2. CHEMISTRY HONORS PROGRAM: Literature Review
VOLCANIC TUFF
A REVIEW ON THE GEOCHEMICAL ANALYSIS OF VOLCANIC TUFFS
AUTHOR:
MEGHOMITA DAS
SYBSC-249
UID-132161
3. Introduction:
This paper is dedicated to describe the basic geochemistry of volcanic tuffs ejected from magma
reservoirs from across the world in different tectonic settings. A basic summary of each of the
research paper is provided here with a special focus on the analytical methods conducted on the
samples and conclusions obtained.
Volcanic tuff, as per Encyclopedia Brittanica, is defined as a relatively soft, porous rock that is
usually formed by the compaction and cementation of volcanic dust or ash. Tuffs are classified
as vitric, lithic or crystal when the tuff is composed of glass, debris of pre-existing rock or crystal
chips respectively. They exhibit different textures as well as chemical and mineralogical
compositions. They are not defined by any particular age range. Rather, they are spread across
the entire geological time scale. Due to the dynamic agents present on Earth, most of the older
tuffs have undergone greater extents of metamorphism which is identified based on the mineral
assemblages.
At times, the hot glass fragments ejected during an eruption consolidates or adheres the volcanic
ash, forming welded tuffs or ignimbrites. Also, the erupted tuff indicates the nature of the magma
chamber underneath. As the underlying magma fractionates, the erupted tuff represents the
changing chemical conditions. Thus, the evolution of the magma can be studied by considering
the ejected tuff’s chemical composition.
Several analytical methods have been developed to study these ejected matter. Some of the
methods are mentioned below:
X-Ray Diffraction Method (XRD) is based on the principle that the atomic planes of a
crystalline substance cause the incident beam of X-ray to interfere with one another as they leave
the substance, forming a diffraction pattern which is used to identify the mineral present. It is a
non-destructive method of chemical analysis. The idea behind this method is: “Every crystalline
substance gives a pattern; the same substance gives the same pattern; and in a mixture of
substances each produces its own pattern independently of the others.”
Energy dispersive X-Ray Spectroscopy (EDS) helps to identify the elemental composition of
the sample by identifying the characteristic X-ray emitted by each element when the sample is
bombarded with a high energy beam of electron or proton. When the incident beam hits the
sample which is at ground state, an electron can be ejected from the inner shells of the atom
which causes an electron from the higher shells to drop down to the lower shell to stabilize the
atom, emitting a characteristic X-ray in the process.
Mass Spectrometry (MS) provides information regarding the structure of the atom in the
sample by producing multiple ions from the sample, separating them based on their charge-to-
mass ratio by the application of the magnetic and electric fields and then plotting the relative
abundance of these ions with respect to the charge-mass ratio. Another variant of this method is
the Inductively-coupled plasma Mass spectrometry (ICP-MS), which ionizes the sample by
inductively-coupled plasma and then uses mass spectrometry to separate the ions. It is used to
4. detect trace elements in the sample and used in situations where the concentration is as low as
parts per quadrillion.
Scanning Electron Microscopy (SEM) is a type of electron microscopy that produces images of
the sample by scanning with a focused beam of electrons. The images thus obtained helps in
measuring grain sizes of the minerals present in the sample.
X-Ray Fluorescence (XRF) is the emission of secondary characteristic X-rays (fluorescent in
nature) from substances bombarded with high energy X-rays or gamma rays. It is mostly used for
chemical and elemental analysis. The XRF spectra thus obtained helps to identify the unknown
minerals present in the sample.
The following section contains summary of each paper particularly with respect to the
geochemical aspect of each paper and their respective conclusions.
5. Petrology and Oxygen Isotope Geochemistry of the Pucón Ignimbrite -
Southern Andean Volcanic Zone, Chile: Implications for Genesis of Mafic
Ignimbrites
Michael McCurry and Keegan Schimdt highlighted the occurrence of mafic tuffs in Volcan
Villarrica in the Southern Andean Volcanic Zone. Mafic (rocks containing dark-colored,
ferromagnesian minerals) tuffs are rare, though mafic components in silicic or intermediate tuffs
are quite common. Using geochemical analytical methods, the authors tried to develop a model
that could explain such mafic tuff eruptions.
