Geochemistry of the Hawi lavas, Kohala Volcano, Hawaii

15 Pages·2004·1.53 MB·English

Contributions to
Contrib Mineral Petrol )8891( 99:90-104
Mineralogy and
Petrology
(cid:14)9 galreV-regnirpS 8891
Geochemistry of the Hawi lavas, Kohala Volcano, Hawaii
Steven R. Spengler and Michael O. Garcia
Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Honolulu, HI ,22869 USA
Abstract. Hawi lavas form the late stage alkalic cap on from basalt to mugearite (which was thought to be the
Kohala Volcano and range in composition from hawaiite dominant rock type of the cap, > 90%), with a significant
to trachyte. New, detailed field mapping of Kohala and eruptional hiatus separating the two. On Kohala, for in-
reinterpretation of previously published age data suggest stance, an apparent age gap of about 100 Ka between shield
that there was no significant eruption hiatus between the and cap lavas was suggested (McDougall and Swanson
Hawi and underlying Pololu shield lavas as was previously 1972). The age and compositional gaps of the Kohala-type
suggested. Mineral and whole-rock chemical data are con- caps are difficult to reconcile with current models for the
sistent with a crystal fractionation origin for the hawaiite thermal history of a Hawaiian hotspot (e.g., Wise 1982).
to trachyte compositional variation observed within the Why would the volcano stop erupting and then restart with
Hawi lavas. Plagioclase, clinopyroxene, Ti-magnetite, oliv- a different composition?
ine and apatite fractionation are needed to explain this vari- Previous interpretations of the Kohala cap were based
ation. The clinopyroxene fractionation may have occurred on reconnaissance mapping and limited chemical analyses
at moderate pressure because it is virtually absent in these (Steams and Macdonald 1946; Macdonald and Katsura
lavas and is not a near liquidus phase at pressures of less 1964; Malinowski 1977; Sibray 1977; Furst 1982). The pur-
than 8 Kb. Plagioclase would be buoyant in the Hawi haw- pose of this study is to re-examine the Kohala cap to deter-
aiite magmas so a mechanism like dynamic flow crystalliza- mine: )1 the relative proportions of the rock types present;
tion is needed for its fractionation and to account for the 2) the causes for the compositional variation among the
virtual absence of phenocrysts in the lavas. Hawi lavas are cap rocks; and 3) the chemical and stratigraphic relation-
distinct in Sr and Nd isotopic ratios and/or incompatible ships of the shield lavas to the cap lavas. We mapped the
element ratios from the Pololu lavas. Thus they were de- western and northern portions of Kohala Volcano in detail
rived from compositionally distinct sources. Compared to as part of the U.S. Geological Survey's Big Island Map
other suites of Hawaiian alkalic cap lavas, Hawi lavas have Project. The southern and eastern parts of Kohala were
anomalously high concentrations of phosphorus and rare mapped by S.C. Porter (University- of Washington). This
earth elements. These differences could be due to greater paper combines the field and geochemical data from both
apatite content in the source for the Hawi lavas. studies.
Regional geology
Introduction Kohala Mountain si the northernmost and oldest of the five vol-
canoes comprising the island of Hawaii (Fig. .)1 Reconnaissance
The chemical composition and character of eruptive prod-
mapping by Stearns and Macdonald )6491( del to the subdivision
ucts from the alkalic stage of development of a Hawaiian of Kohala iavas into two stratigraphic units: the Pololu and the
volcano are distinctly different from that of the underlying Hawi volcanic series. Pololu lavas constitute roughly 99 percent
shield lavas. These late lavas are mildly alkalic and moder- of the subaerial volume of the volcano and are dominantly tho-
ately to strongly evolved (hawaiite to trachyte) in contrast eitiiel in composition. The upper 001 meters of Pololu lavas -xe
to the dominantly tholeiitic basalts of the shield. Volcanism posed in Waipio Valley show a progressive up-section increase
is typically strombolian producing thick flows (i.e., 2-5 m in alkalinity and incompatible element content (Feigenson et .la
.)3891 The overlying Hawi lavas form a cap of up to 351 m thick
vs. typically < 1 m for the shield lavas) which are erupted
which thins abruptly away from the summit region, forming mas-
from steep-sided, 50-100 m high cinder cones.
,evis relatively thick (2-5 m) flows ranging in composition from
Macdonald and Katsura (1962, 1964) delineated two
hawaiite to trachyte (Stearns and Macdonald .)6491 In this study,
types of alkalic caps on Hawaiian volcanoes: the Haleakala aphyrie lavas were found at and near the top of the Pololu section
and Kohala types. The Haleakala type is characterized by which have a composition intermediate between Pololu and Hawi
a basal transition zone of interbedded basalt and hawaiite lavas. They are termed here transitional Pololu.
overlying the tholeiitic shield lavas and is composed pre- Hawi volcanism saw mildly explosive in nature, producing
dominantly of hawaiites (>90%). In contrast, the Kohala large, steep-sided cinder cones (which predominantly follow the
type is characterized by an abrupt change in composition trend of the pre-existing rift zones formed during Pololu activity)
and voluminous lava flows (up to 061 x 601 m3). Morphologically
Offprint requests to." .M Garcia the Hawi lavas are distinguished by their hummocky surface and
19
155~ ' 155" 55'
I I
MAUNA KEA KOHALA
~ ,MUIVULLA SULAT
E& NILLAROC LLIF
~ lWAH
SCINACLOV NACLOV SCI
~ , "~'?",'?1
ULOLOP
,TLU~/F sb si no nwod edis ~ SCINAOI LOV
651 ~ 551 ~
~ ~0
~ L 19~
T
N
r , , ,
KM
Fig. 1. Geologic map of the northern portion of the island of Hawaii. Kohala Volcano is subdivided into two units: Pololu and Hawi
lavas. Flows for each of these units are shown by the upper pattern in the boxes; cinder cones are shown by the lower pattern.
