Nature and cause of compositional variation among the alkalic cap lavas of Mauna Kea Volcano ...

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Contributions to
Contrib Mineral Petrol )8891( 100:383-397
Mineralogy and
Petrology
(cid:14)9 galreV-regnirpS 8891
Nature and cause of compositional variation
among the alkalic cap lavas of Mauna Kea Volcano, Hawaii
H.B. West t , M.O. Garcia t, F.A. Frey z, and A. Kennedy z
1 Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI ,22869 USA
2 Department of Earth, Atmospheric, and Planetary ,secneicS Massachusetts Institute of Technology,
Cambridge, MA ,93120 USA
Abstract. Most Hawaiian basaltic shield volcanoes are reduced volcanic activity, apparently reflecting a decreasing
capped by moderately to strongly evolved alkalic lavas supply of magma (Feigenson and Spera 1981; Wise 1982;
(MgO < 4.5 wt. %). On M auna Kea Volcano the cap is dom- Clague 1987; Spengler and Garcia 1988); hence, this stage
inantly composed of hawaiite with minor mugearite. Al- represents a marked change in the development of Haw-
though these lavas contain dunite and gabbroic xenoliths, aiian volcanoes.
they are nearly aphyric with rare olivine and plagioclase Mauna Kea is an excellent volcano for the study of
phenocrysts and xenocrysts. The hawaiites are nearly ho- alkalic cap lavas because the stratigraphy has been well-
mogeneous in radiogenic isotope ratios (Sr, Nd, Pb) and documented (Porter 1979; Wolfe 1987) and the lavas are
they define coherent major and trace element abundance generally very fresh due to: (1) their young age (<0.3 Ma;
trends. These compositional trends are consistent with seg- Porter 1979), (2) exposure at high elevation (up to 4.2 kin),
regation of a plagioclase-rich cumulate containing signifi- and (3) the low rainfall (above 2 km elevation). This study
cant clinopyroxene and Fe-Ti oxides plus minor olivine. focuses on the south rift zone of the Mauna Kea Volcano
Elements which are usually highly incompatible, e.g., Rb, because it has the widest temporal range of lavas from the
Ba, Nb, are only moderately incompatible within the haw- alkalic cap stage. Previous studies (Macdonald and Katsura
aiite suite because these elements are incorporated into feld- 1964; Macdonald 1968) have shown that the alkalic cap
spar (Rb, Ba) and oxides (Nb). However, in the most lavas at Mauna Kea are dominantly hawaiites with rare
evolved lavas abundances of the most incompatible ele- mugearite.
ments (P, La, Ce, Th) exceed (by ~ 5-10%) the maximum In this paper, major and trace element data for 39 lavas
enrichments expected from models based on major ele- and radiogenic isotopic data for 8 lavas are utilized to eval-
ments. Apparently, the crystal fractionation process was uate the spatial and temporal compositional variations of
more complex than simple, closed system fractionation. The alkalic cap lavas and to model the processes causing compo-
large amounts of clinopyroxene in the fractionating assem- sitional variations within the alkalic cap suite. There is no
blage and the presence of dense dunite xenoliths with CO2 systematic relationship between temporal or spatial varia-
inclusions formed at minimum pressures of 2 kb are consis- tion and the composition of alkalic-cap lavas. Nevertheless,
tent with fractionation occurring at moderate depths. Crys- compositional variations of these lavas define coherent
tal segregation along conduit or magma chamber walls is trends and there is very little variation in Pb, Sr and Nd
a possible mechanism for explaining compositional varia- isotope ratios. These coherent compositional trends can
tions within these alkalic cap lavas. largely be explained by fractionation of a plagioclase-rich
assemblage containing significant amounts of clinopyrox-
ene, Fe-Ti oxides and olivine. The major element composi-
tional trends and presence of dunite and gabbroic xenoliths
are consistent with fractionation at pressures of at least
Introduction
2 kb.
Hawaiian volcanoes follow a well established evolutionary
path (Macdonald et al. 1983). The bulk of each volcano
Regional Geology of Mauna Kea
(95-99%) is presumed to be tholeiitic basalt although less
than 20% of the total thickness is exposed at any Hawaiian Mauna Kea si the highest of the five principal volcanoes on the
volcano. On most Hawaiian volcanoes (e.g., Mauna Kea, island of Hawaii, rising to a height of 4205 m (Fig. .)1 Most
Hualalai, Kohala, Haleakala, West Maui, Waianae) a rela- (87.5%) of the volcano si submarine and a minimum volume -itse
tively thin sequence (10-300 m thick) of evolved alkalic la- mate si 5.2 x 401 km 3 (Bargar and Jackson .)4791 gnidliub-dleihS
volcanism appears to have been concentrated along two principal
vas "cap" the tholeiitic shield-forming lavas. These lavas
rift zones (east and west). A third minor rift zone trends south.
are predominantly hawaiites and mugearites with minor or
Vents for alkalic cap lavas are not confined to the rift zones and
no benmoreites and trachytes. Relative to the shield build-
are widely distributed on the volcano at elevations of 2 km or
ing stage, the alkalic cap stage is a period of substantially greater above aes level (Fig. .)1
Stratigraphically, the lavas of Mauna Kea have been separated
Offprint requests to: H.B. West into two units: a lower "shield-building" ,ecneuqes designated the
384
0 20
Fig. 1.Map of the northern half of the Island of
Hawaii showing the distribution of lavas from
the five volcanoes comprising the island. Mauna
Kea is subdivided into the shield lavas of the
Hamakua Group dedahs( )aera and the alkalic
cap lavas of the Laupahoehoe Group (unshaded
area). Vents for the Laupahoehoe lavas are
shown by the open circles (after Stearns and
Macdonald 1946 and Porter 1979)
AGE ROCK UNITS its that enabled Porter (1979) to divide the Laupahoehoe Group
into six stages (Fig. 2). Wolfe (1987) recently completed a detailed
VOLCANIC GLACIAL
mapping study of the Laupahoehoe lavas and he proposed only
two units: Pleistocene (combining 5 of Porter's units) and Holocene
o LOALOAN (which is identical to Porter's Loaloan unit). Porter's (1979) stratig-
O
Z raphy is utilized here in an attempt to evaluate the temporal evolu-
tion of Laupahoehoe lavas. Because we found no systematic com-
positional changes with eruption age, our interpretations are not
affectd by the alternative stratigraphic interpretations.
w ......... ~<~?.i~:~ ! Laupahoehoe volcanism probably began ~ 65 ka (Wolfe, 1987)
O {ii~-~i~i:~:~-~<~:::,,~.. -","
and the most recent eruption was about 4.4 ka ago (Porter 1979).
