Transition from mildly-tholeiitic to calc-alkaline suite

20 Pages·1998·0.61 MB·English

JournalofVolcanologyandGeothermalResearch86(cid:14)1998.117–136
Transition from mildly-tholeiitic to calc-alkaline suite: the case of
Chichontepec volcanic centre, El Salvador, Central America
Silvio G. Rotolo a,), Francesca Castorina b
a DipartimentodiChimicaeFisicadellaTerra(CFTA),ViaArchirafi36,90123Palermo,Italy
bDipartimentodiScienzedellaTerra,Uni˝ersita`LaSapienza,PiazzaleA.Moro5,00185Rome,Italy
Accepted30May1998
Abstract
TheChichontepecvolcanoisaPlio-Pleistocenecompositevolcanothateruptedlavasrangingfromhigh-aluminabasalts
to dacites. It experienced a caldera-forming paroxysmal eruption during the early Pleistocene. Pre-caldera lavas are mildly
tholeiitic and they evolved mainly by low pressure crystal fractionation, notwithstanding the fact that most mafic lavas
(cid:14)low-MgO high-alumina basalts. retain traces of polybaric evolution. Conversely, post-caldera lavas, which are mainly
pyroxeneandesites,areclearlycalc-alkaline,havingevolvedbyopen-systemcrystalfractionation.Sr–Ndisotopicdataand
trace elements characteristics indicate that the same mantle source was involved in the petrogenesis of these series.
Modelling the AFC process showed that it did not play any role in the petrogenesis of these rocks; a crystal fractionation
modelisconsideredtobemorerelevant.Aslightvariationinthefractionatingassemblagecouldhavecausedthetransition
from an early mildly tholeiitic trend to a late calc-alkaline one. Mineralogical evidence, mass-balance calculations and
elementalchemistrysupportthishypothesis,assumingthatthegreateramountofpyroxeneontheliquidusisattheexpense
of plagioclase; this would have prevented the trend in iron enrichment. q1998 Elsevier Science B.V. All rights reserved.
Keywords:CentralAmerica;ElSalvador;andesites;high-aluminabasalts;Al-spinel;radiogenicisotopes
1. Introduction Nicaragua and Boqueron in El Salvador (cid:14)Ui, 1972;
Fairbrothers et al., 1978., are rare among lavas in
Tholeiiticandcalc-alkalinesuitesdonotnormally
Central America, and, in particular, along the Sal-
coexist in volcanic arcs. Usually, immature island
vadorian volcanic front. These lavas are mostly an-
arcs are tholeiitic in character, while active volcanic
desites and seldom high-alumina basalts.
arcs, which have developed on the continental litho-
This study is concernedwith the magmaticevolu-
sphere, are mostly calc-alkaline. In some regions,
tion of the Chichontepec volcano (cid:14)El Salvador.,
however, there is a close spatial and chronological
which evolved from an early, mildly tholeiiticrtran-
associationbetweentholeiiticandcalc-alkalineseries
sitionalstagetoaclearlycalc-alkalinestage, through
(cid:14)Baker et al., 1994.. Tholeiitic lavas, such as those
a paroxysmal plinian phase. We have focused on the
emitted by the Masaya and Granada volcanoes in
mineralogy, geochemical and petrological aspects of
pre- and post-caldera lavas. We here propose a sim-
)Correspondingauthor.E-mail:[email protected] ple crystal fractionation model to explain magma
0377-0273r98r$19.00q1998ElsevierScienceB.V.Allrightsreserved.
PII: S0377-0273(cid:14)98.00076-6
118 S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136
evolution at the Chichontepec volcano. Furthermore, minous in El Salvador (cid:14)130 km3., after the Santa
we suspect that an increase in pyroxene in the frac- Ana volcano (cid:14)265 km3; Carr et al., 1981.. It is
tionation assemblage was responsible for a transition located approximately 50 km east of the capital city,
from a tholeiitic to a calc-alkaline trend. San Salvador. The remnants of an older volcanic
center lie immediately West of the main volcanic
edifice (cid:14)Fig. 1.. They consist of a series of pro-
2. Geology and the eruptive history of the Chi- nouncedhillsarrangedinasemicircle(cid:14)LaCarbonera
chontepec volcano hills..
