Hawaiite
Category: Volcanic
Type sodic trachybasalt
Commons Hawaiite is a sodic variety of trachybasalt that is macroscopically very similar to basalt. Unlike basalt, which is predominantly black or dark gray, hawaiite is light gray. The rock was originally defined by the American geologist Joseph Iddings in 1913 as an olivine-bearing variety of basalt in which the normative plagioclase has an oligoclase or andesine composition. Currently, hawaiite is chemically defined as an alkali basalt–trachybasalt with a high Na2O content that exceeds the K2O content (Na2O – 2,0 > K2O) (Le Maitre et al., 2002).
Name origin Joseph Iddings named the rock after the largest island of the Hawaiian archipelago—Hawaii (Iddings, 1913). The island is part of the state of Hawaii, the 50th state of the USA.
Locality Mauna Kea, lava flow near the town of Puako, Hawaii, Hawaiian Islands, USA (sample courtesy of J. Lexa)
GPS: 19° 56' 49.69" N, 155° 50' 0.61" E
Major minerals Ca – plagioclase forming phenocrysts (An57.6Ab40.6Or1.8), microphenocrysts (An55Ab43.9Or1.9) and groundmass (An45.9-55Ab43.3-51.6Or1.7-2.6), clinopyroxene in intergranular spaces between plagioclases in the goundmass (Wo38-44En39-50Fs6-17), olivine as microphenocrysts (Fo75.5) and in the groundmass (Fo61-71), magnetite in the groundmass (Wolfe et al., 1997).
Accessory minerals Ilmenite and apatite in the glassy groundmass.
Classification Hawaiite is an alkaline rock rich in Na2O. It is classified in the TAS diagram for volcanic rocks based on its SiO2, Na2O and K2O content (Le Bas et al., 1986). In this diagram, it falls within the trachybasalt field; since the combined alkali content of the sample exceeds 5 wt. %, it can be further classified based on the specific alkali contents. The value of Na2O – 2.0 is 2.79, which exceeds the K2 content (1.89 wt. %); consequently, the trachybasalt can be more precisely classified as hawaiite. Similarly, the TAS diagram allows for the distinction of mugearite and shoshonite as varieties of basaltic trachyandesite, and benmoreite and latite as varieties of trachyandesite. In the older classification by Cox et al. (1979, based on the same principles as the TAS diagram—hawaiite occupies a distinct field. However, that classification does not include potassic varieties of volcanic rocks such as potassic trachybasalt, shoshonite, and latite.
Colour Light-grey.
Structure Compact, aphanitic.
Granularity Fine-grained rock.
Texture Trachytic to pilotaxitic.
Alterations Olivine microphenocrysts altered to reddish-brown iddingsite. Iddingsite is primarily a mixture of amorphous silicates—resulting from the breakdown of olivine—and goethite. It was named after the American petrologist Joseph Paxson Iddings (1857–1920).
Petrographic characteristics Trachybasalt is a basaltic volcanic rock with a mineral and chemical composition intermediate between trachyte and basalt. Historically, the term was used for basaltic volcanic rocks containing labradorite and alkali feldspar. Currently, it is defined chemically using the TAS diagram (Le Bas et al., 1986). Trachybasalt is a basalt with a high alkali content (5–7 wt.%) and is subdivided based on the predominance of a specific alkali into hawaiite (Na2O – 2,0 > K2O) and potassic trachybasalt (Na2O – 2,0 < K2O). The mineral composition of hawaiite includes Ca-plagioclase (andesine), anorthoclase, olivine, augite, and sometimes biotite. The sample originates from a lava flow of the Mauna Kea volcano (Laupahoehoe Volcanics of Pleistocene and Holocene age (Wolfe and Morris, 1996a,b)), a 25 km3 unit composed of hawaiite and mugearite (90%) and benmoreite (10%) located near the town of Puako. The island of Hawaii is home to five shield volcanoes, characterized by very gentle slope gradients. Their evolution proceeds through four stages: pre-shield, main shield, post-shield, and post-erosional. Each of these stages can be distinguished based on magma composition, eruptive activity, and volcanic evolution. Mauna Kea is currently in the post-shield stage of its evolution, a phase marked by the slower production of more alkaline magma compared to the main shield stage, which is characterized by voluminous outpourings of tholeiitic basalt.Lava accumulation during the post-shield stage of a volcano's evolution is approximately 10 times lower (0.002 km3/year for Hualalai, adjacent to Mauna Kea) than during its main shield-building stage (Moore et al., 1987). The shield volcano Mauna Kea is one of five active volcanoes on the island of Hawaii. Its last eruption occurred 4,000 years ago. The island's most active volcano is Kilauea, which, along with Mauna Loa, is currently in the main shield-building stage. Hawaiite may contain plagioclase phenocrysts (1–3 mm) and microphenocrysts (< 1 mm) that share the same orientation as the plagioclase in the groundmass, producing a faint surface sheen that is highly atypical for basalt. The hawaiite sample is aphanitic, lacking plagioclase phenocrysts visible to the naked eye with a distinctive sheen resulting from the tiny plagioclase grains.