Based on air photo interpretation, there were about 65 distinct
o
I eruptions during the upper and middle stages of the Laupahoehoe
Group and about 6 eruptions during the uppermost stage. This
HANAIPOEAN yields an average of about 1 eruption per 1250 years for the upper
and middle stages and about 1 eruption per 1500 years for the
uppermost stages. The lower Laupahoehoe stage eruption rate
O
o POLl AH UA N . ..~.!.~:::" could not be determined because many of its vents are covered
by later lavas and glacial sediments. Recent estimates for Kohala
are similar (but less frequent) than the Laupahoehoe estimates 1(
Q_
eruption per 1900 years; Spengler and Garcia, 1988). The lower
LILOEAN
eruption rate for Kohala is consistent with a much larger propor-
tion (~43%) of highly evolved lava, mugearite to trachyte
...,.:.....:.....................
(Spengler and Garcia, 1988). Eruption rates for alkalic-cap lavas
from Mauna Kea are much lower than those for an active tholeiitic
"-x
shield like Kilauea (1 eruption every 1 to 4 years; Klein, 1982;
2; Dzurisin et al. 1984) and significantly lower than the eruption rate
< 1( eruption per 50 years; Moore et al. 1987) of alkalic basalts on
I Hualalai volcano which has not yet reached the alkalic cap stage.
This trend of decreasing eruption rate with age must reflect move-
Fig. 2. Generalized stratigraphic sequence for volcanic and glacial ment of the volcano away from the mantle "hot-spot".
rock units from Mauna Kea Volcano, Hawaii (after Porter 1979)
Sample Locations
Hamakua Group, and an upper alkalic cap stage, designated the
Laupahoehoe Group (Stearns and Macdonald 1946; Porter 1979). Thirty-nine Laupahoehoe Group lavas were collected from the
The Hamakua Group lavas range in age from 80 to greater than summit and south rift zone of Mauna Kea and were analyzed
150 ka (Wise 1987) and in composition from tholeiite to alkalic for major and trace elements (Table .)1 These samples were collect-
basalt with some ankaramites and high Tit2 hawaiites (Frey et al. ed only from cinder cones and flows which Porter (1979) placed
1987). Laupahoehoe lavas are well exposed on the upper slopes within one of the six volcanic stages. All stages were sampled so
of the volcano and within some gulches on the east flank. The that compositional evolution of the alkalic cap lavas could be eval-
total thickness of Laupahoehoe Group lavas is unknown. Estimates uated. In addition, the collected lavas span a wide range in eleva-
of thickness vary from as little as 10 m on the lower flanks of tion (2255 m) and distance from the summit ((~14 km) and thus
the volcano (Stearns and Macdonald 1946) to as great as 550 m represent a wide spatial distribution. Vents from the various stages
at the summit (assuming the lavas filled a caldera; Porter 1972). are not evenly distributed on the slopes of the volcano. Lavas
The upper portion of Mauna Kea has been glaciated and deposits from the youngest and oldest stages tend to be concentrated on
from the glacial activity are interstratified with lavas from the Lau- the flanks of the volcano; lavas from the middle stages are most
pahoehoe Group. These glacial deposits are time-stratigraphic un- common near the summit.
385
Table 1, Major and trace elements contents of Laupahoehoe lavas, Mauna Kea Volcano, Hawaii. Stages are in order of increasing
age; samples within a stage are in order of decreasing MgO
Loaloan
M1-5 M1-18 M1-4 M1-19 M1-6 M1-17 M1-14 M1-15 M1-3 M1-2 M1-20 M1-7 MI-ll M1-8 M1-13 M1-12
SiO 2 51.22 50.77 50.64 50.78 50.77 50.84 50.67 50.75 50.67 50.58 51.90 52.03 52.38 52.56 52.58 52.83
TiO2 2.59 2.63 2.60 2.60 2.56 2.58 2.60 2.59 2.56 2.58 2.32 2.19 2.18 2.23 2.21 2.05
A1203 17.31 17.22 17.08 17.19 17.12 17.16 17.13 17.10 17.12 16.88 17.12 17.18 17.18 17.57 17.29 17.28
F%O3 11.17 11.34 11.23 11.28 11.11 11.30 11.26 11.34 11.27 11.12 10.59 10.52 10.48 10.63 10.58 10.14
MnO 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 12,0 0.21 0.23 0.23 0.23 0,23 0.24
MgO 3.97 3.95 3.94 3.93 3.92 3.91 3.88 3.88 3.86 3.81 3.52 3.40 3.36 3.33 3.31 3.11
CaO 6.69 6.72 6.67 6.65 6.63 6.69 6.73 6.67 6.60 6.60 6.22 6.14 6.13 6.08 6.17 6.03
Na20 4.50 4.56 4.53 4.63 4.48 4.52 4.61 4.11 4.69 5.1I 5.t4 4.79 4.83 4.66 4.84 4.84
NazO(NA) 4.74 4.84 4.54 4.75 5.11
KzO 1.96 1.92 1.94 1.95 1.96 1.95 1.95 1.95 1.95 1.95 2.13 2.14 2.16 2.15 2,17 2.16
P2Os 0.88 0.90 0.88 0.90 0.88 0.90 0.91 0.90 0.92 0.87 0.95 1.04 1.04 1.06 1.08 1.13
SUM 100.51 100.23 99.73 100.13 99.65 100.07 99.67 99.51 99.86 99.71 100.10 99.66 99.97 100.50 100.46 99.81
F%O3/FeO 0.43 0.77 0.38 0.66 1.09 - 0.86 0.52 0.97 0.45 0.79 - 0.41 11.1 1.30 -
H20 + 0.30 0.37 0.28 0.32 0.23 - 0.38 0.24 0.21 0.30 0.27 - 0.40 0.53 0.38 -
H20- 0.16 0.14 0.11 0.10 0.08 - 0.19 0.09 0.10 0.15 0.14 - 0.20 0.26 0.13 -
CO2 0.03 0.03 0.06 0.09 0.08 - 0.11 0.03 0.09 0.03 0.11 - 0.10 0.06 0.04 -
Rb 33.6 35.5 34.6 36.4 37.9 35.2 33.7 34.7 35.9 36.0 39.0 37.2 42.6 40.6 41.2 35.5
Sr 1281 1254 1280 1269 1273 1275 1262 1261 1260 1265 1222 1242 1244 1241 1244 1241
Ba 627 618 634 645 631 648 630 670 643 639 652 695 703 697 695 736
Sc 9.0 - 8,7 8.8 7.6 - 6.7
V 17 74 71 74 72 70 75 80 73 68 60 45 46 49 43 42
Cr 3.5 2.6 2.4 3.4 4.9
Ni 3 7 5 10 5 5 9 15 8 8 6 6 2 8 6 15
Zn 129 139 135 129 125 137 141 129 135 129 139 133 148 148 127 138
Ga 22.4 22.3 23.2 22.2 22.2 22.8 22.5 21.3 23.2 21.7 22.5 23.5 23.0 21.9 23.5 22.7
Y 45 44 44 44 44 44 44 44 44 43 44 48 48 48 48 49
Zr 487 466 489 474 481 481 479 474 507 495 503 550 536 537 534 561
Nb 62.2 62.6 63.2 62.4 61.0 63.8 63.8 61.8 62.3 63.6 66.7 68.4 69.8 67.5 67.9 71.3
Hf 10.3 9.9 10.I 10.8 12.3
Th 3,4 3.9 3.9 4.6 4.4
La 50.8 51.5 49.9 53.4 63.6
Ce i23 122 119 135 155
Ce(XRF) 115 117 120 111 117 121 119 116 117 114 128 133 136 128 132 143
Nd 66.6 65.0 62.0 70.6 82
Sm 14.0 13.9 13.5 4_41 16.6
Eu 4.44 4.60 4.56 4.56 5.07
Tb 1.97 1.70 1.75 96_1 2.16
Yb 3.41 3.22 3.31 3.26 3.88
Lu 0.45 0.46 0.44 84_0 0.57
Analytical 1 using l~6Nd/144Nd=0.72190. Pb isotopic ratios are normalized
to an SRM 981 value for 2~ /2~ of 2.1671 (Todt et al.