Chichontepec is a paired volcano whose eastern
Chichontepec (cid:14)or ‘Volca`n de S. Vicente’. is an crater(cid:14)elevation:2180m. appearsto be morphologi-
andesitic, composite volcano, the second most volu- cally younger while the western crater (cid:14)elevation:
Fig.1.GeneralizedgeologicalmapoftheChichontepecvolcaniccentre.Samehorizontalscaleinthecrosssection;elevationinmeters.
S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136 119
2105 m. seems older. Several parasitic vents, which mainlyplagioclase,olivine, clinopyroxene,and mag-
appear to be the most recent in the whole complex, netite, which are set in a black microcrystalline
are located on the lower flanks of the volcano, matrix.
mostly on the northeastern side. As a consequence, The silicic dome of Nahuistepe is located 8 km
this area recorded a progressive eastward migration westwards of the Chichontepec summit cones. It is
of the focus of volcanism during its lifetime. composedofrhyolitescontainingphenocrystsof pla-
The whole edifice rises inside a late-Pliocene gioclase, orthopyroxene and hornblende, in a white,
extensional structure (cid:14)20 to 30 km wide., the so- glassy, variably devitrified, groundmass. Although
called Central Graben, that runs along the length of the eruption of this dome falls within the time span
El Salvador and is roughly parallel to the Pacific of pre-caldera lavas, as based on a single KrAr date
coast. The graben cuts through a Tertiary volcanic on plagioclase of 1.7"0.3 Ma, there is no field
basement, which locally consists of basalt and an- evidence for a comagmatic association between rhy-
desite lavas, tuffs and agglomerates. Tertiary vol- olites and La Carbonera lavas.
canic rocks lie in turn on a thick Mesozoic sedimen- La Carbonera volcano experienced a pyroclastic
tarycoverandonaPaleozoicmetamorphicbasement eruptionthatledtothecollapseoftheedifice,thereby
(cid:14)WilliamsandMeyer-Abich,1955;PichlerandWeyl, giving rise to a caldera, which is regarded to as the
1973; Wiesemann, 1975; Carr, 1976.. The northern La Carbonera caldera (cid:14)CEL, 1992.. Caldera rims are
margin of the Central Graben is marked by a series preserved only in the western side of the edifice
of E–W-trending faults with displacements of up to along the La Carbonera hills. They form a gently
300 m. The southern margin is much less evident, curved structureof cinder cones and lava flows (cid:14)Fig.
having been partially buried by recent Chichontepec 1..
lavas. It is represented by a series of minor faults
which extend westward through La Carbonera hills 2.2. The caldera stage
towards lake Ilopango.
The youngest Chichontepec lava flows are cov- Intercalated between La Carbonera lavas and the
eredby3000-year-olddaciticashes(cid:14)‘Tierrablanca’., overlying post-caldera andesites, there is a pyroclas-
which have been emitted from the nearby Ilopango tic sequencewhosethicknessis approximately25 m.
volcano (cid:14)about 20 km northwestwards.; there is no Pyroclasticrockscropoutmostlynorthwestfromthe
evidence of younger eruptions. Its current state is Chichontepecvolcano,buttheyhavealsobeendrilled
one of solfataric activity, characterized by low-tem- under the post-caldera lavas (cid:14)Fig. 1. in the SV-1
perature fumaroles and hot springs, which are lo- deep geothermal well (cid:14)CEL, 1992. on the northern
cated on the northern side of the volcano (cid:14)Aiuppa et slopes of Chichontepec. A few pyroclastic sections
al., 1997.. have been found in the southwesternsector and have
all been heavily reworked by post-depositional pro-
cesses.
