Garnet-fassaite calc-silicate nodule from La soufriire. st ...

American Mineralogist,

Volume 65, pages 302-305, 1980

Garnet-fassaitecalc-silicatenodulefrom La soufriire. st. vincent

JosEpH D. DEvINE AND HARALoUn SlcunDSSoN

Graduate School of Oceanography, (Jniversity of Rhode Island

Kingston, Rhode Island 02881

Abstract

The mineralogyof an unusualcalc-silicatenodule brought to the surfaoeby the 1902eruption of La Soufrierevolcanoon St. Vincent (LesserAntilles arc) is described.It consistsof the

crystalloblasticmineral assemblagefassaitepyroxene + grandite garnet + wollastonite +

anorthite t calcite.The pyroxeneis more aluminous(Al2O3: 19.66weight percent)than any

previously describedfrom terrestrialrocks, with about 45 mole percent Ca-Al Tschermak's

molecule,37 mole percentdiopside,and 15 mole percent Ca-Fe3* Tschermak'smoleculein

solid solution. Unit-cell data have been calculatedon the basisof X-ray powder diflractometry datafor the fassaite

(a:9.104, D: 8.828,c:5.2MA, F: 106.95.;V:426.49A3) and

the grandite (grossularrr-andraditerr;a: ll.905A V: 1687Ai).We infer a paragenesisrelated to skarn-typemetamorphismof calcareoussediment.

Introduction

The 1902 eruption of La Soufridre volcano on St.

Vincent in the Lesser Antilles arc brought a variety

of nodules to the surface, including calc-silicate

metamorphic xenoliths (Lacroix, 1949; Sandrea,

1949) and cumulus nodules (Lewis, 1964, l973a,b).

We describe here a fist-sized calc-silicate nodule

(+V37472), found in the pyroclastic flow deposits of

the 1902 eruption, which consists of the mineral assemblage fassaite pyroxene + grandite garnet * wollastonite + anorthite t calcite. It is the only known

occurrence of a garnet-bearing calc-silicate xenolith

associated with this volcano. The nodule also bears a

fassaite pyroxene with about 45 mole percent of CaAl Tschermak's molecule in solid solution.

Mineralogy

The texture of the nodule is mainly crystalloblastic

but in places becomessubpoikloblastic with garnet

or pyroxeneenclosingsmaller anhedralgrains of the

other silicate minerals. Becausethe silicatesin the

nodule generally appear to be interlocking, sequencesof crystallization cannot be reliably inferred

on the basis of texture..Calcite occurs as thin (-10

micron) layers separatingsilicategrains and is never

observedto be completely enclosedby any silicate

mineral. Bordersbetweensilicatemineralsare generally sharp,often planar, and show no sign ofreaction

between grains. Where thin layers of calcite occur

between wollastonite and other silicates. however.

0/J0.3--OO4)K

/ 80/0304-0302$02.00

the border betweencalcite and wollastonite is more

serrate, giving the appearancethat calcite has replaced the wollastonite. Small irregular patches of

calcite locally extend from intergranular layers into

wollastonite grains. Other silicate minerals have

sharp borders where they are in contact with calcite

layers.

Apart from small silicate mineral inclusions and

calcite,grain sizesare in the range0.2-0.8mm. Table

I showsthe mode of one thin sectionof the nodule

which is believed to be representativeof its assemblage.

The fassaite pyroxene in the nodule is richer in

alumina than any specimenobservedso far in terrestrial rocks. Table I showsthe mean composition of

19 electron microprobe analysesof the pyroxene.

The mean AlrO, content is 19.66 weight percent

(range 17.16Io 20.94weight percent).Knopf and Lee

(1956) describeda similar skarn-typepyroxene containing 15.75 weight percent AlrO, (the most aluminous pyroxenereportedpreviouslyfrom terrestrial

rocks).

The pyroxenegrains are dark greenin hand specimen and light golden brown in thin section.Anomalous (Berlin blue) interferencecolors appearnear extinction under crossednicols. The axial angle (+) is

about 60-70". Typical pyroxene cleavageis moderately well developed.Neither twins nor exsolutionlamellae were observed.

The pyroxene showsa nearly stoichiometricCaO

DEVINE AND SIGURDSSON: GIRNTET-FASSAITE NODULE

content, implying that all M2 sites are filled by Ca.

We have used the method of Papike et al. (1974) to

calculate relative proportions of Fe2* and Fe'* from

our microprobe data. The results show that over 25

percent of tetrahedral sites are filled by Al and that

over half of the Ml sites are occupied by trivalent

species (Al and Fe'*).

Norms calculated accordi.g to the conventions of

Yoder and Tilley (1962, p. 366-367) show that less

than haH of the pyroxene's normative components lie

within the pyroxene quadrilateral. Tschermak's

molecule accounts for the bulk of the norm with

about 45 mole percent Ca-Al Tschermak's component (CAT) and 15 mole percent Ca-Fe'* Tschermak's component (FTS). The diopside corrponent is

about 37 mole percent of the norm.