Younger and coeval lavas from Mauna Kea abut Kohala on its southern flank. Major roads are shown by solid lines. Geology by
M. Garcia, S. Porter and S. Spengler after H. Stearns (Stearns and Macdonald 1946)
blocky flows in contrast to the relatively smooth surfaces of the transitional Polohi lavas has confirmed that the age gap separating
pahoehoe and aa flows of the Pololu lavas. Polohi lavas are also Pololu from Hawi lavas lies within analytical uncertainty of the
commonly overlain by well developed reddish weathering surfaces, measurements (i.e., ages, 28 Ka vs. analytical, 4-10-30 Ka). The
especially on the lower flanks of the volcano. common occurrence of soil horizons between Pololu and Hawi
Reported potassium-argon age dates for 17 Hawi lavas range flows simply reflects a decrease (rather than cessation) in eruptive
from 60 to 254 Ka (McDougall 1969; McDougall and Swanson activity since large parts of Kohala (especially the lower flanks)
1972). The oldest reported age (254 Ka, HW-20; McDougall and remained uncovered for relatively long periods of time before being
Swanson 1972) is for a sample with a lower K20 content buried by the more episodic eruptions which typified Hawi activity.
(1.46 wt.%) than any other definitive Hawi sample collected in Among the dated Hawi lavas there is no correlation of rock
this study. Although this flow has the appearance of a Hawi flow type with eruptive age (Fig. 2). Furthermore, at localities where
(i.e., massive, thick and aphyric), it is overlain by a plagioclase- relative stratigraphic position could be determined, no consistent
phyric Pololu flow. In addition it has a low P2Os content trend in composition with age was observed. Thus, the Hawi lavas
(0.83 wt.%) and lacks modal apatite, features that distinguish it were probably products of numerous discrete magma batches rath-
from all other Hawi lavas. Thus sample HW-20 should be excluded er than products of a progressively fractionating, long-lived magma
from the Hawi age range. Recalculation of the ages of these old body.
K-At age dates using the set of decay and abundance constants There are approximately 88 mappable Hawi vents (this is a
recommended by the IUGS Subcommission on Geochronology minimum estimate since some vents within the caldera complex
(Dalrymple 1979) yields a new age range of 62-232 Ka for the were probably buried by later activity), which erupted over a time
Hawi. Pololu lavas now range from 260 to 455 Ka (using only span of approximately 170 Ka. The continuity in ages for analyzed
samples with uncertainties of less than 30 Ka). This reduces the Hawi rocks (Fig. 2) suggests that volcanic activity was relatively
age gap between the two groups to only 28 Ka. Additional K-At constant and that an eruption occurred on average about once
age dating by G.B. Dalrymple (pers. comm., 1987) of lavas collect- every 1900 years. This eruption frequency for Kohala is similar
ed during this study from the uppermost Pololu section and of to the range estimated for Mauna Kea alkalic cap lavas (1250-
92
3.0 Benmoreites are typically more porphyritic and lighter in color
(b
than the hawaiites and mugearites and are distinguished by the
2.5 POLOLU presence of amphibole (kaersutite). Andesine is the predominate
phenocryst phase with subordinate amounts of kaersutite, olivine,
~o d:~ o HAWl O
magnetite and rare microphenocrysts of augite. Phenocrysts of
20 kaersutite are usually rounded with opacite reaction rims. Many
amphiboles have completely opacitized. The groundmass is com-
~o
posed largely of untwinned, subhedral to anhedral grains of oligo-
O
5.1 clase. Apatite predominantly occurs as microphenocrysts in associ-
O ation with magnetite although it is also rarely found included in
x/ amphibole. Olivine grains are subhedral and are almost entirely
IO
altered to iddingsite. Biotite is also seen as an accessory phase
in some samples. Anorthoclase is present as small groundmass
05- I I ~ ~ - grains.
i o--------~
I I ~ / ~ ~---3:4-~
Mineral compositions
0 o.I 02 0.5 0.4 0.5
AGE(Mo)
New analyses of phenocyrst and groundmass olivine and magnetite
Fig. 2. K-Ar age dates for Kohala Volcano lavas versus whole-
grains were made using the Cameca microprobe at the University
rock, K20 wt.%. Horizontal lines give 2___ sigma error for ages.
of Hawaii (see Garcia et al. 1986 for methods). These analyses
For symbols without lines, the error is less than the size of the
in conjunction with other microprobe analyses of mineral phases
symbol. Data from McDougall (1969), McDougall and Swanson
observed in Hawi lavas (Sibray 1977; Malinowski 1977; Nicholls
(1972), Lanphere and Frey (1987) and G.B. Dalrymple (unpubl.).
1985, pers. comm.) form a large body of mineral compositional
Symbols for transitional Pololu lavas are half filled. For Hawi
data for crystal fractionation modeling. Microprobe analyses of
lavas there is no temporal variation in K20 content
kaersutite and biotite can be found in Sibray (1977).
