Dennis+-+Maui+alkali+basalt

=Alkalic Basalt - Maui, Hawaii=

Dennis Chung University of Miami Revised on April 25th, 2010.

1. Introduction
A basaltic rock was collected on a coast in West Maui, Hawaii. The purpose of this report is to describe the composition and texture of the rock hand sample and its thin sections, and to give interpretations of the tectonic environment in which it was formed.


 * 1.1 Geography and geology of Maui**

Maui is the second largest island in the state of Hawaii (Figure 1.1), situated in the Pacific Ocean and in the Pacific Plate. The location where the hand sample was collected is indicated in Figure 1.2.

The island consists of two low gradient shield volcanoes, called the West Maui Volcano to the west (1.6 Ma) and Haleakalä Volcano to the east (0.8 Ma). The two volcanoes were once separated, but the accumulation of basaltic rocks through numerous eruption formed an isthmus that joins the two volcanoes to a single island.

enlarged in Figure 1.2 (Google Map).
 * Figure 1.1**: State of Hawaii. The red box indicates the location of Maui,

satellite image.
 * Figure 1.2**: Maui. The red arrow indicates the location of where the rock was sampled. Google Earth

mantle plume is responsible for the growth of the oceanic island on the right hand side. The oceanic islands on the left hand side is subjected to net erosion and will eventually become submarine sea mounts.
 * Figure 1.3**: A schematic diagram showing a typical oceanic island chain. The

The Hawaiian Islands, including Maui, are oceanic islands produced from submarine volcanic activity, which is initiated from a narrow rising plume of mantle called a hot spot. At present, the hot spot is located under the island of Hawaii, and is causing eruption of magma through the volcanoes.

The mechanism of generating hot spots is uncertain, but they are responsible for the growth of intra-plate oceanic islands. Melting is generated through the process of decompression as the mantle rises. The eruption of magma through the crust causes the accumulation of basaltic rock, which gradually grows into a submarine volcano and eventually become a volcanic island as it reaches above the sea surface. Because the hot spot is relatively stationary through the geological time, the movement of the overlying Pacific Plate consequently causes the apparent migration of the hot spot and produces a chain of oceanic islands, with the active volcanic islands at the youngest end of the chain. This phenomenon is observed in the Hawaiian Ridge, which is schematically shown in Figure 1.3. Those volcanoes that have moved away from the hot spot become extinct. These volcanoes, however, can become active again during their rejuvenated stage. Nevertheless, once an oceanic island stops erupting and growing, it is subjected to net erosion. It disintegrates and eventually becomes a submarine sea mount. At present, Haleakalä Volcano is still active, but the West Maui Volcano has already undergone the rejuvenated stage between 610,000 and 385,000 years ago ([|USGS, 2001]).


 * 1.2. What is a basalt?**

[|Basalt] is a very common mafic extrusive volcanic rock, usually grey or black. Because basalt is formed from rapid cooling of the magma, it has an aphanitic or porphyritic texture. In general the main composition of a basalt are [|plagioclase] (NaAlSi 3 O 8 to CaAl 2 Si 2 O 8 ), [|clinopyroxene] (XY(Si,Al) 2 O 6 ), [|olivine] ((Mg ,Fe ) 2 SiO 4 ) and [|quartz] (SiO 2 ) or [|nepheline] (Na 3 KAl 4 Si 4 O 16 ).


 * Figure 1.4**: Basalt tetrahedron.

Basalt is classified into different types depending on the the saturation of silica (SiO 2 or quartz). This is depicted in the [|basalt tetrahedron], which has six components, quartz, clinopyroxene, nepheline, olivine, plagioclase and orthopyroxene. The tetrahedron has two critical planes (critical planes of SiO 2 saturation and undersaturation), separating basalts of the different degrees of silica saturation.