As a first approximation this pyroxene may be

viewed as a solution between diopside and CAT. de

Neufville and Schairer (1962) report that the experimental limit of CAT solution in diopside is about 20

weight percent at I atm pressure. The solubility limit

of CAT in diopside may be increased in two ways.

Firstly, Hijikata (1973) has demonstrated experimentally that complete solid solution along the join can

be attained at a pressure of I I kbar (1020"C). Secondly, Wood (1975) has shown that addition of Fe'*

to a pyroxene solid solution increases CAT solubility.

We suggest that addition of Fe'* as FTS in the Soufridre pyroxene has also promoted increased CAT solubility. Increased CAT solubility in the Soufridre pyroxene due to high pressure is considered unlikely

because the required pressure is geologically unreasonable for the inferred source region.

Unit-cell parameters of the pyroxene were calculated using X-ray powder diffractometer data (a :

9.705,b: 8.828,c: 5.2A4A,F : 106.95";unit-cell

volume :426.49A'\. The data of two runs were averaged for the calculation. A Si calibration standard

was used; no unit-cell refinement program was applied. The unit-cell dimensions differ from those expected for ideal solid solution between CAT and

diopside: the c axis is compressed and the B angle enlarged. The unit-cell volu:me is also smaller than that

expected from ideal solid solution (by about one percent), implying a small nr:gative volume of mixing.

The grandite garnets in the nodule are golden

brown in hand specimen and light honey brown in

thin section. They are isotropic but show anomalous

interference along fraclures which cross-cut the

grains. The composition tiTable l) shows that nearly

all X positions are filled by Ca; this also implies that

nearly all iron is Fe'* arLd confined to Y positions.

Table I

Composition of minerals in the V37472 nodule

Anorthite

Calcite

Fass ai te

crandite

sio2

ri02

A1203

3 8 . 0 9 i 8 6 )l

0 72(I0.)

19.b6(85)

38.s1(26)

0.i8(7)

1 6 . 8 4( s i )

49 68(6s)

0 01(ll

1 4s(96)

43 62(17)

00

36 31(20)

036

002

065

FeO*r

Nlno

ugo

8 82(57)

0 14 (4)

7.26143)

7.46(73)

0.28(2.)

0.s2(3)

0.0

0 r7(2)

0.2r(s)

0.0

0.04(0)

0.0

00

0.49

0.35

Cao

Na2O

K20

24.88(3I)

0.03(1)

0.01(r)

3s 44(60)

o 04(1)

0.0

46 47(62)

0 02(l)

0.0

19 71(s9)

o 0s(2)

00

s2 92

o.o

00

Total

99 60(95)

99 87

98.01

99 13

54-79

#anals

3I

3C

l9

Ilodal e,

I'iollastonite

22

73.6

05

and glass

standards;

ndneral

using

analyses

were acconplished

Nlicroprobe

(1968)

alpha

correcBence and Albee

using

was acconplished

data ieduction

(Fingcr

and

PDP-11 niniconPuter

by a slavcd

applied

autonatically

tlons

lladidracos

, It 72) .

lParentheslzc,l

in

terro

38.09(86)

2Feo*

the estinated

represent

figures

to

for

the value

units

cited

of ]east

%.

indicates

an e s d of 0.86 wt

= all

iron

calculateal

as

(e

deviation

staDdard

imcdiate

left,

their

s d)

thus

Feo

Hence, we infer that the garnet's composition may be

expressed as grossularrr-andraditer, (mole). There is

very little pyrope component, most of the Mg in the

nodule having partitioned into coexisting fassaite

(Dffil"o- : 0.07). X-ray powder diffractometer data

were used to calculate the unit-cell dimension of the

grandite (a: ll.905A; unit-cell volume : 1687A').

Nearly pure wollastonite (Table l) occurs as clear

vitreous elongated grains showing good cleavage and

as anhedral mineral inclusions in garnet or pyroxene.

Wollastonite is the only mineral to show complex intergrowths with intergranular calcite. It is uncertain

whether calcite is actually replacing wollastonite or if

the opposite is true, but optically invisible intergrowths of extremely small calcite grains and wollastonite could conceivably account for the low totals in

microprobe analyses of wollastonite.

The anorthite (>Annn) in the nodule is colorless in

thin section; K is present in only trace amounts

(Table l). Anorthite is only a minor constituent and

is most frequently observed enclosed by garnet. Albite twins and one other unidentified twin set are

present. The low alkali content of the anorthite (and

of the nodule in general) is a curious feature of this

xenolith.

Calcite appears as thin intergranular layers in the

nodule and as a possible alteration product of wollastonite, but is only a minor constituent of the nodule.

It is low in Mg (Table l).

Paragenesis

By analogy with other well-documented skarntype occurrences of coexisting grandite garnet and

3M

DEVINE AND SIGURDSSON:GARNET-FASSAITE

NODULE

Table 2. V37472nodule bulk composition

position observed, but the relative compositional homogeneity of the minerals and th€ crystalloblastic

v37472

Nodul e

texture of the nodule strongly suggest that the final

nodule:

bulk comp.

