REPORT
Chemical Analysis of an Ignimbrite from Battleship Rock, New Mexico
A. Dominguez, M. Gunnarsson, B. Mulvihill
Department of Geosciences, University of Michigan, 1100 N. University Avenue, Ann Arbor, Michigan 48104, U.S.A.
ABSTRACT
The Jemez Mountains are home to the Battleship Rock Tuff. This tuff has a volume of approximately
2000 km3 and was formed around 55-60 ka. An ignimbrite sample was collected from Battleship Rock
and analyzed using SEM and EDS in order to identify its mineral components. The chemical analysis
revealed the presence of ferrohornblende, magnesiohornblende, anorthoclase, hypersthene, wustite, and
celadonite. The matrix of the ignimbrite was found to contain a mixture of mainly O, Si and Fe along with
other trace elements. We determined that several other minerals were contained in the ignimbrite by using
optical microscopy analysis. The mineral compositions were then compared to ignimbrites in the region
that were previously analyzed by Self et al. (1988) and Self et al. (1991). Lastly, the discrepancies
between the minerals in the studied sample and those in other sample studies were used to assess
differences in magma compositions in different parts of the Valles Caldera.
INTRODUCTION
The Jemez Mountains are located in northern New Mexico and sit on the western portion of the
Rio Grande Rift. This rift is intersected by the Jemez Line which is oriented northeast and consists of
volcanic centers of Late Cenozoic age. These volcanic belts erupted around 10 Ma during two main
pyroclastic events that emitted and deposited large amounts of pumice and ash of rhyolitic composition.
This created the Bandelier Tuff, a unit of ignimbrites with a thickness of approximately 300 m, as well as
creating both the Pajarito Plateau to the east and the Jemez Plateau to the west (Burton 1982). The Jemez
Plateau is part of the larger Jemez Volcanic Field.
The Jemez Volcanic field contains about 2000 km3 of volcanic material of rhyolitic, basaltic and
andesitic compositions. The formation of such a caldera and the great magnitude of the volumes of ash
that were released signify that there must be a magma chamber feeding it from beneath (Goff et al. 1986).
As further evidence of this, studies done by Dondanville (1978) have shown that this region is very hot at
low depths. Battleship Rock is located within the Valles Caldera, which is in the Jemez Volcanic Field.
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Figure 1: Map of the Valles Caldera, with location of Battleship
Rock emphasized. Adapted from Goff et al., 1986.
The younger Battleship Rock Tuff was carved from an ignimbrite deposition that occurred around
55-60 ka. Its characteristic shape was created by the convergence of two rivers in the area, the Jemez
River and the San Antonio Creek, which drain east and west of the Valles Caldera, respectively (USFS).
Ignimbrites, such as those found in Battleship Rock, are welded ash-flow deposits (sometimes referred to
as pyroclastic flows). Pyroclastic flows are extremely hot, fast-moving clouds of gas and rock fragments
that have been expelled from a volcanic vent. They are driven down-slope at great speeds because they
have a greater density in comparison to the surrounding atmosphere. Due to the violent nature and mixed
composition of the pyroclastic currents by which these deposits are formed, ignimbrites tend to be very
poorly sorted and may be friable or lithified as tuff. Ignimbrites are largely dependent on the makeup of
the magma by which they were formed. Most ignimbrites form from magmas of rhyolitic or dacitic
composition. Ignimbrites can contain alkali feldspar, biotite, quartz, sanidine, hornblende and pyroxene as
phenocrysts. This combination can result in variegation of colors, and a gradient in compositions and
densities in ignimbrites.
RESULTS
Careful analysis of the compositions of the minerals studied under the Scanning Electron
Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) allowed for the identification of
several minerals in the thin section under study. The matrix (Figure 2) consists of volcanic glass, which is
isotropic under cross-polarized light (XPL), with major compositions, by weight percent, of oxygen
(33.88-35.67%), silicon (33.25-18.98%), iron (12.26%), magnesium (9.30%), potassium (5.83-8.61%),
and aluminum (6.93%). Trace amounts of titanium (3.57 %), zinc (3.82%), and chlorine (1.63%) were
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also detected but were generally ignored in the determination of the chemical composition, and in the
mineral identification.
(a)
(b)
Figure 2: Scanning Electron Microscope (SEM) images of the Battleship Rock Ignimbrite (a, b). The red squares
emphasize the matrix of the ignimbrite, which consisted of many elemental components, such as oxygen, silicon,
and iron.
Mineral 1 (Figure 3) was determined to be anorthoclase, and mineral 2 (Figure 4) was found to be
magnesiohornblende. Both minerals are igneous and metamorphic rocks. Anorthoclase ((Na,K)AlSi3O8)
is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member
exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi3O8 and between 64
and 90 percent of NaAlSi3O8 (Wikipedia). Mineral 3 (Figure 5) is an iron oxide with composition (in
weight %) of Fe (67.37%) and O (21.73%). It has traces of Mn (1.37%) and Ti (3.00%). This mineral was
identified as wustite but is not normally found in ignimbrites. Wustites have been located at
Scharnhausen, near Stuttgart, Baden-W’urttemberg, and at Buhl, near Weimar, Hesse, Germany. They
have also been found in Greenland, on Disko Island, near Uivfaq and Kitdlit (Web Mineral). The
presence of wustite is peculiar and it does not seem logical for it to be in an ignimbrite. Thus, the wustite
could be a xenolith in the ignimbrite.
