The distribution of Ga and Rb in coexisting groundmass and phenocryst phases of some basic volcanic rocks

QeoohimloD et Coamochlmlca

Acts,1972,

Vol. 36, pp. 303 to 327. Pergsmon Preas. Printed in Northern Ireland

The distribution of Ga and Rb in coexisting ground phenols phases of some ba& volcanic rocks

and

ROWER J. GOODMAN Bondar-Clegg & Company Ltd., 764 Belfast Road, Ottawa, Ontario, Canada

&&a&--A study has been made of the distribution of Rb and Ga in basic vol~auic rocks aud their constituent grouudmass and mineral phases. partition coefficientsfor Rb and Ga have been determined between pyroxene and groundmass and plagioclase and groundmass. The diadochy of Rb aud K in mineral phases is discussed in terms of raudom adsorption and site oueup~oy of the cations. The geo~ern~t~ of Ga is discussed in relation to both Al and Few, and the available evidence suggests the GalAl coherence is not as close as has hitherto been assumed. INTRODUCTION UNTIL recent years most studies on the distribution of trace elements in igneous rocks were concerned only with observed concentrations in the crystalline phases. However two recent papers by BURNS and FYFE (1966) and WHIYTAKER (1967) have stressed the importance of the coexisting liquid in the interpretation of traceelement distribution patterns. A limited number of measured partition coefficients have been reported in the literature in the past (WAGER and ~CEE~, 1961; EWARTet al., 1968; ONUMAet al., 1968; HI~UCHI et d., 1969; EWART and TAYLOR, 1969; GRXFFM. and MURTHY, 1969; BERLIN~~~HENDEIRSON, 1968,1969; PHILPOTTS and SUHNETZLER, 1970) and the present paper is designed to present some data pertinent to the distribution of gallium and rubidium in selected phenocryst phases and the coexisting silicate melt. PREVIOUS

WORK

from an excellent review of the geochemistry of gallium by SHAW (3957) little work has been carried out on gallium ~stribution in silicate rocks and minerals in recent years. NIGHTINGALE (1962) presented some gallium results from Skaergaard rocks and constituent mineral phases and commented on the diadochy of gallium with both ferric iron and aluminium. ~nsiderable data has been ac~rnu~a~d in the past on the relationship between K and Rb in whole rocks in an attempt at a more complete understanding of the processes governing the formation and differentiation of magmas. However it is only recently with papers by GRIFFIN and MURTHY, 1969; PHILPOTTS and SCRCNETZLER, 1970; GILL and MURTHY, 1970, tha;t the significance of studying the distribution pattern of K and Rb in the constituent phases of rocks has emerged. Apart

SELECTION

OB MATERIAL

Basaltic lavas oontaining well developed phenocrysts of plagioclase, olivine and clinopyroxene in an essentially aphyric groundmass were selected for study. All phenocryst phases studied were essentially zone-free enabling one to assume approximate total equilibrium to exist between the groundmass and phenocryst phases. 303

304

ROGER J. GOODMAN SAMPLE

DESCRIPTIONS

16448 Alkali-basalt, Azores. Fresh porphyritic basalt with largely unzoned plagioclase phenocrysts (30%) in a fine-grained holocrystalline groundmass (68 %) consisting of mostly plagioclase, pyroxene and opaque minerals. Very minor phenocrystal olivine and magnetite. 16457 Fresh porphyritic basalt consisting of unzoned plagioclase Alkali-basalt, Azores. phenocrysts (28 ‘A) and minor pyroxene (0.6%) in a fine-grained holocrystalline groundmass (71.5 %) of plagioclase, pyroxene and opaque minerals. 16540 Alkali-basalt, Azores. Unzoned fresh plagioclase (26 %) and pyroxene (1.5 %) phenocrysts in fine-grained holocrystalline groundmass (72~5%) of plagioclase, pyroxene and opaque minerals. 16561 Ankaramite, Azores. Very fresh porphyritic basalt of unzoned pyroxene (12x), olivine (13 %) and plagioclase (8 %) phenocrysts completely free of inclusions in a glassy groundmass (67 %) of mainly plagioclase, pyroxene and minor opaque minerals. 16566 Ankaramite, Azores. Fresh distinctly porphyritic basalt containing slightly zoned pyroxene (12 %), olivine (20%) and plagioclase (1%) phenocrysts in glassy unweathered groundmass of plagioclase, pyroxene and minor opaque minerals. 18269 Alkali-basalt, Azores. Fresh, vesicular porphyritic basalt consisting of unzoned pyroxene (4 %), plagioclase (3 %) and olivine (5 %) phenocrysts in fairly coarse groundmass (88 %) of plagioclase laths, pyroxene and accessory opaque minerals. 18290 Alkali-basalt, Azores. Fresh porphyritic basalt with phenocrysts of olivine (4 %), pyroxene (22%) and plagioclase (13%) in fine-grained holocrystalline groundmass (61%) consisting of mainly plagioclase, pyroxene and minor opaque minerals. Slight zoning occurs in pyroxene phenocrysts. 18294 Alkali-basalt, Azores. Weathered porphyritic basalt with phenocrysts of pyroxene (5 %), olivine (11%) and plagioclase (4 %) in a fine-grained holocrystalline groundmass (80 ‘A). Pyroxene and plagioclase phenocrysts are largely unzoned, but olivine phenocrysts are highly altered and zoned. 18299 Alkali-basalt, Azores. Vesicular porphyritic basalt with well-developed phenocrysts of pyroxene (4 %), plagioclase (33 %) and olivine (3 %) in fine-grained holocrystalline groundmass (60%) of pyroxene, plagioclase and opaque minerals. Pyroxene and plagioclase phenocrysts are free of zoning, but show signs of alteration. The olivine phenocrysts are highly weathered and corroded. 18301 Trachybasalt, Azores. Vesicular, porphyritic rock with phenocrysts of pyroxene (14x), olivine (5 %) and plagioclase (9 %) in a light-coloured holocrystalline groundmass (72 %) of mainly plagioclase with accessory pyroxene and opaque minerals. Pyroxene and plagioclase phenocrysts are unzoned and quite fresh, but the olivine phenocrysts are highly corroded.

