The document analyzes variations in magmatic compositions between ocean island chains created by hot spots, including the Hawaiian Islands, Galapagos Islands, and Society Islands. Harker diagrams and rare earth element profiles of volcanic rocks from each region show they have similar ocean island basalt signatures overall but with some differences. Hawaiian basalts show a slightly depleted signature while Society Island basalts indicate a shallow, garnet-free source. Galapagos basalts display two distinct series suggesting an evolving mantle source or interaction with different country rocks over time. The compositions provide evidence that mantle source depth and composition, rather than plate tectonic setting, have the greatest influence on resulting magma chemistry.

1. 2016
Petrologic Significance of
Varying Magmatic
Compositions on Hot
Spot Islands
SHANNON BROOKS

2. 1
Petrologic Significance of Varying Magmatic Compositions on Hot Spot Islands
Abstract
The Hawaiian Islands, Society Islands (including Tahiti and Bora Bora), and the
Galapagos Islands are all ocean island chains in the Pacific Ocean and on or near the Pacific
plate. While these three regions are all ocean island chains resulting from hot spot volcanism,
their igneous compositions vary in subtle, but observable manners. I hypothesize that this
composition is due to source depth and variations within that source more so than the country
rock through which the magma travels on its way to the surface of the overriding oceanic plate.
These variations can be determined through analysis of isotopic and REE patterns. Using data
from Georoc, I will show the various similarities and differences of the igneous rocks within
these regions in order to determine the validity of this claim. The Hawaiian Islands are
characterized by an alkaline basalt composition that has an REE content similar to that of a
primitive mantle source, however, they are depleted in CaO and enriched in NiO, suggesting a
source that is garnet-bearing and possibly near the CMB (Herzeberg, 2006). However, research
on the island of Oahu in the Koolau region have found anomalously high isotopic values of
Zr/Nb that suggest a very deep mantle source that is possibly interacting with a small amount of
the molten metallic outer core (Norman and Garcia, 1999). I hypothesize that this indicates that
the Koolau region is indicative of a very deep mantle source than is typical of an OIB, and this is
what has created the unusual signature of Hawaiian basalts. The islands of Tahiti and Bora Bora
have REE signatures that suggest a more felsic alkali composition that is likely due to a shallow,
garnet-free, mantle source (Tracy, 1980). The REE patterns within the Galapagos island rocks
appear similar to those of the Hawaiian rocks and evidence collected by UNC researchers at
Universidad San Francisco de Quito-GAIAS Institute on San Cristobal show garnet xenocrysts in

3. 2
the bedrock of the highlands which indicate a deeper mantle source moving through the garnet
stability zone. Using data from a similar deposit in Siberia as an analog, I will propose that these
garnets are deposited from the rapid ascent of magma from deep sources that preserves the
stability of these garnets (Harangi et al., 2000). These data could suggest the presence of mantle
plumes originating at various depths, or could indicate the degree to which mantle convection
plays a role in hot spot formation.
Keywords: Hawaii, Galapagos, Society Islands, mantle plume, CMB, garnet
Introduction
The Hawaiian Islands are in the center of the plate with ancient islands, seamounts, and
atolls stretching to the Northwest as far as the eastern coast of Japan. The Society Islands are a
smaller chain of islands in the congested area near French Polynesia midway between Australia
and South America with the Galapagos lying just off the west coast of the latter. Both the
Hawaiian and Society archipelagos lie within the Pacific plate while the Galapagos is on the
Nazca, with ancient seamounts and atolls spanning as far north as the Cocos showing a past
interaction with the Cocos-Nazca spreading center (Neall and Trewick, 2008). The oldest island
attributed to the Hawaiian hotspot is believed to have formed roughly 25 Ma, which would make
it the oldest Pacific hot spot island chain, however, Hoernle et al. (2002) attributes some volcanic
structures and igneous complexes near the Pacific coasts of Costa Rica and Panama to the
Galápagos hot spot that may have been formed between 20 and 71 Ma. Regardless, Maupiti is
the oldest island in the Society Island chain at between 3.9 and 4.5 Ma (Uto et al., 2007), making
the chain the youngest studied for this project. Data were collected from the Georoc database and
looked at oxide contents of igneous rocks within the regions as well as isotopic values and REE

4. 3
compositions. These were analyzed in Harker diagrams and REE profiles to attempt to determine
the depletion/enrichment of magma sources which could help to indicate their depth and
composition. Previous research is also included to take advantage of chemical analyses that were
impossible to attain within the scope of this project. The purpose of this paper is to ascertain
whether significant differences exist between the magmatic compositions of the geologic regions
studied and to hopefully inspire further research in this area.
Geologic Settings
Maui, Tahiti, Bora Bora lie within the Pacific plate while the Galapagos Islands spread
across the Nazca near the East Pacific Rise with the oldest islands attributed to its plume
spanning as far North as the Cocos plate. All of these islands are ocean islands created by hot
spot volcanism. The geology of each chain varies, but the overall composition of the volcanic
rocks are very similar.
Maui
Figure 1 Map showing the islands of the state of Hawaii. Maui is the second youngest island just NW of the main island of
Hawaii. The hot spot is located toward the NE corner just off the coast of the main island, indicating the current WNW
movement of the Pacific plate. (Image: lonelyplanet.com)

