K-Ar age of the East Peripheral kimberlite at De Beers Mine, Kimberley, R.S.A.

Abstract

The kimberlite pipe at De Beers Mine, Kimberley is a complex, multiple structure. Three concordant argon-40/argon-39 age spectra have been obtained from samples from the 720 m level in the East Peripheral kimberlite. Each spectrum is dominated by a major plateau-feature between 84 and 89 Ma and none shows significant evidence of either excess argon or argon loss discrepancy. The available evidence suggests that the K-Ar age of normal magmatic emplacement-cooling of the East

LIMA U–Pb ages link lithospheric mantle metasomatism to Karoo magmatism beneath the Kimberley region, South Africa

“…This interpretation is also supported by a ∼170 Ma Re-Os model age for a MARID xenolith from Kimberley (Pearson et al, 1995a) and a ∼180 Ma Pb isotope model age for LIMA macrocrysts in the nearby Jagersfontein kimberlite (Griffin et al, 2014). The forma- (Allsopp and Barrett, 1975;Batumike et al, 2008;Davis, 1977;Fitch and Miller, 1983;Smith et al, 1989). The Barkly West orangeites (or Group II kimberlites) were emplaced in close proximity to the Kimberely cluster ( Fig.…”

“…6) are significantly older than the emplacement ages of the host Kimberley kimberlites (81-90 Ma - Allsopp and Barrett, 1975;Batumike et al, 2008;Davis, 1977;Fitch and Miller, 1983;Smith et al, 1989), as well as orangeite clusters in the region (Barkly West, 118-120 Ma;Kroonstad, 130-135 Ma;Swartruggens, 142-145 Ma -Phillips et al, 1998 and references therein - Fig. 1).…”

“…A variety of dating techniques (i.e. zircon U/Pb, perovskite U/Pb, phlogopite Rb/Sr, phlogopite 40 Ar/ 39 Ar) have provided emplacement ages between 81 and 90 Myr for the Kimberley kimberlites (Allsopp and Barrett, 1975;Batumike et al, 2008;Davis, 1977;Fitch and Miller, 1983;Smith et al, 1989). The Kimberley kimberlites are well known for hosting a range of mantle xenoliths with distinct metasomatic styles (e.g., Giuliani et al, 2012Giuliani et al, , 2013bGiuliani et al, , 2013cGiuliani et al, , 2014Grégoire et al, 2002;Pearson et al, 2003;Rehfeldt et al, 2008).…”

40 Ar/ 39 Ar ages in mantle xenolith phlogopites: determining the ages of multiple lithospheric mantle events and diatreme ascent rates in southern Africa and Malaita, Solomon Islands

“…Zartman et al 1967;Paul et al 1975;Allsopp & Roddick 1985;Phillips et al 1998Phillips et al , 1999. However, K-Ar measurements and 40 Ar/ 39 Ar vacuum furnace step-heating on larger phlogopite grains were found to be complex and were interpreted as being contaminated by excess 40 Ar (Lovering & Richards 1964;Zartman et al 1967;Brookins 1969;Kaneoka & Aoki 1978;Fitch & Miller 1983;Allsopp & Roddick 1985;Phillips & Onstott 1986, 1988, denned as 40 Ar in excess of radiogenic 40 Ar ( 40 Ar*) produced by the radioactive decay of 40 K. The presence of excess 40 Ar in mantle xenoliths was assigned to high partial pressures of Ar (P Ar ) preceding or coinciding with kimberlite eruption (e.g. Kaneoka & Aoki 1978;Fitch & Miller 1983;Phillips 1991).…”

“…However, K-Ar measurements and 40 Ar/ 39 Ar vacuum furnace step-heating on larger phlogopite grains were found to be complex and were interpreted as being contaminated by excess 40 Ar (Lovering & Richards 1964;Zartman et al 1967;Brookins 1969;Kaneoka & Aoki 1978;Fitch & Miller 1983;Allsopp & Roddick 1985;Phillips & Onstott 1986, 1988, denned as 40 Ar in excess of radiogenic 40 Ar ( 40 Ar*) produced by the radioactive decay of 40 K. The presence of excess 40 Ar in mantle xenoliths was assigned to high partial pressures of Ar (P Ar ) preceding or coinciding with kimberlite eruption (e.g. Kaneoka & Aoki 1978;Fitch & Miller 1983;Phillips 1991). In other words, the larger phlogopite grains from xenoliths and megacrysts commonly yield apparent ages older than the kimberlite eruption, despite the fact that phlogopite does not retain argon for geological periods above its maximum closure temperature of c. 480 °C (Giletti 1974), using the equation of Dodson (1973).…”