American Mineralogist, Volume 66, pages 741-745, I98l
A pyrope-spinel(alkremite) xenolith from Moses Rock Dike:
first known North American occurrence
EreNe R. Paooveut
Department of Earth and Planetary Sciences
M assachusetts Institute of Technology
Cambridge, M assachusetts 02 I 39
AND ROBERT J. TRACY
Department of Geology and Geophysics
Yale University, New Haven, Connecticut 06520
Abstract
Pink pyropic garnet (Py 80 Gr 8 Alm 12)and black Cr-poor spinel (Mg/[Mg + Fe] : 0.84;
Cr2O3: l0,tt.Vo) occur in subequalamountsin a 3.5 cm alkremitexenolith from MosesRock
Dike, San Juan County, Utah, a kimberlite pipe. Primary garnet and spinel are more mag.nesian than these phasesin similar xenoliths from kimbertte pipes in South Africa and the
U.S.S.R. Subsequentalteration and hydration along cracks and grain boundariesproduced
secondaryminslals that account for lessthan 5 percent of the rock. The presenceof ironbearing kornerupine in the secondaryminslnl assemblagesuggeststhat alteration occurred at
elevatedtemperaturesand crustal pressures.
Introduction
An unusual xenolith from Moses Rock Dike. a
kimfsdils pipe on the southeasternedge of Monument Valley, Utah (McGsfshin, 1968;McGetchin
and Silver, 1972),contains pink, gem quality pyroperich garnet and black spinel as primary phases.Unlike other deep-seatedinclusions (such as peridotite
and eclogite) which are typically extensively altered
to greenschistfaciesminerals at this locality, the garnet-spinel"alkremite" (a mnemonicname of Russian
origin for a rock enriched in Al, Si, and Mg (Pooomslsnko, 1975)),containslessthan five percent of
secondaryninerals. The secondaryminerals include
kornerupine (Seifert, 1975),Mg-rich chlorite, zoned,
pink Cr-bearing corundum, garnet with more calcium and less magnesium, Cr-enriched spinel and
Fe-Ni sulfide.
Occurrence and associations
The primary garnet (G l) is anhedral, equant and
colorlessin {hin-sss1i6n.I.t has an averagegrain size
of 5 mm. The primary spinel (Sp l) is anhedral,
l $o2.oo
m03-{o4x/ 8r/ 0708-074
brown in thin-section,and averages10 mm in grain
size (Fig. 1). A striking feature of the spinel is the
presenceof finely exsolved rutile plates on (111)
planes(Fig. 2). Extremely small euhedral inclusions,
possibly of corundum, occur in spinel as well. The
garnet, in contrast,is inclusion-free.
The formation of secondarymins14l5has occurred
along cracks and recrystallized spinel margins. Tan
to colorless Mg-rich chlorite is the most abundant
secondary phase. It is effectively isolated from primary spinel either by a rim of secondarydark brown,
Cr-spinel (Sp 2), or by bladed, colorlesskornerupine
which grows perpendicular to Sp I edges (Fig. 3).
Spinel2 variesin bulk compositionparticularly with
respect to titanium and chromium. These variations
simply may reflect the composition of adjacent
phases.In this paper, spinel 2a is the more chromerich variety, while spinel2b is the more titanium-rich
variety. Pale to deep pink subhedral corundum,
zoned with respect to Cr, occurs less frequently as
rims and slasft fillings in Sp I (Fig. 3). Corundum is
always found to be intergrown with grains of Sp 2a
and thus separatesSp I from chlorite along grain
741
742
PADOVANI AND TRACY: PYROPE-SPINEL XENOLITH FROM MOSES ROCK
Discussion
Other alkremite occunenceshave been reported
from kimberlites in South Africa and the U.S.S.R.
(Nixon et al.,1978). Thesesamplesrangein sizefrom
mm-size chips to nodules of tens of centimetersin
maximum dimension (Ponomarenko,1975;Nixon et
al.,1978). The MosesRock alkremite is about 3.5 cm
in maximum dimension. The other alkremites differ
from the Moses Rock sample by having late stage
amphibole,diopside,phlogopite,serpentine,and carbonate and by occasionallyexhibiting inclusions of
olivine and spinel in garnet (Nixon et al., 1978;Ponomarenko, 1975).Cumulus textures have been observedin somesamples(Nixon et al., 1978\,however,
Fig. L Textural relationsin alkremite. Light material (arrows) the texture of the Moses Rock alkremite is interalong grain boundariesis the alteration assemblagethat includes lobate granoblastic and definitely metamorphic in
chlorite, kornerupine, secondarygamet, corundum and secondary
nature (Moore, 1970). The Mg content in pyrope
spinel. Rutile exsolution lamallae can be seen in severalspinel
grains in the field of view. The texture seen in this from the MosesRock alkremite is one of the highest
photomicrograph is similar to that illustrated by Nixon et a/., reported from this rock type. In addition, it is one of
(1978).Scalebar is 2 mm. 1 : primary spinel;2 : primary garnet.
the most pyrope-rich garnets analyzed from Moses
Rock (McGetchin, 1968).