2.1. The pre-caldera stage. The Plio-Pleistocene La
One ignimbrite (cid:14)5 to 10 m thick. directly overlies
Carbonera ˝olcano
La Carbonera lavas, marking the beginning of the
pyroclastic sequence. This deposit is composed of a
The earliest edifice is called La Carbonera. The brownishyellow,poorlywelded,unsortedmixofash
beginning of the activity of La Carbonera volcano is and pumice clasts (cid:14)f30–40 vol.%. and less abun-
poorly defined. Limited whole-rock KrAr dates on dantlithiclapilli(cid:14)f10–20vol.%.. Theignimbriteis
two samples (cid:14)samples 98 and 14. yielded ages of separated from overlying pyroclastic rocks by a
2.2"0.4and1.2"0.2Ma,respectively.Duringthis brown-coloured paleosol (cid:14)1 to 2 m thick., which in
timespan, mildlytholeiitictotransitionallavasrang- turn is overlain by a dacitic pumice-fall deposit (cid:14)0.6
ing from low-MgO high-alumina basalts (cid:14)MgO-5 to 1.3 m thick.. The latter is composed of highly
wt.%; Al O )19 wt.%. to dacites(cid:14)SiO s63 to 76 vesiculated pumice lapilli with minor basaltic–ande-
2 3 2
wt.%. were emitted from the La Carbonera edifice. sitic lithic fragments. This sequence continues with
The lavas are porphyritic, containing phenocrysts of alternatingcoarsepumice-fallsandashdeposits(cid:14)2to
120 S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136
3 m thick.. Higher up the sequence continues with basalts. The composition of olivine phenocrysts is to
surge deposits (cid:14)0.2 to 0.8 m thick. made up of be foundin the Fo rangewitha CaO contentof
74–80
well-developed plane-parallel beds, containing ash- 0.10–0.25 wt.%. They frequently show resorption
sized particlesand accretionarylapilli. The top surge (cid:14)embayments. or thin, pigeonite rims. Al-rich spinel
layer is cut by an erosional surface which, in turn, is (cid:14)Al O s50 to 55 wt.%. phenocrysts are typical of
2 3
overlainbyasequence(cid:14)2to3mthick.ofalternating pre-calderaserieswithmaximumgrainsizesofupto
coarse-grained layers containing variable amounts of 1 mm. The crystals are rounded to lobate and they
pumice and lithic fragments and ash layers. The are not found as inclusions in other phases. Back
whole pyroclastic sequence terminates with a 2-m- scattered electron examination has not shown zoning
thick paleosol. or exsolutions, except in one case of a corona of
The explosive phases, as recorded by the deposits magnetite micrograins around pleonaste. Anorthite
described above, led to the collapse of the La Car- (cid:14)An . is abundant among phenocrysts, together
89–93
bonera volcano and to the formation of the La with a less calcic plagioclase (cid:14)An .. Both are
82–88
Carboneracaldera. The pattern of isopachsof dacitic freeofinclusions,dustyzonesorresorptionsurfaces.
pumice deposit locates the vent position inside the Plagioclasemicrophenocrysts(cid:14)An .haveamean
55–62
annular structure outlined by La Carbonera hills. FeO) content (cid:14)1.1 wt.%., which is distinctly higher
than post-caldera plagioclase microphenocrysts
2.3. The post-caldera stage. The Pleistocene Chi- (cid:14)mean FeO)s0.6 wt.%. (cid:14)Table 1.. The characteris-
chontepec ˝olcano tic features of pre-caldera plagioclases are: (cid:14)1. The
very slight zoning in anorthite crystals (cid:14)DAn core–
The renewed volcanic activity after the pyroclas-
rimiscommonly2–3mol%.. (cid:14)2.Thenormalzoning
tic stageformeda considerablevolume(cid:14)130km3. of patterns of other plagioclase crystals (cid:14)i.e., cores
thick(cid:14);20m. andlobatelavaflows, whichbuiltup An , rims An , microphenocrystss
the Chichontepec edifice inside the La Carbonera 82–88 77–84
An .; (cid:14)Fig. 2, Table1.. (cid:14)3. Theabsenceofdusty
caldera.Lavasonthenorthernandsouthernflanksof 55–62
zones. Augite phenocrysts (cid:14)Wo En Fs .
the volcano were probably emitted from a central 33–36 40 23–25
have a low Ca-Tschermak component, ranging from
vent, while those of the eastern flank were emitted
0.4 to 1.0 mol%. Pigeonite is present in the ground-
from a parasitic vent, which is located to the east of
mass and its presence is typical for pre-caldera lavas
the summit craters.
