Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions
-
Bernardo Cesare
,
Madhusoodhan Satish-Kumar
Abstract
Precise chemical composition, including Fe3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mössbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T > 800-850 °C and P = 5 ± 1 kbar.
The full analyses were normalized on the basis of [O12-(x+y+z)(OH)xClyFz]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H2O-poor (OH = 0.86 pfu), has XMg = 0.49 and Fe3+/Fetot ≤ 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites.
In the host granulite, fine-grained biotite is Fe3+-free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and XMg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of XMg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, XMg, and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant.
These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti4+ + 2O2- = (Fe,Mg)2+ + 2OH- and the “fluorination” Ti4+ + 2O2- = (Fe,Mg)2+ + 2F-. The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Why is amazonitic K-feldspar an earmark of NYF-type granitic pegmatites? Clues from hybrid pegmatites in Madagascar
- Metamorphosed Ordovician Fe- and Mn-rich rocks in south-central Maine: From peri-Gondwanan deposition through Acadian metamorphism
- Boralsilite, Al16B6Si2O37, and “boron-mullite:” Compositional variations and associated phases in experiment and nature
- Prograde muscovite-rich pseudomorphs as indicators of conditions during metamorphism: An example from NW Maine
- Wagnerite in a cordierite-gedrite gneiss: Witness of long-term fluid-rock interaction in the continental crust (Ile d’Yeu, Armorican Massif, France)
- Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions
- Multiple titanium substitutions in biotites from high-grade metapelitic xenoliths (Euganean Hills, Italy): Complete crystal chemistry and appraisal of petrologic control
- Proto-polymorphs of jimthompsonite and chesterite in contact-metamorphosed serpentinites from Japan
- Silicate garnet: A micro to macroscopic (re)view
- Monazite occurrence, chemistry, and chronology in the granitoid rocks of the Lachlan Fold Belt, Australia: An electron microprobe study
- Single-crystal 40Ar/39Ar age variation in muscovite of the Gassetts Schist and associated gneiss, Vermont Appalachians
- Single-crystal X-ray studies of trioctahedral micas coexisting with dioctahedral micas in metamorphic sequences from western Maine
- Tourmaline chemistry and the IIIB site
- The octahedral sheet of metamorphic 2M1-phengites: A combined EMPA and AXANES study
- Crystal chemistry of phlogopite from Vulture-S. Michele Subsynthem volcanic rocks (Mt. Vulture, Italy) and volcanological implications
- Beneath the Stillwater Complex: Petrology and geochemistry of quartz-plagioclasecordierite (or garnet)-orthopyroxene-biotite ± spinel hornfels, Mountain View area, Montana
- Correlation between crystallization pressure and structural parameters of phengites
- The modular structure of dovyrenite, Ca6Zr[Si2O7]2(OH)4: Alternate stacking of tobermorite and rosenbuschite-like units
- Environmental parameters affect the physical properties of fast-growing magnetosomes
- In situ Raman spectroscopy measurements of MgAl2O4 spinel up to 1400 °C
- Vibrational properties of δ-AlOOH under pressure
- Mössbauer spectroscopic study of synthetic leucophosphite, KFe2(PO4)2(OH)·2H2O
- Rhönite in Luna 24 pyroxenes: First find from the Moon, and implications for volatiles in planetary magmas
- New high-pressure B2 phase of FeS above 180 GPa
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