High Ni in Archean tholeiites

Tectonophwm,

411

187 (1991) 411-419

Elsevier Science Publishers

B.V., Amsterdam

High Ni in Archean Nicholas Max-Planck-Institut (Received

tholeiites

T. Arndt

ftir Chemte, Postfach 3060. D-6500 Matn:, FRG *

June 1, 1989; revised version accepted

July 5. 1989)

ABSTRACT Amdt. N.T.. 1991. High Ni in Archean 411-419.

tholeiltes.

In: J.L. Le Mou?l (Editor).

Beyond

Plate Tectonics.

Tectonoph,vsm.

187:

Archean tholeiites generally have h&her Ni, Co. Cr and Fe than most younger tholentes with similar MgO contents. These characteristics cannot be attributed to high T or P batch melting in the Archean mantle. because, although such melts are enriched in siderophile elements, they have higher MgO than normal tholeiites. As primary melts fractionate to lower MgO. they lose Ni, Co and Cr. Nor can the differences between Archean and younger tholeiites be attributed to secular variation in mantle compositions because Archean komatiites have Ni. Co, Cr contents similar to modern (Gorgona) komatiites. It is auggesied that the high siderophile element content of Archean tholeiltes results from mixing of either komatiitic with basaltic magmas. as might occur in an ascending, melting mantle plume or column, or of komatilte and more evolved rocks. as may take place when komatiite encounters and assimilates crustal rocks

Introduction Archean tholeiitic basalts are reported to have higher Ni, Co, Cr and Fe contents than younger tholeiites. Glickson (1971) observed that Ni contents in tholeiitic basalts in the Archean Yilgarn Block of Australia are higher than in modern oceanic basalts. and similar claims have been made by Gill and Bridgwater (1976) for the Ameralik dykes in Greenland; by Jolly (1975) for basalts in the Abitibi belt in Canada; by Jahn et al. (1980) for Finnish volcanics; and by Nesbitt and Sun Fig.

(1976), Gill (1979) and Condie (1984) in broader surveys. There is also general agreement that the Cr, Co and probably Fe contents of Archean volcanics are higher than in their younger equivalents (e.g. Glickson, 1971; Condie, 1984). In this paper I reconsider the differences in siderophile elements abundances in Archean and modern basalts, review some of the theories that have been

1. Histogram

elements

from Condie.

island

continental

propose

basalts

concentrations

basalts;

flood basalta:

to account

= mid-ocean

RI/? = rift

for these

a new explanation

address:

35042 Rennes

0040.1951/91/$03.50

lnstltut

de Geologie,

UniversltC

Rennes

Elsevier Science Publishers

ridge basalt;

basalts;

CFB =

differences,

and

for the enrichment. in Archean rocks:

Figure 1 illustrates some data that have been used to support the concept that Archean basalts

1,

Cedex, France.

0; 1991

basalts (data

IAB = island arc basalts.

Siderophile element enrichment the evidence * Present

of siderophlle

with those in modem

1984, table 1). MORB

OIB = oceanic

proposed

comparing

in Archean

B.V.

NT

412

have higher concentrations than

modern

has&s.

of siderophile

Shown

the average Ni, Co, Cr. and Fe contents normalized Condie

the

show conspicuous

from

five different

illustrated tents

compared

a similar

of Archean

enrichment

with modern

in

ferences

between

The modern

regimes.

Gill (1979)

obvious.

between

the Ni con-

Ni contents

ridge basalts (MORB)

Ascension

mid-ocean

modern

come

basafts

with relatively

oceanic (Weaver

from

(OlB)-White Naldretr Arndr Archean

and Vetter (1984):

et al. (1979). Feigenson

and Turner and Jenner

(1977). Arndt

(I?%),

age and Corgona

2a. but

plotted

at :t smaller

agram shows that komatiites are no

arc basalts

(lABI--Thorpe

et al. (1983). Weaver et al. (1987); Archean (unpublished

Arndt

and Nesbitt

data):

(;orgona data are from Briigmann

(1982).

(1982). Barnes (1985). Smith and Erlank et al. (1987):

scale

so that

Archean

Morris

(1982). Arndt

picrite

data

in

et al. (1984). tux

and

et al. (1982). Rautenschlein

et al. (1983): oceanic

hasalts-Hallberg

data from Neshitt (1986). Arndt

from

Weaver et al. (1987).

