SMITHSONIAN CONTRIBUTIONS TO THE

EARTH SCIENCES

NUMBER 1

George SwitZer Partially Melted

and William G. Melson Tr • -r» i •

Kyanite rxlogite

from the

Roberts Victor Mine,

South Africa

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ABSTRACT

Three specimens have been studied of the rare kyanite eclogite nodules in kimberlite from the Roberts Victor mine, South Africa. All are essentially the same

with the primary assemblage: kyanite, omphacite, garnet, diamond (in one sample),

chrome diopside, and rutile. There is also present a fine-grained secondary assemblage

that appears in two forms: (1) primary omphacite altered to a mixture of plagioclase,

clinopyroxene, and possibly glass; and (2) thin layers along omphacite, kyanite, and

garnet grain boundaries. These layers have a clear-cut igneous texture and consist of

plagioclase microlites with glass or devitrified glass, or plagioclase microlites and

subhedral augite, with or without glass. Hornblende, spinel, and calcite are accessories,

and analcite fills vesicles. Corundum and mullite occur at the margins of kyanite

grains.

The glass in the secondary assemblage has a composition roughly equivalent to

what one might expect if it was derived by incongruent melting of omphacite, followed

by partial crystallization. Omphacite at one atmosphere pressure begins to melt at

about 1030° C and melting is complete at about 1260° C. At 30 kilobars (O'Hara and

Yoder, 1967) melting begins at about 1570° C and is complete at 1600° C. Thus,

sudden pressure release of an eclogite at high temperature could cause partial'melting

of omphacite.

These kyanite eclogites clearly contained an interstitial melt that has been rapidly

cooled. Evidence points to this melt having been generated mainly by partial melting

of primary omphacite rather than by introduction of an externally derived melt. The

partial melting may have occurred in response to one of the following three processes

or some combination of them :

1. Increase in temperature at constant pressure.

2. Introduction of water into the eclogite at constant temperature and constant

total pressure.

3. Release of pressure at constant temperature.

The third process seems to offer the most reasonable explanation for the partial

melting.

Introduction

Of the several rock types found as nodules in kimberlite in South Africa, one of the most interesting and

the least abundant is the kyanite eclogite from the

Roberts Victor mine.

The first description of this rock seems to have

been in a letter from Professor Beck of Freiburg,

Germany, to E. H. V. Melville, who read the letter

at the 16 July 1906 meeting of the Geological Society

of South Africa (Melville, 1907). Further descriptions

have been published by Johnson (1907) and Williams (1932). In these studies one of the most inGeorge Switzer and William G. Melson, Department of Mineral Sciences, Smithsonian Institution, Washington, D.C.

20560.

triguing features of the rock, namely evidence of

partial melting, was overlooked.

The purpose of this paper is to describe the evidence of partial melting and to discuss possible causes.

The primary mineral assemblage of these kyanite eclogites is also of special interest and will be dealt with

in another paper.

Three specimens of kyanite eclogite were available

for study, USNM 87375, 110752, and 111060. The latter two were recently donated by the Geological Department of De Beers Consolidated Mines, Ltd.,

through Mr. D. Du Toit.

Evidence of partial melting is so pronounced in the

Roberts Victor mine kyanite ecolgite that it was in

these it was first recognized. Once having been rec1
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

ognized, other eclogites (without kyanite) examined

from the Roberts Victor mine revealed that some

of these too have interstitial glass, possibly derived

by partial melting. A study of the secondary assemblage

of normal (non-kyanite bearing) eclogite from the

Roberts Victor mine is underway and will be reported

on separately.

Mineralogy

The Roberts Victor mine kyanite eclogite is made

up of distinctly different primary and secondary mineral assemblages. The primary assemblage is a course

aggregate of garnet, omphacite (largely altered), and

kyanite, with diamond (in one specimen), chrome diopside, and rutile as accessories. Microprobe analyses

made possible the elucidation of a complex, fine-grained

secondary assemblage that appears to be largely the

result of partial melting of primary omphacite. Table

1 lists the primary and secondary mineral assemblages.

OMPHACITE.—This primary mineral played a key

role in the generation of the secondary assemblage.

Data on relicts of unaltered omphacite (Figure 1) are

given in Table 2.