Figure 3: Anorthoclase
Figure 4: Magnesiohornblende
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Figure 5: Iron oxide with compositions (in weight %) of Fe (67.37%) and O (21.73%). It has traces of Mn (1.37%)
and Ti (3.00%). It was identified as wustite, but the mineral is not found in ignimbrites.
Mineral 4 (Figure 6) and Mineral 5 (Figure 7) were identified as celadonite and ferrohornblende,
respectively. Ferrohornblende is generally found in granite, granodiorite, and metabasalt and is also
common in amphibolite and schists. Ferrohornblende also forms a solid solution series with
magnesiohornblende (Web Mineral). Mineral 6 (Figure 8) was determined to be hypersthene, a mineral
found in alkaline igneous rocks and meteorites (Web Mineral). Mineral 8 has an intermediate composition
that lay between enstatite and ferrosilite and is found to be anorthoclase (Figure 9), which occurs in hightemperature sodic volcanic and hypabyssal rocks.
Figure 6: Celadonite
Figure 7: Ferrohornblende
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Figure 8: Hypersthene
Figure 9: Anorthoclase
Identification of minerals under the optical microscope revealed the presence of other minerals.
The large angular minerals in Figure 10, seen as blue-white in XPL (b), are plagioclase feldspar. The
undulating clasts are pumice and the dark brown matrix is volcanic glass. They are both extinct in XPL.
In Figure 11, the large sub-angular mineral is plagioclase feldspar surrounded by a volcanic glass matrix.
Its twinning is clearly seen under XPL (b).
(a)
(b)
Figure 10: Plane Polarized Light (PPL) and Cross-Polarized Light (XPL) image of some typical minerals found in
the Battleship Rock Ignimbrite. The large angular minerals are plagioclase feldspar. The undulating clasts are
pumice and the dark brown matrix is volcanic glass.
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(a)
(b)
Figure 11: Image of some typical minerals found in the Battleship Rock Ignimbrite shown under (a) Plane Polarized
Light (PPL) and (b) Cross-Polarized Light (XPL). Large sub-angular mineral is plagioclase feldspar. Twinning is
clearly seen under XPL (b).
The mineral in the center of Figure 12 (b) (yellow-orange; XPL) is biotite. The small, angular
minerals (white in XPL and PPL) are quartz. The light brown, large sub-angular clasts are pumice pieces.
The dark-brown mineral, just above the biotite, is hornblende. The dark brown matrix mineral (extinct
under XPL) is volcanic glass. In Figure 13, the bladed, light brown minerals (light tan in XPL) are
hornblende. The white minerals; seen as bright white are quartz. The undulating, large clasts, which are
extinct in XPL, are pumice pieces.
(a)
(b)
Figure 12: Image of some typical minerals found in the Battleship Rock Ignimbrite shown under (a) Plane
Polarized Light (PPL) and (b) Cross-Polarized Light (XPL). The mineral in the center is biotite. The small, angular
minerals are quartz. The light-brown, large sub-angular clasts are pumice pieces. Dark-brown mineral above the
biotite is hornblende. The brown matrix is volcanic glass.
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(a)
(b)
Figure 13: PPL (a) and XPL (b) images of the ignimbrite sample used for this study. The dark brown
matrix is volcanic glass. The bladed, light brown minerals are hornblende. The white minerals are quartz. The
undulating, large clasts, are pumice pieces.
DISCUSSION
Several of the identified minerals match those detected by previous studies (e.g. Self et al., 1988;
Self et al., 1991). A study done by Self et al. (1988) identified plagioclase, hornblende, biotite, quartz,
glass and phenocrysts. There were also traces of clinopyroxene, orthopyroxene, and oxides in El Cajete
micas. In their other study, they also identified plagioclase, hornblende, biotite, oxides, quartz, glasses
and phenocrysts with traces of clinopyroxene, orthopyroxene, as well as apatite and zircon.
Another interesting aspect of the study of the Battleship Rock ignimbrite was that it contained
flattened pieces of pumice, which some refer to as fiamme (Figure 14). This revealed that the volcanic
material that formed the tuff was so hot that it was cemented together as soon as it settled to the ground.
The pumices were also so soft due to the high temperatures that as the volcanic material settled, the
pumice flattened with the long axis perpendicular to the direction of settling. This process formed the
distinctive flame-like feature of the ignimbrite, making it not just an ignimbrite, but also a piperno, the
specific name for fiamme-bearing tuff (geology.com). The high-temperature deformation can be seen in a
macro-scale as well as in the micro-scale (lineations visible in Figures 2(a), 10 and 12). Although piperno
are not technically metamorphosed (Davis, U Arizona), they can be considered to have been somewhat
metamorphosed because they have some metamorphism-related minerals.