The distribution of Ga and Rb in some basic volcanic rocks

305

18304 Alkali-basalt, Azores. Highly altered porphyritic basalt with pyroxene (5 %) end plagioclase (26 %) phenocrysts in fine-grained groundmass. Phenocrysts and groundmass are highly altered. 18368 Ankaramite, Azores. Fresh porphyritic basalt with fresh well-developed phenocrysts of olivine (9 ‘A), pyroxene (9 ‘A) and plagioclase (5 ‘A) in a glassy groundmass (77 ‘A) of plagioclase, pyroxene and opaque minerals. All phases are very fresh and completely free of zoning. 18362 Ankaramite, Azores. Fresh, porphyritic basalt with fresh well-developed phenocrysts of olivine (22 %) and pyroxene (5 %) in a glassy groundmass (73 %). Phenocrysts are completely fresh and free of zoning. 17826 Alkali-basalt, Juan Fernandez. Fresh porphyritic basalt with phenocrysts of plagioclsse (11%) and minor olivine (0.6%) in a fine-grained holocrystalline groundmass (88.5 %). The plagioclase phenocrysts are slightly altered, but free of zoning, whilst the olivine phenocrysts are weathered and corroded. 17830 Alkali-basalt, Juan Fernandez. Quite fresh porphyritic basalt with fresh phenocrysts of plagioclase (16 %) and corroded pyroxenes (2%) in a fine-grained holocrystalline groundmass of plagioclase, pyroxene and opaque minerals. Phenocrysts are largely unzoned but do contain numerous small inclusions. 17834 Alkali-basalt, Juan Fernctndez. Fresh porphyritic basalt with phenocrysts of pyroxene (6 %), olivine (13 %) and plagioclase (6 %) in a fine-grained holocrystalline groundmass (75 %) of plagioclase, pyroxene and opaque minerals. Some zoning occurs in the plegioolase and pyroxene phenocryst and the olivine phenocrysts are highly altered and corroded. 17837 Alkali-basalt, Juan Fernandez. Altered porphyritic basalt with altered plagioclase (9%) and olivine (0.6 %) phenocrysts in a fine-grained groundmass of plagioclese, pyroxene and minor opaque minerals. Plagioclase phenocrysts are essentially unzoned, but do contain numerous small inclusions. 17853 Alkali-basalt, Juan Fernandez. Fresh porphyritic basalt with phenoorysts of plagioclase (12 ‘A) and olivine (3 %) in a fine-grained groundmass (85 %). The plagioclase phenocrysts are fresh, unzoned but contain some small inclusions. The olivine phenocrysts are altered and corroded. 17885 Oceanite, Juan Fernandez. Porphyritic basalt with large well-developed olivine phenocrysts (50%) in a fine-grained vesicular groundmass. Olivine phenocrysts are fresh, largely unzoned with narrow brown reaction rims, but contain abundant small opaque inclusions. These inclusions cannot be fully eliminated during mineral separation. 17983 Oceanite, Juan Fernandez. Porphyritio basalt with large rounded olivine phenocrysts (47 ‘A) in a vesicular groundmass (53 %). The olivine phenocrysts are essentially fresh and unzoned with a brown reaction rim. Small opaque inclusions in the olivine phenocrysts are abundant.

