinoin X)

Grouo

Field,

42

t abulaf)

Group

Kishb H-30-3

67 81

lherzolite

spine1 (dike

91 147

2L)

Group 11 spine1 rlinopyroxenitc (dike in 3L) Group I spine1 webstetite

KHEZ-5P

I

Group

10070

lherzolite

spine1

KH82-3P KHEZ-4

equsnt

Iherzolite

I!_)

KH82-3L

KH82-5%

spine1

(rosrse

64

87 46

equenif

t abuler) I

Iherzolite

ep,nel

ovroxenite

spine1

(coarse 10008

spine1 lherzotite

froarse I

(Coarse

pyroxen i t e

KHEZ-2P

1 amphibole

lherrolite

1 spine1

107

IYK

Rasalts

it e

pyroxenite

kimberlites

RUCK

NUIIvak

II

pyroxen

basalts

SAHPLC

New Hexico

KH?6-1P

nlkallr

Wash

Xenollths Kilbourne

from

1851

117’” 122 41 54”

68”

I8

40 25”’

21 66 36

29 34 43”

32

33

Kimberlites Afrjra Phloqopite

989

766

Kimberley 114945/13

194

89

110592 t 1059f

Phloqopite Phloqapite

430 697

I?(1598

Garnet

201 258 139**

11*945114 fltV45/21

lherzolite Garnet Iherzolite PhloqoQite-qarnrt

252 352

118 132

110610

Phloqopite

114945/22

lherzolite Phloqopite

367

210

113020/2 11312w3

Ecloqite Kysnite

525

229

lherzotite eclogite lherzolite ecloqite eclooite

346 332 1096 91 239

130 117 35 89

Jeqersfontein, fR8

Mstsoku, l.Bn-9 MM-1

South

24712

Africa Garnet

qernet

lherzolite

lherzoli

te

lteotho

ftm-171 1 (UH-19)

*l&t “tiers +%uplicate

,

South

South Afrira Phlogopite therzoltte

Garnet Garnet fran basaltic analyses are

99 164

lherzolite lherzolite Volcanism of different

Study Project samples of

(1981) the cane

xenolith.

With regard to the acid-washed splits, it can be seen that anhydrous Group I spine1 Ihenolites typitally contain 15-50 ppm C (60-l 80 ppm CO*) (Fig.

These

are

the

pairs

of

values

plotted

1n Fig.

1B.

2). The few amphibole-bearing lherzoiites included in this study have slightly higher C contents (40-95 ppm). Such concentrations are considerably lower

E. A. Mathez, V. J. Dietrich and A. J. Irving

1852

00

a*

120

rio

me&

Total Carbon (ppm) FIG. 2. Total C abundances for acid-washed ultramafic xenoliths from basalts. Datum for sample 10070 not shown (see text).

than previously published analyses would suggest to be typical of mantle rocks (c.$, HOEFS, 1965; BASALTIC VOLCANISMSTUDY PROJECT, I98 1). As pointed out above, at least some of these earlier rn~~rnen~ must have included an adsorbed atmospheric component as well as the calcite and the acid-insoluble C fractions. It is also evident from Fig. 2 that the average C contents of Group II Al-augite pyroxenites is slightly higher than the average for Group I rocks. especially when amphibole-bearing varieties are excluded from the latter group. It is not clear that this difference is either significant or statistically meaningful. however. The samples listed in Table I include several lherzolite-pyroxenite pairs which occurred together as composite xenoliths at Kiibourne Hole. Only for one of these pairs is the pyroxenite significantly more C-rich than the coexisting lherzolite, and this particular sample (KH-82- 1P) is clearly con~minat~ with substantial amounts of meteoric or volcanic calcite. Therefore, C abundances are not related to the degree of chemical evolution of mantle peridotites, at least in terms of their modal or major element compositions. One of the lhenoiites, namely sample 10070 from Nunivak Island, Alaska, is noteworthy because it contains so much more C (-500 ppm) than any of the other sixty-five xenoliths fmm basalts in this study, including six others from Nunivak Island (Table 1f. As reported by RODEN rrai.(1984). 10070 is one of several Nunivak xenoiiths characterized by low incompatible trace element contents, relatively flat chondrite-normalized REE patterns, and La/Yb ratios less than those of chondrites. It is thus similar