ALTERED OMPHACITE.—The omphacite in these

rocks is altered almost completely to a peculiar yet

characteristic gray to greenish-gray, nearly opaque

product. The grain size of the alteration product

ranges from submicroscopic to a birefringent aggregate

in which at least two phases can be seen but not positively identified by optical properties or microprobe

analysis (Figures 1, 2). Some areas that exhibit a

blotchy appearance in ordinary light, under crossed

nicols show curved, stellate plagioclase crystals, giving

a texture very similar to that frequently seen in

quenched silicate melts (Figure 2).

Peaks on an x-ray diffraction pattern can be accounted for by plagioclase and cl.inopyroxene but the

composition of these phases cannot be determined by

this method. The presence of glass as a third phase is

suspected but has not been proven.

Electron microprobe analyses of the alteration

product are given in Table 2. Areas selected for

analysis were those that in thin section were extremely

fine grained, and the electron beam was widened to

20^. Comparison with the analyses of fresh omphacite

shows that the reaction was essentially isochemical.

SECONDARY IGNEOUS ASSEMBLAGE.—Most of the

minerals in the secondary assemblage occur as thin

layers along omphacite, kyanite, and garnet grain

boundaries (Figure 3) and, in part, replace the priTABLE 1.—Mineralogy of Roberts Victor mine kyanite

eclogite

Primary assemblage Secondary assemblage

Garne t (grossular - almandine-pyrope)

Omphacite

Diamond (noted only in

111060)

Chrome diopside (noted only

in 110752)

Rutile (noted only in 87375)

Altered omphacite (plagioclase +clinopyroxene+

glass?)

Plagioclase (An29)

Augite (only in 87375 and

111060)

Fassaite (only in 110752)

Spinel

Hornblende

Glass

Analcite

Calcite

Corundum

Mullite

TABLE 2.—Omphacite and altered omphacite from Roberts

Victor mine kyanite eclogite

Fresh omphacite Altered omphacite

111060 110752 110752« 87375 *

SiOj

TiO2

A12O3

Fe 2O8

FeO

MgO

CaO

Na 2O

K2O

a

y

Z A c

2V( + )

Ac

Jd

Ts

Hd

Di

52.1

0.3

17.5

0.3

1.2

7.2

11.9

6.8

0.3

1.664] Na

1.6721± 0.002

1.688J

36°

70°

2

46

14

3

35

53.3

0.3

17.9

0.4

1.4

6.4

12.1

6.4

0.2

3

42

14

4

36

56.0

0.3

18.6

1.9

6.6

11.3

5.0

0.3

56.4

0.4

20.2

2.3

4.2

9.2

5.2

0.0

1

 Analyses by electron microprobe.

2

 Based on other analyses of similar omphacites Fe assigned

on basis that 0.8 of the total Fe is Fe2+. Only specimen 111060

contains enough fresh omphacite for determination of optical

properties.

3

 Average of five analyses. Range: SiO2 55.4—56.6; TiO2

0.3-0.3; A12O3 18.1-19.0; Fe 1.5-1.5; MgO 6.2-7.0; CaO

10.8-12.3; Na2O 4.5-5.5; K2O 0.3-0.3. All Fe taken as FeO.

4

 Average of four analyses: Range: SiO2 55.8-56.8; TiO2

0.3-0.4; A12O3 20.1-20.5; Fe 1.6-1.9; MgO 3.8-4.6; CaO

8.6-9.7; Na2O 4.7-5.5. All Fe taken as FeO.
NUMBER 1

FIGURE 1.—Relicts of omphacite (white) in altered omphacite (gray). USNM 111060.

mary omphacite. Under high magnification the layers

have a clear-cut igneous texture (Figure 4) showing

plagioclase microlites with glass or devitrified glass,

or plagioclase microlites and subhedral augite, with

or without glass. Hornblende, spinel, and calcite are

accessories, and analcite fills vesicles. Where the

secondary igneous assemblage cuts garnet it is frequently composed entirely of augite, plagioclase, and

spinel, without glass, and has ophitic texture.

Data on the minerals of the secondary igneous assemblage follow.

Plagioclase: This mineral has typical microlite

habit and twinning, little or no zoning, and

composition An29.

Augite, fassaite, and hornblende: Augite, a major

mineral of the secondary assemblage, in thin section

is subhedral and colorless. The augite in specimen

87375 is Al-rich and Ti-poor as compared with

normal augite. In specimen 110752 the "augite" contains 18.3 percent A12O3, placing it much closer to

fassaite than augite in composition, although the CaO

content is a little low for this mineral. (Tabulation

of fassaite analyses in Deer, Howie and Zussman,

1963, shows CaO«*25 percent). To our knowledge

such a fassaite-like pyroxene has not been reported

in a similar paragenesis.