(cid:14)Fig. 3., while it is almost absent in other groups
Lavasaremostlytwo-pyroxeneandesites(cid:14)SiO s
2 (cid:14)except in the post-caldera lava, sample 117. (cid:14)Table
57 to 63 wt.%., ranging from basaltic andesites
1.. Orthopyroxene is rare, being present only in the
(cid:14)SiO s52to57wt.%.todacites,whilethoseofthe
2 most evolved samples (cid:14)Wo En Fs ., and
parasitic cones are mostly dacites. All post-caldera 3 59–68 37–28
amphibole is absent.
lavas are highly porphyritic, containing phenocrysts
Lavas from the Nahuistepe dome are represented
of plagioclase, orthopyroxene, clinopyroxene, mag-
by white coloured, glassy and variably devitrified,
netite, very rare ilmenite, and "hornblende, which
rhyolites with plagioclase occurring as euhedral phe-
are all set in a grey, glassyrmicrocrystalline matrix.
nocrysts (cid:14)An . and microphenocrysts (cid:14)An .. The
Abundant crystal clots and enclaves in pre-caldera 40 20
latter is the most important phase (cid:14)13 vol.%., while
lavas distinguish them from post-caldera ones.
orthopyroxene (cid:14)1 vol.%., hornblende (cid:14)2 vol.%. and
biotite (cid:14)1 vol.%. are rarer.
3. Petrography
3.2. Post-caldera la˝as
3.1. Pre-caldera la˝as
The post-caldera lavas are characterized by the
Pre-caldera lavas are mostly composed of por- presence of abundant glomerocrysts, microgranular
phyritic(cid:14)30–33%phenocrysts., pilotaxiticlavas(cid:14)Ta- enclaves and coarse-grained gabbroranorthositic
bles 1 and 2 . with the most representative samples xenoliths. The phenocryst content is higher than
of this group comprising low-MgO high-alumina pre-caldera lavas, in the 37–42% range. Olivine is
S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136 121
Table1
Mainpetrographiccharacteristicsofpre-calderaandpost-calderalavas
Pre-calderalavas Post-calderalavas
Zoninginplagioclase Normal:high-aluminabasalts Eitherreversed(cid:14)0.52–0.71.
phenocrysts(cid:14)mean X core–rim. s0.93–0.82;otherrockss0.88–0.77 ornormal(cid:14)0.52–0.47.
An
X ofplagioclase Lowerthanphenocrysts Frequentlyhigherthan
An
microphenocrysts(cid:14)range. (cid:14)0.62to0.55. phenocrysts(cid:14)0.57to0.75.
MeanFeO)(cid:14)wt.%.in 1.1 0.6
plagioclasemicrophenocrysts
Zoninginorthopyroxene Normal(cid:14)0.68–0.59. Eitherreversed(cid:14)0.62–0.70.
phenocrysts(cid:14)mean X core–rim. ornormal(cid:14)0.63–0.59.
En
X oforthopyroxene Lowerthanphenocrysts Frequentlyhigherthan
En
microphenocrysts(cid:14)range. (cid:14)0.60to0.53. phenocrysts(cid:14)0.67to0.56.
Low-Capyroxeneingroundmass Pigeonite Orthopyroxene
Olivine Oftenpresent(cid:14)55%oflavas. Quiterarelypresent
(cid:14)18%oflavas.
Al-richspinel Present Absent
Hornblende Absent PresentfromSiO )63%
2
Crystalclots,cognateinclusions, Crystalclotsonly, Abundant(cid:14)15%oflavas.
xenoliths butveryrare(cid:14)-2%oflavas.
Table2
Modalanalysesofthemostrepresentativelavas(cid:14)800to1700pointscountedforeachsample.