Using electron-microprobe analysis (similar to scanning electron microscopy), the authors
conclusively defined the nature of the Pucon Ignimbrites as mafic which is in accordance with
the general bulk chemical composition of the rocks of the area indicating the presence of a mafic
magma chamber in the South Andean region. By comparing the occurrence of trace elements
such as Barium, Lanthanides, Strontium and Scandium (and their respective concentrations) with
the standard values established for magma chamber present in this area, the chemical identity of
the Pucon Ignimbrites was determined. Further, compatibility and incompatibility test with
respect to the substitution of the elements was conducted to standardize the results. Delta
Oxygen studies were conducted to identify the amount of water content in the pre-eruptive
magma chamber. The results indicate a SiO2 content of 53% which makes it intermediate to
basic in composition. The rock samples are Magnesium-Aluminum rich and contains high
amounts of Iron. (Percentages mentioned in the paper).
The occurrence of two types of crystal grains indicate a mixing condition in the magma chamber
prior to eruption. The delta oxygen studies indicate that this occurrence can be attributed to the
variable water content in the magma reservoir.
Also, the voluminous eruption of the mafic tuffs was due to fractional crystallization processes
occurring in the reservoir without convective mixing within the chamber since the upper part of
the magma chamber was density stabilized due to the dissolved water contents. Fractional
crystallization can be considered as a possible cause for the enrichment of the mafic component
in the reservoir. The primary source of this water was from dehydration of the wall-rock
fragments and from diffusive water transfer from a deeper convecting magma.
6. The Petrology and Geochemistry of Tuffs and Tuff breccias of the Thirtyone
mile Mountain area, Thirtynine mile volcanic field, Central Colorado
This paper by Lisa Dameron, published in 1990 in the Abstract Volume of the first Keck
research symposium in Geology, tries to identify and locate three particular tuff units present in
the mentioned area through geo-chemical analysis.
The Thirtyone Mile Mountain lies in the south east portion of the Thirtynine Mile volcanic field,
5km southwest of the village of Guffey, Coloradao, USA.
The three tuff units were identified initially on the basis of petrographical analysis by preparing
thin sections of the samples collected and analyzing it under an optical microscope for the
presence of mineral grains on the basis of their optical properties and also to determine the
texture of the given tuff units.
Four samples were considered for a geochemical analysis which were chosen on the basis of
their homogeneity and the probability of occurring as clastic materials. A standard rhyolite
sample was also analyzed for comparison. Results were plotted on a Total Alkali/Silica diagram
(TAS) which is used to identify the composition of volcanic rocks based on the alkali and silica
content present within the rock. These samples were plotted alongside previous samples with
known values collected from nearby areas. The samples of the given area has a higher SiO2
content and they also have a lower FeO content as well compared to the rocks of the surrounding
area. By comparing the Co, Ni and Cr contents of these samples with respect to SiO2 content, the
tuffs from the Thirtyone Mile Mountain area have depleted mafic components and an enriched
SiO2 content.
Respective plots and graphs are provided in the paper. The exact geochemical tests that were
conducted is not mentioned in the paper.
Based on the angular shapes of the crystal grains seen in the sample, the author predicts that the
eruption in the Thirtyone Mile Mountain area was violent and cataclastic. From petrographical
and geochemical analysis, the author concludes that the tuffs from this area are chemically
similar to the tuffs from nearby areas and hence emanated from the same magma source.
Alteration of certain mineral grains seen under the microscope indicates the presence of
hydrothermal activity.
7. Tuff beds in Kurnool subbasin, southern India and implications for felsic
volcanism in Proterozoic intracratonic basins
This paper authored by Dilip Saha and Vikas Tripathy, published in 2012, reports the occurrence
of tuff beds in the Kurnool Sub-basin for the first time and explains the possible causes and
tectonic models behind the occurrence of such volcanism within intracratonic basins.
The whole study is done on a comparative basis by comparing tuff beds seen in other
intracratonic basins of Proterozoic age such as Vindhya Basin and the Chattisgarh Basins in
India.