In specimen 110752 the fassaite is more coarsely

crystalline than the augite in the other two specimens.

It occurs either as monomineralic rims around garnet,

intergrown with plagioclase, or ophitically enclosing

plagioclase. In thin section it is colorless.

Hornblende, a minor constituent in all three specimens, is subhedral to euhedral, brown and pleochroic

brown to light brown in thin section.
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

FIGURE 2.—Quench texture in altered omphacite. Crystals are plagioclase. Grossed nicols.

USNM 87375.

Microprobe analyses of these three minerals are

given in Table 3.

Spinel: Minute, dark green octahedrons of spinel

are common in the secondary igneous assemblage, particularly where the igneous assemblage is in contact

with garnet. Its composition by microprobe analysis is

approximately Sp40 He60.

Glass: Local areas of residual glass are a prominent

feature of the secondary igneous assemblage. The glass

fills interstices in plagioclase-augite, is pale brown in

color, and isotropic. Some areas are completely clear,

others contain from a few to abundant microlites, and

some have the typical appearance of devitrified glass.

TABLE 3.—Augite, fassaite, and hornblende from Roberts

Victor mine kyanite eclogites 1

Augite Fassaite

in 87375 in 110752

Hornblende

in 87375

Hornblende

in 110752

SiO2

TiO2

A12O3

Fe

MgO

GaO

Na2O

K2O

51.3

0.6

7.4

8.9

11.8

18.6

0.6

0.0

41.7

0.7

18.3

7. 1

7.5

19.9

1.4

0.0

38.6

2.2

17.4

8.2

13.3

10.4

4.0

0.6

37.4

0.6

19.3

7.4

11.3

12. 1

3.0

1.5

1

 Analyses by electron microprobe.
NUMBER I

The best examples of fresh glass are in specimen 87375.

A microprobe analysis is given in Table 4.

TABLE 4.—Glass in kyanite eclogite from the Roberts

Victor mine l

SiO2

TiO2

A12O3

MgO

CaO

Na2O

K2O

Fe

Weight percent

47.7

0.6

23.1

1.6

1.0

9.7

1.9

4.8

Norm2

or 11.1

ab 39.8

an 4.7

ne 22.7

ol 8.6

mg 4.9

il 1.2

c 3.4

1

 Specimen 87375.

2

 In calculation of norm, iron was assigned one-half to Fe2+

and one-half to Fe3+ (3.1 FeO and 3.4 Fe2O3). With this

assignment of iron the sum is 92.1. The difference is probably

water.

Analcite: White, massive analcite fills vesicles in secondary igneous texture layers. Identification was by

microprobe analysis and x-ray diffraction.

Calcite: Calcite is a rare accessory in the secondary

igneous assemblage.

Corundum: Corundum crystals occur commonly at

the margins of kyanite grains. Identification was by

microprobe analysis and optical properties.

Mullite: A frequently seen phase in thin sections is

a fibrous mineral in kyanite that microprobe analysis

demonstrated to be compositionally identical to mullite

(Table 5).

TABLE

SiO2

A12O3

Total

5.—Mullite in

Mullite in

110752

25.9

73.6

99.5

Roberts Victor

Mullite

theoretical

28.2

71.8

100.0

mine kyanite eclogite

Kyanite

theoretical

36.8

63.2

100.0

In thin section the mullite has the following optical

properties: parallel extinction, length slow, index of

refraction less than that of kyanite.

Incongruent Melting of Omphacite

The garnet and kyanite in these eclogites are not altered, except locally in small volumes along their boundaries. The igneous mineral assemblage and the composition of the interstitial glass suggests that they were

involved in only a minor way in the generation of the

melt. On the other hand, the pervasive alteration of

omphacite and the features of the igneous assemblage

indicate that omphacite played a major role in the

formation of the melt and its melting behavior thus

is of special interest here.

The system nepheline-diopside-silica has been investigated by Schairer and Yoder (1960) at one atmosphere, and the results can be used to qualitatively

interpret partial melting of natural omphacite at low

pressure. Mixtures along the diopside-jadeite join begin

to melt at about 1120° C when these mixtures contain

more than 15 percent diopside. The first liquids

(equivalent to residual liquids) are highly undersaturated. The norm of such a liquid would contain mainly

albite and nepheline, a feature that characterizes the

composition of the glass in one of the kyanite eclogites

(Table 6).