Mineral analyses were made with the University of Hawaii electron
1984).
microprobe (Garcia et al. 1986). Major element contents and se-
lected trace elements (Rb, Sr, Ba, V, Ni, Zn, Ga, Y, Zr, Nb and
Ce) were determined in duplicate at the University of Massachu- Results
setts by X-ray fluorescence (XRF) (Rhodes, 1983). Ferrous iron
was determined by titration, HzO + by heating at 1100 ~ with collec-
Petrography and mineral compositions
tion on anhydrone and COz by HC1 dissolution with collection
on ascarite, all at the University of Manitoba. In addition, 19 The Lauaphoehoe Group samples studied are fresh and
samples were analyzed for Na, Sc, Cr, Hf, Th, and REE by instru- essentially aphyric. They contain rare (0-1.4 vol. %) plagio-
mental neutron activation at MIT (Ila and Frey, 1984). Precision
clase phenocrysts and less than 1.8 vol. % microphenocrysts
of the abundance data is indicated in Table 1; in addition, Na
(Table 2). Plagioclase is the dominant microphenocryst
and Ce were analyzed by XRF and neutron activation (Table 1).
phase with subequal to lesser amounts of olivine and mag-
Nd, Sr, and Pb chemical and mass spectrometric techniques
were adopted from Hart and Brooks (1977), Richard et al. (1976) netite. Extremely rare (<0.1 vol.%) xenocrysts of olivine
and Grunenfelder et al. (1986). SVSr/S6Sr ratios are normalized to (resorbed and kinked) and plagioclase occur in some lavas.
0.70800 for E and A SrCO3 and 143Nd/144Nd to 0.51264 for BCR- The lavas have a trachytic texture and range from sub-
683
Table 1 (continued)
Kemolean Kuupahaan
Mk-1 Mk-2 Mk-4 Mk-3 Mk-5 Mk-6 Me-2 Me-1 Me-6 Me-3 Me-4 Me-5
2OiS 49.46 70.15 08.05 23.15 41.05 14.25 82.94 45.05 04.05 30.15 21.25 87.15
TiO2 2.76 05.2 2.50 84.2 75.2 12.2 58.2 2.80 57.2 15.2 2.32 2.22
sO21A 11.71 52,71 71,71 92.71 78.71 28.71 10.71 23,71 51.71 41.71 53,71 82.71
Fe203 84.11 27.01 07,01 86.01 51.11 30.01 44.11 04.11 24.11 38.01 93.01 02.01
MnO 12.0 12.0 12.0 12.0 22.0 0.22 12.0 0.22 0.22 0.22 0.22 0.22
MgO 4.14 3.77 3.74 37.3 07.3 82.3 4.20 4.16 4.12 3.74 84.3 72.3
CaO 79.6 66.6 6.66 6.62 19,6 11.6 40.7 00.7 89.6 95.6 63,6 83.6
Na/O 4.84 38.4 30.5 4.80 4.80 39.4. 16.4 4.32 4.56 4.60 4.82 61.5
Na20(NA) 4.60 +__ 0,04 4 98.4 36.4 4.56
KzO 28.1 2.06 00.2 80.2 77.1 2.20 58.1 49.1 98.1 10.2 51,2 91,2
sOEP 0.85 0.96 59.0 0.95 01.1 60.1 0.84 19.0 68.0 19.0 0.97 11.1
SUM 41.001 30,001 99,76 100.16 100.24 100.27 99.33 100.61 100.35 99,58 100.18 18.99
FezO3/FeO 0.52 37.0 35.0 59.0 37.0 94.0 0.46 0.57 55.0 84.0 0.50 0.62
HzO + 0.20 0.36 0.32 0.37 35.0 07.0 0.12 0.40 0.40 51.0 0.30 32.0
H20- 91.0 80.0 81.0 90.0 96.0 0.27 11.0 81.0 0.17 0.06 71.0 80.0
2OC 0.10 0.02 0.06 0.06 90.0 50.0 90.0 0.04 0.07 30.0 70.0 0.04
Rb 9.13 38.6 3.73 7.83 25.6 40.7 3.43 35.7 3.43 3.83 5.53 41.3
Sr 4621 0621 5521 0321 1621 0911 3721 6421 2621 8421 5421 0321
Ba 326 256 136 756 656 008 675 006 785 516 156 656
cS 8.9 1.0___ 7.0 2.01 8.9
V 88 76 16 76 06 85 79 78 58 17 46 46
Cr 8 +_ 1 6.5 9.8 9.5
Ni 7 8 01 9 01 4 4 9 21 5 3 7
Zn 941 921 031 031 931 961 611 131 331 821 141 231
Ga 7.22 3.12 21,9 9,12 1.12 23.0 21.4 23.2 22.2 5.32 22.7 22.4
Y 44 74 46 46 05 05 44 54 44 46 84 05
Zr 864 935 115 325 235 995 944 466 334 105 335 565
Nb 2.26 5.36 7.36 4.46 0.86 0.86 3.65 8.06 9.06 8.36 5.76 9.86
Hf 80.66.9 5.21 5.9 9.6
Th 2.3 2.0+_ 4.9 1.3 5.3
La 48.8 +0,8 9.06 45,8 1.84
Ce 021 +1 341 311 711
Ce(XRF) 11 711 911 911 631 241 711 021 811 421 231 141
Nd 66 + 2 0.37 8.26 6.36
Sm 14.53_+0.08 9.41 3.31 1.41
Eu 50.0+_65.4 4.92 83.4 85.4
Tb 10.0+_78.1 09.1 19.1 67.1
Yb 11.0_+62,3 17.3 92.3 31.3
Lu 20.0+_44.0 15.0 34.0 34.0
pilotaxitic to hyalopilitic. The groundmass consists primar- Euhedral plagioclase phenocrysts in Laupahoehoe lavas
ily of plagioclase, magnetite, olivine and clinopyroxene with range only from An57 to Ans9 (Table 3). These composi-
rare apatite. The scarcity of phenocrysts and the fine- tions yield a DK (Ca/Na)plagioelase/(Ca/Na)whol .... k of 1.75
grained nature of the matrix justifies treating the whole- which is higher than values obtained in experiments
rock analyses as representative of liquid compositions. (1.1-1.6) by Mahood and Baker (1986) on similar rocks
