Deformation of peridotite in the mantle and extraction by kimberlite: A case history documented by fluid and solid precipitates in olivine

Tectonophysics,

71

92 (1983) I l-92

Elsevier Scientific

Publishing

Company,

Amsterdam

- Printed

in The Netherlands

DEFORMATION OF PERIDOTITE IN THE MANTLE AND EXTRACIION BY KIMBERLITEt A CASE HISTORY DOCUMENTED BY FLUID AND SOLID PRECIPITATES

HARRY

W. GREEN

Department

IN OLIVINE

II and YVES GUEGUEN

*

of Geology. University of California, Davis, Calif: 95616 (U.S.A.)

Laboratoire de Tectonophysique, (Received

September

Universitt! de Nantes, Nantes (France)

1, 1982)

ABSTRACT

Green

II, H.W.

kimberlite:

and Gueguen,

A case history

S. Cox (Editors), During precipitates deformed

Deformation

an extensive

coprecipitation

study

of peridotite

particularly

Processes

in Tectonics.

of the microstructure

well contains

porphyroclasts,

transport

precipitates

to the surface.

Analysis

recrystallized on virtually by a variety

of olivine: neoblasts,

and further

coarse-grained,

and (c) statically

pipes,

recrystallized

these highly tablets.

indicating

that exsolution

of analytical

techniques

has determined

that the

COs. The deformation

and the

of a detailed

allow refinement

kimberlite

which displays

all dislocations,

construction

to the surface

from

(a) original,

exsolution xenolith

92: 71-92.

xenoliths

spine1 and the fluid phase is probably

allow

by and

in olivine. One specimen

solid phase is chrome-aluminum microstructure

and extraction

in olivine. In: M. Etheridge

Tectonophysics, of peridotite

three generations

(b) dynamically

display

in the mantle

by fluid and solid precipitates

of fluid and solid phases has been observed

All three generations during

Y., 1983. Deformation documented

model

of the P-T

of our earlier diapiric

path

followed

model of kimberlite

occurred

by the genesis.

INTRODUCTION

A great deal has been learned in recent years about the composition of the upper mantle and about deformation, exsolution and melting phenomena in natural and experimental peridotites. In particular, we now know that the uppermost mantle everywhere it has been sampled contains appreciable quantities of CO, (Roedder, 1965; Green and Radcliffe, 1975) and that the presence of CO* in peridotites produces a marked drop of the solidus in the vicinity of 2.5 GPa pressure (Wyllie and Huang, 1975; Eggler, 1976; Wendlant and Mysen, 1980). Also, deformed xenoliths found in both alkalic basalts and kimberlites are accidental; they are not cognate fragments related to their host magma. Moreover, they suffered their * Present address:

Institut

de Physique

du Globe,

Strasbourg

(France)

’

deformation

at rather high strain rates (Goetze,

1979; Mercier, 1974; Kirby

1979) during

and Green,

1980) or during

1979); this deformation fluid phase (Green During

has been

and Radcliffe,

an extensive

xenoliths distinctly

the generation

1975a; Green. of the magma

its extraction

accompanied

1976; Gueguen, (Green

from the mantle

or followed

1975; Gueguen,

study of the deformation

(Mercier,

by precipitation

1979; Kirby and Green, substructure

1977.

and Gueguen,

of olivine

of a 1980).

in peridotite

(Gueguen, 1979) it was discovered that optically irisible precipitates are more abundant in nodules from kimberlite than in those from basalt.

Moreover, the precipitates become increasingly abundant with greater depth of origin and degree of deformation; six out of eight specimens with mosaic textures showed

abundant

ciassification

precipitates

(see Boullier

of peridotites).

Transmission

and

Nicolas.

electron

1975. for the textural

microscopy

(TEM)

studies

of

selected examples of these and other xenohths demonstrate that submicr~~scopic precipitates are even more abundant, and that they consist of a fluid alone or of a fluid plus a solid phase.

From

the microstructure,

a definite

order

and timing

events can be deciphered which places constraints on the processes occurring upper mantle. For delineation of these events, we have selected one sample

of

in the which

demonstrates all aspects of the process particularly well, and we have utilized a variety of techniques of structural and chemical anaiysis. These techniques are: optical petrography, optical and electron-optical microthermometry, scanning electron microscopy (SEM), energy dispersive microanalysis (EDAX), TEM with associated diffraction and microanalysis, Raman microprobe, and ion probe. The sample chosen is a po~hyroclastic garnet harzburgite from the Thaba Putsoa pipe, Lesotho (specimen number LTP-I I from the collection of the Laboratoire de Tectonophysique, Universitt de Nantes, France). This xenolith contains three generations of ohvine

and

a precipitation

detailed

picture

SAMPLE

DESCRI~I#N

sequence

of its deformation,

which,

extraction.

when

all taken

and journey

together,

provide

a

to the surface.

