Anatectic melting in a metapelitic system

Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009
Anatectic melting in a metapelitic system: a fluid and melt inclusion study
Ph.D. candidate: SILVIO FERRERO
Tutor: Prof. BERNARDO CESARE, Prof. EMMA SALVIOLI MARIANI, Prof. ROBERT J. BODNAR
Cycle: XXII
Abstract
The main aim of this PhD project is to characterize the anatectic melt and the coexistent volatiles, produced by partial
melting of metapelitic rocks, through a melt and fluid inclusion study. Studied samples are khondalites from the Kerala
Khondalite Belt, Southern India, and granulitic enclaves from the El Hoyazo, Neogene Volcanic Province, Spain. In the Indian
migmatites, both glassy and crystallized MI (nanogranites) were found in peritectic garnet. This is the first report ever of melt
inclusions in migmatitic or granulitic rocks. Nanogranites are formed by Qtz + Kfs + Ab + Bt, with grain size in the range 50
nm - 5 μm. The lack of crystallization in glassy inclusions can be attributed to the volume effect and/or to the presence of
volatiles. High resolution probe and FESEM analyses on melt inclusions showed a rhyolitic composition for both glassy
inclusions and homogenized nanogranites, with water content up to 3 wt%. In El Hoyazo enclaves, peritectic garnets contain
both MI and FI. Trapped fluid is a mixture of H2O+CO2+N2±CH4. Raman data and microthermometric experiments showed
an unexpected FI re-equilibration during enclaves exhumation.
Introduction and state of the art
The aim of this PhD project is the characterization of anatectic melt and coexistent fluid phase in
metapelitic rocks, through the study of melt and fluid inclusions. Anatexis, with generation of migmatites,
is the link between metamorphism (granulites) and magmatism, and represents one of the typical
processes of low-middle crust. By producing granitoids magmas, this process is the main cause of crustal
differentiation. Anatexis may also change the rheological behavior of the lower crust, with strong
influence on the geodynamic evolution of the terrains that experienced partial melting.
The anatectic melt can be investigated in two different contexts: natural or experimental. In migmatites
anatectic melt is present as crystallized leucosome, but its composition cannot be well constrained,
because of the presence of cumulus phenomena, fractional crystallization or presence of xenocrists.
Therefore, the compositional characterization of anatectic melts has been mainly obtained, until now,
with an experimental approach. Experiments are performed at known compositions and known conditions
(P, T and H2O wt%), but since experimental systems must be simple, they aren’t necessarily
representative of a natural system.
The fluid phase coexistent with melt during anatexis can be trapped as fluid inclusions (FI) in peritectic
mineral phases. Many papers have been published in the last 30 years, based on studies of preserved high
density FI in natural migmatites and granulites. All the reported data account for a mixture of
CO2+CH4+N2±H2O, where CO2 is the main component, followed by H2O. FI studies in high grade rocks
are difficult for many reasons: in fact FI are normally tiny, rare and with a doubtful primary nature in
many cases. Very often FI underwent post-entrapment modifications, due to the retrograde metamorphic
path.
Case studies
The study of possible “undisturbed” anatectic melt and coexistent volatile phases, via fluid and melt
inclusion study, is made possible from the finding of melt inclusions (MI) and primary FI in peritectic
phases in natural samples. MI-bearing samples come from two different geological scenarios: crustal
enclaves in the lavas of the Neogene Volcanic Province (NVP), southern Spain, and the metapelitic
granulites of the Kerala Khondalite Belt (KKB), southern India. These two settings share many common
features such as assemblage, protolith and metamorphic peak conditions. The choice of two different
settings was done to compare two similar rocks with very different post-melting evolution. Time is the
key factor for the preservation of inclusions primary features: while khondalites had a classic, slow post
peak history, lasting tens of m.y., the NVP enclaves were emplaced by an eruption, with a very short up
rise and decompression time (up to a day), and then no time to experience post entrapment modifications.