306

ROGER

J. GOODMAN

0.71 Ankaramite, Hawaii. Porphyritic basalt with phenocrysts of pyroxene (la%), olivine (IS %) and plagioclase (1%) in a fine-grained groundmsss (09 %) of plagioclase, pyroxene and opaque minerals. All phenocrysts are fresh with some degree of zoning and contain small inclusions. B.A.1

Alkalic olivine basalt, Hawaii. Vesicular porphyritic lava with large dark green phenocryste of pyroxene (27 %) and olivine ( 12 %) in a glassy groundmass (61 ‘A). Phenocrysts are relatively fresh and free of zoning, but contain numerous small opaque inclusions. 0.0.1 Cceanite, Hawaii. Porphyritic lava with large fresh olivine phenocrysts (45 %) in a vesicular glassy groundmass (55 %). Inclusions are abundant within olivine phenocrysts. 4 Tholeiitic basalt, Iceland. Fresh porphyritic basalt with phenocrysts of plagioclase (19 %) and minor olivine (2%) in a fine holocrystalline groundmass. The plagioclase phenocrysts are fresh and free of zoning. The olivine phenocrysts are slightly corroded and contain numerous opaque inclusions. 40 Tholeiitic basalt, Iceland. Fresh porphyritic basalt with phenocrysts of plagioclase (5%) and very minor olivine (0.2 %) and magnetite (I*2 %) in a fine gmined holocrystalline groundmass (93.6 %). Plagioclase phenocrysts are zone-free and free of alteration and inclusions. ANALYTICAL

PROCEDURES

The potassium determinations were made on a Philips 1640 spectrometer modified to accept a 2 kW tube. Analyses were made using a Cr tube, PE crystal and a gas flow counter. Standards analysed by neutron-activation analysis and mass-spectrometric isotope dilution were used for the low levels encountered in pyroxenes and the coefficient of variation of the results at the 100 ppm level is around 10 %. For potassium concentrations above 0.5 % the coefficient of variation is better than 4 %. Gallium and rubidium were also made on a Philips 1640 X-ray spectrometer using a MO tube operated at 100 kV and 20 MA, a LiF 200 crystal and a NaI scintillation counter. Accurate assessment of background beneath the respective peaks and matrix corrections were made using the method of CUPPELL et al. (1969). The X-ray determinations of Rb were calibrated using precise standards analysed by mass-spectrometric isotope dilution and analyses have a coefficient of variation of about 10 ‘A at the 1 ppm level. Gallium determinations were calibrated using rock samples anslysed by neutron-activation analysis (NI~HTINGUJE, 1962) and the coefficient of variation of the technique at the 20 ppm level is 6 %. Major elements were also determined by X-ray fluorescence using a powdered pellet and published values of mass-absorption coefficients to effect matrix corrections (HOLY and HABZILTON,1965). Ferric iron was determined by difference from total iron analysis by X-ray fluorescence and ferrous iron analysis by the metavansdate method (WILSON, 1955). RELIABILITY

OF DATA

In addition to analytical errors empirical partition coefficient studies are liable to errors from other sources. The purity of the separated phases is critical in the determination of meaningful partition coefficients. In this respect all phenocryst phases of clinopyroxene and plegioclase were subjected to stringent purification during the mineml separation procedures to obtain a purity of better than 995%. Small opaque inclusions were eliminated by crushing to a fine mesh size ( - 160#), separation by an electromagnet and final purification by hand picking under 8 binocular microscope. Olivine phenocrysts were difficult to purify and all olivines analysed in this work are only 95 ‘A pure.

307

The distribution of Ga and Rb in some basic volcanic rooks

Other errors can arise from alteration and secondary effects, but in general these have been minimked by choosingrelatively fresh unalteredsamples. While admitting the possibleexistence of these errors, the consistency obtained on the partition coefficient values suggeststhat the results are valid within the limitations of the experimental error itself.

RESULTS

The gallium contents of the basaltic whole rocks analysed in this work vary from 12-27 ppm whilst the groundmasses are usually relatively enriched in gallium compared to the whole rocks with concentrations varying from 17.5-26 ppm (Table 1). This is probably predominantly due to the crystallisation of phenocryst phases such as olivine and pyroxene showing an absolute depletion in gallium. The plagioclase phenocrysts often show slightly higher gallium contents than the coexisting groundmasses with values ranging from 17.5-28 ppm. The values of the partition coefficient lc between plagioclase and liquid are very consistent varying from 0.84-1.27 (Table 1). Pyroxenes show absolute depletion in gallium with concentrations ranging from 6-14.5 ppm. The partition coefficient between pyroxene and liquid varies from 0.30-058 with a marked concentration of values occurring around 0.40. The few analysed olivine phenocrysts have gallium contents of around 1 ppm and hence have low partition coefficients ranging from o-04-0.05. Table 1. Distribution of Ge in lavas and constituent phases Sample and locality

k = Ga mineral/ k = Al mineral/ Al liquid Ga liquid

Gfb (ppm)

Azores lavas 16448 R GM Pl

23 26 23.5

8.63 15.96

R GM Pl

22.5 24-5 22

IO-14 8.54 17.12

R GM Pl

23 25 21

9.74 8-50 16.48

R GM Px 01

16.5 23 8-4 1

7-43 8.59 2.33 O-78

R GM Px

12 18 7

5.38 8.49 2-15

R GM Px Pl

20 22 10.8 21

R GM

18 21

16467

16640

16561

16566

18269

18271

1.85

2.36 3.01 1.47

2.01

2.22 2.87 1.29

1.94

2.36 2-94 1.27

O-27 o-09

2.22 2.68 3.61 1.28

0.25

2.23 2.12 3.26

o-31 1.70

2.17 2.30 3.61 1.29

9.75

9.21 9.58 2-99 16.28 8.55 8.77

0.90

0.90

0.84

0.37 0.04

o-39

o-49 0.95

Ga/Al x lo4

2.11 2.39 (Table 1 continued on next page)