Fresh and compact tuff samples were collected from the Owk Shale formation within the
Kurnool sub-basin, carefully avoiding any weathered soil zones. The altered glass fragments
seen in the samples indicate a rock-water interaction during eruption and deposition. Also,
secondary iron enrichment is seen in the samples along with thin encrustations. The rocks were
powdered to conduct the major and trace element abundance test. The FeO content present due to
secondary enrichment was measured separately using titration. For major oxides, the detection
limits are 0.01% and for Mn and Ti oxides, it is 0.001%. For rare earth elements (REE), the
detection limit was fixed at 0.004-1ppm, for other trace elements at 0.5-5ppm except for Cr
(20ppm); Ni (20ppm) and Zn (30ppm). Major elements such as Si, Al, Mg, Ca, Fe, Na, K are
mobile within igneous rocks, hence for a better and sharper analysis the relative abundances of
trace elements such as Ti, Zr, Nb and certain rare earth elements are considered to be
representatives of the parent magma.
The SiO2 content of the tuff beds from Owk Shale is 67-73 w% which makes it rhyolitic (higher
silica content). The alumina content is quite low at 6% and the FeO+Fe2O3 content is at 12-14%.
The secondary iron enrichment is indicated by 11-12% Fe2O3 content. The low amounts of
Na2O, K2O and CaO indicate a leaching environment.
Considering the trace element chemistry, the tuffs from Owk Shale are enriched in Th, U, Ba and
incompatible elements such as Cs, Rb, et cetera. This indicates that the parent magma was
derived from a continental crust source since the continental crust in enriched in incompatible
elements. The samples are also enriched in the light Rare Earth Elements (LREE) than HREE
which is similar to the tuff samples collected from other Proterozoic intracratonic basins.
However the values are lower than the international standards which gives the tuffs from Owk
Shale a unique identity. Several other tests were conducted to cross-check the composition of the
tuff samples.
The trace element chemistry and rare earth element abundances helped the authors to confirm the
composition of the samples as well as determine the composition of the source magma and
suggest a continental crust origin for the parent melt.
8. Mineralogical, geological and isotopic features of tuffs from the CFDDP drill
hole: Hydrothermal activity in the eastern side of the Campi Flegrei Volcano
(Southern Italy)
A.Mormone, C.Troise, M.Piochi, G.Balassone, M.Joachimski, G.De Natale published this paper
in 2014 to report the hydrothermal activity in the eastern side of the Campi Flegrei Volcano
based on analysis done on core samples of volcanic tuffs collected at various depths from this
area. It also estimates a paleo-temperature of the area.
Campi Flegrei is a large volcanic area located in Italy, west of Naples. It is a submerged volcanic
area comprising of 24 craters and volcanic edifices along with fumarolic (vents for volcanic
gases) vents. This area is also called the Phlegraean Fields.
The X-ray diffraction analysis was conducted on the powdered core samples collected from the
area at two particular depths: 443m and 506m. Operating conditions were 40kV and 40mA. The
cores were collected from a 6-inch bore hole using drill bits and circulating water-mud stabilized
with organic additives. Thin sections required for optical studies, scanning electron microscopy
as well as energy dispersive scanning microscopy were prepared using Araldite D (epoxy resin
use to impregnate the samples). Thickness of the thin samples were 30mm. Operating conditions
were 15kV accelerating potential, 50-100mA filament current, 5-10 microns spot size and 50
seconds net acquisition time. For Sr isotopic analysis, pure feldspar crystals were considered.
Feldspars were leached with cold and warm 2.5N HCl for 10 minutes, rinsed in sub-boiling
distilled water and finally dissolved in high purity HF-HNO3-HCl mixtures. Isotope ratios were
statically measured in a Thermal Ionization Mass spectrometer (TIMS). For carbon isotope
analysis, carbonate powders were reacted with phosphoric acid at 70°C and then analyzed using
TIMS.
First off, the calcite grains were identified using back-scattered electron mode in Scanning
electron microscopy (SEM).
