Insights into the petrogenesis of the West Kimberley lamproites from trace elements in olivine

The Miocene lamproites of the West Kimberley region, Western Australia include olivine-leucite lamproites (≤10 wt% MgO) containing olivine and leucite microphenocrysts, and diamondiferous olivine lamproites (20–30 wt% MgO) containing olivine phenocrysts and larger (1–10 mm) olivine as mantle xenocrysts and dunite micro-xenoliths. Olivine phenocrysts and thin (<100 μm) magmatic rims define trends of decreasing Cr and Ni, and increasing Ca and Mn, with decreasing olivine Mg#, consistent with fractional crystallisation of olivine (and minor chromite). Many phenocrysts are zoned, and those with cores of similar Mg# and trace element abundances to the mantle xenocrysts may be xenocrysts overgrown by later olivine crystallised from the lamproite magma. Magmatic olivines Mg#91–92 are estimated to have been in equilibrium with olivine lamproite magma(s) containing ~22–24 wt% MgO. The xenocrystic mantle olivines Mg90–92.5 in the olivine lamproites are inferred from trace element abundances to be mostly derived from garnet peridotite with equilibration temperatures estimated from the Al-in-olivine thermometer (Bussweiler et al. 2017) to be ~1000–1270 °C at depths of 115–190 km. Olivines from the deeper lithosphere are less depleted (lower Mg#, higher Na, Al, P, Ti, Zr etc) than those at shallower depths, a feature suggested to reflect the combined effects of metasomatic re-enrichment of the craton roots (Ti, Fe, Zr etc) and increasing temperature with depth of origin (Na, Al, Ca). The West Kimberley lamproite olivines are not enriched in Li, as might be expected if their source regions contained continental sedimentary material as has been previously inferred from lamproite large-ion-lithophile trace elements, and Sr and Pb isotopes.ALJ acknowledges the support of Australian Research Council Grants DP140103841 and DP140101976 in undertaking the stud

Magmatic haggertyite in olivine lamproites of the West Kimberley region, Western Australia

We report the first occurrence of magmatic haggertyite (BaFe6Ti5MgO19) from the Miocene lamproites of the West Kimberley region of Western Australia. This contrasts with the metasomatic formation reported in an olivine lamproite host at the type locality, Prairie Creek, Arkansas. Haggertyite occurs in the groundmass of a diamondiferous olivine lamproite pipe in the Ellendale field, and within the large zoned Walgidee Hills lamproite where it forms part of an extensive suite of Ba- and K-bearing titanate and Ti-rich silicate minerals. The haggertyite co-exists with chromian spinel, perovskite, and ilmenite in the Ellendale lamproite, and with priderite and perovskite and, in one locality, with priderite, jeppeite, ilmenite, and perovskite, in the Walgidee Hills lamproite. Unlike priderite and perovskite, which are common groundmass phases in the Ellendale olivine lamproites and present throughout the Walgidee Hills lamproite, haggertyite appears restricted in its occurrence and crystallization interval, with sparse ilmenite apparently mostly crystallizing as an alternative phase. In the Walgidee Hills lamproite the haggertyite-bearing assemblage is succeeded by the Ba-titanate assemblage priderite plus jeppeite in the evolved central part of the body. The haggertyite in the main zone of the Walgidee Hills lamproite has an average composition of B a 0.7 K 0.3 1.0 T i 5.0 F e 2.1 3 + C r 0.1 F e 3.8 2 + M n 0.2 M g 0.6 N a 0.1 12 O 19 $\left(\mathrm{Ba}_{0.7} \mathrm{K}_{0.3}\right)_{1.0}\left(\mathrm{Ti}_{5.0} \mathrm{Fe}_{2.1}^{3+} \mathrm{Cr}_{0.1} \mathrm{Fe}_{3.8}^{2+} \mathrm{Mn}_{0.2} \mathrm{Mg}_{0.6} \mathrm{Na}_{0.1}\right)_{12} \mathrm{O}_{19}$and is thus very similar to the original haggertyite described from xenoliths in the Prairie Creek lamproite apart from being poorer in Cr and Ni. Haggertyite in the groundmass of the Ellendale olivine lamproite and the central zone of the Walgidee Hills lamproite, in addition to variations in Mg and Cr, show significant variation in Ti and Fe contents and in calculated Fe3+ and Fe2+. A linear inverse relationship between Ti and Fe, and Ti and Fe3+, indicates that Fe3+ is accommodated by the coupled substitution Ti4+ + Fe2+ Fe3+. A marked trend to higher Fe3+ in the haggertyite in Ellendale 9 olivine lamproite is ascribed to increasing oxidation during crystallization, with fO2 estimated from the olivine- spinel thermometer and oxygen barometer at Dlog FMQ = -1 to +3 at temperatures of 790-660 °C. The haggertyite in the central zone of the Walgidee Hills lamproite, in contrast, shows a marked trend to Fe2+ enrichment, which is associated with decreasing Fe in perovskite. This is inferred to indicate formation under more reducing conditions, but suficiently oxidized to permit Fe3+ in co-existing priderite and jeppeite. Trace-element analysis by LA-ICP-MS shows the Walgidee Hills haggertyite contains minor amounts of Na, Si, Ca, V, Co, Zn, Sr, Zr, Nb, and Pb, and only traces of Al, P, Sc, Rb, REE, Hf, and Ta. Moreover, the haggertyite is preferentially enriched in certain lithophile (Ba, Sr), siderophile (Mn, Fe, Co, Ni), and chalcophile (Zn, Pb) elements relative to co-existing priderite. Haggertyite crystallization appears to be a consequence not only of the very high Ba, Ti, and K contents of the lamproite, but of relatively high-Fe concentrations and low temperatures in evolved olivine lamproite magma with the Fe3+/Fe2+ ratio determined by the prevailing f O 2. $f_{\mathrm{O}_{2}}.$The new data suggest that haggertyite might also be present but previously unrecognized in the evolved groundmass of other olivine lamproites. Haggertyite is one of an increasing number of new minerals in upper mantle rocks and volcanics derived from the upper mantle hosting large-ion-lithophile and high field strength cations.The authors acknowledge the facilities and the scientific and technical assistance of Microscopy Australia at the Advanced Imaging Precinct, Australian National University, a facility that is funded by the University, and State and Federal Governments. The project was partially supported by ARC Discovery Project DP140103841