Hawi plagioclases have andesine compositions (Table 1). Si-
bray (1977) used long counting times (80-200 s) and up to 20 analy-
1500 years; West et al. 1988) but is a factor of 16 less than the
ses per plagioclase grain to measure the SrO content of Kohala
estimate of Feigenson and Spera (1981). Their rate was based on
plagioclases by microprobe. Resulting distribution coefficients
a two-dimensional analysis and thus, is a minimum. The new ages
range from 3.5-4.5 in hawaiites to 3.5-7.5 in benmoreites (Sibray
provide a much better framework for estimating eruption rates
1977).
than was previously available. With the revised eruption frequency
Olivines show a decrease in forsterite (Fo) content with increas-
and our new volume estimates for typical Hawi eruptive events
ing differentiation and range in composition from Fo74 in the haw-
(~ 100 (cid:141) 10 6 m3), an eruption rate of 4 x 104 m 3 of lava per year
aiites to Fo46 in the benmoreites. The amount of Mn in olivine
is derived. Adjusting for vesicuiarity of the lavas (approximately
increases dramatically from about 0.60 to wt.% in hawaiites up
10%), a value of 3.6 x 104 m3/yr of magma is obtained. This is
to 2.4 wt.% in bermoreites. The predominant opaque phase in
much lower than the eruption rate for Kilauea Volcano (minimum
Hawi lavas is fitaniferous magnetite (21.524.7 wt.% TiO2). Mag-
of 30x 10 6 m3/yr of tholeiite between 1956 and 1983; Dzurisin
netites found in the hawaiites generally have higher titanium and
et al. 1984) or Hualalai Volcano (2 x 601 m3/yr of alkalic basalt
magnesium contents and lower manganese contents than those in
during the Holocene; Moore et al. 1979). The decrease in eruption
the more differentiated Hawi lavas. Pyroxenes are either absent
rates from Kilauea to the late stage alkalic cap of Kohala clearly
or too small for analysis by microprobe in hawaiites and most
reflects the drift of Hawaiian volcanoes away from the locus of
mugearites. Sibray (1977) analyzed a groundmass augite in a mu-
magma production.
gearite (En43Fs13Wo44) and a microphenocryst in a benmoreite
(En31Fs23Wo46)- Their compositions are similar to values reported
Petrography for augites in evolved alkalic lavas on Maul (Fodor et al. 1975).
Apatite compositions are similar over a wide range in rock
Hawaiian late-stage cap rocks are characteristically aphyric composition (Table 2). However, many of the apatites in the highly
(< 1 vol.% phenocrysts). Petrographically, Hawi lavas can be dis- evolved rocks occur as inclusions in magnetite and amphibole;
tingushed from evolved Pololu lavas by: 1) the presence of modal thus their composition could reflect the chemistry of the magma
apatite, which is a reflection of the anomalously high P205 contents at an earlier stage of differentiation. This is further suggested by
of these lavas (up to 2.35 wt.% in the marie hawaiites); and 2) the similar Sr contents of apatites found as inclusions in St-poorer
the almost complete absence of modal clinopyroxene (even in the benmoreites and the groundmass apatites in the Sr-rich hawaiites
groundmass) in Hawi hawaiites and mugearites. The modal abun- and mugearites (Sibray 1977). The distribution coefficient of Sr
dance of apatite in Hawi lavas is quite constant (0.1-0.3 modal in apatite is 2.3 for hawaiites (Sibray 1977). The fluorine content
percent) regardless of the whole rock P205 content. of the apatite is greater in the core than the rim indicating normal
Kohala hawaiites are typically dark grey in color and generally zoning (J. Nicholls 1986, pers. comm.).
glisten in hand sample because of the parallel to sub-parallel align-
ment of groundmass feldspar laths. They have a fine-grained, inter-
granular texture and are darker in color than the more evolved Whole rock analytical methods
alkalic rocks on Kohala (which contain more groundmass plagio-
clase). In both hawaiites and mugearites, rare subhedral to euhedral Great effort was taken to collect only the freshest, least altered
zoned phenocrysts of plagioclase are present. Euhedral to subhed- rocks available from a given flow unit. Major element analyses
ral magnetites and olivines, whose rims are slightly altered to idd- of these rocks (Table 3) were performed using X-ray fluorescence
ingsite, also occur as microphenocrysts and coarse groundmass spectroscopy and CO2 analyses using a CHN analyzer at the US
grains. Apatite (0.1-0.3 modal percent) occurs in two forms; as Geological Survey's Branch of Analytical Chemistry at Lakewood,
tiny needles, colorless to light brown in color, and as rare, euhedral, Colorado. Na20 abundances were also determined by INAA (pre-
pinkish-brown pleochroic microphenocrysts which are commonly cision < 2%; Chen and Frey 1985). The average deviation of Na20
found in contact with magnetite grains. Extremely rare, small, col- values obtained by XRF and INAA for the same crushed powder
orless to pale green needles of clinopyroxene occur in the ground- was 2%. Based on replicate analyses of USGS standard rocks
mass. BCR-1 and BHVO-I, the precision for the major elements is better
93
Table 1. Microprobe analyses of minerals in Hawi lavas used in modeling calculations for hawaiites (H), mugearites (M) and benmoreites
(B)
Olivine Opaque Plagioclase" Clinopyroxene*
H M B a H M B H M B
SiO2 37.30 35.56 34.60 - - - 56.30 59.30 60.40 47.70
TiO2 - - - 24.68 23.84 21.50 - - - 2.42
A1203 - - - 2.08 0.87 0.94 27.40 24.90 25.10 7.98
NiO 0.14 0.10 0.04 . . . . . .
FeO 25.30 32.44 41.10 67.86 70.45 72.96 1.01 1.02 0.38 6.38
MnO 0.60 1.06 2.41 0.85 1.13 1.70 - - 0.12
MgO 35.10 30.85 20.90 3.24 2.49 1.95 - - - 13.80
CaO 0.53 0.50 0.32 - - 8.96 7.70 5.98 19.80
Na20 . . . . . . 5.88 6.51 6.99 0.70
KzO . . . . . 0.42 0.57 0.80 -
P20s . . . . . . . . .
