Ammonium partitioning and nitrogen-isotope fractionation among coexisting micas during high-temperature fluid-rock interactions: Examples from the New England Appalachians

Despite recent advances in the field of N-isotope geochemistry, our underst anding of the behavior of this element in the solid earth remains limited b y a lack of fundamental information regarding the partitioning of ammonium and isotopic fractionation of N among coexisting mineral and fluid phases. Study of N behavior in regionally metamorphosed rocks provides the opportun ity to assess intermineral NH4+ partitioning and N-isotope fractionation am ong coexisting micas during metamorphism and affords an application of the N system as a tracer of high-T fluid-rock interactions. Analyzed mica sampl es range in delta(15)N(air) from +3.3 to +11.9 parts per thousand, and cont ain 9 to 1820 ppm N. The outcrop at Townshend Dam, Vermont, allows examinat ion of N behavior across-strike on a relatively small scale, and samples fr om western Maine demonstrate the effect of varying metamorphic conditions o n N behavior in metapelites. Delta(15)N(bt-w.mica)(delta(15)N(biotite)-delt a(15)N(white-mica)) ranges from -0.9 to + 2.7 parts per thousand (for all s amples from both suites, mean = +0.36 parts per thousand, with 1 sigma = 0. 79 parts per thousand), with samples containing a separate paragonite white -mica phase showing the greatest range (-0.12 to +1.02 parts per thousand; mean = 0.58 parts per thousand, 1 sigma = 1.03 parts per thousand). Thirtee n samples containing only Na-poor muscovite (six from Townshend Dam, seven from Western Maine) have mean Delta(15)N(bt-w.mica) of 0.07 parts per thous and (1 sigma = 0.41 parts per thousand). In both suites, biotite nearly alw ays contains more N than coexisting white mica, but N-w.mica/N-bt also show s some significant scatter (mean N-w.mica/N-bt = 0.46, with 1 sigma = 0.34) . The thirteen samples, containing only a Na-poor, muscovitic white-mica ph ase, have mean N-w.mica/N-bt = 0.39 with 1 sigma = 0.26, similar to that re ported by others for other metamorphic suites containing only muscovite as the white-mica phase. There is no obvious suggestion of equilibrium N-isotopic fractionation amon g coexisting micas at epidote-amphibolite to amphibolite-facies metamorphic conditions, although NH4+ appears to partition systematically among coexis ting biotite and white mica. Significant scatter in both Delta(15)Nb(t-w.mi ca) and NH4+ partitioning (conceivably the result of differential closure t o exchange during cooling or of retrograde replacements) could, however, ob scure observation of small equilibrium intermica fractionations related to the characteristics of the interlayer sites in which NH4+ resides. Samples most unlike the mean in both Delta(15)N(bt-w.mica) and N-w.mica/N-bt contai n abundant chlorite, some of which is likely retrograde, based on petrograp hic observations [chl/(chl+bt) >0.3]. Thus, retrograde replacement of bioti te by chlorite may have been accompanied by fluid-mineral N-isotope exchang e, perhaps involving the production of fine-grained, retrograde N-bearing w hite mica observed petrographically for some samples. It is also possible t hat sampling at scales greater than those of N-isotope equilibrium domains (e.g., across fine interlayers) results in some scatter because of varying relative modal proportions of the two micas in adjacent fine interlayers. A lthough further investigation of the extent of retrograde reequilibration o f mica N systematics is warranted, the observed lack of systematic N-isotop e fractionation among coexisting micas, and the reasonably systematic NH4partitioning data for these phases provide important preliminary constraint s for attempts to model N-isotope behavior in fluid-rock systems. These res ults, and other attempts to calibrate N-isotope fractionation through field studies, point to the conspicuous lack of experimentally determined mica-f luid N-isotope fractionation factors. Copyright (C) 2000 Elsevier Science L td.