Petrology and microstructure

Optical petrography shows typical porphyroclastic textures (Fig. la). Orthopyroxene crystals show contorted shapes, kink bands and minor recrystallization. The

Fig. 1. a. Optical represented

recrystallization, b. Optical subgrain

micrograph

(crossed

by porphyrcclasts

(large,

polarizers)

of xenohth

fractured

crystal

and tablets (T) developed

micrograph boundaries

(crossed

polarizers)

during

LTP-I

at top),

annealing

1. Three generations

neoblasts

of olivine are during

dynamic

recrystallization.

of a single olivine porphyroclast

with very smali precipitates.

produced showing

decoration

Larger spots in upper center are secondary

of all

CO, bubbles

on a healed crack. c. Optical

micrograph

porphyroclast between

of an oxydized

is completely

the two crystals

absent

consists

porphyroclast

in the tablet,

of about

and a tablet. where dislocations

7 pm of serpentine.

The deformation

substructure

are very rare. The grain

of the

boundary

Fig. 2. Electron a. Back-scattered subboundary

micrographs electron

of precipitates image

with the exterior

in olivine.

of the polished

surface

is marked

surface

of a porphyroclast.

by a NE-trending

The intersection

of a

series of spots which are dark on one

side and bright on the other (SEM). b. Detail of a. Dark portions faceted

crystals

of precipitate

images are roughly

circular

craters,

while bright

portions

are

(SEM).

c. TEM mosaic of intersecting

(001) and (100) subboundaries.

Solid phase crystals

occur as triangular

and

TABLE

I

Recrystallized (Goetze,

50-80

grain sire

1975b; Post, 1977)

Tilt wall spacing

Dislocation

(Durham

(Durham

et al., 1977;

Gueguen

and Darot,

et al., 1977)

20-30

80 MPa

MPa

density 1980)

MPa

larger, older olivine crystals are intensely deformed; kink bands and subgrains are present in all porphyroclasts (Fig. lb). The recrystallized olivine has an average grain size of 160 pm, and many grains exhibit the same deformation features as the porphyroclasts. Oxidation of specimens by the technique of Kohlstedt et al. (1976) reveals the dislocation substructure (Fig. lc): (100) tilt walls are dominant, their average spacing in both the primary and recrystallized grains is 12 pm and the average free dislocation density is 8 - lo6 cm -*. This substructure reflects the dominance of the high temperature glide systems (Ok/) [lOO). Using these microstructural parameters as paleostress indicators, one gets the results of Table I (Gueguen, 1979). We conclude from these data that the rock was deformed and dynamically recrystallized under a stress of 50-100 Mpa *; the lower stress indicated by the free dislocation density probably reflects a brief annealing episode during which some dislocations were annihilated or climbed into walls. This annealing episode is confirmed by a third generation of olivine grains in the form of faceted tablets which contain only rare dislocations and are clearly post tectonic (Fig. 1).

hexagonal

platelets

indicating

a topotoxial

parallel

with some straight bubbles

always

Only

show

the larger

curved

are also decorated

d. Detail of small precipitates out of contrast.

are simultaneously

segments

bubbles

interfaces.

Note

on a (001) boundary lie precisely

against

show straight bimodal

with very small bubbles

The solid platelets

in strong

with the olivine. The fluid phase consists

and some curved boundary

the solid precipitate. dislocations

to (100X and all platelets

relationship

diffraction

contrast,

of equidimensional

bubbles

olivine, but always a planar boundary

segments

size distribution

against

of bubbles,

interface olivine; and

with small

that

free

dislocations

are

(loo0 kV).

viewed parallel

to [OlO]. The boundary

in (100) and the bubbles

have curved

surfaces

against

olivine (100 kV).

* Very recent results,

work on forsterite

suggesting

these estimates. to retain subject

that the stresses

However,

the conclusions to change

single crystals involved

the agreement reached

as improved

(Gueguen

and Darot,

in this and other natural

of the recrystallized

above. The reader

data are collected.

should

grain-size remember,

1982) is in conflict deformations

with the other estimates however,

with these

may be lower than causes us

that such estimates

are

7h

Precipitutes Careful

observation

of subgrain

where with small spots at about (Fig.

lb, c). SEM and TEM

having

two phases,

surfaces

a volatile

boundaries

micrographs

the presence

show that

fluid and a crystalline

shows holes with a faceted

confirms

shows them to be decorated

the limit of resolution

of bubbles

crystal

of the optical these spots

every-

microscope

are precipitates

solid (Fig. 2). SEM of polished

attached

to one side (Fig. 2a. b). TEM

along dislocations,

each bubble

defined by a single, tabular crystal (Fig. 2c, d). The precipitates all dislocations in all three generations of olivine; specifically,

having

a flat side

are found on almost the free dislocations

are decorated as well as those bound into the subgrain boundaries. The maximum size of these composite precipitates is about 0.6 pm diameter so that it is not possible to resolve the two phases by optical

microscopy

Analysis of the solid precipitates Three different methods were used to identify the chemistry and the crystallography of the solid precipitates: X ray dispersive microanalysis, Raman microprobe and electron diffraction. All yielded consistent observations. The first method was used with a scanning electron microscope equipped with an EDAX

system.

