Origin and structural character of hauyness in spinel dunite xenoliths from La Palma, Canary Islands

Two spinel dunite xenoliths (Fo(89.8-91.2) in olivine) from La Palma contain minor amounts (<1%) of a pale-blue sodalite-group mineral with hauyne/lazurite chemistry. Selected-area electron diffraction (SAED) patterns of this phase indicate a cubic unit cell with dimensions 9.12 +/- 0.02 Angstrom, and space group P (4) over bar 3n. Superstructure spots along three <110> directions are common, implying commensurate or incommensurate modulations along <110> directions. Raman spectra show peaks typical of both lazurite and hauyne. It is concluded that the mineral has a structure intermediate between those of pure lazurite and pure hauyne, and it is here referred to as hauyne(ss). The hauyne(ss) occurs together with strongly nepheline-normative glass in thin veinlets (<0.1 mm), in interstitial glass pockets, and as inclusions in olivine porphyroclasts. To our knowledge lazurite or hauyne has not previously been described in mantle rocks. The hauyne(ss) is strongly depleted in REE and most other highly lithophile elements relative to the coexisting glass, whereas D-mineral/glass for Sr is approximate to 1.0, and D-Eu higher than the other REE. The hauyne(ss) crystallized from a melt now present as phonolitic glass, probably in response to rapidly decreasing pressure during transport of the xenoliths to the surface. The coexistence of hauyne(ss) and FeS-rich sulfide globules in some samples suggests slightly more oxidizing conditions than for samples in which the glass contains sulfide globules alone.