Total 98.97 100.51 99.37 98.76 98.78 99.05 99.97 100.00 99.65 98.90
* From Mahood and Baker (1986) for a hawaiite magma at 1150 ~ C and 8 kb
From Sibray (1977)
Table 2. Microprobe analyses of apatite grains in Hawi lavas, Kohala Volcano, Hawaii
Rock Type Hawaiite Mugearite Benmoreite Traehyte
Sample # H~---74 b H-84~3 a H 22-74 b H-84-25" H-28-74 b H-84-62 a H-84-61 a
SiOz 0.14 - 0.17 - 0.08 0.17
FeO - 0.77 - 0.95 - 0.53 0.97
MnO - 0.08 - 0.10 - 0.13 0.13
MgO 0.49 - 0.48 - 0.43 0.38
SrO 0.40 - 0.40 - 0.39 - -
CaO 52.70 53.88 53.80 53.36 53.50 53.27 53.18
PzO5 40.5 40.92 41.20 40.53 41.10 40.80 40.02
F 4.64 3.75 3.59 3.45 4.20 2.00 3.68
1C 0.48 - 0.44 - 0.30 - -
Total 98.87 99.89 98.30 99.09 98.60 97.24 98.53
Trace elements(ppm)
La - - 475 n 790 n 490 n
Ce - - - 530 n - 1990 1025
Pr - - 110 ~ - 140 n 173 n
Nd - - 635" - 920 865
Analysts : a J.Nicholls (University of Calgary)
b Sibray (1977)
The symbol "n" denotes analyses near the detection limit
than 1% for SiO2, A1203, FeO, CaO, K20, and TiOz, 2% for Sr, Zr and Zn, 2.0-3.5% for V and Nb, and about 25-30% for
MgO and 5% for N20 and P205 (T.L. Wright 1986, pers. comm.). Ni and Cr (due to the low concentration of these elements in this
Samples were selected for trace element analyses (Table 4) by standard) for replicate analysis of standard BCR-I (Rhodes 1983).
INAA and XRF on the basis of freshness as determined by petro-
graphic analysis of thin sections (degree of alteration of mineral
phases) and low whole-rock volatile content (loss on ignition at
Petrogenesis
900 ~ C). REE, Hf, Ta, Th, Sc, Na, Cr and Co abundances were
determined by INAA at MIT. Chen and Frey (1985) reported that
The isotopic variation among Hawi lavas is small (Fig. 3).
the average standard deviations for five replicate INAA analyses
Nine out of ten samples are within analytical error for Sr
of an alkalic lava from Haleakala ranged from 1 to 3% for Sc,
isotopes and five out of six for Nd isotopes (Hofmann et al.
Co, Na, La, Sm, 4 to 6% for Nd, Eu, Tb, Cr and 7 to 12%
for Hf, Ta, Th, Lu and Yb. Abundances of V, Cr, Ni, Y, Zn, 1987; Lanphere and Frey 1987). The isotopic variation does
Ba, Zr, Rb, Nb and Sr were determined by X-ray fluorescence not correlate with the age or composition of the lavas. Thus,
at the University of Massachusetts. Analytical precision of XRF given the small and apparently random variation in isotopic
analysis at the University of Massachusetts is 0.6-1.0% for Rb, composition, the observed chemical variation (i.e., hawaiite
94
Table 3. Major element analyses of representative Pololu and Hawi lavas from Kohala Volcano, Hawaii a
Pololu formation Hawi formation
P-84--11 P-84-9 P-85-44 P-85-45 T-85-t7 T-85-16 H-84-11 H-84-3
2OiS 48.70 08.15 47.80 45.80 47.70 47.30 47.60 49.30
TiO2 88.1 82.2 63.3 89.2 38.3 25.3 20.3 36.2
3021A 04.01 00.41 00.41 05.41 09.41 07.61 09.51 06.61
FeO 46.11 77.01 20.21 64.21 55.21 66.11 57.11 86.01
MnO 0.17 0.17 81.0 81.0 0.22 91.0 32.0 0.23
MgO 09.51 19.6 60.7 7.80 01.5 4.99 4.59 49.3
CaO 66.8 00.11 07.01 05.01 99.7 7.66 65.7 7.04
Na20 16.1 2.24 2.77 2.66 95.3 11.4 58.4 00.5
KzO 0.25 0.32 0.80 0.76 43.1 74.1 95.1 67.1
P205 0.17 0.22 0.50 0.39 0.77 39.0 82.2 69.1
LOI 0.04 10.0 0.20 94.1 10.1 10.0 10.0 0.09
CO2 <0.01 <0.01 <0.01 0.16 0.02 <0.01 <0.01 <0.01
Total 83.99 99.72 99.39 86.99 20.99 45.89 99.37 32.99
Normative mineralogy
Ne 0.0 0.0 0.0 0.0 0.0 5.1 2.2 1.1
Hy 7.2 8.91 8.5 6.9 9.4 0.0 0.0 0.0
D.I. b 1.51 20.9 1.62 9.32 37.4 5.24 48.6 8.15
a See text for methods and analytical uncertainty
b Differentiation Index (normative quartz, albite, orthoclase and nepheline)
Table 3. (continued)
Hawi formation
H-84-64 H-84-67 H-84--25b H-84-2 H-85-25 H-84-10 H-84-21 H-84-79 H-84-62 H-84-61
49.30 49.80 07.15 08.25 08.35 05.45 08.35 00.55 01.75 05.85
2.57 2.67 81.2 30.2 96.1 47.1 29.1 64.1 64.1 71.1
06.61 01.71 09.61 02.71 04.71 04.71 02.71 06.71 08.71 05.71
77.01 23.01 9.52 9.26 42.8 14.8 8.14 15.8 6.84 29.5
0.24 0.22 0.24 0.24 0.25 0.25 0.19 0.24 71.0 81.0
54.3 4.00 92.3 3.10 2.80 2.62 2.44 72.2 45.1 94.1
6.14 7.12 39.5 95.5 18.4 4.77 41.5 4.40 64.3 78.2
10.5 34.4 5.32 06.5 30.6 6.00 06.5 74.5 20.6 6.34
97.1 28.1 2.02 2.12 2.27 2.35 2.56 57.2 70.3 14.3
79.1 36.1 75.1 74.1 32.1 22.1 03.1 30.1 0.89 0.72
0.40 10.0 0.35 10.0 0.40 10.0 0.57 32.0 0.76 60.1
<0.01 0.02 0.02 <0.01 <0.01 <0.01 <0.01 10.0 <0.01 <0.01