Sample: Pre-caldera Post-caldera Dome
rock-type:
14 73 98 1 36 117 161 85
high- dacite high- andesite dacite basaltic hybrid rhyolite
alumina alumina andesite lava
basalt basalt (cid:14)mingled.
Plg pha 40.1 7.8 31.3 24.0 28.3 30.8 19.1 12.5
Plg mphb 33.5 17.9 35.1 14.3 13.9 2.8 4.5 3.0
Ol ph 4.0 4.0 tr
Ol mph tr 2.0
Cpx ph 1.0 0.5 1.2 3.0 4.2 3.8
Cpx mph 12.4 2.9 14.5 0.2 0.1 0.9
Opx ph 6.3 3.0 3.4 1.8
Opx mph 1.6 0.3 0.5 1.3 0.7
Hbl 0.8 0.5 2.0
Bt 1.0
Qz 2.0
Oxides ph 0.8 0.1 1.0 2.1 2.0 0.3 0.7
Oxides mph 8.2 2.8 13.0 2.1 2.6 1.0 13.3 0.3
Crystalspercent 100.0 32.0 100.0 50.7 54.0 44.8 45.5 22.2
Gdmc percent 68.0 49.3 46.0 55.2 54.5 77.8
aPhenocrysts(cid:14))0.5mm.;bmicrophenocrysts(cid:14))0.05mm.;cgroundmass(cid:14)-0.05mm..
Bt:biotite;Cpx:clinopyroxene;Hbl:hornblende;Ol:olivine;Opx:orthopyroxene;Plg:plagioclase;Qz:quartz.
122 S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136
Fig.2.Feldsparternarydiagram.Eachpointrepresentsoneanalysis.Samplenumbersarereportedontheleftofeachtriangle.
quite rare and, when present, it is almost completely material. Plagioclase microphenocrysts are in the
resorbed. Plagioclase is present in two types that An range. It is important to note that microphe-
57–75
could be distinguished by their size and the pres- nocrysts are often enriched in the An component, as
encerabsence of inclusions: (cid:14)1. Medium-sized phe- compared to phenocrysts (cid:14)Fig. 2, Table 1..
nocrysts (cid:14)0.5–1.0 mm. either normally zoned (cid:14)cores: Orthopyroxenephenocrystshaveacompositionin
An ,rims:An .orreversely(cid:14)cores:An , the Wo En Fs range. They often show
54–63 47–54 62–51 2–3 62–73 35–24
rims: An .. In the most evolved samples they reverse zoning (cid:14)i.e., Mg-richer rims. and are fre-
72–69
host tiny apatite needles. (cid:14)2. Complexly-zoned quently jacketed by clinopyroxene. Groundmass pi-
megacrysts(cid:14)upto4mm.withcalciccores(cid:14)An ., geonite is almost completely absent while orthopy-
65–70
often consistingof many annealed crystals. Near-rim roxene is present. Clinopyroxene is an augite
zones are often sieved, either by glass, which is (cid:14)Wo En Fs . showing either normal or reverse
42 42 16
rhyolitic in composition, or by fine-grained matrix zoning (cid:14)predominant.. The Ca-Tschermak compo-
S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136 123
(cid:14)1. Fine-grained mafic enclaves occurring as
holocrystalline to hypocrystalline fine-grained en-
claves (cid:14)0.5 to 1.5 mm.. They show intersertalrhy-
alophitic textures, with subhedral orthopyroxene and
euhedralrsubhedral plagioclase laths. When glass is
present (cid:14)5 to 10 vol.%., it forms variably devitrified
pale brown pools enclosing plagioclase. The enclave
textures suggest that they are tightly packed crystal-
rich mushes, that is, depositional cumulates frag-
ments,asdefinedbyIrvine(cid:14)1982..Mineralcomposi-
tions of enclaves are comparable to those of hosting
lavas (cid:14)e.g., pyroxene in enclavess Wo
2–3
En Fs ; pyroxene in hostsWo En
61–64 35–37 2–3 59–62
Fs ; plagioclase in enclavesAn , plagio-
36–38 65–75
clase in hostsAn .. Thus, they are probably
50–58
Fig. 3. Pyroxene compositions for pre-caldera (cid:14)43 analyses. and cognate.