Usage The rock can be used in construction and as a decorative stone in both interior and exterior architecture. Some of the statues on Easter Island—the moai—are carved from hawaiite.
Literature Cox, K.G., Bell, J.D., and Pankhurst, R.J., 1979: The Interpretation of Igneous Rocks, George Allen and Unwin Press, London, 450 pp. Iddings, J.P., 1913: Igneous rocks. Wiley, New York. Vol. 2, 685 pp. Le Bas, M.J., Le Maitre, R.W., Streckeisen, A. & Zanettin, B., 1986: A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram. Journal of Petrology, 27, 3, 745 – 750. Le Maitre, R.W., Streckeisen, A., Zanettin, B., Le Bas, M.J., Bonin, B., Bateman, P., Bellien,i G., Dudek, A., Efremova, S., Kelle,r J., Lameyre, J., Sabine, P.A., Schmid, R., Sørensen, H., Woolley, A.R., 2002: Igneous Rocks: A Classification and Glossary of Terms. Recommendationas of the International Union of Geological Sciences. Subcommision on the Systematics of Igneous Rocks. Cambridge University Press, Cambridge, UK, 236 pp. Wolfe, E.W., Morris, J., 1996a: Geological map of the Island of Hawaii: U.S. Geological Survey Miscellaneous Investigations Series I-2524-A, scale 1:100,000. Wolfe, E.W., Morris, J., 1996b: Sample data for the geological map of the Island of Hawaii: U.S. Geological Survey Miscellaneous Investigations Series I-2524-A, scale 1:100,000. Wolfe, E.W., Wise, W.S., Brent Dalrymple G., 1997: The Geology and Petrology of Mauna Kea Volcano, Hawaii – A Study of Postshield Volcanism. U.S.Geological Survey Professional Paper 1557, Washington, 129 pp.
Photomicrographs
Randomly distributed plagioclase – Pl microphenocrysts form a pilotaxitic texture. In places, they show signs of alignment parallel to the lava flow, creating a typical trachytic texture in which volcanic glass is present between the plagioclase laths. The rock also contains randomly distributed olivine – Ol and magnetite – Mt microphenocrysts. The olivine is altered into a reddish-brown mixture of iddingsite and goethite, while the magnetite appears black under both plane-polarized and cross-polarized light because it is opaque and does not transmit light. Occasionally, the hawaiite also contains biotite – Bt microphenocrysts. Augite – Aug forms only very small grains in the spaces between the plagioclase and the glass.
Normative composition
Hawaiite is a rock slightly undersaturated in silica. The low silica content is compensated by the formation of normative olivine – ol from all provisional hypersthene – hy´. The remaining silica undersaturation was compensated by converting a part of the provisional albite – ab´ into normative nepheline – ne and actual albite ab. However, the normative nepheline – ne content is very low (0.5 wt. %). Magnesium content is reflected in normative olivine – ol and diopside – di. Calcium content is evident from the presence of normative diopside – di and anorthite – an. Elevated alkali content is manifested in high levels of normative albite – ab and orthoclase – or. The entire TiO2 content is assigned to normative ilmenite – il. For the calculation of normative composition, the total iron content—determined as Fe2O3 (11.64 wt. %) was partitioned into Fe2O3 (35%) and FeO (65%).