5. 4
Maui is one of the younger islands within the Hawaiian Emperor Chain and is part of the
principal “windward” islands along with Hawaii, Lanai, Molokai, Oahu, and Kauai. The
volcanoes in this chain are mainly shield volcanoes with their shape being modified by erosion
as each island moves away from the hot spot and the volcanoes on it become extinct. The hot
spot has been productive in this spot for approximately 47 my (Tarduno and Cottrell, 1997) and
has created islands that extend from the middle of the Pacific plate 2700 km west toward Japan.
Many of these have been eroded to the point that they are nothing more than atolls and
seamounts. As most of these islands, atolls, and seamounts are from now extinct volcanoes, the
only historical eruptions have occurred on Maui and Hawaii (Langenheim and Clague, 1987).
The composition of the lava and igneous rocks is largely tholeiitic to alkaline basalt with olivine
and plagioclase phenocrysts in some regions.
Galapagos Archipelago
Figure 2: Map of Galapagos Islands showing proximityto S. America (Image:worldatlas.com)

6. 5
The Galapagos Islands constitute the some of the oldest and most active ocean island
volcanoes in the world. The islands have arisen from a platform on the East Pacific Rise within
two fracture zones that trend north-northwest and east-west. The western volcanoes are
composed of a differentiated tholeiitic basalt, the southern are more magnesium-rich alkaline
basaltic, and the northeastern volcanoes are a combination of the two likely caused by separate
periods of magmatism as well as cooling rates and depths (Naumann et al., 2001). This differs
from the Hawaiian basalts, I believe due to the proximity to the East Pacific Rise as well as the
belief that the hot spot has moved over the course of its activity. As the alkaline basalts are
derived from a deeper source than the tholeiitic basalts, they show less differentiation. Garnets
have been found in the bedrock of some islands which could correlate this deep source beyond
the stability field of these dense minerals. As is regions of Siberia where almandine garnets can
be found in igneous rocks, this could also indicate a more violent and rapid ascent of the magma
that would be necessary to remove the garnet and transport them without deterioration (Harangi
et al., 2000). The discovery of these garnets shows that the magma is traveling through a garnet-
bearing region, but this region may or may not have an overall effect on the final magma
composition.

7. 6
Tahiti and Bora Bora (Society Islands)
Figure 3: Map of the Society Islands and regional significance (Image:worldatlas.com)
Tahiti and Bora Bora are islands within the Society Islands chain in French Polynesia.
Tahiti is a large, high island with an extensive barrier reef surrounding it, while Bora Bora has
been eroded to the point where it is almost considered an atoll. Their movement coincides with
the Hawaiian chain as they are both on the Pacific plate and the creation of new islands indicates
its northwestern movement of about 110mmyr-1 (Neall and Trewick, 2008). Approximately 5 my
passes from island creation to its drowning, which helps to indicate the rate of plate movement
and erosion. However, due to the amount of carbonate accretion on the atoll and seamount
structures, this timeline can be difficult to pinpoint. The lavas found on these islands ranges from
alkaline basalt to tholeiitic, like those found in the Hawaiian and Galapagos islands.
These volcanoes are similar in structure and composition, with some exceptions made in
the Galapagos Islands given their creation along the East Pacific Rise. These shield volcanoes
are known for basaltic lava that is enriched in incompatible elements with olivine and plagioclase
phenocrysts being more common in Hawaiian basalts.

8. 7
Methods
For this project, I used data acquired through the Georoc database. I searched for ocean
island chains within the Geologic Settings heading and constricted my data to volcanic rocks for
each region. Since the data sets were so enormous for these regions, I restricted the data further
to only include those collected on the islands of Maui (Hawaii), Tahiti and Bora Bora (Society
Islands), and Fernandina (Galapagos). This made the data sets more manageable, however, they
were still very large. I compiled Harker diagrams of these data comparing the TiO2, Al2O3, MgO,
and K2O contents of the volcanic rocks between the regions to the SiO content. I then used their
REE content values to create REE profiles for each region in order to determine whether they
exhibited depleted or enriched signatures which could help to determine depth and composition
of the source. For example, curves in the REE diagrams could indicate whether garnet was
included in the source and if that garnet was melted or not, and this is very helpful in
determining the depth of the source as garnets form deep within the mantle and are diagnostic of
a certain range of pressure and temperature conditions depending on their Fe and Mg content.
It was also necessary for me to review past research in order to get a complete view of the
processes involved in these regions. There has been many projects looking into the geochemistry
of these ocean islands within the past two decades, so there were many papers available for
study.
Results
Harker Diagrams