to LIL eiementdepIet~ xenoiiths from Kiiboume Hole (JAGOUTZ et al., 1979: IRVING, 1980), Dreiser Weiher (STOSCH and SECK, 1980) and other localities. some of which are also included in this study. Although fluid inclusions are no more abundant tn 10070 than in most C-poor lherzoiites. the rock contains numerous, piaty, opaque grains In the size range lo- 100 pm on grain boundaries md crack surfaces. This material is probably carbonaceous in view of the C data. We can offer no explanation for the anomalous C content of this rock. The unusual Nunivak sample aside, concentrations of C in mantle xenoliths in basalts are signiticantl! lower than estimates of the C content of the bulk Earth (350 ppm according to the chondrittc Earth model of GANAPATHY and ANDERS, i 974: The Group I peridotites are generally viewed as residues from which partial melts have been extracted and the Group II rocks as magmatic accumulates (e.g. IRVING. 1980). The low C contents of both rypes of xenoliths may result because C behaved as an incom _ patible element during the partial melting and crystallization processes. Alternatively, C may have been depleted in large regions of the upper mantle during an earlier differentiation episode. The second alternative is consistent with recent studies of rare gases suggesting that certain parts of the upper mantle degassed within the first several hundred million years of Earth history (HART t’l ~1.. 1983: ~L&RF et al.. 1983). Yenoliths _fiom Hualalai

The xenoliths from the 1801 Kaupulehu flow oi’ the Hualalai volcano are of particular interest because they are known to contain many more fluid inclusions than xenoiiths from most other localities (ROEDDER. 1965). Indeed, except for xenoiiths of kimberlites, some of those from H&alai are the most C-rich rocks included in this study (Fig. 21. The Hualalai xenoliths are also distinct petrologically and chemically from the common Groups I and II nodules. Their spinels are much more Cr-rich, relict cumulate textures are evident in some samples, and the suite includes gabbroic rocks {JACKSON,1968; JACKSO;Ud al..198 1). They are believed to represent an upper mantle or deep crustai cumulate sequence, and the similarity of some of their rare gas isotopic compositions to those of MORB has led to the suggestion that they accumulated beneath spreading centers (KYSER and RISON, 1982). Carbon abundances in the Hualaiai suite arc clearly related to rock texture, modal composition and style of deformation. The highest C concentrations are found in the wehrlites. These rocks exhibit a distinctive porphyroclastic texture, with porphyroclasts of clinopyroxene in a sub-porphyrociastic matrix of “primary” deformed oiivines surrounded by smalter “sec.. ondary” recrystallized oiivines (see KIRBY and C;REEF‘~, 1980). In the case of oiivine, deformation results

C in peridotite

mainly in recrystallization, in the development of planar extinction discontinuities, and in the formation of subgrains (KIRBY and GREEN, 1980). The pyroxenes contain many more microfractures and fluid inclusion arrays (representing annealed microfractures) and therefore behaved more brittly during deformation than the surrounding olivines (Fig. 3). In rocks that are equigranular, such as the dunites, brittle deformation of individual pyroxene grains was seldom so intense as it was for clinopyroxene porphyroclasts