Analyses

were made

on a precipitate

and in the adjacent

olivine

matrix. The superposition of the two spectra (Fig. 3a) shows the presence of Al and Cr (and the absence of Ca) in the solid phase, a fact which is consistent with the bright

contrast

observed

in back-scattered

electron

images

(Fig.

2b) and

which

suggests chrome spine1 as a possible candidate. Similar (micro) analysis by TEM confirmed the Al and Cr peaks. In neither case was it possible to obtain quantitative information. The second again

method

in comparing

Twenty-five

two well-defined

amounts

EDAX spectra

(olivine+

precipitate).

were carefully

precipitate.

lined with a very thin amorphous face developed

against

et al., 1979) consisted

matrix

polished

the 450-750

and in the precipitates.

and washed

cm-’

of any trace of

range in which there are

(the main one at 700 cm- ’ and a secondary

taken (i) between Gray

peaks

the precipitates

indicate

that

The bubble

has been pierced

one

of Fig. 2b. (olivine) and (ii) on

the precipitate

of Al and Cr which are lacking in olivine. but does not contain

b. Detail of composite crystal

(Dhamelincourt

in the olivine

We investigated

peaks for chromite

Fig. 3. a. Superimposed precipitate

microprobe)

obtained

pm thick sections

resin for the analysis.

one

(Raman

spectra

contains

significant

Ca (probe beam 1 pm diameter).

in this very thin foil: it appears

to be

film. Analysis

of the accompanying diffraction pattern indicates that the olivine is (1 I l),, = (IOO),,; against the fluid the (I 11) face is modified by

(100) facets. c. Selected-area

electron

diffraction

pattern

[loo],, = lIltI,,; [OlOl,, = +[1121,,. d. Optical micrograph (crossed polarizers)

of precipitate

in b. The solid precipitate

of the edge of a TEM

foil. Decorated

is spine1 with

subboundaries

trend

1 i

I

Fe

I Ni

Cr

approximately

perpendicular

the foil thickness

was reduced

to the thin edge of the crystal. below about

1 pm.

Most optically

visible bubbles exploded

where

at 565 cm- ’ (Wilkinson,

1973)). However,

since there are also four peaks for olivine

in the same range (465, 546, 602, 604 cmbe safely used. Analysis absent in the matrix. The last method show

a solid

demonstrated

used was classical

precipitate

selected area electron

on the side of a bubble

perforated during the thinning process) solid phase. Using olivine as an internal precipitate

to be isometric,

ship [loo],,, i= [ll I],,:

’ ), only the 700 cm _ ’ chromite peak can

that this peak is present

(which

in the precipitates

diffraction.

but

Figure 3b, c

in this case has been

and the electron diffraction pattern of the standard, the diffraction pattern shows the

with a cell parameter

a = 8.24 A. The topotactic

[OOl],,, = +[l lO],, is the same as that already

relation-

described

by

other authors for spine1 exsolution in various olivines (Champness, 1970; Ashworth. 1979); the close-packed oxygen planes are common to both crystals. We conclude that the solid precipitates are indeed chrome spinel. Anulysis of the fluid precipitutes Various methods also were used for fluid analysis,

but in this case they failed to

reveal the identity of the major components. Raman spectroscopy failed because the bubbles were too small (fluids yield a much weaker signal than do solids (Dhamelincourt et al., 1979)). Optical microthermometry also failed because of the size of the bubbles. With a maximum diameter of 0.6 pm, the bubbles are at the limit of resolution of the optical microscope so that it was impossible to detect any change presence of spine1 platelets by cooling them down to - 150°C. The ubiquitous probably also interfered with this technique. We attempted to extend this method to electron nonperforated

bubbles

require

that

they

microscopy.

are filled

with

HVTEM

an electron

images of -scattering,

amorphous phase (Fig. 2c, d), but observation of these bubbles at temperatures down to - 100°C showed no contrast modification. TEM of perforated bubbles shows them to be empty, with a thin film of amorphous sohd on their surfaces (Fig. 3b). Lastly, optical examination shows that in the very thin (< 1 pm) portions of TEM foils, explosion fractures emanate from larger bubbles (Fig. 3d). To further characterize the fluid phase, ion probe microanalysis was also performed. Charging

effects, multiply

charged matrix ions (e.g. 24Mg2’ ) and multi-atom

particles prohibited analysis of carbon and certain other species (Green, 1979). s2Cr and 27A1 as internal “locators”, we determined that the However, using precipitates contain concentrations of masses 23, 39 (Fig. 4) and 47. By analogy with the prior analysis of secondary CO, bubbles from Hawaiian xenoliths (Green, 1979), we conclude that these signals represent 23Na, 39K and 47Ti (or 3’P’60), and that these elements are in the vapor-deposited amorphous film which covers the surfaces of these bubbles

(Fig. 3b).