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009
At El Hoyazo, NVP (Spain) two main types of granulitic enclaves occur: Grt-Bt-Sil and Spl-Crd. They
experienced two partial-melting events, respectively at 850±50°C, 5-7 kbar and 900± 50°C, 5 kbar. These
enclaves represent a world-unique case study, because of the presence of abundant preserved rhyolitic
glass both as layers and pockets, and as preserved MI in almost all minerals of assemblage. MI
composition is rhyolitic and peraluminous, and has been already investigated from Acosta-vigil et al.
(2007). Inside Grt, Crd and Pl, MI may occur with FI, indicating microstructural evidence of trapping in
conditions of immiscibility. These inclusions represent the first case study ever of analyzable and natural
anatectic “fluid”-meaning both silicatic melt and coexisting fluid phase- in rocks quenched during an ongoing partial melting. Previous studies (Cesare et al., 2007) showed that FI trapped in Crd and Pl are a
mixture of CO2, N2, CH4 ± CO ± H2 , clearly re-equilibrated after entrapment with loss of components. FI
trapped in garnet appeared to be the most likely to have maintained their primary features and have been
characterized in detail in this project.
Khondalites (garnet-sillimanite-cordierite granulite/gneiss) from the KKB are the second case study of
this project. The KKB is one of the most studied granulitic and migmatitic terrain in the world: it
underwent high grade metamorphism, during the Pan African orogeny (c. 520 Ma), at 900°C and 6-8 kbar
(calculated for the central part of the terrain, where the studied samples come from), with evidences of
partial melting. In the rocks from the quarry of Koliokkode the peritectic garnets contain anatectic melt
trapped in MI: this is the first time that melt inclusions are found in classic migmatites.
Methods
FI in El Hoyazo enclaves were characterized via microthermometric study (composition and density),
Micro-Raman Spectroscopy (composition and gas portion density), mass balance calculation (total
composition and total density) and FESEM elemental mapping.
MI in khondalites were analyzed by FESEM imaging in BSE mode and elemental mapping, and EMP
analysis in WDS mode. Homogenization experiments on MI were conducted at controlled atmosphere
(He) with a HT linkam stage TS1500. LAICPMS were performed on both FI from enclaves and MI from
khondalites. All the reported data were collected in collaboration with different institutions: Department
of Geosciences and Chemistry Department at the University of Padova-Italy, Fluid Inclusions Laboratory
of Earth Science Department at the University of Parma-Italy, I.N.G.V. (Istituto Nazionale di Geofisica e
Vulcanologia) in Rome-Italy and Virginia Polytechnic Institute and State University (V.P.I. & S.U. or
Virginia Tech) in Blacksburg, Virginia-US.
Results and discussion
In enclaves from El Hoyazo, FI in peritectic garnets from Spl-Crd enclaves are two-phase (L+V),
spherical to tubular in shape and often contain graphite as trapped phase. Trapped fluid is a mixture of
H2O+CO2+N2±H2S± CH4, with water up to 90 mol%. LAICPMS measurements did not show any salt
dissolved in the fluid In Grt-Bt-Sil enclaves FI in garnet are one phase, low density CO2+N2 mixtures. In
both enclave types, FI form clusters along with MI and mixed inclusions (melt+fluid) at the core of the
garnet, compatible with a primary origin (Roedder, 1984). Because the densities of fluids in FI are far too
low to be compatible with trapping at the estimated P-T conditions of partial melting, the present study
shows that FI have been re-equilibrated after entrapment, with a density decrease, despite many factors
such as primary microstructural position, coexistence with anatectic melt, hardness of the host garnet and
the rapid enclave exhumation. Re-equilibration is more pronounced in Grt-Bt-Sil enclaves with respect to
the Spl-Crd enclaves, with an almost total loss of material. In both cases no clear evidences of leakage are
visible, also at sub-μm scale, as confirmed by FESEM imaging and mapping.
