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.
References
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Manuscript received,June 7, 1979;
acceptedforpublication,September14' 1979-
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