Relative to the bulk rock (assuming Fe + 2= 0.85 molar at temperatures greater than 1075 ~ The plagioclase xeno-
Fe v) groundmass olivine in Mi-5, the most mafic Laupahoe- cryst has a higher An content (72.5) than the euhedral
hoe hawaiite (Table 3), yields a Ko (Fe/Mg)ol/(Fe/Mg)wR grains and it is too calcic to be in equilibrium with the
of 0.31 similar to the experimentally determined equilibrium bulk rock. The opaque minerals are Ti-magnetites. Adja-
value of ~ 0.3 (Roeder and Emslie, 1970). However, olivine cent to rare dunite xenoliths, the magnetites are enriched
microphenocrysts (<0.5 mm) in Mi-5 (Table 3) yield lower in Cr and depleted in Ti (Table 3).
KD's of ~ 0.20. These disequilibrium microphenocrysts are
similar in composition (Fo 81) to strongly resorbed olivine
Whole rock geochemistry
xenocrysts, in a more evolved hawaiite (Mi-2, Tables 1 and
3). Microphenocrysts in Mi-2 yield near-equilibrium oliv- Major elements. The compositions of Laupahoehoe Group
ine/bulk-rock Fe + 2/Mg ratios of ~ 0.27. lavas are typical of Hawaiian alkalic cap lavas and they
783
Table 1 (continued)
Hanaipoean Poliahuan Liloean
Mh-2 Mh-3 Mh-1 Mh-4 Mp-2 Mp-1 Mi-5 Mi-2 Mi-3 Mi-4 Mi-1
SiOz 50.96 52.06 51.94 51.61 50.14 51.31 49.70 49.26 50.47 44.25 52.46 52.30 52.73
TiO2 63.2 92.2 32.2 51.2 2.74 14.2 59.2 2.96 56.2 20.2 2.04 2.04 2.00
Al2Oa 17.14 17.44 17.34 17.10 17.07 17.37 17.06 16.89 17.25 85.71 17.62 17.70 17.43
Fe203 10.62 10.16 10.21 10.73 11.55 10.93 11.83 11.83 11.17 00.01 10.02 10.10 9.94
MnO 0.22 12.0 0.22 0.22 0.22 32.0 12.0 12.0 12.0 0.22 32.0 32.0 0.22
MgO 05.3 04.3 72.3 62.3 4.17 76.3 54.4 4.40 10.4 10.3 30.3 20.3 89.2
CaO 07.6 91.6 6.40 32.6 09.6 47.6 51.7 51.7 39.6 02.6 22.6 21.6 50.6
Na20 87.4 00.5 71.5 61.5 4.26 16.4 4.49 4.56 4.60 20.5 70.5 39.4 74.5
NazO(NA) 10.5 01.5 12.5 4.90 - 4.39 4.46_+0.08 - 4.62-+0.12 5.04_+0.18 4.92 82.5
K20 2.14 81.2 2.19 22.2 88.1 51.2 48.1 28.1 59.1 62.2 62.2 32.2 2.32
P2Os 23.1 39.0 21.1 03.1 58.0 41.1 18.0 0.80 39.0 22.1 32.1 12.1 02.1
SUM 99.74 99.86 100.09 99.98 99.78 100.56 100.49 99.88 100.17 89.99 100.17 99.89 100.34
Fe203/FeO 0.70 34.0 0.66 76.0 0.40 77.0 0.44 0.74 2.34 84.0 0.66
H20 + 0.30 34.0 80.0 82.0 0.10 87.0 0.32 0.44 0.54 57.0 13.0
H20 0.22 0.10 0.09 01.0 50.0 27.0 80.0 81.0 91.0 92.0 0.09
CO2 0.07 60.0 80.0 30.0 0.02 31.0 0.06 90.0 02.0 60.0 10.0
Rb 6.83 8.14 42.6 7.54 0.53 9.24 0.23 4.33 4.83 41.0 1.34
Sr 1223 1222 1220 1105 1283 1206 0621 7821 8711 1180 1169
Ba 066 776 946 776 506 036 265 316 786 396 096
Sc 7.7 5.7 7.0 7.7 8.0 10.6 +0.1 9.3 +0.2 6.6 2.0_+ 6.7 6.6
V 65 35 84 04 78 55 901 48 53 63 83
Cr 0.3 9.5 4.6 6.3 - 3.3 11.0 +0.4 7.4 8.0+_ 3.5 1.0+_ 6 8
Ni 7 7 4 5 21 01 4 8 tl 7 7
Zn 631 031 921 041 321 631 331 921 241 541 351
Ga 8.12 5.12 8.12 23.0 2.22 1.32 9.22 5.12 22.4 3.32 8.22
Y 25 74 05 25 44 94 34 74 35 25 25
Zr 545 745 455 116 442 635 524 754 706 636 526
Nb 7.86 0.76 8.76 9.07 3.95 65.4 4.75 2.16 5.47 4.37 3.57
Hf 2.21 8.11 1.21 6.21 5.11 9.2 3.0_+ 10.2 +0.2 13.1 +0.6 5.31 4.31
Th 9.5 4.4 4.4 4.9 3.4 2.3 2.0_+ 4.3 1.0+_ 8.4 1.0+_ 9.4 0.5
La 0.26 8.45 3.95 7.66 4.75 44.5 +0.7 1.0+_9.05 65.6 -+1.3 7.66 5.76
Ce 551 131 241 651 711 041 801 3_+ 421 3+- 851 7+- 751 551
Ce(XRF) 051 821 141 351 331 901 811 251 351 451
Nd 48 96 57 97 57 3+_95 3+_86 82-+5 08 08
Sm 1.71 5.51 2.71 1.61 1.61 3.41 14.8 1.0+_ 16.4 +0.6 5.71 7.71
Eu 04.5 96.4 70.5 61.5 4.98 4.40_+0.01 4.74_+0.06 5.22_+0.04 02.5 02.5
Tb 23.2 28.1 89.1 29.1 1.85 1.72_+0.08 1.94_+0.15 2.12-+0.23 70.2 60.2
Yb 4.02 93.3 58.3 28.3 3.68 3-12+_0.03 3.33_+0.03 4.10_+0.16 79.3 4.12
Lu 55.0 05.0 0.50 35.0 0.51 0.44_+0.01 0.47+0.02 0.55_+0.04 65.0 55.0
Notes: )1( Two Mi-5 major element analyses are for two independent sets of duplicates. )2( Two Mi-3 analyses are for two separate
crushes of hand-specimen sample
are distinct from the underlying Hamakua shield lavas Trace elements. Abundances of the compatible elements Ni
which are dominantly basaltic (Frey et al. 1987). Specifi- and Cr are very low, typically < 10 ppm, and do not vary
cally, the alkalic-cap lavas range in SiO2 from ~49% to systematically with MgO content, probably because (1)
53% and lie well within the alkalic field on a total alkalis- these low abundance levels are near the detection limits