The authors established that the minerals occurring in the cores, studied and identified with the
help of X-Ray diffraction methods, followed a depth-dependent hydrothermal alteration zoning
with temperatures of the system rising downward. The authors were able to identify the presence
of Feldspar, Pyroxenes, Calcite, Quartz, clay minerals, Magnetite and Apatite using the XRD
spectra. The X-ray pattern of the shallower 443m core contained Biotite and Calcite (20-30%)
and alkali feldspar (>40%). In the deeper 506m core, a diffraction peak occurs at 7.05 Angstrom
indicating the presence of the zeolite mineral, Phillipsite. The XRD spectra also indicates a
higher amount of calcite, gypsum (<5%), barite (<5%) and pyrite (<5%) in the deeper core.
Magnetite occurs in low amounts (<2%).
SiO2 content for the 443m core is 60% whereas it is 63% for the 506m core. Al2O3 content is
around 18% for both the samples. Both the samples contain low amounts of MgO and MnO but
they are extremely enriched in Na2O and K2O contents along with CaO which indicates a
Feldspar-like mineral composition. This is in accordance with chemical data obtained from
nearby areas. Adularia, a microcrystalline variety of feldspar, is identified using EDS analysis.
9. Biotites appear as Ti, Al and Fe-rich type. Calcite occurs as ferroan calcite with Fe occurring
within the range of 0.5-13.9 w% in the 443m sample and 8.15-15.96 w% in the 506m sample.
Pyrite is present in both samples following the standard mole ratios with a slight addition of As
(<2%).
Isotopic analysis data states that the delta C isotope content in the calcites from the 443m core
samples contain values between -0.2% to -0.3% and delta O isotope values between 13.1-13.3%.
For the 506m sample, delta C values range between -1.6% to -2.02% and delta O values range -
between 11.7-11.8%. 87
Sr/86
Sr value for feldspars in the 506m core is 0.707523+/- 8 which
agrees with the standard value of the given area.
The authors used geochemical analysis to establish the mineralogical assemblages seen in the
eastern part of the Campi Flegrei Volcano and a depth-dependent hydrothermal alteration zoning
seen in this area. Also, the authors indicate at a slightly oxidizing to oxidizing-reducing interface
condition in the given area based on chemical data for pyrite.
10. Mineralogy, Geochemistry and Volcanology of Volcanic Tuff Rocks from
Jabal Huliat Al-Gran, South of Jordan (New Occurrence)
This paper authored by Reyad A. Al Dwairi and Suhail I. Sharadqah, published in 2014, reports
the occurrence of tuffs in the Jabal Huliat Al-Gran Volcano in the basaltic eruptions along the
eastern rim of Jordan graben, south of Jordan. It also reports the alteration process seen in this
area, mainly zeolitization.
The tectonic setting of the given area is described as the relative movement between the African
and Arabian plates and the association of the eruptive vents with the regional faults seen in the
area. Explosive eruptive events have been identified in the area.
32 tuff samples were collected from multiple locations within the Jabal Huliat Al-Gran area.
Mineral content and the sizes of the whole rock samples were determined using optical
microscopy and X-ray diffraction studies. XRD analysis was done on a Phillips PW 340
diffractometer operating at 40kV and 30mA with Cu-Kα radiation. Microscopic studies were
done using SEM Quanta 600F which was combined with Energy dispersive X-ray (EDX)
analysis mapping. Analysis for major elements such as SiO2, TiO2, Al2O3, total Fe2O3, CaO,
MgO, Na2O and K2O was done using X-Ray fluorescence techniques.
Thermal analysis tests were also conducted on the zeolites in the sample. It essentially measures
the physical properties of the sample/mineral as a function of its temperature and involves a
variety of techniques. The sample is heated and cooled for such analysis. Differential thermal
and thermo-gravimetric analysis (DTA-TGA) were carried out using a thermal analyzer Luxx
apparatus. Differential Scanning Calorimeter (DSC) was used to study the thermal behavior of
the tuff samples.
SEM results indicated the presence of the zeolite mineral, Phillipsite, which has variable
amounts of O, Si, Al, Na, K, and Ca with a high content of Na, K and Ca. The values were cross-
checked with EDX analysis. The XRD studies indicated the presence of calcite, olivine, and
pyroxene as the non-zeolite minerals in the sample and phillipsite as the zeolite-mineral. Thermal
analysis indicated good stability under high temperature conditions. A temperature interval of
25-1000°C was considered for this analysis. There were 3 distinct mass losses: (a) at 120°C with
a -0.75% mass change attributed to sorbed water; (b) in the range 120-800°C with -5.96% mass
change due to loss of trapped water; (c) in the range of 800-920°C when the substance collapse
occurs with a residual mass of 74.67%. This values are confirmed with DSC values.