"Average

bulk comp.

silicate mineral assemblage observed is an equilib(all CaO

Sedinent"

of section

recalculated

(Poldervaart,

rium

assemblage.

p1 ane

as CaCOr)

1955)

Calcite is considered to be a secondary alteration

product for the following reasons. Firstly, thermodysi0

38.89

44.5

?

Tio,

0 .69

0 .56

0.6

namic analysis of the metamorphic reactions possible

A1.0"

1 8. 4 5

15.03

1 0. 9

in the sirnple system CaO-AlrOr-SiOr-CO, (data of

Feo*l

7.85

6.39

Robie et al., 1978) reveals that prograde reactions beFeO

0.9

tween

calcite and silicates (e.9., grossular garnet) ocF"2os

4,0

cur at relatively modest temperatures relative to

MnO

0.27

0.r7

0.3

those which must have prevailed in the pyroclastic

IugO

4.76

3. 8 8

2.6

CaO

29.06

t9.7

flow which carried the nodule down the flanks of the

volcano. If the extremely thin layers of calcite were

Na2O

0 .04

0.03

1.1

KZO

0.0

0.0

1.9

present

in the nodule at the time of its entrainment,

coz

(18.se)

t3 .4

we feel that they would have reacted, with grandite

Total

99.95

9 9. 9 9

99 .90

for example, to produce a higher-grade metamorphic

assemblage. There is no evidence that this occurred.

Secondly, it seems unlikely that silicate minerals

lF.o* =

all iron calculated

as Feo.

growing at the expense of calcite in prograde metamorphic reactions would leave behind only thin intergranular layers of remnant calcite and yet never

entirely enclose any remnant carbonate grains. It

fassaite pyroxene (Tilley, 1938; Shoji, 1975; Hucken- seems more likely, therefore, that the calcite now in

holz et al., 1974), we conclude that the nodule under the nodule was precipitated from bicarbonate-laden

consideration is a thermal contact-type skarn assem- meteoric water in the time intervening between the

blage, even though the original field relations are not 1902 eruption and the date of sample collection.

in evidence. Specifically, no limestones or other calThe paragenesis of the nodule can be interpreted

careous sediments are exposed on St. Vincent in the in light of phase relations in the simple system CaOvicinity of La SoufriCre. We propose that the nodule AlrOr-FerOr-SiO,

which has been investigated exis derived from calcareous sediments deposited on perimentally by Huckenholz et al. (1974). An isothe submerged arc crest in the St. Vincent region baric (l aLm) T,X section of the grossular-andradite

prior to the build-up of La SoufriCre volcano from join (CarAlrSirO,r-CarFel*SirO,r) shows that comthe sea-floor. Such sedimentation is likely to have oc- plete garnet solid solution exists at temperatures becurred during the Miocene hiatus (Martin-Kaye,

low 798"C. At higher temperatures, however, garnet

1969) after cessation of activity in the Limestone Ca- solid solution breaks down to other more stable

ribbees (eastern arc) and prior to the main phase of phase assemblages.One of these phase volumes conPlio-Pleistocene volcanism in the present arc. The tains the mineral assemblagegarnet + fassaite * wolmost recent eruption of La Soufridre (April, 1979) lastonite (that which is observed in the nodule); it is

brought marble xenoliths to the surface, which sup- stable over the temperature range 935 to I140'C. We

ports the idea of calcareous sediments existing at suggest, therefore, that the Soufriire nodule repredepth in the vicinity of the vent.

sents calcareous sediment which was first metamorThe calculated bulk composition of the nodule (on phosed to a grandite-bearing assemblage (Z <

the basis of modal analysis) is shown in Table 2. Also 798'C); the assemblage was then heated to higher

shown is the bulk composition with all CaO recalcu- temperatures, perhaps during entrainment, and the

lated as CaCOr; this composition is similar to the grandite originally in the assemblage was metamor"average sediment" ofPoldervaart (1955, p. 132) and phosed into the new assemblage grandite + fassaite

presumably is close to the corrposition of the cal- + wollastonite (T > 935"C). Anorthite is interpreted

careous sediment which was decarbonated by ther- as a remnant of some prograde assemblage and calmal metamorphism. Metasomatic processes may cite as a secondary alteration product.

have played a role in determining the final bulk comThe effect on the host magma of the possible asJ I . O /

DE VI N E A N D SI GU RDSSOIT..GIRffET- FA SSAI TE N OD ULE

similation of calcareoussedimentsand incorporation

of CO, into the melt remains an open question.But

escapeof assimilatedCO, from magmaswhich have

stoped through calcareoussedimentscould obviously

contribute to the explosivenessof eruptions. Volcanoes intruding calcareouscountry rocks (e.9., Vesuvius)might be profitably consideredin this light.

Acknowledgments

We thank C. Klein and M. J. Gole for commentswhich improved the manuscript. This work was made possibleby NSF

grant OCE-77-25689.

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Manuscript received,June 7, 1979;

acceptedforpublication,September14' 1979-

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