Late phenomena Three other phenomena

were observed

in relationship

to the precipitates

de-

Fig. 4. Ion microprobe images of an olivine porphyroclast with precipitates. Fields of view (mm diameter). (a) Mass 27(Al); (b) mass 52(Cr); (c) mass 23(Na); (d) mass 39(K).

scribed. These clearly postdate the onset of precipitation and are the last microstructural features to be produced. Firstly, many of the larger bubbles are associated with trains of (100) prismatic dislocation loops (Fig. 5); in most cases each train has been transformed into a helix (Fig. 5b, c) by interaction with a [ 1001screw dislocation (Gueguen, 1979; Kirby and Green, 1980). Rarely, (001) loops are also produced and converted into helices by interaction with [OOl] screws. The loops are produced by differential thermal expansion and/or differential compressibility between host olivine and precipitate. Such loops can be expected when precipitates are large enough so that the differential dilatation becomes equal to or larger than that of two loops. Straightforward calculation shows that this should occur when R ) 0.25 pm, where 2R is the precipitate diameter (Gueguen, 1979). Most larger precipitates occur on subgrain boundaries, yet only a small fraction

Fig. 5. a. Detail of larger precipitates somewhat

darker

by greater

expansion

to intersection

of the fiuid during

with foil surfaces

b. (100) subboundaries. have been turned

on a (100) subboundary.

gray than the fluid. (100) prismatic (TEM,

travel to the surface.

are in poor contrast,

loops have been punched

into the olivine

Only part of each loop appears

(arrows)

due

1000 kV).

Some larger bubbles

into helices (arrows)

The spine1 crystals

dislocation

on one boundary

by interaction

have punched

(100) prismatic

with (1001 screw dislocations.

Smaller

loops which precipitates

81

of these bubbles show loops or helices. On the other hand, virtually all larger bubbles which are not on boundaries have helices around them. The “punching-out” of loops should be especially pronounced around precipitates on screw dislocations because edges can accommodate the dilatation directly by climb but screws cannot. Moreover, any loops punched from bubbles on [loo] screws will automatically be transformed into helices. In polycrystalline olivine deformed at high temperature (1 lOO-1300°C) and high stresses (0.1-l GPa), the majority of free dislocations are [ 1001 screws (Green and Radcliffe, 1972; Zeuch and Green, 1979; Zeuch, 1980), hence most larger bubbles on free dislocations will be on screws, and we would expect efficient punching and helix formation. We similarly would expect punching on the screw dislocations of (010) twist walls, and we find that the [lOO] screws in such walls are frequently transformed into helices. (Interestingly, the [OOl] screws in these boundaries are not similarly transformed, a difference probably due to splitting of these latter dislocations into partials (Gueguen and Darot, 1982; Zeuch and Green, 1979).) Contrarily, in tilt walls composed of edge dislocations, partial compensation of the differential dilatation by local rearrangement due to climb of boundary dislocations (Fig. 5c) will reduce punching. Only helices punched from larger precipitates are resolvable optically in decorated thin sections, The radius of these helices increases with increasing distance from the precipitate (Fig. 5d, e), requiring that dislocation climb was active during loop and helix formation. The second late phenomenon also involves helix formation, but by a different mechanism. Edge and mixed dislocations (both free and bound into tilt walls) are abundantly decorated with very small precipitates (Fig. 5b). Small stresses exerted on these dislocations after precipitation in many cases have caused them to bow out between the precipitates. If the stresses are sufficiently large, the dislocations can break away from the smallest precipitates (A and B in Fig. 5b). If not, the bowed-out segments can climb and cross slip into an irregular helix, which itself can be further decorated by continuing precipitation (Fig. 6a). The third and final late phenomenon is the formation of secondary bubble arrays

are abundant on the dislocations

of the other boundary. Bowing out between precipitates,

cross slip have broken some of the dislocations

plus climb and

away from the smaller precipitates (A and B) and again

produced helices (TEM, 1000 kV). c. Local rearrangement prismatic

of boundary dislocations

loop formation.

Both the dominant

around larger precipitates

is frequently seen in lieu of

b = [lOO] edges (fainter contrast)

and the subordinate

b = [OOl] edges (stronger contrast) in this (100) tilt boundary show significant climb in the vicinity of the two larger bubbles. The b = at each dislocation

[ IOO]dislocations also show, deflection around the small bubbles precipitated

intersection

(TEM, 1000 kV).

d, e. Conical helices in a neoblast (d) and a tablet (e). Prismatic Ioops and helix segments

formed at

elevated temperature can climb as well as glide away from the source, resulting in noncylind~cal (optical micrographs).

hehces

in healed cracks (Fig. 1b, Fig. 6) (Roedder, bubbles

in secondary

arrays are generally

cases they can be seen to consist Brownian

motion

inclusions

in the literature.)