post-calderalavas(cid:14)74analyses.. (cid:14)2. Coarse-grained anorthositic enclaves (cid:14)2 sam-
ples.. They consist of hypidiomorphic granular
anorthosites, containing 90 vol.% of coarse-grained
nent is higher than in pre-caldera lavas (cid:14)mean value (cid:14)-7mm.subhedralplagioclase(cid:14)An , unzoned.,
90–93
s2.6 mol%.. The presence of brown hornblende, interstial olivine (cid:14)Fo . and Al–Ti-spinel (cid:14)Al O s
78 2 3
rimmed by fine-grained magnetite and glassy mate- 11 to 12 wt.%.. They are characterized by a greater
rial, is typical of post-caldera evolved lavas. Horn- compositional difference compared to the host lavas,
blendeisaMg-hastingsite(cid:14)X s0.66–0.72.witha (cid:14)plagioclase in enclavesAn , plagioclase in
Mg 90–93
low K O content (cid:14)0.3–0.4%. and a NarK ratio of hostsAn ., while olivine and spinel are absent
2 57–62
approximately 5.6–9.0. in the lavas. The lack of any evidence of equilib-
Enclaves are relatively common in post-caldera rium, of post-caldera lavas with an anorthitic plagio-
lavas (cid:14)about 15% of lava samples contain enclaves., clase strongly suggest that these enclaves are cumu-
while they are almost absent (cid:14)i.e., -2%. in pre- lates, which are unrelated to hosting lavas. Both
caldera lavas. Their length ranges from 0.5 to 1 cm groups of enclaves show a very small degree of
across.Wehavedividedtheenclavesintotwogroups: resorption or reaction with the liquid and absence of
Table3
Thermobarometricresults
Sample Group Rock-typea T (cid:14)8C. T (cid:14)8C. T (cid:14)8C. P (cid:14)kbar. P (cid:14)kbar.
1 2 3 1 2
1 pre-caldera and 966 980 1005 1.5 1.4
36 pre-caldera and 931 980 993 1.6 2.0
117 pre-caldera ba 993 – – 3.0 2.1
122 pre-caldera and 974 930 961 3.0 1.5
124 pre-caldera and 953 – – 2.8 2.0
14 post-caldera hab 1000 1025 1075 5.4 2.3
163 post-caldera and – 920 909 4.5 –
85 Rhyolitedome rhy – 800 798 1.8 0.7
aand:andesite;ba:basalticandesite;hab:high-aluminabasalt;rhy:rhyolite.
T: temperature calculated using cpx–opx pairs (cid:14)Wells, 1977.; T and T temperature calculated using mt–ilm pairs according to
1 2 3
BuddingtonandLindsley(cid:14)1964.andGhiorsoandSack(cid:14)1991.,respectively.
Pressure estimates based on plg–liquid equilibria (cid:14)Kudo and Weill, 1970; see text for details.. P: phenocrysts—whole rock; P :
1 2
microphenocrysts—groundmass.
124 S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136
4. Thermodynamic constraints
4.1. Geothermobarometry
Amongthevariousgeothermometersbasedonthe
shape of the orthopyroxene–clinopyroxene miscibil-
ity gap, we have chosen the calibration of Wells
(cid:14)1977.. Althoughassociated errors are large (cid:14)"608C
according to the author., this method is considered
particularly accurate in the temperature range of
andesitic magmas (cid:14)Wallace and Carmichael, 1994..