A

1853

in the wehrlites. Therefore, because the inclusions and microfractures contain C-rich phases, the variation in bulk-rock C abundances among members of the Hualalai suite essentially reflects modal amounts of clinopyroxene and homogeneity of grain size. These observations imply that the wehrlites became enriched in C relative to other Hualalai peridotites during or after their deformation. Xenoliths from kimberlites All peridotite and eclogite nodules recovered from kimberlites have extremely high C contents (Table 1). Taken literally, these data would suggest that those deeper portions of the mantle through which kimberlites migrate are more C-rich than the shallower regions sampled by most basaltic magmas. Indeed, “primary” graphite in addition to diamond are well known in both eclogite and peridotite xenoliths in kimberlites (e.g., LAPPIN and DAWSON, 1975; ROBINSON, 1979). However, some of the C in these xenoliths also exists as calcite and carbonaceous matter (PASTERIS, 198 1) associated with typically pervasive serpentine veinlets. The serpentinizing fluids almost certainly originated from the host kimberlites themselves in view of the fact that carbonates and serpentines are common “primary” phases in many of them. Although our preparation procedures probably allow crude distinction between calcite and other forms of C, it is not possible to distinguish introduced C that is now in elemental form from “indigenous” carbonaceous matter. Therefore, the concentrations of C in xenoliths from kimberlites are probably not representative of mantle abundances. In the samples we have examined, most veinlets in olivines consist of calcite and chrysotile. In some samples, both magnetite and carbonaceous matter are also present. An assemblage consisting of these four phases together with the absence of Mg- or Ferich carbonates restricts serpentinizing fluids to extremely HrO-rich compositions under any reasonable set of P-T conditions (e.g., see TROMMSDORFFand EVANS, 1977). CHARACTERIZATION OF CARBON-RICH PHASES

B FIG. 3. Transmitted light photomicmgraphs (crossed nicols) of (a) polycrystalline olivine and (b) part of a clinopyroxene porphyroclast from a Hualalai wehrlite showing the differences in fluid inclusion and crack densities between the two minerals. Note that the scales of the photomicrographs are different. In addition, the thin section in which the olivine exists is approximately 75 pm thick, whereas that in which the pyroxene exists is 25 pm thick. The differences in both scale and thickness give the appearance that the difference in density of brittle deformation features in the two minerals is less than it actually is. Extinction discontinuities separate regions of slightly different orientations in the deformed and partially recrystallized olivine. The grains along the edge of the clinopyroxene are recrystallized olivines generally devoid of fluid inclusions, kink bands, or subgrains.

Carbonates For nearly all xenoliths for which the acid-washing procedure reduces C abundances significantly, textural evidence clearly indicates an external source for the carbonate. For example, in rocks from Kilbourne

Hole, which is in a desert environment where caliche is observed, calcite-filled microcracks are common. Despite our attempts to avoid it, some of the carbonate was probably included in our analyzed samples. Similarly, xenoliths (as well as their host breccias) from Salt Lake Crater contain large amounts of carbonate @‘NEIL et al., 1970), most of which occurs in veinlets. An example is shown in Fig. 4a, where the veinlet is occupied by Na-free calcite in contact

1854

t. A. Mathez. V. J. D~etnch and A. J. Irvmg

A

B FIG. 4. Nephelinitic glass (dark) of host basalt ~rn~sitio~ in contact with calcite (white) in a veinlet in spine1 Iherzolite UM-13 (Salt Lake Crater). The fact that the calcite is Nafree suggests that it is later than the Na-rich glass rather than having formed contemporaneously with it. (bf Magnesite daughter crystal on the wall of a bubble within olivine from a Canary Islands spine1 Iherzolite (ROVETTAand MATHEZ, 1982). Such crystals presumably result from reaction of CO,-rich fluid and olivine during cooling of the xenolith. The length of the bar in the bottom of the photo is I pm.

with nephelinitic glass of host basalt composition. The jux~~sition of M-rich glass and Na-free calcite implies that the fatter is not a magmatic product but may have formed simply as a result of hydrothermal activity in a cooling lava (e.g.. see KOS-I-ERVON GROOS, 1975). This is consistent with isotopic data of O’NEIL ef a!. (1970), who showed that the Salt Lake Crater rocks contain large meteoric components of C and 0. Given the highly friable nature of most of these and other xenoliths, it is difficult to imagine how carbonate-bearing meteoric fluids would not penetrate along many grain boundaries and microfractures, regardless of their size. Furthermore, the volcanic en~~ironment is C-rich simply bccatise most juvenile