bubbles

1965; Green

of two fluid phases;

within a liquid. (It is these bubbles

are CO, (Roedder,

and Radcliffe,

larger than the precipitates

The optical properties

a vapor bubble that are commonly

definitely

1965; Bilal, 1978; Murek

1975). The and in many

is in constant called fluid

show that the secondary

et al., 1978). Loops punched

from secondary bubbles show no climb, and never display precipitates. An important point to notice is that both primary and secondary clean; no serpentine develops at the fluid-olivine interface.

bubbles

are

83

Fig. 6. a. Stereo pair of hehces produced slip. The b = [OOI] dislocations irregular

helices in which the dislocations

the original

position

faint contrast

of the dislocation.

edge and mixed dislocations

longer images and are in strong

have broken The b = [ IOO]

away from smaller dislocations,

and they clearly show the early precipitates

after the hehces formed

(TEM,

b. Optical

(crossed

principal

from decorated

show generally

micrograph secondary

bubble

(shorter

and, in addition,

by climb and cross

contrast.

precipitates

They consist of which now mark

images), however,

are in very

small precipitates

which grew

1000 kV). polarizers)

of secondary

array crosses decorated

subgrain

CO,

bubbles

boundaries,

on healed the secondary

cracks. bubbles

Where

the

are larger

(arrows).

DISCUSSION

Synthesis

of results

The foregoing data provide a clear picture of the processes affecting this rock during the period just preceding its extraction from the mantle, and during its

journey

to the surface

features

to the degree

recrystallization that it reached

during

in the enclosing observed rapid

the surface

the magma (Green, of low temperature

kimberlite.

indicates

that

The preservation

of deformation

the rock was undergoing

flow at the time it was picked

dynamic

up by the magma

after not more than a few days (probably

and

much less) in

1976: Gueguen, 1977, 1979; Mercier, 1979). Moreover. the lack (1 lO} slip bands and the presence of annealing recrystallization

indicate that no significant deformation occurred during the trip to the surface. The annealing almost certainly occurred in the magma because of the requirement of a very short time between the end of deformation and quenching to temperatures below which dislocation rearrangement and grain boundary migration are inhibited. We have no definite evidence as to the time of onset of precipitation, but the presence continued cannot

of precipitates in the until after the cessation have commenced

boundary

before

(Fig. 7, p. 89). Fluid

repeated

attempts

third generation crystals shows that it clearly of annealing recrystallization. Spine1 precipitation the xenolith

crossed

the garnet/spine1

precipitation

is harder

directly

the fluid within

to characterize

to constrain

peridotite

because

the primary

despite

bubbles,

the

only direct information we have is that they contain the same traces of alkalies which are found in the secondary CO, bubbles of other xenoliths * (Green, 1979). The fluid is clearly under high pressure because the bubbles explode when the enclosing olivine is sufficiently thin. Also, penetrated bubbles show no evidence of glass, so the viscosity of the fluid must be low. The secondary bubble arrays (which the optical properties clearly identify as CO,) show a marked tendency for larger bubbles to form along the line of intersection of the healed cracks and precipitatedecorated subgrain boundaries. The failure of electron microscopy to detect any crystallization of the bubble contents down to - lOO”C, indicates that if the primary (precipitated) bubbles are CO,, their internal pressure exceeds 1.5 GPa. This rock originated at a pressure of approximately 6 GPa. hence such fluid pressures are quite possible. We conclude that the primary bubbles also probably are filled with CO,. It appears, therefore, that neither of the precipitate phases is stable under the conditions

of origin of the xenolith

(Fig. 7); all of the precipitation

had to take place

during transport to the surface. The habit of the spine1 flakes suggests that they may have nucleated on pre-existing bubbles: they take their basic triangular lamellar shape and orientation from the topotaxy constraints imposed by the olivine, but the occasional asymmetrical development of the modifying (100) faces (Fig. 3b) suggests that one (11 I} face was free to modify during growth into the fluid. whereas the other was constrained to remain planar by the low interfacial energy of the semicoherent { 11 I} interface with olivine. Precipitation was clearly a continuing process. This is suggested by the bimodal size distribution of precipitates on subgrain boundaries, and it is demanded by the * Note presence

added

in proojI

of carbon

Preliminary

analysis

by electron

within the fluid precipitates.

energy

loss spectroscopy

(EELS)

confirms

the

85

complicated

interaction

between

precipitates

loops and helices could not begin had

fallen

largest

sufficiently

bubbles.

to generate

During

tion. Others,

however,

journey

crossed

the requisite

precipitates

precipitates

would

beside them, reflecting

curve)

The observed

and

increase

of dislocation and the pressure

concentrations

around

be expected

continued in helix

the

to form.

this earlier precipita-

on the helices

at all. Thus, precipitation

the carbonate-out

to the surface.