We avoided pairs with disequilibrium textures (cid:14)such
as Mg-rich rims, overgrowths, etc.., preferring, in-
Fig. 4. Tyf calculated by magnetite–ilmenite pairs. Filled stead, to choose contiguous pairs, as close as possi-
O2
circles: pre-caldera lavas; empty circles: post-caldera; filled ble to the Xopxrcpxs1, as a constraint for equilib-
squares: rhyolitic dome; stars represent solutions of Eq. (cid:14)1. (cid:14)see Mg
rium. We obtained pre-eruptive temperatures (cid:14)using
text..
the rims of phenocrysts from 15 selected pairs. that
cluster around 9508C for andesites and up to 10008C
quench textures. These observations support a slow for pre-caldera high-alumina basalt (cid:14)Table 3..
cooling in a nearly thermal equilibrium between The Fe–Ti oxide geothermometer was applied to
enclaves and the host liquid. coexisting mt–ilm pairs, preliminarily checked for
Table4
Mineralphasesusedinthermodynamiccalculationsof f accordingtoEq.(cid:14)1.(cid:14)seetext.
O2
Sample: Pre-caldera Post-caldera
14r358Mt 14r359Ol 14r360Opx 122r309Mt 122r316Ol 122r315Opx
SiO 0.09 37.22 51.48 0.06 37.93 52.72
2
TiO 3.77 0.02 0.30 10.15 0.01 0.22
2
Al O 3.16 0.21 0.63 1.75 0.06 2.04
2 3
Cr O 0.02 0.00 0.00 0.00 0.04 0.00
2 3
FeO 85.67 30.73 20.63 81.08 26.10 16.69
MnO 0.38 0.62 1.13 0.79 0.69 0.63
MgO 1.13 29.45 21.18 1.50 36.33 24.78
CaO 0.13 0.25 3.00 0.02 0.16 2.01
Na O 0.00 0.00 0.16 0.00 0.00 0.06
2
Total 94.40 98.50 98.58 95.41 101.32 99.15
X 0.891 0.713
Mt
aa 0.891 0.713
Mt
X 0.596 0.689
En
ab 0.578 0.656
En
X 0.326 0.260
Fs
ac 0.316 0.248
Fs
X 0.624 0.706
Fo
ad 0.389 0.498
Fo
aActivityofmagnetiteinspinel: a sX (cid:14)Xsmolefraction; asactivity..
bActivityofenstatiteinorthopyroxMetne: aFe3sO4,(cid:14)TXi–Mt X X2 .0.5.
En Mg,M2 Mg,M1 Si,TET
cActivityofferrosiliteinorthopyroxene: a s(cid:14)X X X2 .0.5.
Fs Fe,M2 Fe,M1 Si,TET
dActivityofforsteriteinolivine: a sX .
(cid:14)En.enstatite;(cid:14)Fs.ferrosilite;(cid:14)Fo.FoforsteMritge2;Si(cid:14)OM4,tO.lmagnetite;(cid:14)Ol.olivine;(cid:14)Opx.orthopyroxene.
S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136 125
(cid:14)Johnsonetal., 1992.atthetemperaturefixedbythe
geothermometers. We ignored the P effect because
of the small DV of solids participating in the reac-
tion. Activities of various components in the mineral
phases used in the calculations (cid:14)Table 4. were taken
from the activity models of: (cid:14)a. Kerrick and Darken
(cid:14)1975. for olivine and (cid:14)b. Carmichael et al. (cid:14)1977.,
for enstatite, ferrosilite and magnetite.
Lnf was then calculated according to:
O2
lnf s(cid:14)a .2(cid:14)a .12r(cid:14)a .6(cid:14)a .6lnK
O2 mt en fo fs
Estimates of logf with this method (cid:14)stars in
Fig. 4. are in broad aOg2reement with those calculated
by the Fe–Ti oxide method.
The Kudo and Weill (cid:14)1970. plagioclase-liquid
Fig. 5. Plagioclase–liquid equilibria. KDCa–Na were calculated empirical geothermometer was used for an indirect
between groundmass (cid:14)i.e., chilled liquid. and microphenocrysts.