SiO2 plot (Fig. 3). They are dominantly hawaiites with rare and (2) some lavas are contaminated with Ni-rich, disaggre-
mugearites; note that the trends for the Laupahoehoe and gated dunite xenoliths. Sc abundances are also low,
Hamakua lavas are offset in Fig. 4. In terms of normative <15 ppm, and systematically decrease with decreasing
components they contain approximately 70% feldspar and MgO (Fig. 5b). The most dramatic trend is for V which
range from hypersthene normative (<10%) to slightly decreases precipitously from 110 ppm in the most MgO-rich
nepheline normative (<2.5%). With decreasing MgO con- samples to ~ 30 ppm in the low MgO samples (Fig. 5 b).
tent, CaO, FeO T and TiO2 contents systematically decrease Abundances of the incompatible elements, Th, Zr, Hf,
and SiO2, A1203, Na20, K20 and PeOs contents systemati- Nb and LREE vary positively with P205 (Fig. 6) and en-
cally increase (Fig. 5). Sample Mk-5 deviates from the gen- richment factors (most evolved lava/least evolved lava) are
eral trend in K20, A1203 and P205. The largest decrease 1.42 to 1.56. Sr and Ba abundances are less variable. Sr
is for TiO2 (~32%) and the largest increase is for P2Os abundances are high (1105 to 1134 ppm) and decrease with
(~63%). increasing PgOs content. Ba is incompatible in these rocks
388
Table 2. Modal mineralogy of representative Laupahoehoe lavas.
I I I I I I I i I I
Modes (volume %) are based on 1000 point counts on each sam-
ple. Phenocrysts (ph) are >0.5 mm. Microphenocrysts (mph) are 8
0.1-0.5 mm
6 LAUPAHOEHOE~
Sample Olivine Plagioclase Magnetite Ground-
mass
ph mph ph mph ph mph
v
M1-2 - 0.7 - 0.6 - 0.3 98.4 O #4
M1-3 - 0.2 - 0.9 - - 98.9 +
M1-20 - 0.t - 0.2 0.3 99.4 O ea
1:3
Me-2 0.1 0.2 0.5 0.6 - - 98.6 Z
Me-6 - 0.1 1.4 0.2 0.1 0.4 97.8
Mk-1 - 0.1 0.3 0.3 - 0.7 98.6 2
Mp-1 - - 0.6 1.0 0.1 0.8 97.5
Mi-3 . . . . 0.9 99.1
Mi-5 0.2 - - - 99.8
0 44 I 46 I t 48 I I 50 I I 25 I I 54 I
OiS 2 .tw( )%
Table 3. Microprobe analyses of minerals in Laupahoehoe Group
Fig. 3. Alkalis - SiO2 plot for Mauna Kea lavas. Data for Ha-
lavas, Mauna Kea Volcano, Hawaii
makua shield lavas (Frey et al. 1988) straddle the tholeiitic-alkalic
boundary (solid line from Macdonald and Katsura 1964), whereas
Olivine
the alkalic cap (Laupahoehoe Group) lavas lie well within the al-
kalic field. Note, there is no compositional overlap between shield
Mi-2 Mi-5
lavas and these alkalic cap lavas
mph mph mph Xeno mph gm
SiO2 37.78 37.65 38.07 38.27 38.86 37.71 80 I I I I I I I
FeO 22.74 22.91 23.11 17.99 17.75 24.16
MgO 39.25 39.55 38.90 43.30 43.55 37.75 ETIEROMNEB
Total 99.77 100.11 100.08 99.56 100.16 99.62 x 60
A~k 'EOHEOHAPUAL
C~
Fo% 75.5 75.5 75.0 81.1 81.4 73.6 Z -
Z O TLASAB
Plagioclase ~< 40
ETIRAEGUM ETIIAWAH
F-
Mi-2 Mi-5 ,,Z ,
cr"
t.d
Xenocryst DL _ A~OKAMAH
ph mph-0.15 mm rounded-0.3 mm mph-0.35 mm N 2o
SiO2 53.66 53.40 49.86 53.40
A1203 29.26 29.44 32.21 29.65
02
FeO 0.59 0.76 0.51 0.65 0 0 ~ I , r , , 40 I ~ i 60 I i I i 80
CaO 11.40 11.55 14.47 11.56
NazO 4.44 4.28 2.94 4.26 ETIHTRONA %
K20 0.29 0.28 0.13 0.24 Fig. 4. Differentiation Index (DI = normative Q + Or + Ab + Ne +
Ks + Lc) versus An% in normative plagioclase; a classification dia-
Total 99.64 99.71 100.22 99.76 gram from Coombs and Wilkinson (1969). The alkali cap is dom-
inantly composed of hawaiites and distinct from the underlying
An% 57.6 58.8 72.5 59.1 shield lavas which range from basalt to hawaiite (Frey et al. 1988)
Fe-Ti Oxides
up to about 1.1 w% P205 (Fig. 6). Although most samples
Mi-2 Mi-5
define coherent trends among incompatible element abun-
dances, samples Mh-2 and Mh-4 have anomalously high
mph mph mph gm mph gm Dunite
PzO5 (e.g., P205 versus MgO, Ce, Ba and Nb). Sample
0.1 mm 0.2 mm 0.15 mm 0.1 mm beside
Mk-6 has anomalously high Ba and sample Mk-5 has
anomalously low K20 and Rb (Figs. 5 and 6). Some of
TiO2 17.34 18.56 16.70 19.49 16.89 19.25 13.60
A1203 4.94 4.35 4.95 3.61 3.13 3.61 5.35 these geochemical deviations represent post-magmatic alter-
CrzO3 0.00 0.04 0.03 0.05 1.85 0.04 6.50 ation processes; e.g., low K and Rb in Mk-5 probably re-
FeO 68.34 68.90 67.67 67.07 67.47 67.47 63.83 flects alkali mobility indicated by the occurrence of biotite
MnO 0.55 0.70 0.65 0.74 0.64 0.67 0.21 in tension gashes.