Punching

were formed stress

show only the later precipitation

and still others have no precipitates xenolith

until precipitates

this time smaller

Many helices show “ghost”

and helices.

started through diameter

themselves,

early (after the much

of the

with increasing

distance from the source of the larger helices requires climb of their edge dislocation portions and therefore also implies that the temperature was still elevated. At still lower temperatures (and probably greater stress concentrations), fractures were generated which were filled with fluid (probably from exploded bubbles) and then were healed to give the secondary bubble arrays. Lastly, some of these secondary bubbles

also punched

Comparison

loops, but these show no climb or precipitates.

with other xenoliths

It is important to emphasize that this specimen has been chosen for extensive study, not because it is unique or bizarre, but rather because it exhibits almost the entire spectrum

of substructures

we have seen in the few hundred

xenoliths

we have

studied over the last decade. Thus, it allows delineation of a particularly clear picture of upper mantle processes. The precipitation history depicted here, for example, makes

it likely

that

the increase

of precipitate

abundance

with greater

depth

of

origin in xenoliths from kimberlites probably reflects an increasing solubility of carbon with increasing pressure. Alternatively, it may be telling us that the abundance of carbon increases with depth in the subcontinental mantle. We also see a marked contrast between this precipitation history and that deduced for xenoliths from alkalic basalts. In the present case, fluid precipitation succeeds deformation; it takes place in the magma. In the xenoliths from basalts, however, the precipitates are almost

never visible

optically

and precipitation

takes place during deformation;

it

usually terminates before the end of dynamic recrystallization (Green and Radcliffe, 1975; Kirby and Green, 1980). Another difference is that the precipitates in spine1 peridotite xenoliths from basalts consist only of fluid. This could be simply a pressure effect on solubility, but it also might indicate that the partitioning between garnet and olivine for Al and Cr is significantly different than for spine1 and olivine. Both suites of xenoliths record the same late decompression phenomena, except that climb of punched loops and helices appears to be absent in xenoliths from basalt. Also, grain boundary bubbles are commonly seen in xenoliths from basalts (Green and Radcliffe, 1975; Green, 1976), but the ubiquitous presence of a serpentine film on grain boundaries of nodules from kimberlite precludes observation of their boundaries.

The discovery

of spine1 exsolution

this has been observed. been observed

Exsolution

in terrestrial in xenolithic by internal

of spinels

is by no means

the first time

of a wide range of compositions

has

olivine (Deer et al., 1962: Arai, 1978). lunar olivine (Bell

et al.. 1975) and meteoritic also reported experimentally

in this xenolith

olivine (Ashworth.

1979). Spine1 lamellae

in olivine

were

olivine by Roedder (1965), and magnetite was produced oxidation of olivine by Champness (Champness, 1970). In

all cases which have been studied, the same topotoxial relations as reported here were demonstrated or implied. Although precipitation of spine1 in olivine usually is accompanied

by simultaneous

exsolution

of a silicate

and development

of a sym-

plectite (Bell et al., 1975; Arai, 19781, Ashworth (1979) found essentially pure FeCr,O, exsolving alone from meteoritic olivine. He postulated that this may imply some

high temperature

solubility

of Cr in tetrahedral

sites. We similarly

lack a

silicate co-precipitate. but the high pressure origin of our rocks and the evidence carbon solubility make us hesitate to assume stoichiometry. We presume

for the

majority of the Al occupied tetrahedral sites and Cr resided in octahedral sites, but since we cannot quantify our data we can go no further. We can conclude from the former studies, however, that solubility of Al and Cr is significant at high temperatures and low pressures, and that the precipitation is principally a down-temperature reaction.

The precipitation

of CO,, however, is largely a down-pressure

reaction.

Sohbility of carbon

The observations presented here indicate that the carbon-bearing phase which precipitates is CO,. This is also the phase commonly found in secondary bubbles in xenoliths

(e.g. Roedder,

1965; Bilal, 1978), but primary

On the other hand, kimberlite

pipes are frequently

dolomite

diamond

is extremely

or graphite-bearing,

rare. and

these phases are also occasionally found in the xenoliths themselves. The lower oxygen fugacity implied by the native carbon phases is supported by the recent work of Arculus and Delano (1981) who reported intrinsic oxygen fugacities of mantle minerals

at iron-wustite.

