Advancing  U-­‐Pb  high  temperature  thermochronology  by  combining  single  grain  and  intra-­‐grain  dating  Thesis  director:  Richard  Spikings  PhD  student:  Andre  Navin  Paul    The  main  aim  of  this  project  is  to  advance  high-­‐temperature  U-­‐Pb  thermochronology  by  applying  it  to  accessory  phases  extracted  from  young  (Mesozoic)  crystalline  rocks,  and  inverting  U-­‐Pb  age  data   to  generate  continuous   thermal  history  paths  at   temperatures  higher  than  350°C.    U-­‐Pb  dates  of  apatite,  rutile  and  titanite  will  be  combined  with  grain  size  and  diffusion  parameters   to  generate  plausible   thermal  history  solutions  by   inverting  U-­‐Pb  age  data  using  a  controlled  random  search  method  (Markov  Chain  Monte  Carlo,  and  a  Bayesian  approach;   E.g.   Figure   1).   The   accessory   phases   will   be   separated   from   Triassic  leucosomes   of   a   high-­‐temperature   metamorphic   belt   within   the   Northern   Andes.   We  will  combine  i)  ID-­‐TIMS  dates  of  a  range  of  grain  sizes,  ii)  laser  ablation  multi-­‐collector  inductively   coupled   plasma   mass   spectrometry   (LA-­‐MC-­‐ICP-­‐MS)   intra-­‐grain   dates  (Figure  2),   and   iii)   cathodoluminescence,  back-­‐scattered  electron   imaging  and  LA-­‐ICP-­‐MS   trace   element   mapping   to   qualitatively   and   quantitatively   constrain   crystal  heterogeneity,  respectively.    

 

 

   Thermochronological  techniques  have  significantly  contributed  to  our  understanding  of  geological   processes   since   the   early   1970’s   because   they   are   capable   of   accurately  quantifying  variations   in   temperature,  with   time.  Most   techniques  are  sensitive   to   low  temperatures  (<350°C),  and  thus  are  limited  to  investigating  the  thermal  histories  of  the  upper  crust.  This  study  represents  an  important  contribution  to  Earth  Sciences  because:  i)  High   temperature  (>350°C)  U-­‐Pb  thermochronology  provides  earth  scientists  with  a  tool   to   generate   continuous   t-­‐T   paths   for   the   lower   and   middle   crust,   which   will  significantly   increase   our   understanding   of   a)   how   lower   crustal   rocks   exhume   to   the  surface  (e.g.  pure  or  simple  shear  during  extension?),  b)   the   tectonic  stability  of   lower  crust  and  cratons,  and  c)   the   tectonic  history  of  active  margins  over   long  time  periods  

(e.g.   500   Ma),   during   which   they   may   have   experienced   numerous   terrane   collision  events  and  a  substantial  quantity  (e.g.  >15km)  of  exhumation.  ii)  This  will  be  the  second  study   to   combine   ID-­‐TIMS   and   LA-­‐MC-­‐ICP-­‐MS   ages   of   apatite,   which   can   be   used   to  derive   theoretical   thermal   history   paths,   and   assess   the   accuracy   of   those   paths.  Furthermore,  corroboratory  data  from  both  techniques  would  confirm  that  Pb  is  lost  by  thermally   activated   diffusion,   confirming   the   use   of   apatite   U-­‐Pb   ages   as  thermochronometers.    The  Northern  Andes  will  be  used  as   the  study  region  because  they  represent  a  superb  natural  laboratory  to  test  the  U-­‐Pb  thermochronological  method.  The  tectonic  history  of  the   region   has   been   extensively   studied   by   Richard   Spikings   and   other   authors,   and  numerous   40Ar/39Ar   and   lower   temperature   thermochronological   constraints   have  been   published.   Previous   studies   that   utilised   the   U-­‐Pb   thermochronological   method  applied   it   to   Precambrian   rocks.   This   study   will   be   one   of   the   first   to   apply   it   to  Phanerozoic   rocks,   and   will   therefore   face   the   challenge   of   generating   sufficiently  precise  U-­‐Pb  ages  from  minerals  with  low  ratios  of  radiogenic  to  common  Pb.