magmatic gases contain high concentrations uf CC>: (e.g., CERLACH, 198&&b). It might be anticipated, therefore, that volcanic gases might penetrate along cracks as they form and that CO? trapped in lava during initial eruption or C-rich phases retained b> other means would be subject to r~ist~b~tjon b> post-eruption processes. Thus, an immediate source of C should always exist, regardless of focal meteoric conditions. Truly “indigenous” carbonate. which appears IF consist only of dolomite and magnesite, is present in some xenoiiths. Its “indigenous” nature is based on habit. It includes dolomite in ~iymine~lic inclusions in mantle minerals in xenoliths from kimberlites {~~~~c~l~ and BESANCON,1973; HERVG and SMITH, I98 1; HUNTER and SMITH, 1QSI ). In addition. magnesite has been identified as a daughter mineral rn fluid inclusions in a spine1 lhertolitc from the Canary Islands (ROVETTA and MATWEZ, 1982) (Fig. 4b). Carbonates in these habits are extremely rare, however, and in n? samples that we have studied do they constitute more than a trivial fraction of the C-rich phases. In the above discussion, it is assumed that the leachable C fraction is calcite. However, in only a few nodules has carbonate actually been observed. Its presence in others is based on the assumption that the acid-washing procedures have no effect on other forms of condensed C. This is supported b: the fact that acid-washing does not influence Cm abundances in some of the most C-rich rocks. such as those from Huaialai, which contain abundant fluid inclusions as well as carbonaceous matter. In summary. the ease with which it is removed by acidivashing suggests that the leachable C fraction is mainly calcite, and where it has been observed. calcite has clearly been introduced from sources external to the xenoliths.

The existence of carbonaceous matter in xenoliths from alkalic basalts was reported by MATHEZ and DELANEY ( 198 I), and since then its presence has been confirmed in each of the 1I samples from X localities examined by the microprobe techniques described in that work. What is presumed to be carbonaceous matter is observable optically ~‘1nearly all xenoliths. This material occurs along grain boundaries and microcracks and on the walls of secondary fluid inclusions. Some of the micron- to submicronsize primary inclusions reported from several xrnoliths {GREEN and GUEGUEN, 19831 are also believed to contain CO2 and carbonaceous matter (H. W. GREEN. pers. commun., 1983). Not all secondary fluid inclusions contain condensed carbons, however, and although their distribution has not been examined cursory observation suggests that systematically, C-bearing and C-free inclusions do not O*‘ZUTr:in~~ tiomi~ but comprise distinct arrays,

C in peridothe

Surfaces such as cracks, grain boundaries, and cleavages which are exposed by fragmentation of the rock are typically coated with a C film. This is apparent from Fig. 5, in which a C-bearing natural crack surface is exposed and intersects the C-free polished surface. WENGELER et al. ( 1982) and OBERHEUSER et al. (1983) reported that C-rich surface layers develop on synthetic periclase and natural olivine crystals and proposed that their formation results from extremely rapid diffusion of C within the crystal structures. The surfaces they studied were produced by polishing and thus were initially C-free. The contrast in C concentrations between the polished and crack surfaces exposed in Fig. 5 indicates that

the material on the latter is actually a separate phase (i.e., a film) and not a surface layer of the silicate (in this case olivine) rich in dissolved C formed by adsorption or intercrystalline diffusion. As pointed out by DUBA and SHANKLAND ( 1982), if C exists as an interconnected grain boundary film in the mantle, then it should influence electrical conductivity. Many of the surfaces on which carbon films now exist may have formed during quenching and/or decompression of the xenoliths. However, on preexisting surfaces, carbonaceous matter might have formed by condensation from a vapor under mantle conditions that in principle should be readily predictable thermodynamically. The existence of a grainboundary C-rich phase in the upper mantle is consistent with some estimates of mantle oxidation states that place at least part of it in the field of graphite stability (e.g., RYABCHIKOV et al., 1981; ARCULUS and DELANO, 1981; EGGLER, 1983).