The report mentions the chemical compositions of three zeolitic tuff samples. The most abundant
oxides are: SiO2, Al2O3, CaO, MgO and Fe2O3; whereas TiO2, MnO, K2O and Na2O are present
only in small quantities. Average values for SiO2 is 39.3% and for Al2O3, it is 9.7% (weight %).
Samples with high iron, MgO and CaO contents are rich in volcanic glass and calcite. Those
containing higher amounts of SiO2 and Al2O3 are rich in zeolites and clay minerals.
The authors conclude that the zeolites from this area show high thermal stability and
geochemical studies indicate that these are tuffs have undergone weathering to form zeolites.
11. Petrology, Geochemistry, Petrogenesis and Reactivation of Volcanic Tuffs at
Dair El-Kahif Area, NE-Jordan
This paper authored by Ibrahim A. Bany Yaseen, Zayed Al Hawari and Abdullah A. Diabat,
published in 2010, explains the tectonic setting of this area, dominant mineral phases seen in the
area and the conditions and compositions of the magma chamber underlying the Dair El-Khalif
area in north-east Jordan. The study area is essentially a strato-volcano, 7km north east of Dair
El-Khalif and the area is characterized by pyroclastic products. The tectonic setting of this area is
along the Dead Sea Transform (DST) fault which is 1100km long major plate boundary between
the Arabian and the African Sinai plates that accommodates most of the compressional and
tensional processes seen in this area.
11 samples were collected from the upper, middle and bottom parts of the tuff section. Minerals
were identified using electron micro-probe and the chemical analysis for major and trace
elements on the whole rock samples were done using an atomic absorption spectrophotometer. In
Atomic absorption spectrophotometer (AAS), the atoms in the sample absorb definite quantities
of energy and get excited (mainly due to electronic transitions). This energy absorption
corresponds to a particular wavelength which is characteristic for a particular element and hence,
the relative concentrations of individual elements present within a sample can be determined.
Optical microscopy studies were also conducted by preparing thin sections of the samples.
The volcanic tuff consists of fragmentary particles as observed under microscope. The tuff is
composed of olivine, clino-pyroxene, plagioclase, magnetite and ilmenite. The average modal
composition is 49 vol% plagioclase, 25 vol% clinopyroxene, 21 vol% olivine and 5 vol%
magnetite and ilmenite.
Analysis of Olivine shows that it is enriched in Mg content which makes it fall on the Forsterite
side of the Olivine series. The clinpyroxenes are considered as titanium-augites. They are rich in
Ca and poor in Fe and exhibit zonation. The average Al, Cr, Na and Ti abundances are 6.1, 0.61,
0.04 and 2.93%, respectively. The clinopyroxenes have low concentrations of Na2O and MnO as
well. The plagioclase shows variation in total SiO2 (51.92 - 54.63) and Al2O3 (28.31-30.59)
contents. Average Ca and Na concentrations are 11.7% and 4.54% respectively. TiO2, FeO and
MnO are present in limited amounts. Magnetite and Ilmenite are present in low concentrations.
Since the Fe content is low, the magnetite is called a titano-magnetite.
Overall, the authors concluded that the tuffs originated from an alkali basaltic magma (under-
saturated in Silica content) formed by a smaller degree of partial melting which erupted
explosively from fissure-like vents. The MgO content indicates that the parental magma
underwent different degrees of fractionation. The parent magma was also rich in volatiles as
concluded from chemical analysis. The contents also exhibit homogeneity. The authors predict
that the magma originated in the upper mantle from a peridotitic source as indicated by the
incompatible elements.
12. Geochemistry and Rb-Sr dating of the Muruvik rhyolite tuff,
Trondheimsfjord, Central Norway
David Roberts published this paper in 1987 reporting the chemical composition of volcanic tuffs
from the Ordovician Lower Hovin group at Muruvik, Trondheimsfjord and the related tectonic
setting. An attempt had been made at isotope dating but with inconclusive results
The samples have been collected from the western Trondheim region. The author has described
the region with respect to stratigraphic units. But overall, the study area belongs to the larger
Trondheim Nappe complex. The rhyolite tuff considered for this study is 80-120m thick situated
in NE-SW ridge between Muruvik and Hommelvik.