MgO 4.70 4.52 5.55 4.52 5.89 5.07 6.44 REE abundances for the 19 samples analyzed span a
narrow range and have significantly higher LREE/HREE
Total 96.57 97.07 95.45 95.48 95.87 96.11 95.93
ratios than the shield lavas; Eu anomalies are absent
983
I I I I I I I I
53 ~+ + Loaloan x Hanaipoean
8.0
0 0 o Kemolean .~ Poliahuan
52 %x
17.5 Dz + :: Kuupahdan * Liloean
o ea~
< 3/(
17.0
+
o
O5
65
J 120
49 I I I I
I I I I I I I I
7.0 5.5
I00
xx +
0 65 0 5.0 * 0 x 0 +
8 XI3 +%q~ 0 80
x + Z E
+ z ~ (cid:12)9 0-
6.0 5.4 {3.
> 60
+
I I I I r I I
4O
0.21
L2
+ ~
115 %
2O I I I I
y llO ~ 2.0 15
105
1.8
0
I0.0
I0
I I I I I t I I E
50 Y C 3.1 x x {3.
#
+ A CO
c,125
0 0 5-
2c
+~
2.0 9.1
0
5.~ z.o I I 4.0 I 50 30 I I 4.0 I I so 2.0 3.0 I I 4.0 I I 5.0
MgO MgO
MgO (wt.%)
Fig. 5a. Major oxide - MgO variation diagram for Laupahoehoe lavas. Each time-stratigraphic group has a different symbol (legend
in upper right panel); note coherent trend defined by samples; .b Abundances of the compatible elements, Sc and V, versus MgO
for Laupahoehoe lavas
(Fig. 8). Within the alkalic cap lavas LREE/HREE ratios tios than the shield lavas at comparable S?Sr/86Sr ratios
increase with decreasing MgO and increasing P205 (Li- (Hegner et al. 1986).
loean-Mi suite in Fig. 8 a). La/Yb and Ce/Y (which is analo-
gous to a LREE/HREE ratio) change by ~20% (Figs. 8
Discussion
and 9).
cinegoidaR isotopes. Sr, Nd and Pb isotopic data for 8 sam- Temporal and geographic compositional variations
ples which span the age and the compositional range of The Laupahoehoe lavas show no consistent compositional
Laupahoehoe lavas (Table 4) define a relatively restricted variation with age or location. In particular, lavas within
range which is slightly larger than analytical error, e.g. each of Porter's (1979) Laupahoehoe stage, including the
87Sr/86Sr from 0.70335_+4 to 0.70351_+4. These small Holocene unit which is common to both Porter's (1979)
variations in isotopic ratios do not correlate with composi- and Wolfe's (1987) subdivisions of the Laupahoehoe lavas,
tional variations or age. Sr, Nd and Pb isotopic ratios of range widely in degree of differentiation (as measured by
Laupahoehoe lavas overlap with those of the underlying MgO content, incompatible trace element abundances and
shield lavas (Table 4). However, the alkalic cap lavas trend AlzO3/CaO) and in degree of silica saturation (as indicated
to less radiogenic Pb ratios than the shield lavas. These by normative hypersthene and normative nepheline con-
results contrast with West Maui data where the alkalic cap tent; Fig. 9). The same is true for distance from the summit
lavas have slightly higher 2~176 and 2~176 ra- of the volcano. Over a 13.5 km distance from the summit,
093
I I I I i i i i i I I 6.1 I I I I I I L I I I I I I I I
160
1300 + 4.1
O 2.1
oo21~ O x 8 o41 + +0 o E~ f 0.1
x D ++
+ I i L I I I I I I L I I I I I
I lO0 (cid:141) 200 /RANGE OF Mi-1,5,4 (I)
x
800 I I i O I I I I I I t I I Jl 3okF- I00 MMii--25 uo----uo
E
+ 7o + -4 X Z 3d 50
o 70C ~+ (cid:141) 4~x (cid:141) 0 I
rrl +X x &
RANGE FOR
5 20 II SHIELD LAVAS
6O A
60C
~D
I I I I I I I I I I I -?- I0
65O
Lo Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
2.2 x+D $-+(cid:127)O Lx X O X RARE EARTH ATOMIC NO
Fig. 7. Lower: Comparison of chondrite-normalized REE abun-
O 5~
Z 1~550 X- dances in Hamakua shield lavas (range of 11 tholeiites and alkalic
2.C DO basalts from Maulua Gulch, Frey et al., 1988) to chondrite-normal-
ized Laupahoehoe lavas. Data are shown for Liloean (Mi) series.
450 The 41 other Laupahoehoe lavas (Table )1 fall within the range
1.8
0 of the Mi samples.
I I I I I I I I I I I
Upper: REE in evolved Laupahoehoe hawaiites (Mi ,1 3, 4) com-
(cid:141)
pared to the least evolved hawaiite (Mi .)5 Note the convex down-
+ X 55
x +9,+ D ward pattern
4O k?
+~@
+ + ~
50
~+ +:~
5 %+
30 30 3.0
45 §
0 El2+++ + +
I I I I I I I I I
0.8 O.I t2 0.8 0.1 2.1 18.2
P205 P205
Fig. 6. Abundances of incompatible elements versus P2Os. Except
for Sr, these elements are positively correlated with P205 content. o
Marked deviations from coherent trends are K20 and Rb in Mk-5 8 2.6
which contains biotite in tension gashes and Ba which is inexplica-
bly high in MK-6 C
4C
2.4
there is an elevation change of over 2.2 kin. Despite this
significant difference in elevation, there is no correlation
of composition of the Laupahoehoe lavas with distance +
from the summit (elevation). In contrast, there is a strong 0.5: i
correlation of vent location (i.e. proximity to the summit)
with composition at Mauna Loa and Kilauea volcanoes.