A

B FIG. 5. (a) A secondary electron photomicrograph of an exposed fracture in an ohvine of spine1 lherzolite from the Kishb field, Saudi Arabia. The lenath of the bar in the bottom of the photo is 100 pm. (The-rectangular shadow is C deposited by cracking of hydrocarbonsfrom the microprobe atmosphere by the electron beam as it was rastered over the Al-coated sample surface.) (b) Beam scanning photomicrograph for C X-rays over a portion of (a). The spot density

is proportional to C concentration. Note that C exists as a distinct film on the crack surface. Note also that the regions of high C contents correspond to distinct grains visible in (a). These grains appear opaque in transmitted light and are partially chlorinated.

1855

In addition,

extremely rapid diffusion of C along surfaces is a process known to occur under certain conditions and may happen in rocks at high temperatures and pressures as well. Unfortunately, observable petrographic relationships do not provide a great deal of insight into either how or when these carbon films develop. Discrete platy lumps of carbonaceous matter are also common on grain boundaries and cracks. Several such grains are visible in Fig. 5. These may be what KURAT et al. (1980) referred to as the “black dots” on oiivine in some of the Kapfenstein xenoliths and to which they attributed anomalous geochemical characteristics. In SEM photomicrographs, the lumps typically exhibit non-crystalline morphological characteristics consistent with their having amorphous structures on the scale of 103-IO4 A or less. Our preliminary electron microprobe studies indicate that many are partially chlorinated and have associated with them anomalously high concentrations of other elements, notably S, K, and Ca. Chlorinated carbons have been found in xenoliths from Hualalai, Kilboume Hole, and Massif Central and are probably ubiquitous in other suites as well. The presence of carbons in chlorinated as well as elemental forms is consistent with the previously reported observation that subtle differences exist in the shapes of the characteristic C X-ray peaks generated from graphite and carbonaceous matter in peridotites (see Fig. 7 of MATHEZ and DELANEY, 198 1). The precipitation of graphite from CO*-rich vapor is accompanied by relative enrichment of the latter phase in “C (BOTTINGA, 1969). Consequently, if the carbonaceous matter in the mantle rocks condensed from a vapor with an initial isotopic composition similar to that of the mantle (6°C = -5 to -9k, as estimated from type I diamonds [DEINES, 1980]), then the 6°C of the condensed phase should be lower (and that of the coexisting vapor should be. higher) than the mantle value. In fact, such a relationship has been observed by WATANABE et al. (1983) for C phases in a Hualalai dunite. Using selective separation

E. A, Mathez. V. J. Dietnch and ‘4..I.irving

1856

techniques, they measured 613C values of -3.2% and -20% for the vapor and condensed C fractions,

respectively. Their data imply that most, if not all. of the carbonaceous matter and vapor presently in the rocks originated from a common parental mantle fluid. TRACE ELEMENT ABUNDANCES AND C-RICH FLUIDS It has been suggested that the lithophile trace element abundances in mantle rocks are determined by a distinct component (component B of FREY and GREEN, 1974) which is carried by a mobile, volatiferich fluid or melt phase. Several lines of evidence have suggested that this phase may be a COJ-rich fluid. First, STOXH (1982) found that among olivines separated from single spine1 lherzolite xenoliths, caystals with numerous CO1-rich fluid inclusions possess 20-40 times the bulk light REE abundances compared with those lacking visible inclusions. Second. there is indirect experimental evidence that CO&ch lluids can dissolve substantial amounts of REE at mantle pressures and that Iight REE are more soluble than heavy REE (WEN~~~~T and HARRISON, 1979). Therefore, we have examined the correlation between bulk C and REE abundances for samples for which both types of data exist. The trace element data include those published in the Basaltic Volcanism Study Project ( 198 1) and new INAA analyses obtained by P, Salpas at Washington University. Before discussing these data, it is necessary first to address the question of the effects of acid-washing on major and trace element abundances. During the preparation of samples for C analysis, it was noticed that samples washed in I N HCI became distinctly yeIIow compared with those washed in water. suggesting that components other than calcite were being affected by the acid-washing treatment. In fact, analyses of the leachates by ICP spectrometry show that significant amounts of K, Na. Sr. Ca, Mg. Fe, and Ni are removed by acid-washing. Furthermore, the acid-washed splits are systematically depleted in La compared with those washed in Hz0 (Table 2). These results confim those of previous studies on the effects of acid-washing on ultramafic rocks (e.g.. ZARTMAN and TERA, 1973; BASU and MURTHY, 1977: EHRENBERG, 1982; ZINDLER et al.. 1983), which demonstrated that even weak acids can remove K, Rb, Ba. La, Ca, Al, and the elements which enter olivine. and can change Sr and Pb isotopic compositions. The specific sites occupied by the acid-soluble trace elements are not well known. For example, the relative decrease in La from its original concentration caused by acid-washing bears no obvious relationship to the observed decrease in C. This implies that the leachable La and C fractions are not associated together in the same phase. Based on analyses of vein calcites in xenoliths from minettes of the Colorado Plateau. EHRENBERGf 1982) also concluded that cap