15 of the 18 samples were considered for geochemical analysis. The 3 additional samples were
collected during a pilot study conducted by the author. Emphasis was given to collect fresh, non-
weathered samples. Major and trace elements were analyzed on powdered samples using Phillips
XRF. For major element chemistry, the rock samples were melted with lithium tetraborate 1:7.
Trace elements were analyzed using pressed rock powders and ferrous ion, H2O+, H2O-, CO2
were determined using wet chemical methods.
Using the Total Alkali/Silica diagram, the composition of the volcanic tuffs is confirmed as
rhyolitic in nature. Even though the tuffs have undergone metamorphism, they haven’t been
altered to greater extents and hence, retain the original properties. SiO2 content of these tuff
samples is around 72.35%. The chemical data obtained for these samples were compared with
data obtained for tuffs from nearby areas which also includes volcanic tuffs from Sweden as
well. Higher K2O content makes these tuffs high K rhyolite tuffs. These values are homogenous
throughout the sample range. Immobile elements such as Nb and Zr indicate that a sub-duction
event was involved. Based on further chemical analysis, it was concluded that these tuffs were
calc-alkaline in nature which indicates eruption along active continental margins. The samples
also show a higher abundance of trace elements such as Ba, Rb and Sr. This is in accordance
when compared with rhyolitic tuffs seen in the western parts of North and South America. The
calc-alkaline nature and the high K character indicates that the eruption was along a destructive
plate boundary. If we consider rhyolite tuffs from the interior of the continents, the chemical data
are starkly different. Hence, the author conclusively proves a tectonic setting involving
continental margins for the formation of these tuffs.
13. Radiometric dating and geochemistry of tuff horizon from a mammal bearing
lacustrine sequence, Miocene Bicorp Basin, eastern Spain
This paper authored by P.Anadon, A.Vasquez, J.M.Mitjavila, R.Utrilla, N.Lopez Martinez was
published in 1995 to report the occurrence of a tuff horizon in the lower parts of the lacustrine
sequence seen in the basinal sequence of Bicorp basin. This tuff horizon was inter-bedded with
mammal bearing horizons and this allows the authors to date the possible volcanic event that led
to formation of these tuffs. The aim of this paper is to determine the age of the tuff horizon,
explain its geochemical features and place the volcanic event that lead to the formation of this
horizon within the Neogene volcanism episode of the Western Mediterranean.
This basin is an ENE-WSW elongated basin located between the eastern Iberian range and the
Prebetics.
X-ray fluorescence spectroscopy was conducted on the whole rock samples of the tuff bed that
yielded a K2O content of 2.8%. 100g of cleaned tuff samples were used for whole rock analysis
and for separation of 1-2g of mineral concentrates. The fraction used for dating was crushed,
sieved and homogenized. It was treated with very dilute HF and HNO3 to remove carbonates,
secondary clay minerals, devitrified glass fragments and zeolites. Zeolites were further removed
by heavy liquids and thus a concentration of primary K-feldspar fraction was obtained. For
geochemical analysis, the samples were analyzed using ICP-MS after fusion for whole rock
analysis; after total digestion for trace elements and Instrumental Neutron Activation Analysis
(INAA) for trace elements, including rare earth elements. X-Ray fluoroscopy was done on
pressed pellets for Pb, Ga, Sn, S, Nb and Rb. Thin sections were also made for optical
microscopy. SEM studies indicate a fine-grained matrix within the samples. XRD analysis shows
that the tuff contains mainly Na-sanidine and a zeolite of heulandite-clinoptilolite group. Further
EDX analysis shows that the sanidine crystals have high K contents and noticeable amounts of
Na. The zeolite contains Ca and K with minor amounts of Mg which is equivalent to heulandite
composition. The high k feldspar seen in this area is known for its extreme purity, almost the K
end member of the feldspar series. The presence of Na in the sanidines indicate a volcanic origin
for this particular tuff.