BVSP (1981).

the

komatiIsland, The di-

with concentrations

(1972). Nesbltt

(1982), Jahn Ed al. (1980. 1982). Redman

data). (b) MgU versus Ni plot &wing picritrs.

basalt

the Ni contents of Archean higher than in the Gorgona

et al. (1977). Sun et al. (1979). Schilling

and some modern

flood

compositions of Archenn and “modern” ites (the Tertiary Iavas from Gorgona Echeverria, 19X0) can also be plotted,

and Nesbitt {19X2). Smith and Erlank

and Arndt Island

island

(MORB)-Langmuir

et al., 1987). the

continentai

Fig. 2. iaf Xi contents uf Archean &oieiitts icirclesj plotted as a function of MgU content, and compared modern basal& Sources of data include: ail suites-BVSP (1981); continental flood hasaits (fFB)-Duncan ridge basalts

high

such as

sarily to the composition of the Archean mantle. Figure 2b shows data similar to those in Fig.

island basalts (OIB) and to a lesser extent, continental flood hasalts (CFB). These characteristics are in general accord with the observations of Gill (1979) and Condie (1954), in that when the predominant basalt types in each age group are com-

(1985). Fodor

islands

less

tents of Archean basalts might owe their origin to peculiarities of the environment of melting beneath Archean greenstone belts. and not neces-

generally higher than in modern MORB and IAB. but overlaps concentrations in modern oceanic

mid-ocean

basalis

sequences such as the Karoo (Duncan et al., 1984). These observations suggest that the high Ni con-

plotted against MgO content, and compared with Ni abundances in modern basalts. It can be seen that at any given MgO content, Ni contents of Archean basalts vary widely. The total range is

Hawkesworth

not considered

et al., 1979) and Hawaii (Feigensen

et al., 1983) and

in another greenstone

belts (data sources are given in the figure caption),

(1985);

Archean in Ni con-

in Fig. 2 are due

and Archean

from

and Gough

Azores (White

and island arc basalts (IAB).

Figure 2a shows the Ni abundances set of tholeiitic basalts from Archean

with

from sources

basalts

tectonic

and modern

IAB

a differertce

by Gill (1979) and Condie (1984) have been taken into account. which make the systematic dif-

contrast basalts

and

The differences

to the fact that data

from

hasalts

MORB

belt tholeiites).

tent is apparent.

of basahs.

of 60, taken

all four elements

(modern

greenstone

are

(1984, table 2). In this representation,

Archean

to a Mg number

pared

elements

in the diagram

AKNDT

compositions and Jenner Clarke

and Sun (1976),

and Keays {1985),

of knmatiites

and Sun {1976), Jahn Duncan

and basaits

of

et al. 11980. 1982),

(1986) and Arndt

(1970).

et al.

island basalts

(unpublished

er al. (1984) and

HIGH

‘J, IN AKCHF-ZN

komatiites.

Data

413

THOLEIITES

from both komatiite

on a single array, essentially

suites

a fractional

plot

crystalli-

zation trend that projects

to Ni contents

those of modern

basalts,

and lower than those in

most

basalts.

Archean

picrites,

such as those

and the Karoo province, the Archean

Modern from

Baffin

similar

to

indistinguishable.

but

and

Not only are the modern

they plot

noncumulate

majority

of Archean

Bay, Hawaii

of some OIB and CFB also argues against variation.

Although

phile elements

komatiites.

for high Ni, Co, Cr and Fe

out.

lustrated

lower

komatiites

on a trend

to Ni contents

ruled Previous explanations

these arguments.

of Archean

extends

plot on the same trend as

and modern

Fig. 2 reinforce Ni contents

basalts. minor

that

those

of a

The high Ni content enrichment

in the Archean

the more

than

pronounced

mantle

secular

of siderocannot

be

differences

il-

in Figs. 1 and 2 must have another

cause.

in Archean tholeiites Explanations for elevated siderophile abundance in Archean tholeiites fall groups:(a)

The high concentrations

ment of siderophile the basalts in the

elements Archean

elements into two

reflect emich-

in the source(s) of upper mantle (e.g.