ROUE~ J. GOODMAN

308

Table 1. (continued) k = Ga mineral/ k = Al mineral/ GalAl x lo” Ga liquid Al liquid

Sample and locality R GM Px Pl

22 8.5 28

7-19 7.64 1.63 15.11

N GM Px

17 25.5 11.6

8.28 7.83 2.25

R GM Px Pl

21.5 23 8.8 26

9.26 8.05 2.11 15.99

R GM Px Pl

25 24 11 27

10.13 9.23 2.62 14.32

R GM Px Pl

27 25 10 28

8.76 7-30 1.78 14.94

R GM Px

13.5 19.5 8

5.79 7.69 1.58

R GM Px

13.5 20 6

6.44 7.20 1.78

Hawaiian 1.avas R c.71. GM Px

19 23 7.8

7.42 8.09 2.43

R GM Px

19 25 14.5

6.38 8.73 3.61

R GM 01

15 19 1

4.97 7.81 o-59

18290

18294

18299

18301

18304

18358

18362

B.A.I.

O.C.I.

18.5

Juan Fernandez lavas 17826 R 22-5 GM 24 Pl 22 17830

R GM PI

22-5 23 22

8.86 8.28 16.53 8.51 8.24 16.56

o-39

l-27

0.45

0.38 l-13

0.46 l-13

0.40 1.12

0.41

0.30

o-34

0.58

0.05

0.92

O-96

0.21 1.98

2.57 2.88 5.21 l-85

o-29

2.05 3.26 5.16

O-26 I.99

2.32 2.86 4.17 1.63

0.28 1.55

2.47 2.60 4-20 1.89

0.24 2.05

3.08 3.43 5.62 1.87

o-21

2-33 2.54 5.06

0.25

2.10 2.78 3.37

0.30

2.56 2.84 3.21

0.41

2.98 2.86 4.02

0.08

3.02 2.43 l-69

2.00

2.54 2.90 1.33

2.01

2.65 2.79 I.33

The distribution

309

of Ga and Rb in some basic volcanic rocks Table 1. (oontinzced)

Sample and locality

k = Ga mineral/ Ga liquid

Ga (ppm)

R GM Px Pl

21.5 26 9 24

8.41 8.16 I.94 15.77

17837

R GM Pl

24.5 24.5 23

9.81 9.21 16.80

17853

R GM Pl

24 26 22

8.48 8.02 16.43

17885

R GM 01

17.6 23.5 -

7.08 8.02 0.76

17983

R GM 01

16 23 1

7.21 8.18 O-89

Icelandic lavas R GM Pl

20 20 20

9.28 9.02 17.44

17.5 18.5 17.5

9.10 8.88 17.68

17834

40

R GM Pl

0.35 0.92

0.94

0.85

k = Al mineral/ Al liquid

Ga/Al

x 10’

0.24 1.94

2.56 3.19 4.64 1.52

1.82

2.50 2.66 1.37

1.92

2.83 3.24 1.43 2.47 2.93

0.09

0.04

l-00

0.95

0.11

2.22 2.81 1.12

1.93

2.15 2.22 1.15

1.99

1.92 2.08 0.99

The rubidium contents of the basaltic whole rocks vary from 5-82 ppm with groundmasses giving a relative enrichment compared to the whole rock values, with concentrations ranging from 6-89 ppm (Table 2). The plagioclase phenotrysts show a strong depletion in Rb with values ranging from l-2-20 ppm, the highest value being found in the plagioclase separated from the trachybasalt (sample 18301). The value of the partition coefficient k between plagioclase and liquid varies from 0.05-0.23, again the highest value being found in the plagioclase separated from the trachybasalt (sample 18301). The pyroxene phenocrysts are also severely depleted in Rb with values ranging from 0.2-1.8 ppm and the partition coefficient from O*OOPO*O8. Table 2. Distribution Sample and locality Azores lavas 16448 R GM Pl 16457

R GM Pl

K

Rb

(%)

(ppm)

1.41 1.61 0.245

41 48 2.4

1.21 1.49 0.121

34 41 2.4

of Rb in lavas and constituent phases k = K mineral/ K liquid

0.16

0.08

k = Rb mineral/ Rb liquid

K/Rb

0.05

344 335 1021

0.06

366 363 604

(Table 2 continued on next page)

310

ROUER J. GOODMAN Table 2.

