Deciphering the Trace Element Characteristics in Kilbourne Hole Peridotite Xenoliths: Melt–Rock Interaction and Metasomatism beneath the Rio Grande Rift, SW USA

摘要

Significant differences between bulk-rock lithophile trace element budgets and the sum of the contributions from the constituent minerals are common, if not ubiquitous, in peridotite xenoliths. Notwithstanding the possible contributions from fluid inclusions and grain-boundary glass and micro-phases, it is often difficult to reconcile the bulk-rock incompatible element budgets with those of the silicate phases. In the absence of modal metasomatism this discrepancy is often attributed to the ‘catch-all’ yet often vague process of cryptic metasomatism. This study presents comprehensive petrological descriptions, major and trace element and Sr–Nd isotope data for variably metasomatized bulk-rock peridotites, host basalt, and constituent peridotite mineral phases from spinel lherzolite and harzburgite xenoliths from the Kilbourne Hole volcanic maar, New Mexico, USA. Similar measurements were also made on hand-picked interstitial glass from one of the most highly metasomatized samples in an attempt to unravel the sources, effects, and relative timings of multiple metasomatic events. Reaction textures around clinopyroxene grains are evident in the most metasomatized samples. These are accompanied by films of high-SiO₂ interstitial glass, which is not in equilibrium with the primary silicate minerals. Trails of glassy melt inclusions terminate in these films against which the margins of the primary minerals appear partially resorbed. Furthermore, different styles of high field strength element fractionation [e.g. (Nb/Ta)_N vs (Zr/Hf)_N] are evident in the bulk-rocks and the clinopyroxenes that they host. In all of the Kilbourne Hole peridotites analysed, hand-picked, optically clean clinopyroxenes preserve a more unradiogenic Sr isotope signature than the corresponding bulk-rock. Hand-picked interstitial glass from KH03-16 reveals the most radiogenic 87Sr/86Sr of any component recovered from these xenoliths (87Sr/86Sr = 0·708043 ± 0·00009; [Sr] = 81 ppm). Similarly, the 143Nd/144Nd of the glass is amongst the most evolved of the peridotite components (143Nd/144Nd = 0·512893 ± 0·000012; [Nd] = 10 ppm). However, the host basalt (87Sr/86Sr = 0·703953 ± 0·00012; 143Nd/144Nd = 0·512873 ± 0·000013), similar in composition to that of the nearby, contemporaneous, Potrillo Volcanic Field basalts, contains nearly an order of magnitude more Sr and more than three times more Nd ([Sr] = 655 ppm; [Nd] = 34 ppm) than the interstitial glass. Despite the high Sr and Nd contents of the host basalt the evidence for basalt infiltration is scant, although the effects of melt–rock interaction, both in antiquity and more recently, are preserved in several xenoliths. Mixing between clinopyroxene and the host basalt cannot account for the full range of bulk-rock Sr–Nd isotope ratios; nearly half of the xenoliths require an additional component that could involve varying amounts of interstitial glass. In detail the behaviour of Sr and Nd isotopes has been decoupled, requiring multiple, temporally distinct, metasomatic events. Several bulk-rock samples require a further, as yet unidentified, component to explain the bulk-rock trace element mass balance and Sr–Nd isotope composition fully, implying that at least three episodes of melt–rock interaction, refertilization and metasomatism must have occurred prior to the arrival of the xenoliths at the surface in their host maar deposits.