The different types of basalts depicted in the tetrahedrons are (1) quartz tholeiite, (2) olivine tholeiite and (3) alkalic basalt. //Quartz tholeiite// has a chemical composition to the right of the critical plane of SiO 2 saturation, and is described as silica oversaturated. This type of basalt is characterized by the the dominance of clinopyroxene and plagioclase, and a smaller proportion of quartz, orthopyroxene and olivine in the groundmass. //Olivine tholeiites// are basalts having a composition in the volume between the critical planes of SiO 2 saturation and undersaturation. This type of basalt can be described as SiO 2 saturated. It has a composition similar to the quartz tholeiite, but olivine is slightly more abundant. Quartz tholeiite and olivine tholeiite are often referred to //tholeiitic basalt//. //Alkalic basalt// has a composition to the left hand side of the critical plane of SiO 2 undersaturation, and can be said to be SiO 2 undersaturated. This indicates that it does not contain quartz or orthopyroxene, but nephaline (a type of [|feldspathoid], i.e. silica depleted feldspar) is present. In many cases, [|augite] (Ca,Na)(Mg,Fe,Al)(Si,Al) 2 O 6 phenocrysts are abundant (the abundance of augite is a good indicator for alkalic basalt). In some cases, nephaline can be absent.

Laboratory experiments showed that the critical plane of SiO 2 undersaturation acts as a thermal divide. This means that neither can a SiO 2 undersaturated melt produce a tholeiite, nor can a SiO 2 saturated melt produce a alkalic basalt. Silica saturated melt is produced under low pressure and high degree of melting. This suggests that tholeiitic basalt is produced by melting of the upper mantle. Silica saturated melt is produced under high pressure and low partial melting of the mantle, suggesting that alkalic basalt is produced from the deep mantle.


 * 2.1 Hand sample descriptions**

The rock in Figure 2.1 is an alkalic basalt collected on a shore of West Maui, Hawaii, and has a dimension of about 8 cm x 5 cm x 5 cm.

The rock has a highly porphyritic texture (i.e. a bimodal distribution of grain size) with a dark grey fine-grained groundmass, and augite (black) and olivine (orange brown) coarse-grained phenocrysts. The olivine phenocrysts are orange brown instead of the "normal" green is because of alteration. The phenocrysts have a size ranges from 1 to 12 mm diameter and are anhedral in shape.

arrows indicate the augite and olivine phenocrysts and the fine-grained groundmass.
 * Figure 2.1**: A rock hand sample of alkalic basalt collected in on the coast of West Maui .The

The thin section (Figure 2.2) reveals the following proportion of minerals.

45% augite (75% of which are phenocrysts, and 25% of which is in the groundmass) 20% olivine (75% of which are phenocrysts, and 25% of which is in groundmass) 20% plagioclase laths in groundmass 10% clinopyroxene in groundmass 5% opaques in groundmass





the fine grained groundmass. Scale bar is 1 mm. **(a)** Plain polarized light; **(b)** Cross polarized light.
 * Figure 2.2**: A view of the thin section of the alkalic basalt. Arrows indicate the augite and olivine phenocrysts, and


 * 2.2 Thin section descriptions**


 * - Phenocrysts**

Augite – The predominant phenocrysts of the rock are augite. It is a type of clinopyroxene, as indicated by its inclined extinction under crossed polarized light (XPL) and its distinctive cleavage. The latter makes augite distinguishable from olivine. In addition, it has a slight tanned color under plain polarized light (PPL) and a 3rd order blue, yellow color under XPL. The augite phenocrysts are anhedral and rounded, suggesting dissolution after the crystals were formed.

Olivine – Olivine has a slightly lower birefringence color (2nd order). However, olivine is colorless in PPL and has fractures instead of cleavage. Like the augite phenocrysts, they are also anhedral and rounded in shape, suggesting redissolution after the crystals were formed. In addition, they have brown reaction rims at the edge of the crystals, indicating the presence of chemical alteration.

A detailed view of the phenocrysts in thin section is shown in Figure 2.3.

phenocrysts. Olivine phenocrysts are surrounded by reaction rims (marked as "r.r." in the figures). The three parallel arrows indicates the cleavage direction of the augite phenocryst. Scale bar is 1 mm. **(a)** Plain polarized light. **(b)** Cross polarized light.
 * Figure 2.3**: A detailed view of the augite and olivine phenocrysts in thin section. Note the anhedral shapes of the


 * - Groundmass

Olivine - Olivine is also present in the groundmass. Similar to the olivine phenocrysts, the crystals also have reaction rims surrounding them. Because of their small sizes (< 250 µ m diameter), most of them have been completely altered into a brown looking minerals, but some of them still contain a very small olivine core surrounded by a reaction rim.