TABLE 6.—Norms of primary omphacite and secondary

glass in Roberts Victor mine kyanite eclogite

Omphacite

in 111060

Omphacite

in 110752

Glass in

87375

or

ab

an

ne

di

ol

il

mg

c

ab + ne + di

1.7

24.6

16.4

17.9

33.0

5.2

0.6

0.5

75.5

1. 1

30.4

19.7

12.8

31.3

3.6

0.6

0.7

74.5

11. 1

39.8

4.7

22.7

4.2

1.2

4.9

3.4

62.5

Melting experiments* by the authors on omphacite

from a Roberts Victor mine eclogite show that melting begins at about 1030° C and is complete at about

1260° C. This lowering of the liquidus and solidus temperatures compared to the synthetic system (Schairer

and Yoder, 1960) is probably due to the FeO content

of the natural omphacite, the only important con1

 Charges placed in 60 Pd 40 Ag crucibles, or Pt crucibles

above 1200° C, and crucibles sealed in evacuated silica

glass tubes; vertical quench furnace, Pt 10 Rd thermocouple

calibrated against Au and synthetic diopside standards; furnace temperature readings ±2° C; liquidus and solidus temperatures ±10° C.
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

FIGURE 3.—Layers of the secondary igneous assemblage along grain boundaries of altered

omphacite (black) and garnet (white). USNM 87375.

stituent not included in the synthetic system. Omphacite also melts incongruently at high pressure, but

detailed phase diagrams have not been worked out.

O'Hara and Yoder (1967, fig. 8) have shown that at

30 kilobars omphacite begins to melt at about 1570°

G and is completely melted at 1600° G. The composition of first liquids from the melting of omphacite at

high pressure has not been determined, but it is

reasonable to assume that they will contain considerable

normative nepheline and albite.

Cause of Partial Melting

These kyanite eclogites clearly contained interstitial

magma that has been rapidly cooled. Two hypotheses

may be offered to account for the presence of this

magma:

1. Introduction of an externally derived melt from

the kimberlitic magma.

2. Partial melting of the primary assemblage.

We believe that the latter hypothesis is much more

likely, based on the following evidence: (1) the pervasive occurrence of the secondary igneous assemblage

in very thin layers along all grain boundaries, combined with no more nor less of the assemblage toward

the margin of the nodules; and (2) the composition

of secondary assemblage can be accounted for by partial incongruent melting of the primary omphacite.

The partial melting may have occurred in response
NUMBER 1

FIGURE 4.—Closeup view of a secondary igneous assemblage layer showing plagioclase microlites with interstitial glass. The bordering material is altered omphacite. USNM 87375.

to one of the following three processes, or to some combination of them:

1. Increase in temperature at constant pressure.

2. Introduction of water into the eclogite at constant temperature and constant total pressure.

3. Release of pressure at constant temperature.

The merits of each of these hypotheses are discussed

in the following sections.

INCREASE IN TEMPERATURE AT CONSTANT PRESSURE.—This mechanism might be invoked, for example, by considering immersion of an eclogite nodule

in kimberlitic magma. Based on the present state of

knowledge of the melting of omphacite at high and

low pressure as reviewed earlier, one cannot categorically choose between partial melting at high or low

pressure to explain the decomposition of the omphacite in these kyanite eclogites, if indeed partial melting

did occur at constant pressure. Actually, the nature

of the nodules argues against this interpretation,

which assumes slow heating (and slow cooling) under

constant pressure. If the specimens had undergone

such a history, it seems likely that the partial melting

along grain boundaries would have caused disaggregation of the nodule. Such disaggregation obviously

did not occur.

INTRODUCTION OF WATER AT CONSTANT TEMPERATURE AND CONSTANT TOTAL PRESSURE.—The presence

of hornblende and analcite in the secondary igneous
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

FIGURE 5.—Mullite (M) and corundum (C) on the margin of a kyanite grain (K). The lines

in kyanite are twin lamellae. USNM 87375.

assemblage shows conclusively that the interstitial

magma did contain dissolved water. Indeed, the

vesicles observed in specimen 87375 show that, at some

stage in the crystallization of this magma, the water

content became high enough for the water vapor

pressure of the magma (combined with the vapor

pressure of other dissolved volatiles) to equal the

total pressure.