97.84 41.99 99.04 99.42 98.92 72.99 68.89 79.89 53.89 98.10
Normative mineralogy
0.0 0.0 0.0 0.0 0.8 0.0 0.0 0.0 0.0 0.0
7.4 8.5 6.1 3.0 0.0 3.0 2.7 9.0 5.01 6.7
2.15 48.3 9.55 9,95 8.36 7.46 5.26 6.26 0.96 8.37
to trachyte) was modeled with the simplest possible mecha- cally analyzed. Hawaiite flows cover approximately 70%
nism, fractional crystallization. of the area, mugearites 26%, and bermoreites and trachytes
The relative abundances of rock types in a suite of rocks 4%. No basalts were identified. Based on field observations,
is the classic test for fractional crystallization. Hawi lavas the average thickness of hawaiite flows is 3 m, mugearite
show a strong negative correlation between the differentia- flows is 5 m and bermoreite and trachyte flows is 7 m. This
tion index and the number of mappable flow units of a yields relative volume abundances of 57% hawaiite, 35%
given differentiation index (Fig. 4). Roughly 85% of the mugearite and 8 % bermoreite and trachyte. These new data
area covered by Hawi lavas have been remapped and chemi- are in sharp contrast to estimates by Macdonald (1963)
95
Table 4. Trace element analyses of representative Pololu and Hawi lavas from Kohala Volcano, Hawaii a
Pololu formation Transitional Hawi formation
P-84-11 P-84-9 P-85-44 P-85M5 T-85-17 T-85-16 H-84-11 tt 84-3
Rb 3.6 3.8 3.9 8.4 25.3 31.4 28.0 35.9
Sr 222.7 281.9 511.9 492.2 685.4 1193.9 1854.4 1720.5
Ba 68.5 65.4 278.8 219.4 395.3 526.6 611.6 663.3
Sc 27.0 33.5 28.7 29.2 20.2 12.9 9.1 7.6
Cr 965.0 344.0 285.2 135.6 t6.3 1.6 1.4 1.5
Co 74.1 41.7 71.6 71.3 53.1 55.5 15.7 13.6
Ni 690.3 102.0 138.4 149.6 35.0 10.1 14.3 8.7
V 221 .I 275.1 283.5 277,8 215.7 147.8 53.2 37.9
Zr 106.7 138.5 273.3 208,6 365.3 347.9 337.4 373.8
Nb 8.6 9.2 28.6 21,2 40.7 48.4 52.0 55.3
Hf 2.6 3.4 6.5 5,1 8.2 7.6 7.7 8.2
Ta 0.45 0.49 2.98 1.69 2.82 3.94 2.93 3.19
Th 0.4 0.8 2,6 1.0 3.4 3.0 3.2 4.2
La 7.33 8.25 25.t4 20.33 36.0 42.8 55.6 57.1
Ce 18.7 24.2 64.5 50.45 90.5 100.5 149.5 143.2
Nd 13.4 16.5 38.5 31.4 50.9 53.7 88.3 82.9
Sm 3.85 4.92 9.26 7.81 11.92 12.19 20.15 18.35
Eu t.39 1.78 2.90 2.54 3.68 3.70 6.22 5.64
Tb 0.67 0.81 1.20 1.12 1.62 1.46 2.35 2.13
Yb 1.67 2.08 2.56 2.16 3.02 2.74 3.66 3.36
Lu 0.23 0.30 0.37 0.31 0.44 0.36 0.50 0.49
Y 20.5 26.4 34.4 28.9 40.8 37.3 55.6 51.7
See text for methods and analytical uncertainty
Table 4. (continued)
Hawi formation
H-84-64 H-84-67 H-84~25b H-84-2 H-85-25 H-84-10 H-84-21 H-84-79 H-84-62 H-84-61
31.4 31.9 42.1 47.0 42.6 53.2 53.5 59.3 67.8 81.6
1650.0 1538.5 1775.2 1747.0 1650.2 1615.2 1115.0 945.2 781.7 647.7
731.7 726.9 817.4 816.4 790.9 823,2 710.6 816.4 621.6 579.7
7.5 8.3 6.3 5.8 4.8 4.8 5.6 5.0 4.0 3.5
3.8 1.2 nd nd 2.6 nd 2.1 4.3 5.4 2.3
13.3 16.3 9.7 8.2 nd 6.8 9.8 8.6 1.7 4.7
5.0 17.2 19.7 11.3 4.4 15.5 11.7 nd 12.0 11.5
36.8 56.3 27.0 17.6 9.4 12.5 34.1 nd 12.3 14.9
417.8 435.4 446.5 458.8 532.8 531.5 586.4 801.3 694.1 743.7
59.4 63.5 64.1 64.5 77.7 74.4 75.1 81.4 88.6 93.0
9.1 9.1 9.6 10.1 11.1 11.0 12.2 16.3 14.2 15.7
3.41 3.52 3.60 3.84 5.7 4.1 4.2 4.7 5.0 5.5
4.2 4.1 4.6 4.5 5.0 4.8 6.3 6.9 7.5 8.4
61.9 89.4 62.8 63.5 72.4 65.1 67.3 72.4 85.7 63.5
149.6 140.3 150.7 155.1 176.9 163.5 144.9 174.7 137.9 146.9
85.8 87.5 78.4 80.3 91.7 79.3 71.6 80.5 84.5 64.8
19.74 18.82 17.89 17.57 18.6 16.6 15.3 16.4 16.1 12.6
5.89 5.77 5.50 5.47 5.4 5.0 4.2 4.8 3.8 2.8
2.29 2.33 2.19 2.11 2.07 1.94 1.83 1.84 2.45 1.46
3.41 4.63 3.77 3.59 4.06 3.6 3.1 3.9 3.3 3.8
0.56 0.64 0.52 0.49 0.53 0.50 0.46 0.57 0.46 0.57
56.9 77.4 53.5 51.7 51.4 49.0 41.5 47.4 43.4 43.5
of rock-type distribution for Hawi lavas (i.e., 1% hawaiite, Hawi lavas show a well-defined trend in major element
58.5% mugearite, 0.5% trachyte and 40% alkalic basalt). variation from hawaiite to trachyte. Figure 5 shows the ob-
These new rock-type abundances represent the most thor- served major element variation in Hawi lavas, using K20
ough characterization of the alkalic stage of a Hawaiian as a measure of the degree of differentiation (KzO is the
volcano and are consistent with a crystal fractionation mod- most incompatible major element oxide analyzed). These
el for the Hawi lavas. variation diagrams allow qualitative constraints to be
96
I I I I greater amounts of plagioclase fractionation in the deriva-
.515020 tion of the more evolved mugearites and bermoreites.