Plagioclase - The groundmass plagioclase are present in laths (about 100 µ m long, 30 µ m wide). Under PPL, the plagioclase laths are colorless and have a low relief, hence it is difficult to distinguish out individual laths. Under XPL, however, individual plagioclase laths show polysynthetic twins.

Pyroxene - Pyroxene in the groundmass shows a 2nd to 3rd order color. They are about 50 µ m in diameter, much smaller than the altered olivine in the groundmass. Cleavages are difficult to see because of their small size, but in some rare occasions, they are visible.

Opaques - Opaques are the accesories and appear as black in both PPL and XPL. They are crystals that do not transmit light in both PPL and XPL. The crystals are anhedral in shape and has an average diameter of about 10 µ m. These opaque grains are metal oxides, and are scatted throughout the thin section. **

A detailed view of the groundmass in thin section is shown in Figure 2.4.

olivine; **r.r.** - reaction rim; **cpx** - clinopyroxene; **plag** - plagioclase. Scale bar is 0.5 mm. **(a)** Plain polarized light.
 * Figure 2.4**: A detailed view of the groundmass in thin section. Note the following features: **ol** - olivine; **a.o.** - altered
 * (b)** Cross polarized light.


 * 3.1 Rock Classification - alkalic, picritic basalt**

This is a mafic rock because olivine, clinopyroxene and plagioclase are the dominant minerals. Its porphyritic texture suggests that it is an extrusive rock, hence a basalt (Figure 3.1). Because the rock was produce from the oceanic island, we can call it an oceanic island basalt (OIB). Knowing it is a basalt, then what kind of basalt is it? The major clue is the presence of a large proportion of augite phenocrysts. Augite, (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al) 2 O 6, is the major indicator for alkalic basalt, and hence this is an alkali basalt. In addition, the presence of plagioclase in groundmass (rather than as phenocrysts) and the absence of quartz suggest that it is not a tholeiitic basalt, though it is interesting to that nepheline is absent when the basalt tetrahedron suggests it to be present.

Moreover, a name of "picrite" can be given to this particular rock. The definition of a picrite is a high-magnesium olivine basalt which contains yellow-green olivine and dark brown augite phenocrysts.


 * 3.2 Tectonic environment - Oceanic Islands**

Because the rock sample is an igneous and was collected in West Maui, an assumption can be made that it was erupted from the West Maui Volcano. Melting is generated through decompression as the mantle rises with the hot spot plume. The porphyritic texture of the rock indicates two stages of crystallization. The olivine and augite phneocrysts were first formed. The large sizes of the phenocrysts suggest that the process of crystallization was slow as magma cooled slowly. However, the melt was then brought towards the surface and cooled relatively quickly, leading to the crystallization of the fine-grained groundmass. During the process of crystallization, the augite and olivine phenocrysts redissolved into or interacted with the magma, evident by their anhedral shapes and the reaction rims around the olivine crystals.

Oceanic island basalt is very diverse in chemical composition. This is due to the different environments (pressure and temperature conditions) in which the melts were generated.

Picrite, a type of alkalic basalt, is the product of melt that is generated deep in the mantle made of mostly peridotite. The extent of melting needed to produce picrite is also lower than to produce tholeiitic basalt. Laboratory experiments suggest that picrites are generated at depth of near 50 to 80 km with about 20% melting (Wyllie, 1998).

Alkalic basalt only composes about 5% of basalt on a typical oceanic island. The majority (about 95%) of basalt of an oceanic island is tholeiitic. The variation of basalt compositions is due the changes of melting environment at different petrological stages of an oceanic island life history. The petrological stages are (1) alkalic pre-shield stage, (2) tholeiitic shield stage, (3) alkalic post-shield stage and (4) rejuvenated stage (Figure 3.1).


 * Figure 3.1**: Petrological stages of a typical oceanic island; **(a)** alkalic pre-shield stage; **(b)** tholeiitic shield stage;
 * (c)** alkalic post-shield stage; **(d)** rejuvenated stage. Figure adapted from USGS.

