Dear Matt, Mike and Nigel,

Many thanks for giving me fruitful suggestions on the issue, and Matt for a very detailed analysis on the various possibilities that exist in interpreting monazite growth in high-grade rocks. I am aware of the majority of the mentioned references and also the interpretations. But the issue that I was looking at is whether monazite can grow during prograde granulite facies metamorphism e.g. in the T range of 700-800 oC as the muscovite dehydration melting and the first biotite melting reactions are crossed in the pelites with heating! Matt rightly inferred this.

My query stemmed from the observations in monazites from the c.1.6 Ga granulites of the CITZ. Some of these monazites preserve a pre-melting core of Paleoproterozoic age. These are ovoid to ellipsoidal in shape with high Y (Y2O3=1.5-2 wt%) and low Th (ThO2=1.5-3 wt%) contents. These cores are variously altered, but maintaining their original shapes. At least two alteration zones, radial from the surviving core are recognisable: (a) the innermost one (zone 1 for example, is still high in Y (as in the core) but with significant enrichment in Th (ThO2=6-7 wt%). Commensurate with textural observation, relatively high HREE of zone1 is similar to that of the unaltered core, implying that zone1 is genetically linked to the Paleoproterozoic core. (b) the outermost alteration zone (zone 2)in contrast shows progressive depletion in Y (Y2O3→ 0)but with high Th content as in zone 1. The unaltered and altered cores are variously resorbed and overgrown by a new monazite which has an euhedral crystal shape. This variety of overgrowth monazite is compositionally distinct by being enriched in Th (ThO2=8-10 wt%) but depleted in Y (Y2O3=0). Despite error in monazite chemical ages, mean monazite ages of the alteration zone and overgrowth monazites show younging fron zone 1 (1614 Ma) through zone 2 (1602 Ma) to overgrowth (1595 Ma), consistent with textural predictions.

While the overgrowth monazite can be correlated with crystallisation from partial melt at TMax condition or immediately following TMax along the retrograde path (as Mike has suggested), to me the interpretations for alteration zone monazites 1 and 2 in terms of simple cooling from TMax (or involving a number of punctuated melt crystallisation stages) may not be that straight forward. 

My interpretation for alteration zones 1 and 2 are: (a) zone 1 monazite equilibrated with melt generated by muscovite dehydration melting (high Y, no Grt stable); (b)zone 2 monazite equilibrated with the first appearance of peritectic Grt, marking initiation of biotite melting. It is in this text, I am thinking whether alteration zones 1 and 2 monazites which are the products of dissolution-reppt of Paleoproterozoic monazite (which I think is of detrital origin) can be equated with prograde monazite growth!

I am aware of the limitation of the interpretation. There are textural and compositional supports that old monazites are dissolving in melt, but equally monazite is also growing in equilibrium with melt as Harlov's recent experimental work on dissolution-reppt of monazite (Harlov et al., 2011, CMP) predicts. The question is can we consider these alteration zone monazites to have grown along the heating path? I am aware of a few publications which predicted this for zircon (Vavra et al., 1999; Williams, 2001; Rubatto et al., 2001; Hermann and Rubatto, 2003).

Sorry for this rather longish mail and for your patience.

With best wishes,

Santanu 

----- Original Message -----
From: "Michael Brown" <[log in to unmask]>
To: [log in to unmask]
Sent: Saturday, March 10, 2012 6:45:42 AM
Subject: Re: [geo-metamorphism] Monazite growth in anatectic melt

Hi Santanu

I agree with Matt Kohn that a lot of the complexity relates to 
crystallization along the high-T retrograde path down to the elevated 
solidus for residual rocks. One point we made in the following paper, is 
that ages vary because the final solidus T varies according to how 
residual the rock is i.e. how much melt it lost on the prograde path. So 
with slow cooling at 1C/Ma, a 10-20C difference in solidus equals a 20 
Ma difference in age.

See: Reno, B.L., Piccoli, P.M., Brown, M. and Trouw, R., 2012. In situ 
chemical dating of monazite from the Southern Brazília Belt, Brazil. 
Journal of Metamorphic Geology, 30, 81-112 (Article first published 
online: 20 SEP 2011 | DOI: 10.1111/j.1525-1314.2011.00957.x).

Regards,
Mike


On 3/9/12 7:51 AM, Santanu Kumar Bhowmik wrote:
> Dear friends,
>
> Apology for multiple posting. I am looking for reference on monazite dissolution and reprecipitation at the onset of partial melting in metapelites. I am particularly interested in studies which documented new growth of monazite, coinciding with muscovite dehydration melting. Are there specific compositional attributes in monazite and associated minerals which are suggested to constrain the mentioned melting process? Is it possible to relate an increased cheralite content in the newly produced monazite with the participation of plagioclase in the muscovite melting reaction,
>
> muscovite + plagioclase + quartz →K-feldsparss + aluminosilicate + melt?
>
> With best wishes,
>
> Santanu
>
> ----------------------
> Dr. Santanu Kumar Bhowmik
> Professor
> Department of Geology&  Geophysics
> Indian Institute of Technology
> Kharagpur-721 302
> India
> Ph: 91-3222-283390(O), 283391(R), 279766(R)
> Mobile:91-9434041791
>

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