Table 2.

Comparison of La ,?ontentx l? h31:t"' and acid-washed samples i++.; rourtesy of P. saipas\ ---_l-____.-._. ---._I-_. .__ I 1.P C'ONCENTRATL:'ih:p~a:

.--~YL-78-3 (Hualalall :14358/l (Hualalai) Ai4389/1S !Hualalai) T;LTRA 5 (Hualalai) !14854/3 (Dreiser Weiher: 114854/4 (Dreiser Weiher! ‘,'T-83-1 iVulcan's Thronei FRB247/2 iJagersfontein) LBM-9 IMatsokui LBM-11 (Matsoku)

watel.,"i_ washed was:;ci! ..I _l__l_-_-_l n,64 I$ : :,i *;.8 3 ,..!!H4 .-: “‘1Sh ': I.6 Z.38 l.50 . 3’: : .:.I& ".C .. t:, 7.44 ?.05* :.17* ‘;:

*Basaltic

Prolfct

SAMPLE

__._. ._..__-._

Volcanism

Stildy

bonate contaminants cannot account for the labile La component. Our leachate analyses demonstrate that dissolution of small amounts of olivine occurs during acid-washing, but this cannot explain observed decreases in either La or elements such as Al, Ca, Ba, and alkalis. Although dissolution of clinopymxene could contribute to this decrease, we surmise that elinopyroxene is not affected signi~cantly by acidwashing because both clinopyroxene-~ch and clinopyroxene-poor xenoliths show similar decreases in bulk trace element abundances. it would appear that the labile trace elements exist elsewhere; possibly they are concentrated on the surfaces of major minerals. are associated with acid-insoluble carbonaceous matter on grain boundaries, or reside in interdial glass. Such glasses are ubiquitous in our samples and are likely to be relatively soluble in weak acids. 111 addition, the glasses tend to be dispersed along grain boundaries and thus have relatively large surface areas. Although little is known about the trace elemenr characteristics of glasses in spine1 lherzolite xenoliths. they are relatively evolved in terms of their major element compositions and U contents (KLEEMAN CY lil., 1969; HAINES and ZARTMAN, 1973; FREY and GREEN. 1974; IRVING and MATHEZ, 1982) and are probably La-rich as well (F. A, FREY, pers. commun.. 1984). A comparison of C concentrations in acid-washed splits with chondrite-normalized La/Yb ratios ([La/ Yb1c.n.) of H&)-washed splits for 20 spine] lherzolites is presented in Fig. 6. (A similar plot using acidwashed splits for the trace element data would not be significantly different from this), Several points are apparent: (a) With the exception of the C-rich Nunivak xenolith (sample 10070), those samples which arc: most depleted in light REE (from Kilbournr Hole, Cochise Crater and San Quintin) also possess the lowest bulk C contents (12-24 ppm). (b) For other samples, [La/Yb]c.n. is not related In any simple way to bulk C content. This is cuntrar! to our previous assertion, which was based on a more limited set of data (MATHEI q’i iii, !WZ Thm.