Sample and locality 16540

16561

16566

18269

18290

18294

18299

18301

18304

18358

18362

(E)

Rb

k = K mineral/

(ppm)

K liquid

R GM Pl

0.86 l-05 0.154

24 30 2

R GM Px

0.78 1.10 0.073

20 28 1.8

R GM Px

0.43 0.91 0.025

16 28 0.9

R GM Px Pl

1.31 1.54 0.010 0.2015

34 40 0.3 3.0

R GM Px Pl

l-30 2.00 0.004 0.425

34 56 0.2 4.2

R GM Px

1.84 1.81 0.045

56 60 0.8

R GM Px Pl

1.69 l-93 0.012 0.40

51 55 0.6 5.0

R GM Px Pl

2-73 2.78 0.041 0.915

82 89 1.4 20

R GM Px Pl

1*51 l-98 0.02 1 0.487

45 63 0.4 3-o

R GM Px

0.90 1.42 0.02

25 43 o-5

R GM Px

1.25 1.56 o-04

34 46 o-3

0.41 0.69 0.051

15 18 1

Hawaiian Isvct c.71 R GM Px

(continued)

0.15

0.05

0.03

0.006 0.13

0.002 o-21

0.02

0.05 o-21

o-015 0.33

0.01 0.25

o-014

0.026

o-074

k = Rb mineral/ Rb liquid

K/Rb

0.07

358 350 770

0.06

390 393 406

0.03

269 325 278

O-008 0.08

385 385 333 672

0.004 0.08

0.01

382 357 200 1012 329 302 563

0.09

331 361 200 800

0.02 0.23

333 312 293 458

0.006 0.05

336 314 525 1623

0.01

0.01

o-007

0.06

360 330 400 368 339 1330

273 383 510

311

The distribution of Ga and Rb iu some basic volcanic rocks Table 2. (continwd) Rb

Sample and locality

(ppm)

Juan Fernandez h-was 0.67 17826 R o-75 GM o-1115 Pl

12 14 1.7

R GM Pl

O-76 0.81 O-1400

14 17 l-5

R GM Px Pl

0.65 O-88 0.02 O-1525

8 13 1.0 l-4

R GM Pl

O-56 0.60 0.1205

5 6 I.2

R GM Pl

0.80 0.99 0.1730

15 20 1.6

17830

17834

17837

17863

k = K mineral/ K liquid

0.15

0.17

o-03 0.17

0.20

0.17

k = Rb mineral/ Rb liquid

K/Rb

0.12

558 536 656

0.09

543 476 933

0.08 o-11

813 677 020 1089

0.50

1120 1000 1004

0.08

633 495 1081

DISCUSSION (i) Gallium A coherence of Ga and Al has been frequently advocated in the past by several authors: WAGERand MITCHELL(1951); NOCKOLDS and ALLEN (1953,1954,1956); SIEDNER (1965). However a much closer coherence of Ga with FeS+ has been suggested by NI~HTINCJALE (1962) in more recent studies of Skaergsard rocks and constituent mineral phases. The general significance of both coherences can be easily understood owing to similar valency state (3+) and similar ionic radii as shown below for both four-fold and six-fold coordination (Values taken from WHITTAKERand MUNTUS,1970). GP 0.55 A Gavl O-70 A

AIIv 0.47 A AIV1 0.61 A

FeV1 O-73 A

From ionic radii considerations alone a closer coherence of Ga with FeS+ could be expected on the grounds that Ga would prefer to enter a larger Fe3+ site rather than an appreciably smaller Al site. Ga-AI co7z,erence.A fair correlation can be illustrated by a plot of Ga vs Al for the whole rocks (Fig. 1). The Ga x 104/A1ratio varies from 1-67-3-08 with a mean value around 2.5 and these values correspond quite well with the range of 2-2-5.9 found by SIEDNER (1965) working on a diverse suite of differentiated alkaline rocks. The Ga x 104/A1 ratio for groundmasses only varies from 2.04 3.43, and this consistency is probably attributable to occupation of analagous tetrahedral sites in the liquid.

312

ROGER

J. GOODMAN

26-

22 E g

16-

s 14-

I” I05

I

I

I

I

6

7

6

9

Al,

,

I

II

IO

%

Fig. 1. Plot of gallium vs. aluminum for whole rocks. 0 Azores lavas; 0 Hawaiian lavaa; x Juan Fernandezlavas; 0 Icelandic lavas.

Although Ga shows absolute enrichment in plagioclases the Ga x 104/Al ratio is low varying from 0.96-149 (Table 1) showing that Al is enriched over Ga. The preferential incorporation of Al over Ga in plagioclases can be best explained by reference to their relative melting points. GOLDSMITH(1950) synthesised gallium felspars and showed that these have lower melting points than natural aluminous plagioclases. He illustrated this by reference to Na Ga S&O8which melts at 1015°C as compared with albite which melts at 1118°C. Hence melting point criterion would lead us to predict an enrichment of Al over Ga in crystallising plagioclase plenocrysts. Of greater significance with respect to gallium distribution in plagioclases is the fact that the Ga content is apparently not directly related to the amount of Al present. If a close Ga/Al covariance existed in plagioclases, the anorthite rich plagioclases would be expected to be relatively Ga-enriched. In fact the converse is true and Ga tends to show increasing concentration with increasing albite content of the plagioclase (Fig. 2). The observed negative correlation between Ga and Al implies that Ga tends to be preferentially excluded from the more calcic plagioclases of the earlier differentiates. 100

r

60 16

24

20 Ga wm

28

Fig. 2. Plot of gallium vs. anorthite for plagioclase felspars. 0 Azores lavas; x Juan Fernandez lavas; 0 Icelandic lavas.