In Khondalites, garnets contain irregular clusters, up to 2 mm across, of hundreds of MI, from totally
glassy to totally crystallized (called hereafter nanogranites), often negative-crystal in shape. Clusters
never touch garnet borders. Nanogranites are 5-25 μm across and contain a cryptocrystalline aggregate of
Bt+Kfs+Pl+Qtz±Ap. The grain size of crystals in nanogranite is variable, from few tens of nanometer to
several microns. Many inclusions contain trapped phases (apatite, zircon, rutile and titanite) that favoured
the entrapment of the anatectic melt during the host growth (see Roedder, 1984). Often nanogranites show
a diffuse nanoporosity. In partially crystallized inclusions, a differentiated melt (60 to 20% of the
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009
inclusion area) can be identified by FESEM elemental mapping because of 1) the lobate cuspate
geometry, typical of former-melt pockets, 2) the different relative distribution of Na-Ca and Cl,
preferentially partitioned in the residual melt, and 3) EMPA data, since the composition cannot belong to
any rock-forming mineral. Glassy inclusions are usually smaller (2.5–17.5 μm) than nanogranite
inclusions and represent about 15% of the total amount of MI in the studied clusters. They contain an
amorphous phase, identified as a glass by Micro-Raman Spectroscopy, and the same trapped phases
found in nanogranites. EMP analysis provides a rhyolitic composition, high in K2O and low both in CaO
and Na2O. The EMP totals suggest a H2O content (calculated by difference to 100%) <3 wt%. The
occurrence of preserved glassy MI is a striking aspect of our research. The most likely explanation is
related with the different size among preserved glassy inclusions and nanogranites. The two populations
are statistically different, with mean diameter 8 μm for glassy inclusions and 13 μm for nanogranites. We
propose that this difference in size was influential in the crystallization of melt droplets, so that most of
the smaller inclusions remained glassy because of inhibited nucleation. The control of pore size on
nucleation is a well-known phenomenon in aqueous solutions (Putnis et al., 1995), where the finer pores
maintain a higher supersaturation threshold. The same phenomenon is also reported in silicate melts
(Holness and Sawyer, 2008), but critical physicochemical parameters of this process are still obscure.
A consistent set of compositional data were obtained by analyzing homogenized nanogranites.
Measured melting temperature for nanogranites is about 1040ºC, compatible with a granitic melt with a
very low H2O content. In fact, homogenized nanogranites have a rhyolitic composition, very similar to
compositional data from preserved glassy inclusions. The composition of re-melted nanogranites (orange
diamonds in fig.1) plot in a relatively small field in the Q-Ab-Or diagram (Fig.1), overlapping with
analyses from preserved glassy inclusions (black triangles). In fig.1, nanogranites bulk composition is
close to the join Q-Or, testifying that 1) partial melting took place at T in excess of 860°C (if an aH2O=
0.5, coherent with a dehydration melting reaction in high grade metapelites, is assumed), in agreement
with the inferred PT conditions of partial melting (900 °C and 6-8 kbar) and 2) assuming a minimum melt
composition as representative of the anatectic melt is not always correct in the present case study (Cesare
et al., 2009).
Figure 1: Nanogranites composition after homogenization (orange
diamonds) and preserved glassy inclusions (black triangles) in the
CIPW Q-Ab-Or diagram. Black lines: minima and cotectic curves for
the subaluminous haplogranitic system at 5 kbar, with a H2O =1
(solid) and a H2O = 0.5 (dashed). Red lines: curves for the
haplogranite system at 10 Kbar with a H2O =1. Black circle: minimum
melt composition at 10 kbar and a H2O =0.5. Numbers on the right
side of the diagram refer to the T (°C) of beginning of melting at the
cotectic compositions on the Q-Or join.
