Methodology

 

a) Number of samples 

 

This is difficult to say how many samples are needed for a given basin but we can give you examples.

 

  • Sub-Andean Zone of Ecuador and Amazon Basin: 90 samples (substratum + overlying Cretaceous to Pliocene pile)

  • Andes and MAdre de Dios Basin Peru: 100 samples (basement, Paleozoic to Pliocene cover)

  • Western Atlas, including basins and Sahara: 100

  • Zagros - Iran: 80

  • Caucasus + Kura-Rioni basins (2 section against the chain, in Georgia and Russia): 60

  • Tunisia: 30

 

b) Mineral separation for low-temperature thermochronology 

 

  • Selection of key lithologies from key-regions under our supervision (the amount of samples depends on the objectives) 

  • Accurate lithologies are all magmatic rocks (volcanics, plutons, breccias), metamorphics ones, continental sediments (silt, slumps, conglomerates & sandstones).

  • Sample should weight as much as possible and a non-polished 20 microns thin section made 

  • Sample is being defragmented at the grain boundaries using a Selfrag machine

  • Heavy liquids separation (density of 3.1 and 3.3). Apatite mineral has a density of 3.10-3.15 whereas zircon reaches 4.8

  • Iron rich aggregates or minerals are removed with a magnet

  • Apatite and zircon rich fractions will be selected by hand-picking

c) Fission-Track analysis

 

 

  • Apatites are mounted into epoxy, polished and chemically attacked with acid to reveal the tracks due to the spontaneous decay of U238. U235 (more stable) is later investigated through an ICP-MS. Length and width of the U238 tracks are measured because they represent the chemical composition of the apatite to allow thermal modelling with the help of external input parameters.

 

  • For Zircons the principle is the same except that there is no thermal modelling and the acids utilized are much more dangerous (NaOH-KOH). For Fission-Track on apatite and zircon up to 20 crystals should de dated to get an age but more is better.

d) U-Th-Sm/He analysis

 

From the Apatite and zircons separates, the most beautiful, i.e. inclusion free crystals are selected and packed for determining the He content thanks to a quadrupole mass spectrometer whereas U, Th and Sm are determined using an ICP-MS. Up to 5-3 crystals should be analysed to produce a reliable age.

 

e) After collecting the data

 

They are first all put back into their geological context (map, section), age-altitude profiles, constraining denudation and sedimentation rates. Once all information are collected we proceed to the reverse modeling of all thermochronological data to generate the paleo-thermal history of basin fill series.

 

f) Reports, interpretation and conclusions

 

Turn around from sampling to report is roughly 5 months but can be reduced to 4 months. We will produce a detailed report with all data where we will interpret them with some unput on your side because you know better than us the basin you have choosen.