Introduction of small amounts of water along grain

boundaries into an anhydrous eclogite at high temperature and high total pressure would either generate

a small amount of hydrous magma, or lead to the

formation of a new hydrous phase, or both. If the

temperature was sufficiently high, a melt rather than

a hydrous phase would most likely form.

Kimberlitic magmas are known to contain considerable H2O and CO2. Thus water could have been

introduced into the nodule from its environs. The

low K2O and high Na2O content of the residual glass

and the pervasive occurrence of the secondary igneous

assemblage along grain boundaries indicate that, if

anything was introduced from the surrounding kimberlitic magma, it was only water plus possibly other

volatiles. It is also possible that the small amount of

water that must be accounted for was derived internally, for recent work2

 has shown that practically all

"anhydrous" silicates, including pyroxene, garnet,

and kyanite, contain some water.

1

 C. Frondel, personal communication.
NUMBER 1

Water, either externally or internally derived, or

both, certainly played a role in the formation of the

interstitial magma, but at the present time there seems

to be no way to evaluate its importance.

RELEASE OF PRESSURE AT CONSTANT TEMPERATURE.—As discussed earlier, at 30 kilobars omphacite

begins to melt at about 1570° C, whereas at one atmosphere melting begins at about 1030° C. Thus, sudden

pressure release of an eclogite at high temperature

would clearly cause partial melting of omphacite. Even

if the initial temperature was as low as 1150° C and

the pressure release large, some melting would take

place.

A rapid pressure release implies that there would

also be adiabatic cooling because of insufficient time

for the nodules to exchange heat conductively with

their environs; however, for solids the amount of

such cooling is small because of their small A V. Thus,

initially melting would occur, the amount being determined by the temperature at the time of pressure

release, the magnitude of the pressure drop, and the

composition of the omphacite.

It is difficult to conceive how these eclogites could

have remained intact when each grain boundary was

"coated" with interstitial magma unless the time interval between melting and quenching was short. Melting

on release of pressure would be almost immediate. The

heat required for melting would come from the eclogite

itself; that is, no conductive transfer of external heat

into the nodule would be required. Quenching then

took place almost immediately after the partial melting, by cooling in the rapidly ascending kimberlite

magma. Kimberlite magmas are generally considered

to be gas-rich and may undergo rapid cooling during

ascent because of (1) expansion of an initially present

gas phase, and (2) emission of gas from the magma

due to rapid drop in pressure. The actual importance

of this cooling, however, is difficult to evaluate.

Summary

The presence of diamond in one of the specimens

studied indicates a deep-seated origin for the kyanite

eclogite nodules. The mineralogical, textural, and spatial relationships of the secondary igneous assemblage

are consistent with the view that the unusual features

observed in these nodules resulted from emplacement

into the crust in a rapidly ascending kimberlite magma,

which caused first melting due to sudden release of

pressure followed by quenching due to rapid cooling

in rising, expanding gas-rich kimberlite magma.

These nodules are, to our knowledge, the first described examples of partial melting of very deep-seated

rocks. We do not mean to imply, however, that the

partial melting of these samples is necessarily pertinent

to the generation of magma by the partial melting of

eclogite in the upper mantle. On the contrary, the

partial melting described herein is of a very specialized

nature and may be a result of the specific but as yet

quantitatively unspecified conditions existing during

the generation, ascent, and cooling of kimberlitic

magma.

References

Deer, W. A., R. A. Howie, and J. Zusman

1963. Rock-forming Minerals. London: Longmans, Green

and Co., Ltd.

Johnson, J. P.

1907. Notes on Lherzolite and Eclogite Boulders from the

Roberts-Victor Mine. Transactions of the Geological Society of South Africa, 10:112-114.

Melvill, E. H.

1907. Proceedings of the Geological Society of South

Africa. Transactions of the Geological Society of

South Africa, 9:68.

O'Hara, M. J., and H. S. Yoder, Jr.

1967. Formation and Fractionation of Basic Magmas at

High Pressures. Scottish Journal of Geology,

3:67-117.

Schairer, J. H., and H. S. Yoder, Jr.

1960. The Nature of Residual Liquids from Crystallization,

with Data on the System Nepheline-Diopside-Silica.

American Journal of Science, 258-A: 273-283.

Williams, A. F.

1932. The Genesis of the Diamond. London: Ernest

Benn, Ltd.

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