Conclusions
In the present study, a melt and fluid inclusion study was performed to better characterize the anatectic
melt produced by partial melting of metapelitic rocks. In granulitic enclaves from El Hoyazo, this study
have shown that a water-rich phase was present along with anatectic melt, and it is now preserved as
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009
workable FI in peritectic mineral phases. In khondalites, the finding of anatectic melt inclusions in
peritectic garnet, for the first time, allowed us to analyze directly the composition of the anatectic melt,
until now only derived from experimental data and software-based modeling. This project also points out
the possibility to find anatectic melt, preserved as glass in MI, also in slowly-cooled terrains.
References
ACOSTA-VIGIL, A., CESARE, B., LONDON, D., MORGAN, G.B., VI, 2007. Microstructures and
composition of melt inclusions in a crustal anatectic environment: the metapelitic enclaves within El
Hoyazo dacites, SE Spain. Chemical Geology, 237, 450-465
CENKI, B., BRAUN, I. and BROKER, M., 2004. Evolution of the continental crust in the Kerala
Khondalite Belt, southernmost India: Evidence from Nd isotope mapping combined with U-Pb and
Rb-Sr geochronology. Precambrian Research, 134, 275–292.
CESARE, B., FERRERO, S., SALVIOLI-MARIANI, E., PEDRON, D., CAVALLO, A., 2009.
"Nanogranite" and glassy inclusions: The anatectic melt in migmatites and granulites. Geology, 37,
627-630.
CESARE, B., MAINERI, C., BARON TOALDO, A., PEDRON, D., ACOSTA-VIGIL, A. 2007.
Immiscibility between carbonic fluids and granitic melts during crustal anatexis: a fluid and melt
inclusion study in the enclaves of the Neogene Volcanic Province of SE Spain. Chemical Geology,
237, 433-449.
HOLNESS M.B. and SAWYER E.W. 2008. On the pseudomorphing of melt-filled pores during the
crystallization of migmatites. Journal of Petrology, 49(7), 1343-1363.
PUTNIS, A. and MAUTHE, G. 2001. The effect of pore size on salt cementation in porous rocks.
Geofluids, 1, 37 – 41.
ROEDDER, E., 1984. Fluid Inclusions. Min. Soc. Am. Rev. in Mineralogy, 12, 644 pp.
TOURET, J., 1985. Fluid inclusions in migmatites, 265-286. In: Ashworth, J.R., Migmatites,
Chapman&Hall, 286 pp.
SUMMARY OF PhD ACTIVITIES
Courses:
B2 level English course. February 2007, University of Padova.
D.PEDRON: “Raman Spectroscopy”. March 2007, Dipartimento di scienze chimiche, Università di Padova.
R.J. BODNAR: “Fluid in the earth”. June 2007 Memorial University, Newfoundland.
A. LIEBSCHER: MSA/GS Short Course on “Fluid-Fluid equilibria in the crust”. August 2007, Cologne, Germany.
GNP-GIV International School of Isotope Geology. June 2007, Verbania, Italy.
B. CESARE (coordinator): 1° Eurispet School “State-of-the-art analytical and imaging techniques in petrology”, October
2000, Paris, France.
G. WALTON: “Scientific English”, May 2008, Dipartimento di Geoscienze, Università di Padova.
B. CESARE: “Fluid inclusions”. May 2008, Dipartimento di Geoscienze, Università di Padova.
R.J. BODNAR: “Fluids in the Earth's Crust”. Fall 2008, Virginia Tech, Blacksburg, VA – US.
R.J. BODNAR: “Uranium deposits”. Fall 2008, Virginia Tech, Blacksburg, VA – US.
R. TRACY: “Quantitative Metamorphic Petrology”. Fall 2008, Virginia Tech, Blacksburg, VA,US.
E.W. SAWYER & M. BROWN (Organizers): “Working with Migmatites”, MAC Short Course, May 24-25, 2008,
Quebec City – Quebec (Canada).
B. CESARE (coordinator): “3rd Eurispet School - Petrology of the lithosphere in extensional settings”, Eotvos
University, August 21-31, 2008, Budapest - Hungary.