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Figure 14: Fiamme-bearing ignimbrite shown as an example (Andrew Alden, geology.com)
A quick glimpse at the sample SEM and optical microscope images of the sample (Figures 2-14)
revealed that the sample is impregnated with vesicles, which is expected from an ignimbrite. The
chemical analysis also tended to have anomalously high amounts of carbon, which was probably due to
the carbon coating performed on the sample in order to study it under the SEM. Several minerals were
determined with some uncertainty (e.g. wustite). This is understandable because the chemical analysis
result was given by the EDS. Although the EDS gave a fairly good idea of what the ignimbrite contained,
the percent chemical composition output was more qualitative than quantitative; therefore a full chemical
analysis was unable to be performed. This required some flexibility when identifying minerals, not only
when linking the minerals by their percent composition, but also by their occurrence, and localities. This
means that, although all attempts were made to be as accurate as possible in the mineral identification, the
results in this study must be taken with a grain of salt, as the measurements are not ideal. The use of the
optical microscope helped to identify minerals that had not been detected by the SEM but were expected
to be found in an ignimbrite.
The ignimbrite sample from Battleship Rock was found to contain the minerals anorthoclase,
magnesiohornblende, iron oxide, celadonite, ferrohornblende, hypersthene, and volcanic glass through the
SEM analysis. Unsatisfied with the SEM findings, the optical microscope was used to perform mineral
identification. As expected, other minerals that were predicted to be found within an ignimbrite were
identified. The optical microscope analysis allowed for the identification of plagioclase feldspar, pumice
clasts, quartz, hornblende, and volcanic glass. Volcanic glass was able to be identified both in the SEM
and with the optical microscope. Its identification was rather simple, as it consisted of an amalgam of
elements which did not yield a specific mineral when analysed with the SEM. The volcanic glass also
made up a dark brown matrix under PPL which became extinct in XPL. The minerals that tended to have
the smaller clasts, possibly due to a lower abundance, were usually rich in magnesium, iron, or other less
common elements, whereas the larger minerals were generally ignored under the SEM because they were
erroneously assumed to be the matrix. Thus, it seemed like several important minerals were missed or
mistakenly called matrix under the SEM. This was because the clasts were most likely too large to be
identified with the SEM analysis.
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CONCLUSION
Using the scanning electron microscope (SEM), the minerals anorthoclase, magnesiohornblende,
iron oxide, celadonite, ferrohornblende, hypersthene, as well as volcanic glass were revealed. Viewing of
a thin-section of the sample through the optical microscope led to the identification of plagioclase
feldspars, pumice clasts, quartz, hornblendes, and volcanic glass. Several of the identified minerals were
found to have been previously observed in other studies (Self et al., 1988; Self et al., 1991), such as
plagioclase feldspar, hornblende, biotite, quartz, oxides, and volcanic glass. However, unlike the previous
observations, the identification of clinopyroxene, orthopyroxene, apatite and zircon was unable to be
done. This might have been caused by non-homogenization of the lava, thus affecting the ignimbrite
composition, causing discrepancies between the minerals identified within this study’s sample and those
identified by other studies. Another possibility for mineral discrepancies could be due to the poor
reliability of the SEM analysis data and it being fairly qualitative, which may have introduced further
error. The most probable situation, however, is that the study had poor sampling, namely that only one
sample of an ignimbrite was collected in only one site, rather than having a larger collection throughout
the Battleship Rock Ignimbrite locale. Having a larger sample base and performing an analysis on them
would make the analytic study more statistically significant. This finding adds to the original remark that
the ignimbrite itself is not homogeneous.
In conclusion, it is highly recommend that the study be reproduced with a larger sample size
covering the more of the ignimbrite region rather than just Battleship Rock. Also, care should be taken in
noting the discrepancies between mineral compositions at different regions within the Battleship Rock
Tuff in order to gain some conclusive insight of the volcanic history of the region.
REFERENCES CITED
Barthelmy, David. Mineralogy Database. Webmineral, 05 010 2009. Web. 6 Dec 2009.
http://webmineral.com.
Burton, B.W. (1982) Geologic Evolution of the Jemez Mountains and Their Potential for Future Volcanic
Activity. Los Alamos National Laboratory Report, LA-8795-GEOL, 1-33.
Dondanville, R.F., Geologic characteristics of the Valles Caldera geothermal system, New Mexico,
Trans. Geotherm. Resour. Counc., 2, 157-160, 1978.
Goff, F. et al. (1986) Initial Results from VC-1, First Continental Scientific Drilling Program Core Hole
in Valles Caldera, New Mexico. Journal of Geophysical Research, 91, 1742-1752.
http://gdavis.web.arizona.edu/node/50
http://geology.about.com/library/bl/images/blfiamme.htm
http://www.fs.fed.us/
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Self, S., Kircher, D.E., Wolff, J. A. (1988) The El Cajete Series, Valles Caldera, New Mexico. Journal of
Geophysical Research, 93, 6113-6127.
Self, S. et al. (1991) Revisions to the Stratigraphy and Volcanology of the Post-0.5 Ma Units of the
Volcanic Section of VC-1 Core Hole, Valles Caldera, New Mexico. Journal of Geophysical Research, 96,
4107-4116.
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