Mike Dorais' comment reminds me of something I have often thought
about. How DO we really tell the difference between peritectic garnet
(e.g., growth at the expense of a fluid-absent melt reaction like
Bt+Pl+Qtz --> Grt + Opx + Melt) from magmatic or "phenocrystic" garnet
(e.g., actually crystallizing from the melt)? I would be particularly
interested in the textural or microstructural observations that support
these interpretations. In those granitoid systems that do have garnet,
how do we know that it is not peritectic garnet entrained as restite in
the melt? What is the consensus on Grt-bearing leucogranites, in
general? Do most people consider them to be magmatic or "phenocrystic,"
i.e., garnet that crystallized directly from the melt?
Best regards,
Greg
On 9/10/2010 5:11 PM, Mike Dorais wrote:
> Sorry to join the discussion so late, but I have some recent LA-ICPMS
> analyses of REE in garnets from the Cardigan Pluton in New Hampshire
> (USA) that pertain to this subject. The attached pdf shows
> chondrite-normalized plots for core to rim traverses for 2 garnets,
> one that we think is a peritectic garnet, the other a phenocryst.
> There is a core to rim decrease in HREE in the phenocryst forming a
> fan-shaped array compared to the essentially flat HREE patterns for
> the peritectic garnet. Is the fan-shaped HREE pattern produced by
> diffusion or by fractionation of a phase that prefers the HREE, e.g.,
> could the garnet have competed with zircon for the HREE? With respect
> to diffusion, Van Orman et al. (2002) wrote that "there is no
> significant influence of ionic radius on diffusion rates; at each
> temperature the diffusion coefficients for Ce, Sm, Dy and Yb are
> indistinguishable...." If that is the case, it seems that the HREE
> would be no more susceptible to diffusion than the LREE and MREE, yet
> the LREE and MREE in the phenocrystic garnet show a narrow
> compositional range compared to the HREE.
>
> Regards,
> Mike Dorais
> Department of Geological Sciences
> BYU
>
--
Gregory Dumond
Assistant Professor
Department of Geosciences
University of Arkansas
18 Ozark Hall
Fayetteville, AR 72701
Phone - 479.575.3411
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