Alkalic basalt is first erupted during the alkalic pre-shield stage, when initial eruptions causes submarine volcanoes to form. As already mentioned above, these alkalic basalts are the products of deep mantle melting. The only make up 3% volume of the oceanic island. Tholeiitic basalt is then produced in the tholeiitic shield stage as a result of a change to a shallow melting condition. This stage produces the majority (~95%) of the basalt volume on an oceanic island. The alkalic post-shield stage is characterized by the production of alkalic basalt, meaning that melting is shifted back to the deep mantle. The volume of eruption at this stage begins to diminish, as the oceanic island moves away from the hot spot due to the migration of the oceanic plate. The alkalic basalt from this stage makes up about 1% of the island volume. This is followed by quiescent period before it enters the rejuvenation stage. Eruptions during the rejuvenated stage also produce alkalic basalt. At this stage, the volcanic island is away from the direct influence of the hot spot plume.However, the generation of melt in the rejuvenation stage is believed to be isostatic adjustment due to the building of the neighboring active oceanic island. As the active oceanic island grows, it causes the lithosphere in the immediate surrounding to sink, but causes it to rise in regions further away. The rise of the lithosphere causes melting in the deep mantle through the process of decompression and hence alkalic basalt is produced.

previous studies, marked with K-Ar ages. The red arrow indicates sample location for this report. Figure adapted from Tagami //et al//. (2003).
 * Figure 3.2**: Surface geology of West Maui. Crosses indicates sampling locations of

In short, alkalic basalts are produced during the pre-shield, post-shield and rejuvenated stages of the oceanic island life history. The West Maui Volcano (1.6 million years old) has already gone through the four stages of volcanism, hence the alkalic, picritic basalt hand sample can represent one of the three stages of eruptions. The hand sample and the thin section do not show clues of which of the three stages it represents. It is unlikely that the rock sample was formed during the pre-shied stage, because these basalts should have long buried on sea floor by the tholeiitic shield basalt, as suggested in Figure 3.1a. The geographical location of where the hand sample was collected suggests that it is likely to have been erupted during the post-shield stage. Figure 3.2 shows the surface geology of the West Maui Volcano, which is predominantly Honolua Volcanics (alkalic post-shield stage basalt) and Wailuku Basalt (tholeiitic shield stage basalt). The rejuvenated stage basalt only occurs in four isolated localities in the region. The hand sample collected corresponds to the Hololua Volcanics, indicated by a red arrow in Figure 3.2. McDougall (1964) used K and Ar isotopes and determined that Honolua Volcanics, i.e. the alkalic post-shield stage basalt, has an age between 1.21 to 1.91 million years.

4. Conclusions
The rock hand sample was collected in on the coast of West Maui, Hawaii. It is a alkalic, picritic, oceanic island basalt with large phenocrysts of augite and olivine. The fine-grained groundmass contains plagioclase, clinopyroxene, olivine and a minor amount of metal oxides. The porphyritic texture is the result of two stages of crystallization. Augite and olivine phenocrysts first crystallized slowly in the magma chamber, but were then brought towards to the surface and cooled quickly, during which the groundmass crystallized. The anhedral shapes of the augite and olivine phenocrysts and the reaction rims around the olivine crystals suggest redissolution and reactions with the magma occurred during crystallization.

The basalts in Hawaiian islands are the products of melts generated by the upwelling plume of mantle. Alkalic basalts are the products of deep mantle melting, and only make up not more than 5% of the volume of an oceanic island. In a life history of an oceanic island, alkalic basalts erupted during the pre-shield, post-shield and rejuvenated stage. Although the hand sample and the thin section do not suggest which stage the rock was formed, information of West Maui's surface geology suggests that it was formed during the post-shield stage. The rock has an age of between 1.21 to 1.91 million years.


 * References**:

McDougall, I. (1964) Potassium-argon ages from lavas of the Hawaiian Islands. //Geological Society of America Bulletin//, **75**, 107-128.

Tagami, T. Nishimitsu, Y. & Sherrod, D. R. (2003) Rejuvenated-stage volcanism after 0.6-m.y. quiescence at West Maui volcano, Hawaii: new evidence from K-Ar ages and chemistry of Lahaina Volcanics. //Journal of Volcanology and Geothermal Research//, **120**, 207-214.

United State Geological Survey (2001) West Maui's rejuvenated-stage eruption were about 600,000 and 385,000 years ago.//Volcano Watch//. Hawaiian Volcano Observatory. Sept. 13th. Available at: http://hvo.wr.usgs.gov/volcanowatch/2001/01_09_13.html