- Ora
o POLOLU
IWAH o
Major Element Least-Squares Modeling
.515000
- 0
Computer modeling of the major element variation was
-- -- carried out using the least-squares mixing equations of
'13 o
~Z.512980 -- -- Bryan et al. (1969). The fractionation calculations were bro-
- ~D a ~ _ ken up into four intermediate steps, using rocks which had
VID - been analyzed for both major and trace element content.
~Z.512960 Plagioclase, apatite and magnetite were used in the fraction-
3
ation calculations because of their clear fingerprints on the
variation diagrams and their occurrence as micropheno-
.512940 -- -- trysts in the lavas. Olivine phenocrysts are found in most
Hawi rocks and were also used in the calculations.
-
Clinopyroxene (cpx) fractionation was also considered
.512920 I I I
.70350 .70560 .70570 .70580 (despite its nearly complete absence in Hawi lavas) because
87Sr/86Sr the marked decrease in the CaO/A1203 ratio could not be
Fig. .3 dN44X/dNa41 versus rS6S/rS7S isotopic ratios in Kohala Vol- explained by fractionation of apatite and plagioclase alone
cano lavas. Lines gives _=4_ 2 sigma error for isotopic ratios. Data (Hofmann et al. 1987). In addition, a small but consistent
from Hofmann et al. (1987). Transitional Pololu lavas shown by decrease in Sc content with increasing K20 is observed for
half filled squares Hawi lavas (Fig. 6). Since cpx is not a near liquidus phase
at low pressure in hawaiites (as evidenced by the virtual
absence of cpx phenocrysts in these lavas and experimental,
1-atmosphere run products on rocks of hawaiitic composi-
03 L
tion Thompson 1974), cpx fractionation probably oc-
curred at moderate pressures. Additional evidence for high-
er pressure crystal fractionation is that Hawi lava composi-
tions cross the low-pressure normative nepheline/hyper-
sthene thermal divide as differentiation proceeds from haw-
aiite to benmoreite.
iMiiiiiiiiiiiiiiiiiiiiiiiiiiiii!ii
In the modeling calculations a moderate-pressure cpx
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii!i composition calculated from 8 Kb experimental runs by
Mahood and Baker (1986) was used. The use of other com-
iiiii,iiiiiii~.~iii
positions (e.g. Salt Lake Crater pyroxenite 68SAL-7 cpx
which was used by Feigenson et al. 1983), had only minor
45 50 55 60 65 70 75 80
effects on the modeling results. The use of amphibole and
DIFFERENTIATION INDEX other accessory minerals found in the evolved lavas does
Fig. .4 Histogram of differentiation index (normative orthoclase + not improve the quality of fit. Table 5 shows the results
albite + nepheline) for Hawi lavas. The general decrease in abun- of the least-squares calculations using both the four-phase
dance of Hawi lavas with differentiation index si consistent with
observed assemblages and a five-phase (observed-plus-cpx)
a crystal fractionation model for the cause of compositional varia-
assemblages. Note that for three of the four differentiation
tion among the Hawi lavas
steps, there is a large improvement in the calculated residual
when using the five-phase assemblage. In particular the cal-
culated differences in CaO and P2Os are greatly improved,
placed on possible fractionating phases. P205, CaO, FeO, because for the four-phase assemblage the program com-
TiO2 and MgO contents decrease strongly, Na20 and SiO2 pensates for the large observed decrease in CaO by fraction-
contents increase, and A12Oa content slightly increases (up ating out excess apatite. The triangular symbols in the vari-
to about 3% K20) as differentiation proceeds. The continu- ation diagrams (Fig. 5) are the calculated least-square com-
al decrease in FeO and TiOz contents with differentiation positions using the five-phase assemblage. The small magni-
indicates that an iron-titanium-rich phase, presumably ti- tude of the residuals from these calculations suggests that
taniferous magnetite (which is a common microphenocryst fractionation of the observed mineral assemblage and vari-
in Hawi lavas) has fractionated. The decrease in P2Os con- able amounts of cpx is a viable explanation for the observed
tent throughout the series indicates that apatite fractiona- chemical variation. The relatively high Na20 residual in
tion also occurred. The decrease in MgO content indicates the last two differentiation steps is thought to be due to
that olivine and/or clinopyroxene fractionation was impor- Na loss in the benmoreite (H-84-62). It was collected from
tant while the drop in CaO observed could be due to clino- the interior of a 30 m thick flow. Similar flows in Kenya
pyroxene, apatite, and/or plagioclase fractionation. A plot have been shown to have lost Na during cooling (Baker
of CaO/A12Oa vs. K20 shows a negative trend which shal- and Henage 1977). This sample was the freshest benmoreite
lows somewhat at higher K20 contents. A change in slope available and of the major elements only Na seems to have
at about 2.3 wt.% K20 is also noted for other major ele- been affected. Similar results were observed by Baker and
ments in Fig. 5. The slope changes can be explained by Henage (1977) on thick evolved lavas in Kenya.
initially greater amounts of clinopyroxene and apatite frac- Figure 7 shows the relative proportions of fraetionating
tionation in hawaiites and mugearites followed by relatively phases required to derive several evolved Hawi lavas from
97
I I ,_ I 3.0 -'~ I
59- A ~..~
(cid:12)9 e~,, I
56-
4c A 2.0- d,
0 ao Ill(cid:127) 0
03 5,3 - 0
I--
1.0-
ta i I I o I
Jl~
91 -21 :;~,,
I'O
O d0 81 .. .A --; "(cid:127)..