The distribution of Ga and Rb in some basic volcanic rocks

313

Ga-Fe3+ coherence. A good correlation of Ga with Fe3+ is seen in the clinopyroxenes analysed in this work (Fig. 3). In this case Fesf and Ga probably occur in six-fold octahedral coordination in the M.l. clinopyroxene site. No magnetites were analysed in the present study and hence no quantitative data are available to support the Ga/Fe3+ correlation in this phase. Qualitative arguments can be used in support of the Ga/Fe3+ correlation, in that the groundmasses with the highest Ga x 104/A1 ratios have the highest concentration of interstitial magnetite (for example, specimens 18294, I8304 and 17834). It is

-0

I

2 Fe3+,

3

4

%

Fig. 3. Plot of gallium vs. ferric iron for pyroxenes. 0 Azores lavas; 0 Hawaiian lava.

to note that NIGHTINGALE (1962) obtained Ga values exceeding 100 ppm in magnetite separates from basic rocks of the Skaergaard complex in Eastern Greenland. However ilmenite separates from similar basic rocks of the Skaegaard complex gave very low Ga values, indicating a distinct correlation of gallium with ferric iron and not ferrous iron.

interesting

(ii) R ubidiwn Pyroxenes. The K/Rb ratio of pyroxenes shows a high degree of variability from a low value of 200 to quite a high value of 1330 (Table 1). These results show that the K/Rb ratios of some pyroxenes are lower than those of the coexisting melts, i.e. Rb is preferentially incorporated over K in early crystallising pyroxenes. This is difficult to explain by crystal-chemical reasoning where an enrichment of the smaller K ion over the larger Rb ion would be expected. This argument assumes both ions to be entering distinct lattice sites, probably the larger eightfold coordinated Ma (Ca) clinopyroxene site. This appears likely in the case of K, but in view of the very low concentrations of Rb present in the crystallizing system there appears to be no real reason why Rb should not be incorporated along crystal growth planes or occupy crystal defects. DE VORE (1955a, 1955b) has treated the subject of adsorption of trace and minor elements into crystals in detail. He concluded that most trace and minor elements occur along surfaces of

314

ROGER

J. GOODMAN Table 3. Major element analysis

Clinopyroxenes Sample No.

16561

16566

18269

18290

18294

18299

SiO Al,& Fe??, Fe0 MgO CaO

50.3 4.4 1.3 3-9 16-6 21.2 o-75 1.41 o-13

50.1 4.1 1.3 3.1 18.3 21.7 0.43 O-98 0.11

49.3 5.6 1.5 3.5 16.5 21.1 o-55 1.88 o-12

49.4 3.1 2.5 5.5 17.2 20.5 0.38 1.54 0.14

48.6 4.2 3.3 5.0 16.7 20.0 0.45 1.77 0.14

49.2 4.0 2.4 4.7 16.6 20.8 0.41 1.99 0.13

43 51 6

44 49 7

41 48 11

41 48 11

43 48 9

Na,O TiO, MIlO Composition Ca Mg Fe

(Atomic

%)

44 48 8

Plagiochee Sample No.

16448

16457

16540

18269

18290

18299

18301

SiO,

49.5 30-l 0.7 0.1 15.6 3-41 O-29 o-17

47.1 32.3 0.6 0.1 17.3 2.27 0.14 0.12

48.6 31.1 0.6 0.1 16.2 3-00 0.18 o-13

49.2 30.7 0.6 0.1 15.7 3.27 0.24 0.13

51.1 28.5 0.7 0.1 14.6 3.74 0.51 o-20

50.5 30-2 0.7 o-1 14-4 3-44 O-48 o-22

53.4 27-O 0.6 0.1 12.3 4*93 l-09 0.14

84 16 0

79 21 0

76 23 1

72 26 2

73 25 2

62 35 3

A% Fe0 MgO CaO Na,O %O TiO, Composition An Ab Or

(Atomic 75 24 1

%)

crystal growth or occupy crystal defects. Recent work on site populations by Mossbauer spectroscopy (BANCROFTet aZ.,1967, 1968) certainly does not substantiate this extreme view in the case of transition metals. But the idea of adsorption appears far more feasible at very low concentrations, especially for the incorporation of large ions, such as Rb in unfavourable mineral structures such as clinopyroxenes. The large Rb cations could occur as ‘trapped’ inclusions between successive growth layers and would therefore occur as minute inclusions randomly distributed throughout the individual phenocryst. The amount taken up in this manner would be related in psrt to the ability of the Rb cation to form stable bonds with anions at the surface of the crystal. This process could account in part for the more or less random variation of the K/Rb ratio in pyroxenes, particularly the ratios that are lower than those found in the coexisting silicate melt. However dogmatic conclusions must be avoided in the interpretation of these K/Rb ratios in view of the total experimental error on the K/Rb determinations (in general f20%) and other problems such as sample impurity which becomes critical at the fraction of the ppm level.