9. 8
Figure 4 Shows TiO2 content in our volcanic rock data; shows the enrichment of TiO2 in Galapagos and Society Island volcanic
rocks, but depletion in evolved Maui samples
Figure 5: shows a magma depleted in Al2O3 in Society Island basalts
0
1
2
3
4
5
6
4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5
TIO2(WT%)
SIO2 (WT %)
TIO2
Galapagos Maui Society Islands
Expon. (Galapagos) Expon. (Maui) Expon. (Society Islands)
0
5
10
15
20
25
30
4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5
AL2O3(WT%)
SIO2 (WT%)
AL2O3
Galapagos
Maui
Society
Expon. (Galapagos)
Expon. (Maui)
Expon. (Society)

10. 9
Figure 6: evolved Maui samples are depleted in MgO
Figure 7: Strong alkali signatures in Galapagos and Society samples
0
5
10
15
20
25
30
35
40 45 50 55 60 65 70 75
MGO(WT%)
SIO2 (WT %)
MGO
Galapagos Maui Society
Expon. (Galapagos) Expon. (Maui) Expon. (Society)
0
1
2
3
4
5
6
4 0 4 5 5 0 5 5 6 0 6 5 7 0 7 5
K2O(WT%)
SIO2 (WT %)
K2O
Galapagos Maui Society
Expon. (Galapagos) Expon. (Maui) Expon. (Society)

11. 10
REE Profiles
Figure 8: Enriched REE signature typical of OIB shown in this REE diagram (values normalized to chondrite)
Figure 9: normalized REE profile;another typical OIB REE signature
1
10
100
1000
10000
Rock/Chondrite
Galapagos REEs
La Ce Nd Sm Eu Gd Tb Dy Yb
1
10
100
1000
10000
Rock/Chondrite
Society Islands REEs
La Ce Nd Sm Eu Gd Tb Dy Yb Lu

12. 11
Figure 10: REE values of Maui volcanic rock samples - normalized to chondrite values; somewhat depleted REE signature with
some samples displaying a bell curve
Discussion
The Harker diagrams indicate that the Galapagos and Society Island samples were
characterized by more silica and alkaline rich evolved compositions, whereas the evolved Maui
samples were more mafic in composition. Their REE profiles were indicative of typical ocean
island basalts (OIB), however the Hawaiian samples in Figure 10 appear to be slightly depleted.
The bell curve indicated on the Maui sample suggests that this source is garnet-bearing and these
garnets may be interacting with the melt to slightly enrich the depleted mantle source. This could
also be from enrichment of another source unrelated to garnets, however, more data would be
needed to investigate this hypothesis.
The samples taken from the Society Islands shown in Figure 9 indicate a shallow source
that is not garnet-bearing, though the unusual pattern could indicate a reaction with the
surrounding country rock and overriding lithosphere which led to the enrichment of felsic
material and differentiation of more alkaline crystallization products.
1
10
100
1000
Rock/Chondrite
Maui REEs
La Ce Nd Sm Eu Gd Tb Dy Yb Lu

13. 12
The data from the Galapagos samples is in line with the extended eruptive history
proposed by Hoernle et al. (2002) that led to the formation of two distinct series of volcanic
rocks (darker blue series and lighter blue series in Figure 6). The lighter series is defined by a
primitive mantle source similar in composition to the Hawaiian rocks whereas the second (dark
blue) indicates a more evolved source. This could be due to the movement of the Galapagos hot
spot moving over time and interacting with new country rocks or changing geochemistry, or the
evolution of the mantle source throughout the eruption history.
Conclusions
This research suggests that the position of a hot spot within a certain plate will have little
effect on the composition and chemistry of the suite of igneous rocks it produces. These results
show that it is the composition of the mantle source as well as its depth within the mantle that
has the greatest effect on final magma composition. The unusual enrichment of Hawaiian basalts
can be due to the magma interacting with a garnet layer at depth which is partially melted and
included in the composition, or there could be other implications to this magma that is typically
primitive to become enriched in heavier elements. One hypothesis presented by Herzeberg
(2006) is that the magma is coming from a very deep mantle source near the CMB, and it is this
interaction with some of the liquid outer core that is leading to the enrichment. This is a result of
anomalous Zr/Nb enrichment ratios that cannot be explained for a depleted source that is not
being enriched at depth. However, one problem with this idea is that, were it true, we would
expect to find this enrichment over a broader area within the chain, but Herzeberg was only able
to identify it in the Koolau region. More research would be needed to investigate this hypothesis,
but it would provide an interesting addition to the debate about the existence of mantle plumes
and their possible origins.

