Geochemical and experimental studies of the origin of ultrapotassic igneous rock

The geochemistry and experimental petrology of the three ultrapotassic igneous rock groups can be explained by an origin by hybridization of melts from alkali-rich veins and peridotitic wall-rocks. Lamproites form from primary magmas ranging in SiO2 content between about 42 (olivine lamproite) and 53 wt % (leucite lamproite). Source rocks are mica-rich, but poor in Ca, Al and Na in comparison to primitive mantle. High-pressure experiments support the hypothesis that the range of primary lamproite magmas can be explained by difference in their pressure of origin: leucite lamproites form at pressures below 20 kbar, whereas diamond-bearing olivine lamproites originate at more than 55 kbar.

Experiments at the same pressure and temperature with differing fluid compositions (CH4 > H2O, H2O > CH4, CO2 > H2O) show that oxygen fugacity has a stronger effect on mica chemistry than does variation in pressure or temperature. Micas in experiments with H2O > CH4 have higher Al- and Ba-contents and lower Si, and K-contents than those in experiments with CH4 > H2O. Fluorine contents are lower due to the high water activity, and Ti contents higher. At higher fO2, at which CO2 forms an important species in the fluid, Ba- and Ti-contents are appreciably higher.

The experimental results show that the rapid decrease in Al and the high Ti contents of phlogopites in natural lamproites is best explained by fractionation in H2O-rich conditions. The extremely high K/Al of lamproites can be directly attributed to melting of mica in the source rock if melting conditions correspond to the most reducing of the experiments (fO2 = IW. The typically high Ba contents of lamproites must then be attributed to melting of accessory phases in the source. The origin of lamproitic magmas under reducing conditions also explains other chemical and mineralogical characteristics, namely: (i) high Fe2O3 in later generations of spinel and leucite, (ii) the stability of armalcolite, (iii) low C-contents, and (iv) the SiO2-rich composition of leucite lamproites.

A critical assessment of liquidus experiments on ultrapotassic igneous rocks and partial melting experiments on lherzolites shows that the ultrapotassic rocks could not have formed from lherzolitic source rocks. Clinopyroxene and mica are much more common in the liquidus experiments than melting of Iherzolite would allow. Trace elements and isotope measurements provide further arguments against a homogeneous source rock.

The alkali-rich veins in the source regions are the results of earlier magmas which solidified at depth. The veins commonly contain abundant mica and clinopyroxene and may contain accessories which are not stable in the peridotitic wall-rock. Examples are priderite, K-Richterite and LIMA. Olivine is uncommon or absent in the veins, since it is consumed by the crystallization of mica.

The first, strongly alkaline melt of veined peridotite is restricted to the vein assemblage due to the concentration of incompatible elements and volatiles. With increasing temperature this melt becomes hybridized by a component from the wall-rock by means of two mechanisms: (1) Dissolution of wall-rock minerals: The first melt is sucked into the wall-rock due to surface energy minimization, where it dissolves wall-rock minerals (especially olivine and orthopyroxene) at temperatures below the wall-rock solidus. (2) Solid-solution melting. Vein assemblages contain several minerals which consist of extensive solid-solution series (e.g. mica, amphibole, spinel and apatite). These minerals melt progressively over large temperature ranges and thus effectively bridge the temperature gap between the melting points of vein and wall-rock. A melt component from the wall-rock is therefore mixed in with the vein-derived melt whilst the most refractory vein minerals are still stable.

Volcanics with contradictory trace element and isotopic indicators of tectonic setting can be explained by storage of these characteristics in vein assemblages within the, mantle. Veins of differing age will be reactivated together during a later melting event. Ultrapotassic rocks represent melts originating dominantly from the vein assemblage. Continued melting may result in dilution of the vein-derived melt by components from both lithospheric and asthenospheric peridotite. This process explains parallels in trace element patterns between ultrapotassic rocks and commoner basaltic rocks, with the extreme example of alkali-picrites.