316

The distribution of Ga and Rb in some basic vokxnic rocks of phenooryst phases in weight %

18301

18304

18358

18362

c.71

17834

47.8 5.0 3-4 4.7 16.0 20.5 0.64 2.03 0.14

45.1 3.4 10.9 5.9 17-o 16.3 0.41 1.74 0.19

50.1 3.0 2.8 3.4 17.2 21.9 0.44 I.35 0.12

50.6 3.4 1.3 3.5 17.9 22.2 0.44 1.06 0.10

60.7 4.6 2.3 3.9 15.8 21.3 0.43 1.02 0.13

49.8 3.7 2.0 5.5 16.8 20.2 0.52 1-52 0.15

43 47 10

33 49 18

44 48 8

44 60 6

45 47 8

41 48 11

18304

17826

17830

17834

17837

17863

4

40

52.0 28.2 0.7 0.1 13.7 4.41 0.59 0.22

49.0 31.2 o-7 0.1 16.0 2.71 0.13 0.13

49.1 31.3 o-7 O-1 16.0 2.69 0.14 0.16

49.9 29.8 0.7 0.1 15.6 3.51 0.18 O-15

48.7 31.7 0.7 O-l 15.9 2.60 0.15 o-14

61.1 29.1 0.6 o-1 14.6 4.09 0.21 0.15

464 32.9 0.7 0.2 19.3 1.37 0.02 0.07

45.0 33.4 0.7 0.1 19.6 1.18 0.01 0.06

67 31 2

80 20 0

81 IQ 0

75 24 1

81 19 0

71 28 1

91 9 0

92 8 0

PZugioclases From a consideration of ionic radius criterion alone Emenrichment of fhe smaller K+ ion over Rb+ would be expected in early crystallising plagioclases. Additional support for this trend of enrichment is given by reference to the lower meIting points and lattice energies of Rb compounds compared to analagous compounds of K (BURNSand FYFE, 1967). Such an enrichment of K is observed in all analysed plagioclases separated from alkali basalts, the K/Rb ratios varying from 450 to 1633. The arithmetic mean of the KfRb ratios of all plagioclases separated from the Azores and Juan Fernandez basal& is around 885. The present work appears to be in good agreement with the observations on K/Rb ratios in plagioclases by other workers (GRICFPINand MURTHY, 1969; PHILPOTTS and SCBNETZLER, 1970; GILL and MURTHY, 1970). PHILPOTTS and SCHNETZLEE (1970) in a study of plagioclase separates fi-am lavas of such diverse composition as alkali basal&, andesites and rhyodacites, found very variable K/Rb ratios ranging from 281-3610, but in alf except two samples the K/Rb ratio of the plagioolase is greater than that of the

316

ROQER

J. GOODMAN

coexisting melt. As both of these ‘anomalous samples’ are of talc-alkaline parentage, pronounced compositional zoning probably accounts in part for the aberrant ratios. In addition these ‘anomalous’ plagioclases of Philpotts and Schnetzler are the two with lowest absolute concentration of alkalies, where adsorption processes may overshadow the crystallographic effects. The consistently higher K/Rb ratios of plagioclases compared to the coexisting melt suggests site occupation is a more likely event than random adsorption of Rb cations along crystal growth planes. CoNCLUsIoNs

Ga appears to follow both trivalent iron and aluminium. (2) The gallium-aluminium coherence is not as pronounced as has been hitherto assumed, judging by the evidence of the plagioclase felspars. (3) The geochemistry of gallium appears to be dictated by its strong affinities for tetrahedral co-ordination. This explains its overall concentration in plagioclases and magnetite phenocrysts. However if both tetrahedral Fe3+ and A13+sites are available it appears that Ga strongly prefers the former on the basis of its larger size. (4) The overall depletion of Ga3+ in clinopyroxenes substantiates its dislike for six-fold coordination in phenocryst phases. (5) The almost ubiquitous occurrence of a higher K/Rb ratio in plagioclases than in its coexisting melt suggests that K/Rb fractionation by plagioclases may be useful as a reliable indicator of petrogenetic events, such as anorthosite formation. (6) The inconsistency of the K/Rb ratio in pyroxenes suggests petrogenetic interpretations based on K/Rb fractionation by pyroxenes should be made with extreme caution. (7) It is suggested that the process of adsorption as defined by DeVore is probably in general only important at the sub-ppm level, judging by the evidence afforded by Rb distribution in pyroxene phenocrysts. (1)