Glickson, 1971; Gill, 1979). (b) The high concentrations result from different conditions of partial melting in the Archean mantle (e.g. Nesbitt and Sun, 1976; Condie, 1984). Siderophile

element enrichment

in the Archean man-

tle Support for the idea that the Archean mantle was enriched in siderophile elements comes not only from the compositions of Archean tholeiites, but also from apparently high Fe in the lunar mantle (Jagoutz and Wgnke, 1982), and from the “ Pb paradox”; the anomalously high ‘O’Pb/ lo4Pb in modern oceanic basalts. It is argued that the Moon and the Earth were derived from material enriched in siderophile elements, and that since the Archean the Earth’s mantle has become depleted in siderophile elements, as well as in Fe and Pb. as a result of core growth (e.g. Vollmer, 1977; Vidal and Dosso, 1978; Jagoutz and Wgnke, 1982). These ideas were questioned by Newsom et al. (1986) who cited a lack of correlation between Pb isotopic compositions and ratios of siderophile to lithophile elements (e.g. Pb/Ce, Mo/Pr, W/Ba) as evidence against the loss of siderophile elements to the core, and by Briigmann et al. (1987) who cited similar noble element abundances of Archean and modern komatiites as evidence for an essentially constant upper mantle composition throughout geological time. The data plotted in

Unusual melting conditions in the Archean

mantle

There are a number of variants of this explanation. In its simplest form, high temperatures in the Archean mantle degrees of partial magmas enriched

are supported to lead to high melting and to the formation of in siderophile elements. Melting

at high pressures can have the same result (Nesbitt and Sun, 1976). The enrichment is further enhanced by a decrease in partition coefficients with increasing temperature and pressure. Experimental studies by Hart and Davis (1978). Arndt (1977) and Bickle et al. (1977) show that 0;;‘;“” (the olivine-silicate liquid partition creases markedly with increasing

coefficient) temperature,

deand

the same is probably true for Co and Cr. Melts produced at high temperatures will therefore contain higher concentrations of these elements than melts produced

at lower temperatures.

The problem with such arguments is that an increase in T, P or percentage melting results in enrichment not only in siderophile elements, but also in MgO content or Mg number. The relationship between the Ni and MgO contents of mantle melts and the conditions of melting are shown in Fig. 3: a 75% partial melt has indeed four times the Ni content of a 5% partial melt, but it also has a high MgO content; a melt produced at 3 GPa has high Ni, hut also high MgO. The comparisons between the Ni contents of Archean and modern basalts discussed in the previous section were made at specific, relatively low MgO contents. Gill (1979) pointed out that as a highly magnesian primary melt evolves to lower MgO. olivine crystallization extracts Ni so efficiently that when the high temperature-high pres-

N.T. ARNDT

414

MgO (wt%)

(a) Fig. 3. (a) Calculated

compositions

crystallization

of melts

produced

by batch

of such melts. Sources of data and method

melting

in the mantle.

of calculation

(b) The paths

followed

by fractional

are given in the text and Appendix.

sure melt reaches a reference MgO value (e.g. 6% MgO; Fig. 3) it has a lower Ni content than

curvature and a steep slope at the high-MgO end. Because of this curvature, magmas formed by the

typical low temperature-low pressure conclusion applies for all reasonable

mixing of melts have higher Ni contents than batch melts with similar MgO contents (Fig. 6).

melts. This and inter-

nally consistent choices of melting conditions and partition coefficients. (The calculation presented

This observation provides an explanation high Ni of Archean tholeiites; namely

by Condie (1984, table 3) is questionable because the relatively high Ni content of his fractionated high temperature melt stems from the use of an unrealistically low Ni partition coefficient (2.62.8). Calculation using an incremental method in which the partition coefficient is redetermined after each small increment of olivine is subtracted (Appendix) yields lower Ni in evolved liquids, as

parental products

contents. But is the Ni content of mixed magmas sufficiently enhanced to account for the Ni enrichment

illustrated

in Archean

in Fig. 3.)