It is not

clear,

therefore

whether

the

carbon

species

dissolved in olivine was CO,, or whether reduced carbon may have been oxidized during precipitation, analogous to that proposed by Knobel and Freund (1980) for MgO. In our discussions here, therefore, “carbon solubility” simply means dissolution of carbon in some form; the specific defects involved are unknown. The volume of fluid now enclosed in the olivine crystals can be estimated from the average spacing of (100) subgrain boundaries (12 pm), the average diameter of the larger bubbles (0.5 pm), and the average spacing of bubbles on (100) boundaries ( - 3 pm). These figures yield a concentration of 600 ppm fluid by volume. This is a lower limit because the larger bubbles are also found on the less abundant (001) and (010) boundaries. The smaller bubbles, which occur on all boundaries and most free

87

dislocations have been omitted from the calculation. The contribution from these other bubbles will be secondary, however, almost certainly they would not double the estimates. We arrive, then, at a fluid (presumably CO,) solubility of the order of 0.1% by volume if the olivine was saturated when it entered the magma. These concentrations are not large and the fluid is unlikely to have a specific gravity exceeding 1.5. Nevertheless, the implied carbon solubility is difficult to understand, given the very small size of the carbon ion. This same problem arises with the great increase in solubility of CO, in peridotite melts above 2.5 CPa, and we presume that we are looking at a similar (but smaller) effect in olivine. For melt, this has been attributed to the stabilization of the (CO,))’ radical (Eggler, 1976), but the structural implications of such a stabilization are not clear for the melt, and even less clear for olivine. We believe that simple substitutional solution of carbon in the amounts implied here is unlikely; at these high pressures, new point defect types must become important. In this regard, a recent paper by Freund et al. (1980) is of particular interest. These authors have conducted an exhaustive investigation of “pure” MgO crystals and reported a very large concentration of neutral carbon (0.1 atomic % in the bulk, with strong enhancement near surfaces) which is in solution. Their preli~na~ studies of arc-melted synthetic forsterite and natural olivine lead them to believe the same may be true for olivine. The natural crystals they examined are known to contain CO, bubbles and our experience suggests they may contain submicroscopic bubbles similar to those reported here. It is not yet clear, therefore, whether they have detected carbon dissolved in olivine or merely broken bubbles and analyzed the CO, from them. Nevertheless, their rationale for carbon solubility in olivine might apply to its high pressure solubility evidenced here. The new varieties of point defects postulated, especially O- , could have important implications for the mechanical and electrical properties of the upper mantle, as well as for current concepts of the solubilities of foreign cations in silicates

Our observations are consistent with an internal source for the fluid observed. The ubiquitous occurrence of CO2 in mantle xenoliths throughout the world suggests that this is a normal phenomenon, and not evidence of metasomatism preceeding or ac~mpanying the magmatic activity which brings the nodules to the surface. On the other hand, the lack of serpentine on bubble surfaces suggests that the H,O now present in serpentine probably entered the xenoliths during a very late stage of the eruption. MODEL OF KIMBERLITE GENESIS

The observations reported here provide some new pieces to the puzzle of kimberlite origin and intrusion. They can be used, along with the important

xx advances

in knowledge

of the melting

behavior

of the peridotite-CO,

interpretation

of dislocation

and Gueguen,

1974). Figure 7 shows our current

We do not attempt both convective

microstructure,

quantitative

calculations

heat transport

likes by suitably

varying

to update

model (Green

view of the status of this problem.

as we did before because

and shear heating,

the contributions

system and the

our previous

by allowing

one can get whatever

answer one

of the two effects. We have shown that

the purely convective problem gives reasonable numbers, and Goetze (1975a) has shown that the temperature rise implied by the “kinked” geotherm can be produced by shear

heating

alone.

if one makes

his assumptions.

It follows

that any inter-

mediate model can be made to fit the data quantitatively; further calculations. therefore, provide no new information. The “kink” in the geotherm has been questioned on geochemical grounds by a number of authors. The deformation microstructures of these rocks, however, suggest a late deformation at higher than normal stresses which would have to be accompanied by at least some shear heating (Goetze, 1975a; Mercier, 1979). Therefore, whether the magnitude of the steepening is as large as in Boyd’s original paper (Boyd, 1973) is open to question, but the presence

of a kink in the geotherm

appears

to us to be an expected,

not a bizarre

effect. We retain explains undepleted

2000

our basic,

the correlation xenoliths

I

(

1

diapiric,

model

(lherzolites)

,

because

of the change

,

,

from

with

/

,

it is still the only

depleted

the transition

,

,

/

xenoliths from

undeformed

(

1800

EXTRACTION

OF MELT

-

MAGMA

....‘.’

XENOLITH

PATH

FL”lOlZE0

0

2

4

6

8

PRESSURE

Fig. 7. Proposed

model of kimberlite

IO

PATH PATH

12

14

(GPO)

generation

and extraction.

model

which

(harzburgites)

See text for discussion.

rocks

to to

89

deformed

rocks (Boyd,

which envisions

upwelling

must be explained material.