C in peridot&e

ill

17.0

SPINEL LHERZOLITI

a

80 7

.

m

CONCLUSlONS

.

r

l

ANHYDRI

@fHYDROU

9

‘4

4 4 l--J 3

. 2

.

1

.

. l

1

s* 0

20

fluid that migrated through a specific volume of rock and the surface area exposed to such fluid than to the amounts of carbonaceous matter or CO2 that remained in that volume. Furthermore, as has been demonstrated for the Hualalai rocks, C abundances may be strongly influenced by deformation events that may or may not be related to metasomatic ones.

.

8

“5

1857

40

60

80

IO<

Total C, ppm FIG. 6. Variation of chondrite-normalized La/Yb ratio with C abundance for spine1 lherzolite xenoliths from basalts.

samples with C contents of about 40 ppm have [La/ Yb)c.n. ranging from 1.7 through 6.5 to 17.6. Other studies have established that bulk I.a/Yb in spine1 lherzolite xenoliths is inversely correlated with bulk CaO content (e.g., FREY and GREEN, 1974; I3VSP, I98 1: FREY, 1983). Indeed, three of the newlyanalyzed samples (from Vulcan’s Throne and Dreiser Weiher) with relatively high fLa/Yb)c.n. (6.2-7.4) have notably low CaO contents (0.6-I .6 wt.‘%). If some incompatible trace elements are transported by CO*-rich fluids in the mantle, there is little clear evidence of the operation of such a metasomatic process from present bulk C contents of mantle xenoliths. The C-rich but LIL element-depleted Nunivak sample 10070 demonstrates clearly that the abundances of C in mantle rocks are not necessarily controlfed by the same factors which influence incompatible trace element abundances. Neither, for that matter, are the relative enrichments of these incompatible elements related to bulk H or F contents of peridotites as indicated by the presence of amphiboles. It should be remembered, however, that the acid-washing experiments clearly relate high concentrations of some of the labile elements to grain boundaries and other natural surfaces that are presumably accessible to permeating fluid. The possibility exists, therefore, that abundances of these elements in xenoliths may be related more to the amount of

It is evident that a number of complex processes influence the abundance and dist~bution of C in mantIe rocks. In the case of the generally C-rich Hualalai xenoliths, for example, abundances and distribution are influenced by mineralogy and style of deformation. In addition, unknown quantities of C and other elements are probably introduced into xenoliths during their residence in magmas or by meteoric or volcanic fluids that permeated through them at the Earth’s surface. Indeed, carbonates appear to be common contaminants in nearly all xenoliths, especially in those recovered from carbonate-rich kimberlite hosts. The present bulk C contents of spinei lherzolite xenoiiths are not clearly related to their degree of light REE enrichment. Therefore, there is no specific geochemical evidence that C-dominated fluids act as the major carriers of incompatible trace elements in the mantle. However, neither do our data prohibit such a possibility. Mantle rocks that are evolved in terms of modal mineralogy and major element compositions are not significantly enriched in C relative to the more refractory sampIes. Ahhough based on a limited number of samples, it appears that amphibolebearing Iherzolites contain, on average, slightly more C than amphibole-free ones. Condensed carbonaceous matter appears to be ubiquitous in xenoiiths. It is observed as discrete platy lumps and as a film on most natural surfaces. It is not obvious from petrographic observations whether such films occupied surfaces under mantle conditions or formed during subsequent decompression or cooling. Some of the carbonaceous matter is partially chlorinated and is probably geochemically anomalous for other elements as well. ~ckno~,led~~en~s-We wish to thank P. Salpas for generous& providing the INAA analyses and R. J. Arculus, P. Deines, F. A. Frey, and F. Freund for helpful reviews. Many of the samples utilized in this study were provided by the Smithsonian Institution. We are indebted to W. G. Melson, N. H. Banks, Jr., and other members of the Smithsonian staff for their help in acquiring them. We also thank M. F. Roden, F. R. Boyd. R. G. Coleman. and D. M. Johnson for their generous contributions of additional samples. The support of National Science Foundation through grant OCE8 I IOI49 is gratefully acknowledged.

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