This correlation is interpreted as reflecting shallow reservoir _Q
Q.
systems (e.g., Wright and Fiske 1971 ; Rhodes 1987). v
>-
2.5
Compositional variation among L.)
Mauna Kea alkalic cap lavas :
evaluation of a fractional crystallization model
Given the small variations in radiogenic isotopic ratios (Ta-
ble 4) and coherence in the abundance trends (Figs. 5 and 2s
I
6), it is probable that the compositional trends of the Lau- 2.0 5.0 4.0 5.0
pahoehoe lavas were dominantly controlled by a relatively Mg 0 (wt. %)
simple process such as fractional crystallization. The geo-
Fig. 8. Ce/Y and AI~O3/CaO versus MgO. Ce/Y correlates posi-
chemical data can be used to develop and evaluate a frac- tively with La/Yb. Therefore, the plot shows increasing fractiona-
tional crystallization model. Following, are geochemical tion of REE as MgO decreases. These trends require an important
constraints on the fractionating mineral assemblage. role for clinopyroxene fractionation
193
I I I I I I I I I I I I I I )1 The very low Ni and Cr abundances and Mg/Fe ratios
8 o o require fractionation of marie phases such as olivine, spinel
< 1.2 - ~ oCo and pyroxene;
0 0 o ~ o __oo C 2) The systematic increase in AlzO3/CaO with decreas-
o o~)
Q_ 1.0- o 000% ing MgO (Fig. 8) requires a significant role for clinopyrox-
o o o 0 co o 06) ene. Clinopyroxene fractionation also could explain the in-
o I I 8 o I l--I oCO I o I I ~I I I- crease in La/Yb and Ce/Y and decrease in Sc with decreas-
21
ing MgO content (Figs. ,5 7 and .)8
0 0
~-- o o 3) The compatible behavior of Sr (Fig. 6) requires pla-
8-
gioclase fractionation.
~-- ~ o 4) The marked decrease in TiO2 and V with decreasing
4 -0 o
MgO (Fig. 6) requires segregation of Fe-Ti oxides. Because
0%0
-o o
Nb is more compatible than Zr in Fe-Ti oxides (e.g.,
0-5--- -8- -o- _ _~9o~ ~ .... o_.o_o
Y 2-g 0 McCallum and Charette, 1978; Pearce and Norry, 1979),
4 o 0 - - o e ~176176176 the decreases in Nb/Zr and Nb/La with decreasing MgO
are also consistent with significant fractionation of Fe-Ti
2.9-0 o 8 o o(b -- oxides.
0 _0 0 0-- -- 0 0 0 _ 5) The inverse relation between MgO and SiO2 (Fig. 6)
(D o 2.7- 0 o 8 0 8o o o requires subtraction of SiO2-poor phases such as olivine
0
C Jo 5.2 -o o 0 o 0 o o o Q,~176176 O 0 0 0 &% - -- and 6) Fe-Ti The oxides. incompatible behavior of P205 (0.80 to 1.32%)
--0 0 --0 0 -- and absence of apatite as phenocrysts or microphenocrysts
2.3-
I I I I I I Ilflllll suggest that apatite was not a fractionating phase. This
is consistent with the apatite-saturation equation proposed
45- 8 o by Harrison and Watson (1984); i.e. ~2% P205 is needed
0 o Q(cid:127)O O to reach apatite saturation in a hawaiite melt at 1100 ~
40- o The petrographic characteristics of Laupahoehoe lavas
0
o 8 %~ are also important in evaluating a fractional crystallization
0
35 0 0 0 0 0 0 model; e.g., their nearly-aphyric nature establishes that
0 0 0 0 ~ O0 0 .8u^~ 0 each analysis is representative of a melt, and the scarcity
0.3 - 0 of phenocrysts requires very efficient segregation of postu-
lated fractionating phases. However, the rare presence of
I I I I I I I I I I I I I I
Mi Mp hM eM kM IM 0 4 8 21 olivine and plagioclase xenocrysts that crystallized from
melts less evolved than the most marie hawaiites (Tables 2
Old AGE/STAGE Young DISTANCE (km)
and 3) may provide evidence for a range of magma compo-
Fig. .9 Variation in the composition of Laupahoehoe Group lavas
sitions along a liquid line of descent. The absence of clino-
sa a function of ega and distance from the summit of Mauna
Kea. No correlations are evident for incompatible (P205) or com- pyroxene despite geochemical trends requiring clinopyrox-
patible (MgO) elements, AlzOs/CaO ratio or degree of silica satu- ene fractionation (Figs. ,5 7 and 8) is another example of a
ration (normative %Hy or Ne) "clinopyroxene paradox" (e.g., Dungan and Rhodes, 1978).
Table .4 Radiogenic isotope data for Laupahoehoe lavas
selpmaS rS68/rS78 143Nd/144Nd 2~176 2~176 2~176
Youngest
"2-1M 0.70349(cid:127) 0.513054(cid:127) 31 123.81 654.51 498.73
MI-I1 0.70349(cid:127)
91-IM 83307.0 (cid:127) 0.513070(cid:127)
Mk-6 0.70350(cid:127) 0.513004(cid:127) 81 18.358(cid:127) 71 15.478(cid:127) 51 37.952(cid:127)
0.70350(cid:127)
6-eM 0.70343(cid:127) 0.513018(cid:127) 18.332(cid:127) 15.468(cid:127) 37.914(cid:127)
"4-hM 0.70342(cid:127) 0.512973(cid:127) 423.81 034.51 078.73
Mp-I a 15307.0 (cid:127) 0.513016(cid:127) 91 933.81 064.51 649.73
tsedlO
5-iM 0.70335(cid:127) 0.513033(cid:127) 18.322(cid:127) 5 364.51 (cid:127) 37.907(cid:127) 11
Range for 01 64307.0 800315.0 63.81 44.51 788.73
Hamakua Lavas to to to to to
(Frey et .la )8891 16307.0 350315.0 44.81 55.51 960.83
Mean of 2 Pb analyses
a
rS and Nd data with two sigma uncertainty determined at MIT by Kennedy
Pb data determined by S.-T. Kwon at Univ. Calif. Santa Barbara.
