10 research outputs found
Experimental evidence for the preservation of U-Pb isotope ratios in mantle-recycled crustal zircon grains
Zircon of crustal origin found in mantle-derived rocks is of great interest because of the information it
may provide about crust recycling and mantle dynamics. Consideration of this requires understanding
of how mantle temperatures, notably higher than zircon crystallization temperatures, affected the
recycled zircon grains, particularly their isotopic clocks. Since Pb2+ diffuses faster than U4+ and Th+4,
it is generally believed that recycled zircon grains lose all radiogenic Pb after a few million years, thus
limiting the time range over which they can be detected. Nonetheless, this might not be the case
for zircon included in mantle minerals with low Pb2+ diffusivity and partitioning such as olivine and
orthopyroxene because these may act as zircon sealants. Annealing experiments with natural zircon
embedded in cristobalite (an effective zircon sealant) show that zircon grains do not lose Pb to their
surroundings, although they may lose some Pb to molten inclusions. Diffusion tends to homogenize the
Pb concentration in each grain changing the U-Pb and Th-Pb isotope ratios proportionally to the initial
206Pb, 207Pb and 208Pb concentration gradients (no gradient-no change) but in most cases the original
age is still recognizable. It seems, therefore, that recycled crustal zircon grains can be detected, and
even accurately dated, no matter how long they have dwelled in the mantle.This paper has been financed by the Spanish Grants CGL2013-40785-P and
CGL2017-84469-P
Ninety million years of orogenesis, 250 million years of quiescence and further orogenesis with no change in PT: significance for the role of deformation in porphyroblast growth
In situ dating of monazite grains preserved as inclusions within foliations defining FIAs (foliation inflection/intersection axes preserved within porphyroblasts) contained within garnet, staurolite, andalusite and cordierite porphyroblasts provides a chronology of ages that matches the FIA succession for the Big Thompson region of the northern Colorado Rocky Mountains. FIA sets 1, 2 and 3 trending NE–SW, E–W and SE–NW were formed at 1760.5 ± 9.7, 1719.7 ± 6.4 and 1674 ± 11 Ma, respectively. For three samples where garnet first grew during just one of each of these FIAs, the intersection of Ca, Mg, and Fe isopleths in their cores indicate that these rocks never got above 4 kbars throughout the Colorado Orogeny. Furthermore, they remained around approximately the same depth for ~250 million years to the onset of the younger Berthoud Orogeny at 1415 ± 16 Ma when the pressure decreased slightly as porphyroblasts formed with inclusion trails preserving FIA set 4 trending NNE–SSW. No porphyroblast growth occurred during the intervening ~250 million years of quiescence, even though the PT did not change over this period. This confirms microstructural evidence gathered over the past 25 years that crenulation deformation at the scale of a porphyroblast is required for reactions to re-initiate and enable further growth