However,

xenoliths

require

significant

1973). This correlation is a natural consequence of a model of fertile mantle

by ad hoc arguments the stresses implied a reevaluation

process. The uncertainty

of determining

into or through

depleted

lithosphere

for models not calling for transport by the olivine microstructure

of our

earlier

dismissal

in the piezometers,

the strain in these xenoliths

in the deformed

of shear

and the present

makes a unique

but

of solid

diapiric

heating

as a

impossibility

path impossible

to deduce. We here adopt a path closely similar to our original deduction, but we assume somewhat slower upwelling and allow for a shear heating phase at the end. This allows upward transport to provide the heat source for melting as well as the chemical discontinuity between deformed and undeformed xenoliths, and at the same time takes account of the probable shear heating effects during the more rapid deformation associated with the coalescence of the magma and/or its initial upward acceleration. As before, initiated

we have no direct

upwelling;

evidence

of the origin

we assume that it originated

of the instability

by a small perturbation

which

on a shield

geotherm. For lack of any other information, we assume a diapir large enough that its central axis follows an adiabatic path and that it cools by conduction on its margins, producing a temperature profile as shown in Fig. 7 (see Green and Gueguen, 1974 for details). At the time of decompression melting and consequent magma separation, we assume a significant shear heating the diapir, which steepens the “kink” in the geotherm.

contribution in the top of If one wished to place a

greater emphasis on the shear heating term, one could postulate upwelling ing anywhere deeper than A’. The important constraint is that the upwelling reach the solidus general

at B. We have constructed

form of the experimental

our solidus by requiring

originatmaterial

it to be of the

studies of Eggler (1976) and of Wyllie and Huang

(1975), but further constrained it to pass through the 1.5 GPa and 3.0 GPa points of Wendlandt and Mysen (1980) *. The greatest disagreement in the solidi determined by Wyllie and Huang

(1975) and by Eggler (1976) is in the slope at pressures

3.0 GPa; the former workers determined

a slope shallower

above

than that shown in Fig. 7,

and the latter found a steeper slope; we have simply constrained

our solidus to pass

above the deepest xenolith on the Lesotho geotherm of Boyd (1973). We consider this to be the best guess currently possible for the solidus of a garnet lherzolite mantle for which the CO, content exceeds the H,O content by more than an order of magnitude. Upon magma separation near B, the short time required between xenolith incorporation into the magma and kimberlite intrusion suggests an initially adiabatic magma path. Upon entering the magma, Xenolith LTP-11 simultaneously begins to heat up and to move upward. It must follow a P-T trajectory of the form CDE in * Implicit in useOfthis solidus is the assumption that the oxygen fugacity of the mantle is hj& enough to carbonate. If this is not the case, then the shape of the solidus is unknown.* Note a&jed in p,.oof,

stabilize

Fig. 7: the exact path depends upon the size of the xenolith and the rate of ascent of the magma. The lack of appreciable partial melting in Lesotho xenoliths suggests

that

the residence

presumably

time above

begins at point

the solidus

was short.

C’. If CO2 precipitation

exsolved phase probably would be a carbonate considerably greater than the experimentally

Annealing

should

plus enstatite determined

recrystallization

begin immediately,

the

because the pressure is decarbonation curves

(Newton and Sharp, 1975; Wyllie and Huang. 1975; Eggler, 1976). However, the xenolith path must cross the decarbonation curve (point E in Fig. 7); any enstatite + carbonate present in the olivine should react to yield CO, bubbles, and all precipitation at depths shallower than E would be fluid. We have found no evidence to suggest pre-existing carbonate + enstatite precipitates, but at the high temperatures involved, it is conceivable that they existed and have left no textural trace. Upon reaching point F the silicate portion of the magma crystallizes, changing into a fluidized mass of C02, crystals, and xenoliths with a suddenly viscosity; boundary

the eruption probably accelerates. At point H the garnet/spine1 peridotite is crossed and spine1 can begin to exsolve. (The presence of appreciable Cr

in the spine1 suggests that the spine1 stability field may be entered (MacGregor, 1970) allowing the possibility that spine1 exsolution point

the melt reduced

H.) Further

upwelling

and helices being

punched

after exsolution

has begun

out with simultaneous

results

dislocation

at higher pressures commences before in dislocation

climb

loops

producing

the

conical helices (region I in Fig. 7). This will be restricted to temperatures exceeding about 1200°C (Ricoult, 1978). At still shallower depths, J, cracks are generated and healed to produce the low pressure secondary arrays of bubbles. The great expansion of the fluid should efficiently cool the kimberlite to low intrusion temperatures. Moreover, this explosive manner of eruption strongly favors entrance of crustal H,O during or after the final phase of intrusion, resulting in the observed serpentinization of every grain boundary and every crack in every xenolith. This does not preclude the presence of mantle H,O on grain boundaries before intrusion. but the complete lack of serpentinization

around

either primary

or secondary

fluid bubbles

suggests

that the H,O content of the mantle fluid was very low; if the xenoliths contained significant H,O, it must have been strongly partitioned into the “anhydrous” silicates. In summary,

we conclude

that a very simple model is consistent

with all of the

data currently available: The system considered is known to be capable of producing kimberlitic magmas; the observed phases are seen to develop under known stability conditions; and the short times required by the kinetics of olivine recovery are consistent kimberlite

with the field observations intrusion.

concerning

the temperature

and

nature

of

91

ACKNOWLEDGEMENTS

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