Sincethe alkremite xenolith in questionexhibits a
boundaries (seeanalysis Sp 2a, Table 1). A Ti-rich
metamorphic texture, its origin could be other than
type ofsecondary spinel (Sp 2b) is lessabundant and
cumulate. Zoned and unzoned stringers of pyroxemay representsomeinvolvementof rutile-bearing Sp
nite in the ultramafic complex at AriCge, France
I in the secondaryreaction. Subhedral to euhedral
(Conqu6r6, 1977) contain spinel with composition
grossulariticgarnet (G 2) occurs in associationwith
identical to that of the spinel in the alkremite from
chlorite and has been observedgrowing on Cr-spinel
(Sp 2b) rims. Fe-Ni sulfides occur as blebs along
cracksand grain boundariesin associationwith chlorite. Representativeelectron microprobe analysesof
primary and secondaryphasesare given in Table l.
Fig. 2. Rutile exsolution (light, parallel bands) in primary
spinel, which is rimmed by kornerupine, chlorite and secondary
spinel.Note sealedcrackswhich cut exsolutionlamellae(arrows).
These cracks contain secondaryspinel, chlorite, corundum and/or
iron nickel sulfides.Crossedpolars. Scale bar is 0.20 mm. I :
primary spinel; 2 : primary garnet.
Fig. 3. Detail of bladed kornerupine growing perpendicularly
to margin of primary spinel. Note that kornerupine or secondary
spinel effectively isolates primary spinel from chlorite. Dark
mottled grains of spinel 2b are seen along contact between
kornerupine and primary spinel and between chlorite and primary
spinel in the right-hand portion of this photomicrograph.
Secondary grossularitic garnet is always associatedwith chlorite
but occasionallyis observedgrowing adjacentto kornerupineor to
primary garnet. Transmitted light photomicrograph. Plane
polarized ligh1. $sals bar is 0.20 mm. I : primary spinel; 2 :
primary garnet; 3 : kornerupine; 4 : chlorite; 5 : secondary
gamet; 6 : secondary Cr-rich spinel; corundum (arrow).
PADOVANI AND TRACY: PYROPE-SPINEL XENOLITH FROM MOSES ROCK
743
Table l. Compositions of primary and secondary minerals in the Moses Rock alkremite*
G2
GI
Sp 2a
Sp 2b
Korn
chl
s102
4r.93
0.0
41-.46
0.0
0.0
2 7. 3 8
29.86
0.0
Tio2
0.0
0.0
0 .0 7
0.0
7,0
0 .0 3
0.0
0.0
41202
24.29
61.05
22.73
49.30
5 8 .1 7
44.33
22.66
9 6 .1 8
ur^u^
0.53
8.93
0.75
20.45
9. 1 3
L.7t
o.82
3.24
Fe0
6.50
7.30
9.9L
8.20
4.07
L.62
o.64
Mn0
Mon
Ca0
0.20
0.13
0 .5 1
o.34
0.0
0 .0 4
0.0
0.0
23.38
22.t5
L 3 . 75
L9.54
16.98
L6.25
32,73
0 .1 9
3 .1 3
0.0
14,43
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
o.o2
o.23
0.34
0.06
0.34
Total
99.96
9 9. 9 0
101.02
9 9. 7 7
99.82
93.83
8 8. 0 3
1 0 0 .2 5
0xygen
Basis
L2
4
L2
4
4
t4
3
Ni0
t
si
2.937
0.0
2.984
0.0
0.0
1 .008
2.760
0.0
A1
2.003
1.813
L.928
1.553
r . 15 2
r.924
2.470
1, 9 4 3
Ti
0.0
0.0
0 .0 0 2
0.0
o.I25
0 .0 0 r
0.0
0.0
Cr
0.028
0 . t 77
0.o42
0 .4 3 1
0.184
0,050
0. 0 6 0
0.044
Fe
0.380
0 .1 6 0
o.439
0.22r
0.175
0.r24
0 .1 2 5
0.009
Mn
0.01
0.002
0 .0 2 9
0.007
0.0
0.004
0.0
0.0
2.439
0. 8 3 2
r . 4 74
o . 77 8
o.646
0.892
4.5L4
0.0
Ca
o.234
0.0
1.111
0.0
0.0
0.0
0,0
0.0
NI
0.0
0.008
0.0
0.004
0.006
0,001
o.024
0.0
8 .0 3 1
2.992
8.009
2.994
2. 8 8 8
4.004
9.953
1.996
Total
fomula
*Electron nicroprobe analyses of mjor
in tems of oxide weight percent; struclural
and ninor phases in alkrsite
spinel; G2= secondarygrossulariticgamet;
sp 1=prioary
calculationsfor
€cirphaseare
iaclurieri. c l=prlnarygarnet;
=
=
=
CoJ
secondary chlorlte;
Chl
kornerupine;
spinel; Kom
Sp 2a = secondary Cr-rich spiirel; Sp 2b = secondary Ti-rich
cotundun.