R.J. BODNAR: “Fluid Inclusions Techniques ”, Spring 2009, Virginia Tech, Blacksburg, VA-US.
R.J. BAKKER: “Fluid inclusions calculations”, September 26th, 2009, Granada-Spain.
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Scuola di Dottorato in Scienze della Terra,
Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2008-2009
Oral Communications:
ROSSETTI, P.G. & FERRERO, S. 2007. The Zn-Pb deposits of Casario (Ligurian Alps, NW Italy): late Palaeozoic
sedimentary-exhalative bodies affected by the alpine metamorphism. GeoItalia, Rimini (Italy).
FERRERO, S., CESARE, B., SALVIOLI MARIANI, E., PEDRON, D. 2008 Anatectic melt trapped in garnet from
migmatites of the Kerala Khondalite Belt, Southern India: evidence from crystallized and glassy melt inclusions. In:
Quebéc 2008, GAC-MAC joint meeting. Abstracts. Quebec city (Canada).
FERRERO, S., CESARE, B., SALVIOLI MARIANI, E., PEDRON, D. 2008. Anatectic melt trapped in garnet from
migmatites of the Kerala Khondalite Belt, Southern India: evidence from crystallized and glassy melt inclusions. In: 1st
SIMP AIC joint meeting. Abstracts. Sestri Levante (Italy).
Oral Communications as co-author:
CESARE, B., FERRERO, S., SALVIOLI-MARIANI, E., 2009. Nanogranite and glassy inclusions: finding the anatectic melt
in migmatites and granulites. Granulites & Granulites, Hruba Skala Chateau (Czech Republik), July 2009.
CESARE, B., FERRERO, S., BARTOLI, O., BRAGA, R., SALVIOLI-MARIANI, E., ACOSTA-VIGIL. A., MELI. S., 2009.
“Nanogranite” inclusions in peritectic minerals: discovering the anatectic melt in migmatites and granulites. MAPT,
Edinburg (UK), August 2009.
CESARE, B., FERRERO S., BARTOLI O., BRAGA R., SALVIOLI-MARIANI E., ACOSTA-VIGIL, A., MELI S., 2009.
“Nanogranite” inclusions in peritectic minerals: discovering the anatectic melt in migmatites and granulites. GeoItalia,
Rimini (Italy), September 2009.
Posters:
SALVIOLI-MARIANI, E., CESARE, B., FERRERO, S. 2007. Anatectic melt enclosed in garnet and sillimanite: a study
from metapelitic granulites of the Kerala Khondalite Belt. GeoItalia, Rimin (Italy), September 2009.
FERRERO, S., CESARE, B., SALVIOLI-MARIANI, E., BODNAR, R.J., 2009. Textural and compositional study of melt
inclusions (nanogranites) in anatectic metapelites. Granulites & Granulites 2009, Hruba Skala Chateau (Czech
Republik), July 2009.
FERRERO, S., BODNAR, R.J.,CESARE, B. 2009. Primary fluid inclusions in peritectic garnet from granulitic xenoliths, El
Hoyazo, Spain. XX ECROFI meeting, Granada-Spain, September 2009.
FERRERO, S., CESARE, B., SALVIOLI-MARIANI, E., BODNAR, R.J., 2009. Textural and compositional study of melt
inclusions (nanogranites) in anatectic metapelites. 2009 GSA annual meeting, Portland (Oregon-US), October 2009.
Publications:
ROSSETTI, P.G., & FERRERO, S., 2008. The Zn-Pb deposits of Casario (Ligurian Alps, NW Italy): Late Palaeozoic
sedimentary-exhalative bodies affected by the alpine metamorphism. Geodinamica acta, 21/3, 117-137.
CESARE, B., FERRERO, S., SALVIOLI-MARIANI, E., PEDRON, D., CAVALLO, A., 2009. "Nanogranite" and glassy
inclusions: The anatectic melt in migmatites and granulites. Geology, 37, 627-630.
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