Normative minerals
SiO2
TiO2
ZrO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
F
S
CO2
Total
Molar proportion of normative mineral
Molecular mass of normative mineral
Weight % of normative mineral
Oxide
(wt. %)
49.94
2.75
17.22
4.07
6.81
0.21
3.59
6.98
4.79
1.89
0.90
99.16
Molecular
weight
60.08
79.88
101.96
159.69
71.85
70.94
40.31
56.08
61.98
94.2
141.95
Molecular
proportion
0.8312
0.0344
0.1689
0.0255
0.0977
0.0030
0.0891
0.1245
0.0773
0.0201
0.0063
ap
0.0211
0.0063
0.0063
328.68
2.08
il
0.0344
0.0344
0.0344
151.75
5.22
or
0.1204
0.0201
0.0201
0.0201
556.67
11.17
ab´
0.4637
0.0773
0.0773
0.0773
524.46
40.53
an
0.1431
0.0715
0.0715
0.0715
278.21
19.90
mt
0.0255
0.0255
0.0255
231.54
5.91
zvyšky
0.0378
0.0891
di
0.0636
0.0095
0.0223
0.0318
0.0318
225.95
7.18
hy´
0.0950
0.0283
0.0667
0.0950
109.78
10.43
ol
0.0475
0.0283
0.0667
0.0475
159.48
7.58
ne
0.0035
0.0009
0.0018
0.0018
284.11
0.50
ab
0.4531
0.0764
0.0755
0.0755
524.46
39.61
D: 0.0546
Mg/(Mg+Fe2+): 0.702
Total of normative wt. % 99.16
Comment All provisional normative hypersthene – hy´ was used to form normative olivine – ol, and a part of the provisional albite – ab´ was used to form normative nepheline – ne and albite – ab.
Chemical composition
The hawaiite from the Mauna Kea volcano, a sodic variety of trachybasalt, has a low SiO2 content (up to 50 wt.%). However, compared to common basalt, it has increased alkalis (6.68 wt.%), TiO2 (2.75 wt.%), and Al2O3 (17.22 wt.%). The content of volatile components (LOI) is also very low (0.42 wt.%). Regarding trace elements, the hawaiite shows elevated concentrations of Sr (1166 ppm), Ba (529 ppm), and Zr (426 ppm). The Mauna Kea volcano is in the post-shield stage of its evolution, a phase characterized by the production of alkalic basaltic magma. The parent melt of the hawaiite is an alkalic basalt that undergoes intense fractional crystallization within a deep-seated magma chamber (Wolfe et al., 1997). At the onset of fractional crystallization—occurring at pressures of 5–7 kbar—large quantities of clinopyroxene crystallize and are removed from the system. This process leads to the formation of magma with a hawaiitic composition. Subsequent fractional crystallization of this magma (involving the further crystallization of olivine, clinopyroxene, plagioclase, and magnetite) produces a more fractionated (differentiated) magma, shifting the composition from hawaiite through mugearite toward benmoreite (see TAS diagram). Such differentiated magma of hawaiite to mugearite or benmoreite composition rises toward the surface, where it spreads out as lava flows. The source of the magma and volcanism in the Hawaiian Islands is the Hawaii hotspot. The entire Hawaiian archipelago—which includes the largest island, Hawaii—represents a classic oceanic island tectonic setting developing on oceanic crust. Melting originates in deep, enriched mantle zones, as evidenced by the geochemical characteristics of the basalts. As the Pacific Plate moves north-westward over the relatively stationary hotspot, the individual oceanic islands develop sequentially over time. The island of Hawaii, with its shield volcanoes, is the youngest in the chain. Basalts produced in this oceanic island tectonic setting are collectively termed OIB (ocean island basalts). They are classified into two primary magmatic series: tholeiitic and alkaline. Hawaiite belongs to the alkaline series.
Puu Waa Waa, Mauna Kea, Hawaii, USA - sample HW-4
SiO2
49.94TiO2
2.75Al2O3
17.22Fe2O3
4.07FeO
6.81MnO
0.21MgO
3.59CaO
6.98Na2O
4.79K2O
1.89P2O5
0.90LOI
0.42Total
99.15Mg(Mg/Fe2+)
0.48A/CNK
0.76A/NK
1.73
Sample HW-4, Gajdošová 2018, unpublished data.
Puu Ka Pele, Mauna Kea, Hawaii, USA - sample HP-11
SiO2
50.12TiO2
2.86Al2O3
17.17Fe2O3
1.44FeO
9.53MnO
0.22MgO
4.11CaO
7.08Na2O
4.70K2O
1.86P2O5
0.90Total
99.99Mg(Mg/Fe2+)
0.43A/CNK
0.76A/NK
1.76
Wolfe et al. (1997). Total Fe content was partitioned into FeO and Fe2O3 in an 88:12 ratio.
Puu Makanaka, Mauna Kea, Hawaii, USA - sample HR-55
SiO2
49.44TiO2
3.08Al2O3
16.92Fe2O3
1.48FeO
9.80MnO
0.21MgO
4.55CaO
7.39Na2O
4.50K2O
1.76P2O5
0.87Total
100.00Mg(Mg/Fe2+)
0.45A/CNK
0.74A/NK
1.82
Wolfe et al. (1997). Total Fe content was partitioned into FeO and Fe2O3 in an 88:12 ratio.