~z
1% (cid:12)9 I0-- Q ,.xz
-- ~.:'~. (cid:12)9 ~ (cid:12)9 (cid:12)9
8- .A.
61 -A
I I I I
9
"X';~.
4
_ .'. ~ ;.
.
O 3 ,,/ 6-
O
2 blq t D(1:3
4--
I-
I I i 2 I
x~
"4 Fig. 5. Major element variation diagrams
2.0-
~ -- o ~o 0.4 -- 4:~. for Haw(cid:12)9 lavas. Triangles show the
k'3 ,,.--,' average compositions used in the least-
(cid:14)9 15 squares modeling(cid:12)9 Note the well defined
--
i trends for these nearly aphyric lavas for
2 o" " A all oxides except A1203. Oxides are
0.1 0.2 m plotted against K20 because it is the
most incompatible of the major elements.
I
5.0 I 1 i oJ I I All values are in weight%
5.1 2.0 2.5 5,0 :5.5 5.1 2.0 2.5 5.0 5.5
K20 K20
50 HAWAIITE I MUGEARITE BEN T
I I I I I I
0
t I
m.,
POLOLU d..I
40 HAWI > I0
o
~n
2o

~-" 5o
n co. J
<~50
v cD Yr
oc 2o _ iii
~4o
Io- ~ o o o ~ d i H 5O
_ _ i i i I i ~176 6C (cid:12)9 I L I I ~
0O 05 0.1 5.1 2.0 25 50 55 30 90 80 70 60 50 40
K20(wL %) .TW % RESIDUAL LIQUID
Fig. 6, KzO versus Sc variation for Kohala lavas. Data from Hof- Fig. 7. Proportions of mineral phases fractionated as a function
mann et al. (1987), Lanphere and Frey (1987) and this study. The of weight percent residual liquid in sequential modeling of a tra-
low Sc abundance in even the least fractionated Haw(cid:12)9 lavas indicate chyte from a mafic hawaiite (sample H-84-11). Weight percent
substantial cpx fractionation prior to the formation of these lavas of each mineral removed is shown by the vertical distance between
(Hofmann et al. 1987) symbols. Dashed vertical lines separate different rock types. Ben
- benmoreite; T - trachyte
89
I I I I I I I I I I I I I I I 0.8
POLOLU
200
- ~ HAWI o
uJ 0.6
I'-i0 0 U)
r'r ILl
d
d3 o
Z
~" 0.4
0 5C ..0
-r-
9.( 3o
lLI
J_
ILl 0
1_.
_.o 2o 0.2
0 0 ~ / 0 0O
:<
)f.(
IO
0.0 /,-// i I i
8.0 9.0 I0.0 II.0 0.21
I I I t I I I I I I I I I I MOLES La / MOLES K (xl0 -s)
La Ce Nd Sm Eu Tb Yb Lu
Fig. 8. Rare earth element abundances in Kohala Volcano lavas 0.8 I i I i I I I ~
normalized to chondritic average. Field for Pololu lavas includes
all analyzed samples (data from Feigenson et al. 1983; Hofmann
et al. 1987; Lanphere and Frey 1987; this study). Note narrow
range and crossing patterns for Hawi lavas 0.6
U.I
o ...I ~ 0 o
1.0 I I I I I I I I I I I I I I I . Q~- 0.4
Oc 0
UJ
-J
o
wlZ
3_ 0.2
D.. I ~-1-
0.0 P I I i/ I I I I I i I I I
14.0 16.0 0.81 20.0 22.0 24.0 26.0 28.0
MOLES Ce/MOLES K(xl0 -6)
0.8
1.0 I I I I I I I I I I I I
QL Ce Nd Sm uE Tb Yb Lu 0.6
(,')
Fig, 9, Variation in rare earth element abundances of selected Hawi
LIJ
lavas normalized for fractionation (dividing by whole rock K20 o -J
content) and compared to the most marie Hawi lava (H-84-11).
-~" 0.4
Symbols: square, hawaiite (D.L - 51.8); triangle, mugearite (D.I.
- 56.9); open circle, mugearite-benmoreite (D.I.- 64.7); solid circle,
benmoreite-trachyte (D.I. - 73.8). See text for explanation
-o -J
~ 0.2
the most primitive observed Hawi composition, H-84-11.
