Center to margin electron microprobe analyses were performed across 42 plagioclase crystals in samples Hx14b and Hx14n, with a total of ~ 2500 analyses. Plagioclase textural features were examined with Nomarski Differential Interference Contrast (NDIC), following the methods described in Singer et al. (1995). Plagioclases of both samples are strongly zoned, with cores at An86 and rims at ~ An20. Because plagioclase textures and certain zoning patterns among the two samples are different, they will be described separately. For the following description and discussion, plagioclase core compositions are those of anorthite contents between An86 to An70 mol %, compositions in transition zones between cores and rims are those of An70 to An45, and rim compositions are those with
<An45.
Plagioclase crystals in sample Hx14n are almost featureless, with only weak oscillatory zoning and few dissolution surfaces (Fig. 2a-d). Normally zoned euhedral cores (An86 to An75) mantled by normally zoned rims (An45 to An30) characterize plagioclase crystals that are not included in orthopyroxene, hornblende, or phlogopite (Fig. 2a-d). Plagioclase cores and rims are comparable in width, and are separated by a compositional gap of 30 to 35 mol % An. This abrupt compositional shift typically occurs over a distance of less than 100 µm, and it results in a bimodal compositional distribution with one mode at An80-82 and another at An40-42 (Fig. 3). Plagioclase crystals included in orthopyroxene, hornblende, or phlogopite have nearly identical compositions and are normally zoned from An86 to An40, (most compositions between An86 and An75; Fig. 3), suggesting that the three post-cumulus minerals crystallized concurrently.
Plagioclase crystals in sample Hx14b display more complex textural and compositional features such as oscillatory zoning, dissolution surfaces and mineral and melt inclusions (Fig. 2e-f). Plagioclase crystals not included in orthopyroxene, hornblende, or phlogopite, show normally zoned cores from An86 to An70 that are mantled by normally zoned rims with anorthite contents as low as An6 at the edge of some crystals (Fig. 3). Typically, cores (~ 500 µm) are much larger than rims (~ 50 µm) and are separated by a narrow transition zone (~50 µm) in which decreases of up to 40 mol % An occur. Plagioclase crystals included in orthopyroxene are also normally zoned from An86 to An42, but most compositions are between An85 and An70 (Fig. 3). Multiple minor anorthite shifts (<10 mol
%) associated with dissolution surfaces are present in many plagioclase cores. These
secondary features will not be considered in detail, as we are mostly interested in the large compositional differences between plagioclase cores and rims.
Despite the similar post-cumulus histories in which hornblende, orthopyroxene, and phlogopite were produced by late reactions, plagioclase cores of sample Hx14b show numerous oscillatory zones and dissolution surfaces whereas those of sample Hx14n do not. This implies that plagioclases from the two samples may have resided in different magmatic environments prior to being fixed in the cumulus framework. Specifically, it would appear that the numerous oscillatory zones and dissolution surfaces observed in plagioclases of sample Hx14b are characteristic of plagioclase phenocrysts of the interior of a convecting magma chamber (Singer et al., 1995; Hattory & Sato, 1996), whereas the featureless plagioclases of sample Hx14n have been interpreted as evidence of crystallization in a more static environment, such as the partially crystallized zone of a magma chamber (e.g., Brophy et al., 1996). We envision a scenario where plagioclase crystals of sample Hx14b were phenocrysts from the interior of a convecting magma chamber which became captured by its crystallizing margin (sample Hx14n). Such a process has been previously proposed by Loomis & Welber (1982) and by Kuritani (1998) to explain the zoning profiles of plagioclase from plutonic and volcanic environments.
Figure 2. NDIC images and anorthite mol % profiles of plagioclase. Distance in micrometers is from the margin. A. Sample Hx14n. Plagioclase (Pl2n) consists of an euhedral normally zoned core with scarce oscillatory zones and dissolution surfaces. An almost featureless rim mantles the core. Cores and rims are separated by a composition gap of ~ 35 mol % An that occurs in ~ 15 µm. The electron microprobe traverse is marked by the small rounded pits, whereas as the larger pits are the ion microprobe analyses. B. Sample Hx14n. Plagioclase (Pl4n) is four times smaller than the previous crystal but it shows nearly identical textural and compositional features. C. Sample Hx14n. Plagioclase consisting of an absolutely euhedral sector zoned core of virtually constant anorthite composition. Dissolution surfaces and oscillatory zones are only present right before the rim. The rim is normally zoned and lacks dissolution surfaces or
oscillatory zones. D. Sample Hx14n. Plagioclase included in hornblende is normally zoned from An83 in the center to An50 in the margin. Oscillatory zones or dissolution surfaces are only present in the crystal margin where plagioclase composition changes abruptly (~ 50 µm). E. Sample Hx14b. Plagioclase included in hornblende. Note the numerous oscillatory zones and the complex core is very different from the plagioclase crystal of sample Hx14n (Fig.2D). Black thick line marks the position of the electron microprobe traverse. F. Sample Hx14b. Normally zoned plagioclase in contact with interstitial glass. Note the extreme change in anorthite compositions at the last 50 µm.
The complex zoning texture with numerous oscillatory zones and dissolution surfaces is very different from plagioclases of sample Hx14n. Black thick line marks the position of the electron microprobe traverse.
Figure 3. Histogram of plagioclase compositions (total of 42 crystals). Plagioclases not included in Opx, Hbl or Phl (a and b) show a large compositional spectra. The bimodal plagioclase composition distribution of sample Hx14n (b) is due to the compositional gap between cores and rims. Plagioclases included in Opx, Hbl and Phl (c and d) display narrower compositional spectra.