Geochemical analysis was done on two tuff samples to a 100% water-free basis. The analysis
confirms a rhyolitic-intermediate composition for these tuffs. When it is compared to the fields
of other calc-alkaline volcanic rocks in the Western Mediterranean area, there is absolutely no
overlap amongst the tuff compositions. The tuff samples are slightly enriched in Rb, U, Th,
LREE, K, Hf, Zr; very enriched in Nb, Sn and anomalously enriched in Sr. They are slightly
depleted in Ba, Na, Y, HREE and Ti contents and highly depleted in the more compatible
elements with respect to the average continental crust. High amounts of Sr is not related to the
volcanic event, rather it pertains to the diagenesis phenomenon in a lacustrine environment.
Since the geochemical signatures of the tuff sample under consideration does not overlap with
those of the surrounding volcanic fields in western Mediterranean area, the authors conclude that
the tuff must have erupted from an un-located volcano far from the western Mediterranean area
which traveled a large distance and got deposited in the Bicorp Basin of eastern Spain.
14. Future volcanism at Yellowstone caldera: Insights from geochemistry of
young volcanic units and monitoring of volcanic unrest
Guillaume Girard and John Stix, published this paper in 2012, reporting the intra-caldera
volcanism seen within the Yellowstone caldera in order to monitor and predict any future
eruptions in this area. The paper tries to identify the locations where possible future eruption
episodes can occur within the caldera. It uses geochemical techniques on younger volcanic units
of the area in order to predict the mechanism of eruptions in the future and also provide data on
the current state of the magma reservoir.
Yellowstone Caldera is known for three specific caldera-forming eruptions. Although the caldera
doesn’t show any eminent eruption and as of now, has not produced any Holocene eruption, the
whole caldera system exhibits numerous indicators of unrest including the highest volcanic
degassing rates on Earth. For this study, the authors have considered the youngest rhyolite tuff
horizons of Yellowstone: the Central Plateau member.
TitaniQ Titanium-in-quartz geobarometer was used to determine the crystallization depth of the
Central Plateau member quartz crystals using magmatic temperatures and TiO2 activity
calculated on similar Central plateau member units by a previous author. Laser ablation ICP-MS
technique was used for geo-barometry calculations. Despite large uncertainties involved in Ti
concentrations and imprecisions regarding temperature and Ti activity, results give a pressure
range of 5-13kbar, approximating depths of 20-50km in the crust using equations of Thomas.
Using equations of Huang, a pressure range of 0.6-5kbar, approximating depths of 2-20km is
obtained. Even though the continental crust underneath the Yellowstone caldera is 40km thick,
the values are unrealistically high for results obtained from Thomas’ equation. Results obtained
from Huang’s equations coincide with the depth of the current imaged magma reservoir and
hence this is preferred.
The quartz crystals show zoning with brighter cores and darker rims indicating higher Ti
concentration within the core and a lower Ti concentration along the rims. Younger lavas exhibit
progressively lower Ti concentrations. Using this data and geobarometry (using Ti in quartz)
provide a crystallization depth of 8-10km. The authors conclude that the rounded crystals of
quartz and the glass embayments suggest decomposing and rapid heating. No high Ti-
overgrowths indicate rapid ascent of the magma without any storage at shallower levels. Since
the Yellowstone caldera is characterized by rhyolitic tuffs, the authors predict that the future
eruptions could include large rhyolitic flows and/or pyroclastic eruptions without any precursory
signs. Also the abundance of water sources in this area could lead to a phreato-magmatic
eruption violent eruption in the future. (The authors have not considered the extra-caldera
volcanic activity in the area)
15. Conclusions:
Geochemical studies of volcanic tuff is necessary since it indicates the nature and properties of
the underlying magma chamber. The research papers mentioned above deals with volcanic
systems across the world in different tectonic setting and this makes each volcanic tuff
characteristic of a particular region.
Broadly the tuffs can be classified as felsic or mafic based on the SiO2/alkali contents. The first
analysis conducted on any tuff sample involves determination of the nature of the tuff. Next step
involves thorough chemical analysis which is done using techniques mentioned above. The
obtained results are cross-checked with previously established values and in some cases
(Muruvik Rhyolite tuffs), it is compared with similar volcanic systems in other parts of the
world.