estimationof P (cid:14)seeLuhrandCarmichael,1980.,
Filled rectangles: pre-caldera; empty rectangles: post-caldera; H2O
solving the quadratic equations given by the authors,
banded rectangle: rhyolitic dome. Horizontal bars represent the
meanAnvalueofeachsample.Numbersinsidecirclesrepresent andthenfittingpressureforanindependenttempera-
the KCa–Na values. Sample numbers are reported close to each ture estimate. Errors associated with this method
D
verticalrectangle. depend on the accuracy of temperature estimates:
"508Cshiftpressureof "0.7 kbar. Whole-rockand
groundmass compositions were assumed to be repre-
equilibrium according to the method of Bacon and
sentative of the liquid as regards phenocrysts and
Hirschmann (cid:14)1988.. Due to the extreme scarcity of
microphenocrysts, respectively. The estimated P
the rhombohedral phase, only eight pairs were used. was unexpectedly high (cid:14)5–5.5 kbar. for high-HA2n0
Temperatures were estimated using two methods:
phenocrysts of pre-caldera lavas. Microphenocrysts
empirical calibration of Buddington and Lindsley gave consistently lower pressures (cid:14)2–2.5 kbar. (cid:14)Ta-
(cid:14)1964.andthermodynamicsolutionmodelofGhiorso ble 3..
and Sack (cid:14)1991.. Agreement between these two
methods was nearly always within the error experi-
enced by the authors (cid:14)"308C.. Estimated tempera-
tures were as high as 1025 to 10758C for high-
alumina basalt, 920 to 9808C for andesites, and 780
to 8008C for rhyolite (cid:14)Table 3..
The equilibrium between Fe–Ti oxides also al-
lowed the estimation of oxygen fugacity (cid:14)Fig. 4.
withlogf rangingfromy13.2(cid:14)at8008C.toy10.6
(cid:14)at 10008OC2., i.e., 1–2 log units above the Ni–NiO
buffer (cid:14)with the exception of sample 163 which
straddles the NNO line.. We also performed inde-
pendent thermodynamic calculations based on equi-
librium (cid:14)Luhr and Carmichael, 1980.:
6Mg SiO q6FeSiO qO s2Fe O q12MgSiO
2 4 3 2 3 4 3
(cid:14)1.
Thermodynamic data (cid:14)DG8f, DGr8 and lnK. were Fig.6.SiO2vs.FeO)rMgO.Thedividinglinebetweentholeiitic
calculated using the SUPCRT92 computer program andcalc-alkalinetrendswastakenfromMiyashiro(cid:14)1974..
126 S.G.Rotolo,F.CastorinarJournalofVolcanologyandGeothermalResearch86(1998)117–136
Fig.7.Harkerdiagramsformajorandminorelements.SymbolsasinFig.6.Majorelements:theliquidlineofdescentstartsfromsample
98andateachstep5%ofsolidwasremovedfromeachproducedliquid.Mineralproportionsofthefractionatingassemblagewerechosen
bearinginmindphenocrystabundance(cid:14)counted.andphaserelationsfromGroveandBaker(cid:14)1984.:plgs0.75,olivs0.15,spinels0.10,
for SiO2)50.5%; plgs0.67, olivs0.10, cpxs0.10, mts0.13, for SiO2)54%; plgs0.67, cpxs0.10, opxs0.10, mts0.13, for
SiO2)58%.Equilibriummineralcompositionswerecalculatedateachstepaccordingtothefollowingcrystal–liquidexchangepartition
coefficients: olivine KFe-Mgs0.31–0.32; plagioclase KCa–Nas2.5–1.1 (cid:14)where KFe–MgsXxtal)XliqrXxtal)Xliq; KCa–Nas
D D D Fe Mg Mg Fe D
Xxtal)XliqrXxtal)Xliq.;Al-spinel,magnetiteandclinopyroxenecompositionswerekeptconstant.Compositionsofthesolids,whichwere
Ca Na Na Ca
removedateachstep,werecalculatedmultiplyingthewt.%oxideineachphase)wt.proportionofthephase)wt.proportionofthesolid
(cid:14)0.05..Theresultingresidualliquidswerethennormalizedto100%.Theremainingliquidfractionaftereachstepwas:0.95n, nbeingthe
progressivenumberofthestep.Minorelements:stepwiseFClinesweredrawnusingthesamestartingpointandmineralassemblagesof
majorelementmodel.