14. 13
Another interesting result can be found in the Galapagos data. While UNC researchers
(Madelyn Percy and Kayla Seiffert) were able to observe garnets in the bedrock of igneous
deposits on San Cristobal, the REE diagram does not indicate a significant garnet interaction.
This could be due to the suite of rocks sampled, or it could mean that like Harangi et al. (2000)
discovered in Siberian provinces, the mantle source is very deep and ascends rapidly, disturbing
a garnet layer at depth and bringing them up as xenocrysts in the melt before they have time to
interact with it. If this were the case, I would expect to find that the magma is cooling rapidly as
it ascends, which corresponds to a hypothesis presented by Geist et al. (1997) that suggests the
wide variability of compositions and trace element signatures in the igneous rocks of the
Galapagos is from the variability in cooling depths of the magmas.
Overall, this paper shows that there is much more to the geochemistry of an igneous
province than simply its location on the map. Not only does it matter where the hot spot forms,
but it is also important to determine the depth at which it forms in order to identify the types of
rocks the magma will be introduced to as it rises. The data collected and analyzed here indicates
that is the location of the hot spot, along with the depth of its source that has the greater effect on
petrology of the magma formed than the general chemistry of the tectonic plate through which it
travels.

15. 14
Works Cited
1. Harangi, SZ, H. Downes, L. Kosa, CS Szabo, M. F. Thirlwall, P.R. D. Mason, and D.
Mattey. "Almandine Garnet in Calc-alkaline Volcanic Rocks of the Northern Pannonian
Basin (Eastern–Central Europe): Geochemistry, Petrogenesis and Geodynamic
Implications." Journal of Petrology 42.10 (2001): 1813-843. Oxford Journals. Oxford
University Press, 9 Mar. 2001. Web. 13 Apr. 2016.
2. Neall, Vincent E., and Steven A. Trewick. "The Age and Origin of the Pacific Islands: A
Geological Overview." Philosophical Transactions of the Royal Society B: Biological
Sciences. The Royal Society, 3 Sept. 2008. Web. 13 Apr. 2016.
3. Hoernle K, van den Bogaard P., Wener R., Lissinna B., Hauff F., Alvarado G., Garbe-
Schonberg D., Missing history (16-71 Ma) of the Galapagos hotspot: Implications for the
tectonic and biological evolution of the Americas. Geology. 2002;30:795-798.
4. Uto K., Yamamoto Y., Sudo M., Uchiumi S., Ishizuka O., Kogiso T., Tsunnakawa H.,
New K-Ar ares of the Soicety Islands, French Polynesia, and implications for the Society
hotspot feature. Earth Planet. Space. 2007;59:879-885
5. Tarduno J. A., Cottrell R.D. paleomagnetic evidence for motion of the Hawaiian hotspot
during formation of the Emperor seamounts. Earth Planet. Sci. Lett. 1997;153:171-180.
6. Langenheim, Virginia AM, and David A. Clague. "U.S. Geological Survey Professional
Paper." Google Books. N.p., 1987. Web. 22 Feb. 2016
7. Naumann, Terry, Dennis Geist, and Peter Larson. "Evolution of Galápagos Magmas:
Mantle and Crustal Fractionation without Assimilation." Journal of Petrology. Oxford
Journals, 2 Dec. 1997. Web. 22 Mar. 2016.

16. 15
8. Naumann, Terry, Dennis Geist, and Mark Kurz. "Petrology and Geochemistry of Volcán
Cerro Azul: Petrologic Diversity among the Western Galápagos Volcanoes." Journal of
Petrology. Oxford Journals, 22 Nov. 2001. Web. 22 Mar. 2016.
9. Herzeberg, Claude. "Petrology and Thermal Structure of the Hawaiian Plume from
Mauna Kea Volcano." Nature.com. Nature Publishing Group, 13 Sept. 2006. Web. 22
Mar. 2016.
10. Geist, Dennis J., Alexander R. Mcbirney, and Robert A. Duncan. "Geology and
Petrogenesis of Lavas from San Cristobal Island, Galapagos Archipelago." Bulletin. The
Geological Society of America, n.d. Web. 22 Mar. 2016.
11. Geist, Dennis J., Terry R. Naumann, Jared J. Standish, Mark D. Kurz, Karen S. Harpp,
William M. White, and Daniel J. Fornari. "Wolf Volcano, Galápagos Archipelago:
Melting and Magmatic Evolution at the Margins of a Mantle Plume." Journal of
Petrology. Oxford Journals, 11 Apr. 2005. Web. 22 Feb. 2016.
12. Norman, Marc D., and Michael O. Garcia. "Primitive Magmas and Source Characteristics
of the Hawaiian Plume: Petrology and Geochemistry of Shield Picrites." Science Direct.
Elsevier Science B.V., 8 Feb. 1999. Web. 22 Mar. 2016.