Analyses represent average corrposltions of najor and minor phases.
tFornula uormalized to 4 cations.
Moses Rock. In addition, the pyroxenitesat Aridge
contain abundant pyropic garnet associated with
spinel and variable amountsof recrystallizedpyroxenite along the intrusive contacts.Although the garnet
composition in the alkremite xenolith is more pyropic than that of garnet in the pyroxenites, it is entirely possible that the alkremite could have been
produced by a similar reaction between pyroxenite
melt and peridotite in the upper mantle beneath
MosesRock Dike.
Of particular interest in the alkremite from Moses
Rock is the presence of kornerupine, a hydrated
magnesium-aluminum silicate, among the secondary
minerals (seeTable I for analyses).Experimentalresults on the stability of kornerupine (Seifert, 1975)
suggestthat the lower stability limit of Mg-korneru-
pine in the system MgO-SiOr-AlrO3-HrO (MASHrO) is 4.5 kbar and 735'C. However, reactionsinvolving the formation of kornerupine in equilibrium
with chlorite, corundum and spinel, the observedsecondary mineral assemblagein the alkremite, are
bracketed at pressuresat or above 5.5 kbar and at
temperaturesat or above 760oCin the systemMASbracketedby theseP-T
HrO. The stableassemblages
conditions include the mineral pairs corundum-kornerupine, chlorite-kornerupine and chlorite-spinel.
Natural occuffences of kornerupine are found primarily in granulite faciesmetapelites.Seifert (1975)
has estimated conditions necessaryfor the formation
of these naturally occurring kornerupine-bearing
granulitesto be at least9 kbar. However,becausethe
effectsof adding FeO, NarO and B to the MAS-HrO
PADOVANI AND TRACY: PYROPE.SPINEL XENOLITH FROM MOSES ROCK
systemare unknown, the equilibrium pressureof formation of kornerupine in the xenolithic assemblage
could be lessthan 9 kbar and possibly as low as 3-4
kbar.
The secondarymineral assemblagecan be explained by the breakdown of the primary mineral assemblage in the presence of a limited amount of
HrO-rich fluid. An approximate chemical balance,
determinedby least squaresmethodsusing the mineral formulae given in Table l, is given by:
5.42(Spinel l) + 5.72(Garnet)+ 26.92(H,O) s
2.92 (Kornerupine)' + 3.0 (chlorite)
+ 1.80(Corundum)+ 1.88(Garnet 2)
+ 0.99(Spinel2a)
The reaptionwas apparentlyisochemicalfor major
elements although minor elements such as nickel,
chrome, and copper appear to have been introduced
@ig. a). The presenceof theseadditional elementsis
manifested by the occurrence of pentlandite and
chalcopyriteas accessoryphasesfound only in chlorite-rich reaction zones in grain boundary cracks.
The reaction balanced well using the simple system
(MgO)-(SiO,)-(ALOJ or by using a combination of
major and minor elementssuch as (MgO + FeO)(Al,O3 + CrrO,).
It should be noted that the proportions of the primary phasesthat result from this calculation approximates the volumetric proportions of primary garnet
and spinel in the xenolith; the proportions of secondary phases with respect to one another differ only
slightly from those observed in the thin section.
Conclusions
In summary,the occurrenceof an alkremite xenolith at Moses Rock Dike is a further indication of a
complex petrogeneticevolution for the mantle beneath this locality. The prirnary phasesin the alkremite formed at mantle depths,possibly as cumulates,
but more likely as a result of metasomaticreactions
at intrusive contactsbetweenpyroxenitemelt and peridotite. Later alteration along cracks within minerals
and cracksassociatedwith grain boundariesoccurred
through reaction with hydrous kimfsllili. fluids ostensibly at elevated temperatures at deep crustal
pressures.
I Water assumedto be 2 (OH) per formula unit following
Seifert(1975,p.59).
Fig. 4. Graphical representation of the mass balance equation
given in th€ text. Plot is in atomic weight percent showing the
relationship of the chemistry of the primary phasesto secondary
phasesgrowing along cracks and grain boundaries. Notc that the
bulk composition of the rock lies halfway between primary spinel
(S l) and primary garnet (G l) close to the chlorite corner of the
four phase volume representedby chlorite (Chl), secondary spinel
(S 2), secondary garnet (G 2), and corundum (Cor). In this plot,
the reaction is modelled as a simple grain boundary reaction. The
formation of alteration products requires the introduction of H2O
to the primary assemblage.
Acknowledgments
This study was supported by ONR contract No. N00014-760478 and by the Division of Earth SciencesNSF Grant EAR-7723181.Critical reviewsfrom two anonymousrevicwersand from
Jane Pike were most helpful and improved the content of the text.
Discussionswith Frank Spearrelating to the equilibrium between
primary and secondarymineral assemblagescontributed greatly to
the manuscript and are gratefully acknowledged.
.
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PADOVANI AND TRACY: PYROPE.SPINEL XENOLITH FROM MOSES ROCK
745
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