Reproducibility si better than 0.08% per mass unit for 2~162176 and 0.05% for 2~176 and 671~2
293
In order to quantitatively evaluate fractional crystalliza- We used clinopyroxene from an ankaramite for model 1 B
tion as a major process in controlling compositional varia- (Table 5).
tions within the Laupahoehoe Group, we modelled compo- 3) Clinopyroxene and Fe-Ti oxide-bearing gabbroic xe-
sitional variations within several of the Laupahoehoe noliths are common in Mauna Kea hawaiites (Jackson et al.
stages, including the Holocene unit. Similar results were 1982: Fodor and Van der Meyden 1988). Compositions
obtained for each stage. We discuss in detail modelling for of clinopyroxene, plagioclase and Fe-Ti oxides in these gab-
the oldest group (Liloean, Mi-1 through Mi-5) because it bros were used in the fractionation models (model 2 of
has the largest compositional range. Sample Mi-5 is the Table 5). Clinopyroxene in these gabbros is significantly
least evolved Laupahoehoe lava. Samples Mi-1, Mi-3 and less aluminous than other clinopyroxene compositions used
Mi-4 are among the most evolved and sample Mi-2 is inter- in modeling (Table 5).
mediate. These estimates of the fractionating clinopyroxene com-
position vary considerably and enable evaluation of the
Major element modelling. There is little experimental infor- sensitivity of the models to choice of clinopyroxene compo-
mation about equilibrium mineral compositions for haw- sition. The fractionating assemblages used to model the
aiite melts as a function of pressure and temperature; conse- small compositional step from Mi-5 to Mi-2 (4.42%-4.01%
quently, we have not evaluated major element trends during MgO) all yield acceptable fits to the data with similar de-
fractional crystallization or small increments of equilibrium grees of fractionation, 7.2-8.6% (Table 6). Except for K20
crystallization (e.g., Grove and Baker 1984). Rather we all oxides are fit within analytical error. The calculated K20
used a least squares approach assuming equilibrium crystal- is 5-7% too low. This could be explained by fraetionation
lization to model major element variations among the Mi of a more potassium-rich feldspar, by derivation from a
samples. Undoubtedly the fractionation process is more parent with a lower K20 content or late-stage K20 mobi-
complex, but over the compositional range (4.4-3.0% lity. All models require plagioclase as the dominant frac-
MgO) of the hawaiites this approach should be adequate tionating phase with olivine, magnetite and ilmenite in de-
for determining the nature and proportions of fractionating creasing abundance. Exclusion ofilmenite as a fraetionating
phases. phase results in poor mass balances for TiO2. Fodor and
Compositional variations within the Mi series were Van der Meyden (1988) identified two types of gabbroic
modelled in steps: Mi-5 to Mi-2, Mi-2 to Mi-3, and as xenoliths, olivine-gabbro and opaque-rich gabbro, in
a single step of Mi-5 to Mi-3 or Mi-1. In each case mineral Mauna Kea hawaiites. The latter group has model propor-
compositions were chosen to have Fe/Mg DK( = 0.3 for oliv- tions very similar to the calculated fractionating assemblage
ine, ~0.25 for cpx) and Ca/Na DK( ~1.5-1.7 for plag.) (Table 6). In particular, the calculated abundance of Fe-Ti
in equilibrium with the evolved melt. oxides (~ 15-20 wt.% of the crystallizing assemblage) is in
For the Mi-5 to Mi-2 step two sources of olivine, plagio- excellent agreement with the high proportion (11-27 vol.%)
clase and oxide data were used: (a) microphenocryst com- of Fe-Ti-oxides in the opaque-rich gabbroic xenoliths.
positions in Mi-2 (Table 3) and (b) compositions typical Models for the step of Mi-2 to Mi-3 and the sum of
of minerals in Fe-Ti oxide-rich, gabbroic xenoliths in these two steps Mi-5 to Mi-2 plus Mi-2 to Mi-3 were evaluated
hawaiites (Fodor and Van der Meyden, 1988). These xeno- also. Results of the two step model are very similar to the
liths are interpreted by Fodor and Van der Meyden (1988) single step from Mi-5 to Mi-3 (Table 6b). This single step
as cumulates from Mauna Kea magmas. The evolved com- encompasses almost the largest compositional variation
position of the minerals in these xenoliths indicates that among the hawaiites (4.42-3.01% MgO). The composition-
they may have formed during crystal fractionation of haw- al change from Mi-5 to Mi-3 was also satisfactorily mod-
aiite melts. These oxide-rich gabbros contain ilmenite and elled with a fractionating assemblage of plagioclase > clino-
clinopyroxene. Both are absent as xenocrysts or microphe- pyroxene>Fe-Ti oxides>olivine, but this step requires
nocrysts in Laupahoehoe lavas. As discussed above, clino- considerably more fractionation, ~27% (Table 6). Again,
pyroxene is required as a fractionating phase if these haw- the high wt.% of oxides (~ 16% of the crystallizing assem-
aiites are genetically related. Because clinopyroxene (cpx) blage) in these models corresponds to that in the opaque-
is not a phenocryst or microphenocryst phase in the Laupa- rich gabbros. We also modelled the evolution of sample
hoehoe lavas, its composition is the largest uncertainty in Mi-1 from Mi-5. Because Mi-I is slightly more evolved than
the modelling. Three different approaches were used to esti- Mi-3 (Table 1), this model requires slightly more fractiona-
mate cpx compositions: tion, ~ 30% (Table 6).
1) It is known that the stability field for cpx in basic Because the major element discrepancies are small, crys-
melts expands with increasing pressure (e.g., Mahood and tal-melt segregation is a viable process to explain the major
Baker 1986). Therefore, the geochemical evidence in Laupa- element abundance trends among the Liloean (Mi) samples.
hoehoe lavas for clinopyroxene fractionation may reflect An unanticipated major conclusion is that the fractionating
moderate pressure crystallization. Specifically, for hawaiites assemblage contained substantial amounts of clinopyroxene
experimental studies have found that cpx is the liquidus and Fe-Ti oxides. The important role of clinopyroxene
phase only at moderate to high pressures > 8 kb (Thomp- (18-26%) is surprising in that it does not occur as a pheno-
son 1974; Knutson and Green 1975; Mahood and Baker cryst or microphenocryst in these nearly aphyric hawaiites.
1986). We used the results of Mahood and Baker to esti- A possible explanation is that the small amounts of plagio-
mate clinopyroxene composition in equilibrium with haw- clase and olivine phenocrysts and microphenocrysts in Lau-
aiite melts at 8 kb (for details see model 1 A in Table 5). pahoehoe lavas (Table 2) reflect low pressure crystallization
2) Ankaramites occur in the uppermost portion of the and that fractionation of a clinopyroxene-bearing assem-
Mauna Kea shield. These cpx-rich rocks probably formed blage occurred at moderate pressures. Experimental studies
in part by clinopyroxene accumulation (Frey et al. 1988). of hawaiite compositions clearly show that the clinopyrox-