Acknowledgements-The work was carried out during the tenure of a NERC grant at the Department of Geology and Mineralogy, University of Oxford, England. At various periods I have benefited immensely from stimulating discussions with Prof. E. A. VINCENT,E. J. W. WHITTAKERand R. G. BURNS. REFERENCES BANCROFT G. M., BURNSR. G. and MADDOCKA. G. (1967) Applications of the Mossbauer effect to silicate mineralogy-I. Iron silicates of known crystal structure. Geochim. Coamochim. Acta 31, 2219-2246. BANCROFT G. M., BURNSR. G. and STONEA. J. (1968) Applications of the Mossbauer effect to silicate mineralogy-II. Iron silicates of unknown and complex crystal structures. Geochim. Cosmochim. Acta 32, 547-559. BERLINR. and HENDERSON C. M. B. (1968) A reinterpretationof Sr and Ca fractionation trends in plagioclasesfrom basic rocks. Earth Planet. Sci. Lett. 4, 79-83. BERLIN R. and HENDERSONC. M. B. (1969) Distribution of Sr and Ba between the alkali felspar, plagioclaseand groundmassphases of porphyritic trachytes and phonolites. Geochim. Cosmochim. Acta 33, 247-265. BURNSR. G. and FYFE W. S. (1966) Distribution of elements in geological processes. Chem. Geol. 1, 49-56. BUXNS R. G. and FYFE W. S. (1967) Trace-element distribution rules and their significance. Chem. Geol. 2, 89-104.

The distribution of Ga and Rb in some basic volcanic rooks

317

CHAPPELLB. W., COMPSTONW., A~RIENS P. A. and VERNON M. J. (1969) Rubidium and strontium determinations by X-ray fluorescent spectrometry and isotope dilution below the part per million level. Geochim. Cosmochim. Acta 33, 1002-1006. DEVORE G. W. (1955a) The role of adsorption in the fractionation and distribution of elements. J. Bed. 63, 159-190. DEVORE G. W. (1955b) Crystal growth and the distribution of the elements. J. Bed. 63, 471-494. ETVARTA., TAYLOR S. R. and CAPP A. C. (1968) Trace and minor element geochemistry of the rhyolitic volcanic rocks. Central North Island, New Zealand. Contrib. Mineral. Petrol. 18, 76-104. EWART A. and TAYLOR S. R. (1969) Trace element geochemistry of the rhyolitic volcanic rocks, Central North Island, New Zealand; phenocryst data. Con&b. Mineral. Petrol. 22, 127-146. GILL J. B. and MURTHY RA~~AV. (1970) Distribution of K, Rb, Sr and Ba in Nain anorthosite plagioclase. Ueochim. Cosmochim. Acta 34, 401-408. GOLDSMITHJ. (1950) Ga and Ge substitutions in synthetic felspars. J. Qeol. 58, 518-636. GRIFFIN W. L. and MURTHYRAMA V. (1969) Distribution of K, Rb, Sr and Ba in some minerals relevant to basalt genesis. Ueochim. Cosmochim. Acta 33, 1389-1414. HI~UCHI H. and NA~ASAWAH. (1969) Partition of trace elements between rock-forming minerals and the host volcanic rocks. Earth Planet. Sci. Lett. ‘g, 281-287. HOLLAND J. G. and HAMILTONE. J. (1965) Mass-absorption corrections in X-ray fluorescenae analysis of natural igneous rocks and their metamorphic equivalents. Spectrochim. Acta 21, 206-208. NIUHTINUALEG. (1962) The distribution of gallium in Skaergaard rocks and minerals. B.Sc. Thesis, University of Oxford. NOCKOLDSS. R. and ALLEN R. (1953) The geochemistry of some igneous rock series. Ueochim. Cosmochim. Acta 4, 105-142. NOCKOLDSS. R. and ALLEN R. (1954) The geochemistry of some igneous rock series-II. Ueochim. Coeoamochim. Acta 5, 245-285. NOCKOLDSS. R. and ALLEN R. (1956) The geochemistry of some igneous rock series-HI. Ueochim. Coamochim. Acta, 9, 34-77. NORRISHK. and CRAPPELLB. W. (1967) Physical Methods in Determinative Mineralogy (editor J. Zussman), Chap. 4. Academic. ONUMAN., HIGUCHIH., WAKITA H. and NAUASAWAH. (1968) Trace element partition between two pyroxenes and the host lava. Earth Planet. Sci. Lett. 5, 47-61. PHILPOTTS J. A. and SCHNETZLERC. C. (1970) Phenocryst-matrix partition coefficients for K, Rb, Sr and Ba with applications to anorthosite and basalt genesis. Ueochim. Cosmochim. Acta 34, 307-322. SHAW D. M. (1957) The geochemistry

of gallium indium and thallium-a review. Phys. Chem. Earth 2, 164-211. SIEDNERG. (1965) Geochemical features of a strongly fractionated alkali igneous suite. Ueochim. Cosmochim.

Acta 29, 113-137.

WAQER L. R. and MITCHELLR. L. (1951) The distribution of trace elements during strong fractionation of a basic magma, a further study of the Skaergaard Intrusion, E. Greenland. Geochim. Cosmochim. Acta 1, 129-208. WHITTAKERE. J. W. (1967) Factors affecting element ratios in the crystallisation of minerals. Ueochim. Cosmochim. Acta 31, 2275-2288. WHITTAKER E. J. W. and MUNTUS R. (1970) Ionic radii for use in geochemistry. Ceochim. Cosmochim. Acta 34, 945-956. WILSON A. D. (1955) A new method for the determination of ferrous iron in rocks and minerals. Bull. Ueol. Surv. Gr. Br. 9, 56-58.