Similar arguments can also be made for Co, and for Cr, which would be stripped from the melt by fractional crystallization of chrome spinel. Another explanation for the high concentrations of these elements in Archean tholeiites is therefore needed.

for the that the

magmas of the Archean basalts are the of mixing between komatiite and basalt

or less magnesian melts or rocks, whereas the parental magmas of most modern tholeiites are mixtures of magmas with lower MgO and Ni

tholeiites,

and

are there

geologically

reasonable situations in which such mixing could take place? To test the model two types of mixed magmas Mixing

are considered. between mantle melts

Archean tholeiites as the products of mixing between komatiite and other magma or rock types

The eruption temperature of picrites and komatiites greatly exceed those of common basaltic magmas. Modern picrites probably reach the

The foundation of this explanation is given in Fig. 3 which shows the compositions of liquids produced by batch melting in the mantle. The manner in which these compositions were calculated is described in the Appendix. The compositions of batch melts fall on a line with a marked

surface at temperatures close to 1500°C and the most magnesian Archean komatiites probably erupted at temperatures exceeding 1600 o C. Typical eruption temperatures of MORB rarely exceed 1200 o C. This contrast has led to the suggestions in recent papers that the anomalously high magnesium contents of picrites and komatiites reflect

Hl(;H

N, IN ARC Ht.AN

anomalously sources. Bickle

THOLEIITFS

high

temperatures

McKenzie (1988)

komatiites

415

(1984)

propose

in their

and

that

modern

form in “hot jets”

in regions of lithospheric

Campbell

et al. (1989) have developed

the formation

of komatiites

mantle.

According

basalt

and

material

extension

while

a model for

plumes

ascend-

layer deeper

to this model, komatiite

forms by melting and

within

boundary

and

picrites

of mantle

ascending

ing from a thermal

mantle

McKenzie

in the magma

in the hot axial jet of the plume

by melting

in the cooler

5

head of the

10

IS

20

MgO (wt%)

plume. Both

situations

provide

an

environment

in

which highly magnesian magmas penetrate regions in which basaltic magmas are being produced, and under these conditions mixing between the two types of magma is inevitable. The Ni contents of the mixed magmas can be determined by considering two extreme situations: (a) simple mixing between komatiite and basalt end-members: and (b) mixing or pooling of the entire range of magmas produced within the melting region. The former situation might apply when komatiite mixes with basalt in a starting mantle plume; the latter corresponds to the “integrated melts” (Klein and Langmuir, 1987) that form by pooling of melts within a rising, melting mantle column beneath a spreading centre. Figure 4 shows the Ni contents of simple mixed melts and Fig. 5 the Ni contents of integrated melts. The Ni contents of both lie well above the batch-melt curve.

Fig. 5. Composition duced

by pooling

column The

and crystallization of melts

(squares).

paths

equilibrium

paths

in an ascending.

PO = the pressure

followed

by fractional

crystallization

(EC)

of magmas melting

at whxh crystallization

of the 5GPa

pro-

mantle

melting (FC)

starts. and

melt are also

shown.

However, it must be questioned whether this difference in Ni contents is sufficient to account for the high Ni of Archean

tholeiites.

The paths

followed by the two mixed melts as they evolve to lower MgO contents via olivine crystallization are also shown in Figs. 4 and 5. A simple mixed melt with 15% MgO evolves to a basalt with only about 70 ppm at the 6% MgO reference value (Fig. 4 and Table 1); another with 20% MgO evolves to a basalt with about 60 ppm Ni. Similar Ni contents are calculated for the magmas that evolve from the integrated melts shown in Fig. 5. Thus. if the melts evolve solely through fractional crystallization. the enhanced

Ni content

of the mixed melts

are insufficient to account for high Ni in Archean tholeiites. If, however, the melt evolved by equilibrium rather than fractional crystallization, the Ni content of the final liquid is higher. The product of equilibrium

crystallization

of the 15% MgO

simple mixed melt has about 150 ppm Ni: the product from an integrated melt with similar MgO has about 140 ppm. These values are comparable

MgO (wt%) Fig. 4. Composition magma and

produced

basalt

(square) by simple

(7% MgO)

path;

and mixing

melts.

EC = equilibrium

crystallization of komatiite

FC = fractional crystallization

paths

of a

(30% MgO) crystallization path.

to concentrations in Archean tholeiites (Figs. 1 and 2). The presence in komatiite lavas of olivine phenocrysts with unzoned cores and compositions in equilibrium with the host komatiite (e.g. Amdt, 1986) may be taken as evidence that during the ascent and emplacement of these magmas equilibrium crystallization was common. However,

K T ARNDT

416

TABLE

librium

1

Calculated

MgO (wt%) crystallization

5% meltmg at

F(g)

Ni (ppm)

*

from batch melt;

9.5

300

Evolved melt

6.0

80

crystallization

ment

Evolved melt Fractional

6.0

45

crystallization

Parent Evolved melt Equilibrium

Fractional

1140

6.0

50

crystallization

from integrated

19.0

984

6.0

41

Equilibrium

crystallization

Evolved melt Assimilation

from integrated

6.0

Evolved melt Assimilation

32

80

6.0 of granite

Assimilation

into

tholeiites

will increase

the evolved

magmas.