Alteration of tuff to form secondary minerals or zeolites is quite common and is reported in
several papers (Dair El-Khalif, Jordan; Bicorp Basin, Spain; et cetera). Zeolite occurrence is
important in defining any hydrothermal system which is usually associated with volcanic
systems.
Chemical composition of tuffs can be divided into two categories: Major and Trace elements
which includes Rare Earth elements. Major elements includes Si, Al, Na, Fe, Mg, Ca and K in
general. Each volcanic system mainly consists of these elements in variable concentrations and
hence the erupted tuff shows these elemental presence. Trace elements include Ba, Sr, Ti, Mn, et
cetera. These are usually present in minor quantities and at times, the concentrations of these
elements are considered to be representatives for the underlying magma composition since their
low concentrations acts as unique signatures for each volcanic systems.
Since tuffs are light materials which can be easily carried by the wind, they are at times
deposited in faraway lands forming tuff horizons there with chemical signatures that does not
overlap with the surrounding tuff compositions of the area as is the case in the Bicorp Basin in
Spain.
Age estimation of volcanic events is also done by isotope dating of tuffs and then correlating
with the surrounding stratigraphy to determine a sharper age range.
[Presence of compatible and incompatible elements within the tuff indicate the origin of the
magma chamber since incompatible elements are enriched in the continental crust and
compatible elements are enriched in the mantle. Exact amounts of K and trace elements is also
used to identify the setting in which the magma was generated as in case of Muruvik rhyolite
tuffs.
Although similar characteristics between volcanic systems is used for comparison, each volcanic
system has its own identity and character.
16. Acknowledgement:
I would like to thank Mr.Marazban Kotwal, our Chemistry teacher and our honors program
teacher for helping me with the review. I approached him with many doubts and every time he
was gracious enough to solve them and explain them to me.
References:
1. Anadon, P.; Vasquez, A.; Mitjavila, J.M.; Utrilla, R.; Lopez-Martinez, N. (1995):
Radiometric dating and geochemistry of tuff horizon from a mammal bearing lacustrine
sequence, Miocene Bicorp Basin, eastern Spain.
2. Al Dwairi, Reyad A.; Sharadqah Suhail I. (2014): Mineralogy, Geochemistry and
Volcanology of Volcanic Tuff Rocks from Jabal Huliat Al-Gran, South of Jordan (New
Occurrence)
3. Dameron, Lisa M. (1990): The Petrology and Geochemistry of Tuffs and Tuff breccias of
the Thirtyone mile Mountain area, Thirtynine mile volcanic field, Central Colorado
4. Girard, Guillame; Stix, John (2012): Future volcanism at Yellowstone caldera: Insights
from geochemistry of young volcanic units and monitoring of volcanic unrest.
5. McCurry, Michael; Schmidt, Keegan: Petrology and Oxygen Isotope Geochemistry of the
Pucón Ignimbrite - Southern Andean Volcanic Zone, Chile: Implications for Genesis of
Mafic Ignimbrites.
6. Mormone, A.; Troise, C.; Piochi, M.; Balassone, G.; Joachimski, M.; De Natale, G.
(2014) : Mineralogical, geological and isotopic features of tuffs from the CFDDP drill
hole: Hydrothermal activity in the eastern side of the Campi Flegrei Volcano (Southern
Italy)
7. Roberts, David (1987): Geochemistry and Rb-Sr dating of the Muruvik rhyolite tuff,
Trondheimsfjord, Central Norway.
8. Saha, Dilip; Tripathy, Vikash (2012): Tuff beds in Kurnool subbasin, southern India and
implications for felsic volcanism in Proterozoic intracratonic basins
9. Yaseen, Ibrahim A. Bany; Al-Hawari, Zayed; Diabat, Abdullah A. (2010): Petrology,
Geochemistry, Petrogenesis and Reactivation of Volcanic Tuffs at Dair El-Kahif Area,
NE-Jordan.
10. Information on volcanic tuff obtained from www.wikipedia.org and Encyclopedia
Brittanica.
11. Information on the mentioned analytical methods obtained from www.wikipedia.org
12. Information about the chemical classification of volcanic rocks from www.tulane.edu
