Banca de QUALIFICAÇÃO: Luisa Gomes Braga

Uma banca de QUALIFICAÇÃO de DOUTORADO foi cadastrada pelo programa.
STUDENT : Luisa Gomes Braga
DATE: 21/12/2022
TIME: 14:00
LOCAL: Sala de Videoconferência do IG e Plataforma Teams
TITLE:

Petrology of mantle xenoliths from the Canastra-1 kimberlite, SW of the São Francisco Craton: mineral chemistry and isotopic geochemistry (Sr-Nd and noble gases)

KEY WORDS:

mantle xenoliths; mineral chemistry; Sr-Nd isotopes; lithological hybridization; mantle metasomatism

PAGES: 81
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

The diamond-bearing Canastra-1 kimberlite represents a Cretaceous intrusion that belongs to the Alto Paranaíba Igneous Province (APIP), Brazil. This kimberlite hosts mantle xenoliths including garnet-bearing lherzolites, websterites (anhydrous and amphibole-bearing), clinopyroxenites, and eclogites. Magnesium number (Mg#) of lherzolitic olivine (88–91), orthopyroxene (90–92) and clinopyroxene (89–90) are typical of mantle origin. Minerals from the hydrated websterites contain lower Mg# (opx = 74–78; cpx = 78–80) than the anhydrous ones (opx = 90; cpx = 89). The clinopyroxene Mg# of clinopyroxenites is low (86–87), except for one sample that contains high Mg# (91). The Mg# of eclogite omphacite vary from 87 to 89. Pyrope endmember in garnets decreases from lherzolite (71–76) to anhydrous websterite (71–72), clinopyroxenite (62–69), eclogite (49–57) and hydrated websterite (42–50). Amphibole, restricted to the hydrated websterites, is classified as pargasite with Mg# = 75. The rare earth element (REE) patterns of clinopyroxenes display variable enrichment in light-REE (LREE) relative to heavy-REE (HREE). Pargasites exhibit same convex-upward REE pattern observed in the coexisting clinopyroxenes with fractionated LREE/HREE (Ce/YbN = 10.26–57.06). Garnets show normal REE pattern characterized by the domain of high HREE and gentle decrease of LREE (Ce/YbN <0.32). Temperature (T) and pressure (P) estimates demonstrate that lherzolites, anhydrous websterites and clinopyroxenites represent the deepest and hotter mantle column of Canastra-1 lithosphere (T = 1195–1288

°C; P = 41.1–56.4 kbar), whereas hydrous websterites (T = 866–921 °C; P = 23.4–26.2 kbar) and eclogites (T = 1011–1015 °C; P = 33.9–41.9 kbar) samples are shallower and colder. The Sr-Nd isotopic ratios from lherzolites and clinopyroxenites are characterized by intermediate to high 143Nd/144Nd (clinopyroxene = 0.51273–0.51293; garnet = 0.51287–0.51315; whole-rock = 0.51277–0.51292) coupled with enriched 87Sr/86Sr isotopic ratios (clinopyroxene = 0.70560–0.70770; garnet = 0.70704–0.70869). Hydrous websterites represent the more enriched samples with the lowest Nd (143Nd/144Nd clinopyroxene = 0.51196–0.51198; whole-rock = 0.51196–0.51227) and the highest Sr isotopic ratios (87Sr/86Sr clinopyroxene = 0.70894–0.71003; garnet = 0.70947–0.71094). Eclogites are slightly less enriched compared to the websterites, with higher 143Nd/144Nd (clinopyroxene = 0.51232; garnet = 0.51226; whole-rock = 0.51235) and lower 87Sr/86Sr (clinopyroxene = 0.70912; garnet = 0.70842) isotopic ratios. The age average calculated based on four Sm-Nd isochrons yielded 135.7 ± 4.8 Ma, which represents the isotopic equilibrium at the time of the proto-kimberlite melt percolation into the base of the lithosphere beneath the São Francisco Craton. Therefore, we propose that the lithospheric mantle beneath the Canastra-1 kimberlite records cryptic and modal metasomatism. Partial melting process of hydrated oceanic slab fragments promoted the melt-rock interaction and consequent mantle hybridization recognized in shallower pargasite-websterites. In the deep cratonic lithosphere, the metasomatic event is coeval to the early stages of the Gondwana break-up and the consequent opening of the Atlantic Ocean during the Cretaceous. The proto-kimberlite metasomatism occurred immediately before the kimberlite erupted. In this context, the destruction of the São Francisco Craton through thermal mechanic erosion favors its rejuvenation.

BANKING MEMBERS:
Presidente - 1069348 - TIAGO LUIS REIS JALOWITZKI
Interna - 1555904 - CATARINA LABOURE BEMFICA TOLEDO
Interno - 404825 - CESAR FONSECA FERREIRA FILHO
Interno - 1178074 - GUILHERME DE OLIVEIRA GONCALVES
Interna - 2697315 - MARIA EMILIA SCHUTESKY