These

quantitatively,

however,

magma

abundances

processes

in

melts in

the parental

of Archean

cannot

because

in be

Co par-

at high P are poorly known

and

of komatiite

A second mechanism komatiite with basaltic

70

rocks,

of granite

205

55

magma

103

63

364

compositions

an increment

(see Appendix).

50

from Figs. 3-5.

and AFC

happen

parent which process cooling accompanying

by komatiite,

crystallization

may

during

the

passage

of

positions ranging from basaltic to granitoid. Crystallization will accompany contamination. and the liquid evolves via fractional or equilibrium crystallization. Again it is not immediately ap-

crystallization: 6.0

as

involves contamination of or more evolved crustal

komatiite through the basement or lower sections of greenstone belts. The contaminant has com-

by komatiite.

6.0

Evolved melt

paths

calculated

of 1%; equilibrium Ratio of fractionation

paths

numeriby mass

to assimilation

is 0.5. * F-percent

enriched

in high pressure

and admixture

Contamination

crystallization:

using

Cr are strongly

argu-

melt, 5 GPa:

crystallization:

Evolved melt

balance

43

of basalt by komatiite,

Assimilation

Fractional

which

in similar

pressure melt component. As explained by Langmuir and Hanson (1980) high-pressure melts are relatively enriched in Fe and are not depleted in Fe during crystallization.

melt, 5 GPa:

2300

6.0

Evolved melt

Parental

29

150

31

equilibrium

elements

siderophile

Cr contents of fractionated basalts are dominated by largely unpredictable chromite fractionation. The high Fe contents of Archean tholeiites are entirely consistent with the presence of a high-

43

crystallization:

fractional

tholei-

of basalt by komatiite,

Parent

equilibrium

which range

in Archean

are enriched

Co and

tition coefficients

from simple mixed melt: 173

Evolved melt

fractional

19.0

6.0

Parent

Ni contents

and probably

modelled

54

from simple mixed melt:

crystallization

Evolved me1 t

cally

1130

other

tholeiites

hold.

general.

from batch melt:

22.0

the

komatiite.

10

5’S melting at 3 GPa: Parent

ites. For Archean

1 GPa:

Parent

Fractional

to produce

up to the high levels observed

melt compositions

Fractional

paths,

crystallized.

strong zoning in other phenocrysts and the presence of xenocrysts in these and other lavas (Nisbet et al., 1987) indicate that re-equilibration between olivine and the evolving magma was imperfect. Likely crystallization paths for the high pressure magmas fall between the fractional and equi-

is the more likely, but slow contamination in a middle

to lower crust magma chamber probably involves some measure of equilibrium crystallization. As in the previous case, the model involving fractional crystallization produces basaltic melts with relatively low Ni contents, whereas equilibrium crystallization results in basalts with enhanced Ni contents. Examples of four basaltic melts produced by assimilation accompanied by fractional crystallization (AFC) and by equilibrium crystallization (AEC), with a basaltic and a granitoid contaminant, are listed in Table 1. At this stage it might be asked whether mixing is really necessary: could not the Archean tholeiites have evolved simply by equilibrium crystalli-

H,<;H

i-4, IN ARCHFAN

zation

of more magnesian

Fig. 4, shows batch

417

7HOLEIlTF.S

melts

250/O MgO Equilibrium

that

may

produce

Reference

crystallization

with MgO contents

high Ni contents. dance,

equilibrium

do not produce crystallization

komatiite geological

parents?

less than

nature

can it account differences

an evolved

of

/

,

/

I

,

/

,

I

Batch

_,

about

liquid

but such a mechanism

of most Archean

for the trace element

between

I

1

the required result. of more magnesian

grounds: it cannot explain the uniform composition

porphyritic

1500

to

komatiites

and

with

fails on the abunand non-

tholeiites,

nor

and isotopic tholeiites.