Overall, plagioclase is the dominant fractionating phase fol-
0.0 / I I /I I I I I I I
lowed in importance by cpx, then magnetite and much 6.0 8.0 I0.0 0.21 0.41 0.61
smaller amounts of apatite and olivine. The ratio of plagio-
MOLES Nd/MOLES K(xl0 -6)
clase to cpx fractionation increases with decreasing percent
Fig. 10. Molar ratio diagrams illustrating the variation in light REE
residual liquid.
vs. P (both normalized for fractionation by dividing by K) for
Hawi lavas. Lines show the variations predicted for removing apa-
rite with different concentrations of La, Ce and Nd. Open circles
Trace elements
show the observed variation in Hawi lavas, which is broadly consis-
Trace elements can be used as an independent test of the tent with the crudely measured REE concentrations in Hawi apa-
major element modeling. Throughout the Hawi suite, the tites (Table 2). Mafic hawaiites plot in the upper right corner of
highly incompatible trace elements (Zr, Nb, Ta, Th) all each plot; trachytes in the lower left comer
show varying degree of uniform enrichment while the ferro-
magnesian compatible trace elements generally decrease
with differentiation. The low content of Ni in the hawaiites
99
Table 5. Major element, crystal fractionation calculations for Hawi lavas
Parent Hawaiite Mugearite Mugearite Benmoreite
H-84-11 H-84-25b H-84-10 H-84-62
Daughter Mugearite Mugearite Benmoreite Trachyte
H-84-25b H-84-10 H-84-62 H-84-61
of Components 4 5 4 5 4 5 4 5
Residuals"
2OiS 0.02 -0.01 0.09 0.00 0.05 0.02 0.0l 10.0
TiO2 -0.09 -0.13 0.02 --0.03 -0.04 --0.25 --0.05 -0.05
A1203 -0.13 10.0 -0.17 0.00 -0.18 -0.07 0.00 0.00
FeO 0.04 0.05 0.00 10.0 0.02 0.08 0.02 0.02
MnO 0.02 - 10.0 - 10.0 - 10.0 - 10.0 0.06 - 0.04 - 0.04
MgO - - 40~0 0.02 - 0.03 0.00 - 0.07 - 0.05 - 0.02 - 0.02
CaO 0.22 30.0 31.0 0.00 0.18 0.02 0.00 0.00
Na20 0.08 0.18 -0.15 -0.05 0.42 0.24 -0.18 -0.18
K20 -0.01 0.02 50.0 10.0 -0.18 -0.20 -0.01 -0,01
P2Os -0.28 0.03 -0.17 0.00 -0.23 -0.03 0.00 0,00
~r2 0.120 0.054 0.109 0.004 0.336 0.178 0.036 0.036
Fractionation assemblage in percent
Plag 7.01 9.0 10.2 6.8 1.21 3.01 3.8 8.2
Cpx - 5.0 3.0 - 2.6 - 1.0
Ti-Magnetite 5.7 8.5 5.3 3.2 2.9 3.6 2.3 2.3
Olivine 4.5 2.7 3.2 2.1 3.9 1.2 0.4 0.4
Apatite 1.3 2.6 7.1 3.1 7.1 1.2 0.6 0.6
Residual melt 1.67 74.7 82.5 84.6 3.97 80.0 7.88 88.7
a Residuals are the difference between the observed and calculated daughter compositions
(5-9 ppm) suggests that the Hawi lavas had undergone ex- REE content increase with differentiation (West et al. in
tensive olivine and/or cpx fractionation prior to eruption review). The decrease in middle REE concentration in Hawi
of these least evolved Hawi lavas. The low abundance of lavas is probably due to the fractionation of apatite, which
Cr (<4 ppm) and Sc (< 10 ppm) in the hawaiites is indica- preferentially incorporates middle rare earth elements (Lan-
tive of fractionation of clinopyroxene and/or optional of phere and Frey 1987). This is illustrated in Fig. 9, which
Cr-rich spinel. The low abundance of cobalt in Hawi lavas is a plot of normalized REE abundances of Hawi lavas
(2-16 ppm) is consistent with either cpx fractionation (par- divided by the REE abundances of a Hawi mafic hawaiite
tition coefficient, D, =6-11) or appreciable amounts of using K20 content as the normalization divisor to account
olivine fractionation (D =2.8-5.2). for different degrees of fractionation. The resulting curves
The decrease in Sc content from hawaiite to trachyte are mirror images of REE distribution coefficients for apa-
(9-3.5 ppm) requires the fractionation of either cpx or mag- tite co-existing with hawaiite (Watson and Green 1981).
netite; both have partition coefficients for Sc greater than Apatite fractionation would also help explain the lack of
one. The strong depletion in Sr from hawaiite to trachyte a negative europium anomaly in the hawaiite and mugearite
(1850-650 ppm) can be accounted for by both apatite (D= REE patterns despite extensive plagioclase fractionation.
2.2-2.4) and plagioclase (D = 3.5-5.0) fractionation. The de- The decrease in europium abundance expected by the frac-
crease in V (56.3-10 ppm) is predominantly controlled by tionation of plagioclase is masked by the concomitant de-
opaque fractionation and to a much lesser degree by cpx crease in the other middle rare earth abundances due to
fractionation (D = 13, D = 1.3). apatite fractionation (which has a low relative D for Eu).
The Hawi lavas all show strong light REE enrichment Only the most evolved Hawi lavas, such as sample H-84-
(Table 4 and Fig. 8) which is typical of ocean island alkalic 61, show a slight Eu anomaly, consistent with the higher
basalts (e.g., Schilling and Winchester 1969; Zielinski and relative proportions of plagioclase fractionation calculated
Frey 1970). Unlike other Hawaiian alkalic suites the Hawi for the most evolved lavas.
lavas show a decrease in abundance of REE, especially the An additional, quantitative check on the absolute
middle rare earths (Nd, Sm, Eu) as differentiation proceeds amount of the rare earth elements removed from the melt
which leads to crossing REE patterns (see Fig. 8). Hofmann can be made using molar ratio plots. If the variation in
et al. (1987) suggested that cpx fractionation could lower phosphorus and rare earth element contents (excluding eur-
the rare earth element contents in evolved Hawi lavas. How- opium) are predominantly controlled by apatite fractiona-
ever, the alkalic lavas found on Mauna Kea show strong tion, then a plot of P/K vs. REE/K delineates the amount
evidence of roughly similar amounts of cpx fractionation of rare earth element removed. Figure 10 shows the varia-
in their petrogenesis (based on computer modeling of major tion anticipated for removal of various concentrations of
element data and their similar low Sc contents), and their La, Ce and Nd in apatite. The trends of the data are broadly