@article{fdi:010073954,
  title = {{T}he effect of disequilibrium crystallization on {N}b-{T}a fractionation in pegmatites : constraints from crystallization experiments of tantalite-tapiolite},
  author = {{V}an {L}ichtervelde, {M}arieke and {H}oltz, {F}. and {M}elcher, {F}.},
  editor = {},
  language = {{ENG}},
  abstract = {{T}apiolite [{F}e{T}a2{O}6] and columbite-group minerals [({F}e, {M}n)({T}a, {N}b)(2){O}-6] are common {N}b-{T}a-bearing accessory minerals in rare-element granites and pegmatites. {T}heir compositional gap has inspired several experimental studies, but none of them have succeeded in reproducing the parameters that influence the compositional gap. {I}n this study, tapiolite and columbite-group minerals ({CGM}) were crystallized from water-saturated, flux-rich granitic melts at various conditions of pressure, temperature, oxygen fugacity, and {T}i contents. {C}rystals with a size as small as 500 nm were analyzed with a field emission gun ({FEG}) electron microprobe. {T}he results show that temperature, pressure, and {T}i content only slightly affect the compositional gaps between tapiolite and {CGM}, whereas high f({O}2) leads to complete solid solution between a rutile-structured component {F}e3+ {T}a{O}4 and ({F}e, {M}n) {T}a2{O}6. {T}he experimental {CGM}-tapiolite compositional gaps are compared with natural {CGM}-tapiolite pairs from rare-element granites and pegmatites worldwide. {T}his study reveals that the crystallographic structure of tapiolite and {CGM} could be the dominant parameter that influences the position of the compositional gap. {O}rder-disorder in {CGM} and tapiolite is tightly linked to disequilibrium crystallization triggered by supersaturation. {S}ignificant isothermal {N}b-{T}a fractionation is observed inside {CGM} crystals that grow at high degrees of supersaturation. {T}he effect of supersaturation prevails over the solubility effect that is known to increase the {T}a/({T}a+{N}b) ratio in {CGM} and coexisting melts. {T}hus, even if global equilibrium in terms of the solubility of {N}b-{T}a-bearing minerals is attained, the {T}a/({N}b+{T}a) ratio in the crystals may differ significantly from equilibrium. {I}t implies that {N}b-{T}a fractionation in {N}b-{T}a oxides is controlled by crystallization kinetics rather than equilibrium chemical fractionation (or any other processes such as {F}-complexing of {T}a or fluid exsolution) in dynamic systems that can rapidly reach supersaturated conditions. {T}hese results have important implications for the understanding of crystallization processes in highly evolved and pegmatite-forming magmas.},
  keywords = {{C}rystallization experiments ; disequilibrium crystallization ; {N}b/{T}a ; fractionation ; pegmatites ; tapiolite ; {F}rom {M}agmas to {O}re {D}eposits},
  booktitle = {},
  journal = {{A}merican {M}ineralogist},
  volume = {103},
  numero = {9},
  pages = {1401--1416},
  ISSN = {0003-004{X}},
  year = {2018},
  DOI = {10.2138/am-2018-6441},
  URL = {https://www.documentation.ird.fr/hor/fdi:010073954},
}