MgO (wt%)

In

comparison to komatiites, Archean basalts are usually enriched in incompatible elements (e.g. Nesbitt and Sun, 1976; Jahn et al., 1980; Gill, 1979; Redman and Keays. 1985) and have more evolved isotopic compositions (e.g. lower c:Nd f. These characteristics, which can be attributed to a component of low-degree melt in the parental mixed magmas. and/or to interaction with crustal

Fig. 6. Compositions

of mixed.

may be the Ni-enriched and

continental

flood

low MgO content

parents basalts

melts

mid-ocean

that

hi&P.

high MgO melts that

of oceanic (CFB).

and

island basalt (OIB) the mixed.

are the parents

low-P.

of Ni-depleted

ridge basalts (MORB).

rocks, are inconsistent with the concept that the basalts are simply crystallization products of

the base of the oceanic crust. Magmas that form in this environment are mixtures are melts that plot on the flat, low-MgO end of the batch melt

komatiite

curve and have relatively

parental

magmas.

low Ni contents.

High Ni contents of modem OIB and CFB

Conclusion

The mixing mechanisms advocated for the Archean basalts might also explain the relatively high Ni contents in many modern oceanic island

The relatively high concentrations of Ni and other siderophile elements in Archean tholeiites can be explained if the parental magmas result from mixing between komatiite and basaltic or

and continental flood basalts (Fig. 2). There is general agreement that OIB forms in plumes ascending from deeper levels in the mantle (BVSP. 1981),

and,

recently,

a similar

origin

has

been

proposed for CFB (Courtillot et al., 1986). In both situations, the magmas separate from their source peridotites at relatively great depths; in the case of OIB at or near the base of the oceanic lithosphere, and in the case of CFB somewhere between the base of continental crust and continental lithosphere. In both situations mixing between magmas produced by different extents of melting is likely. and in both cases the melts involved have relatively high MgO contents. These mixed melts therefore have compositions that lie above the central portion of the batch melt curve as illustrated in Fig. 6, and as such have elevated Ni contents. In contrast, MORB magmas form by melting within a mantle column that extends to

more evolved melts or rocks. Such mixing have taken place in two plausible geological

could situa-

tions: in an ascending mantle columns or plumes, or during interaction between komatiite and crustal rocks. In both cases the evolution of the parental magmas must have involved some measure of equilibrium crystallization; pure fractional crystallization tents similar

produces basalts to those in modern

with nickel tholeiites.

con-

Acknowledgements The ideas in this paper were clarified during discussions with W. McDonough, A. Rocholl, S. Goldstein and U. Christensen. and helpful reviews were provided by I.H. Campbell, A. Cattell, E.G. Nisbet. M.J. Bickle, K. Condie and anonymous reviewer.

NT

418

Appendix: Calculation of the compositions

of batch

taken

from

assuming

Arndt,

compositions

of the mantle

the experimental

(1980) and Takahashi

studies

ing equation

relating

MgO content

of the melt (MgO, ).

IX;“”

batch

the partition

= (12R,‘MgO,)

and

Green

and adopting

the follow-

(D:;“‘! ) to the

coefficient

(1)

was derived

lower

than

GPa

the calculated

from experimental

those

data

experiments

0;;‘;“”

under-

end but does not materially At a pressure

degrees

of melting

majorite

(Takahashi.

residual

assemblage

is less than

relationships

alter

of 5 GPa, a komatiite and

the residual

1. assuming

between

reported

phases

are olivine coefficient

D~~~“q=O.15

and a steep slope at the high-MgO method

similar

(from

of melting,

described and

calculated

by integrating

Ohtani

by

Klein

numerical

curve:

between

e.g. between

a

method.

trends

The composition distribution

were

two posior

which

MgO contents

yields

slightly

from higher

amount

Arndt

(1986).

O$;““

values (usually

was normalized

new olivine composition

was calculated,

repeated

MgO value was reached.

until the target

crystallization

paths

were calculated of olitine

needed

to reduce

content

of the liquid

to several

reference

values

and from these the Ni contents

was

Equilibrium

by determining.

partition

at

1%) was

and the procedure

the amount

initial and final values. Appropriate

This

at high

to 100%. the

balance,

determined.

a

for crystallization

of olivine

the liquid composition

using

of 0.30 and the equa-

than eqn. 1. is appropriate

levels. A small

subtracted,

calculated

of olivine was calculated

coefficient

D;;‘,“” = (MgO,/124)-2.2

equation,

were

by mass the MgO

between

coefficients

London.

and geochemistry

of komati-

komatiites: Earth

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