Vein mineral assemblage in partially serpentinized peridotite xenoliths from Hyblean Plateau (south-eastern Sicily, Italy)

摘要

A mineralogical study was performed on seven selected peridotite xenoliths, found in diatremic tuff-breccia deposits from Valle Guffari (Hyblean Plateau, Sicily, Italy), in order to investigate evidence for serpentinization. Petrographic examinations of spinel-harzburgites revealed the presence of the pervasive network of composite serpentine veins, suggesting a variable degree of alteration of 50-80%. Veins were characterized by X-ray Powder Diffraction (XRPD), which allowed the identification of chrysotile 2M_(c1) and lizardite 1T. No peaks referable to antigorite polymorphs were detected. Microprobe WDS (wavelength dispersion system) data evidenced a different iron content between chrysotile [average composition (Mg_(2.58)Fe ~(2+) _(0.27)Al_(0.01))_(Σ2.86)Si _(2.06)O_5(OH)_4, Fe/(Fe+Mg) = 0.09] and lizardite [average composition (Mg_(2.49)Fe~(2+) 0.37~(Al) _(0.01))_(Σ2.87) Si_(2.05)O_5(OH) _4, Fe/(Fe+Mg) = 0.12]. Chlorine was detected in both polytypes, with an average concentration of 0.09 wt% in chrysotile and 0.05 wt% in lizardite. A series of cogenetic secondary minerals are enclosed in serpentine veins, described hereunder: a) Sulphides, predominantly S-poor phases (atomic sulphur/metal ratio < 1), which are heazlewoodite [average composition (Ni_(2.75)Fe_(0.09))_(Σ2.84)S_2] and godlevskite [mean composition (Ni_(8.52)Fe_(0.63)) _(Σ9.15)S_8), and to a lesser extent by S-rich sulphides (S/M > 1), such as millerite [mean composition (Ni_(0.95)Fe _(0.03))_(Σ0.98)S] and polydymite-violarite solid solution [mean composition (Fe_(1.38)Ni_(1.69)) _(Σ3.07)S_4]. b) Interstitial dendritic aggregates of Na-rich sylvite, whose molar ratio of NaCl/(NaCl+KCl) (X_(NaCl)) varies from 0.03 to 0.21. c) Widespread veinlets of (Ca, Na, S)-rich phosphates (CaO 50.30-52.70 wt%, Na_2O 1.50-3.10 wt%, P_2O_5 31.76-34.60 wt%, SO_3 7.40-12.60 wt%), whose chemical composition is similar to (Na, S)-rich apatite-(CaOH)-M (CaO 48.70-54.30 wt%, Na_2O 0.10-3.90 wt%, P_2O_5 32.01-40.82 wt%, SO_3 0.40-11.40 wt%). d) Aragonite veins. e) Fe-rich saponite (FeO 5.20-13.50 wt%). f) Chalcedony. The studied secondary mineral assemblage hints that serpentinization reactions were triggered in the ultramafic core-complex, forming the Hyblean basement, by hypersaline aqueous solutions, most likely deriving from seawater, in a temperature range from 250 °C to 350-400 °C at a pressure below 0.2 GPa. Partially serpentinized harzburgite xenoliths can be distinguished on the basis of secondary mineral assemblages. Most of the studied xenoliths contain abundant S-poor sulphides, which are indicative of reducing conditions (f_O from-40 to-32) and high temperature (～ 400 °C; Fleet, 1988), likely referable to the early s2tage of serpentinization. Only two xenoliths include S-rich sulphides, aragonite, saponite, and chalcedony, which are the products of the incipient carbonation and saponitization of serpentine, suggesting a relatively oxidant environment (f_(O2) from-34 to-30) and low to moderate temperature (below 300 °C). The dominance of xenoliths bearing S-poor sulphides, and the occurrence of NaCl-KCl solid solutions, whose values of X_(NaCl) support a temperature range of 200-380 °C, would suggest that serpentine veins and their related secondary minerals were produced before the diatreme eruption (Tortonian, Carlentini Formation). In fact, a post-depositional alteration would imply strongly oxidizing conditions (above the Hematite-magnetite buffer) and low temperature, certainly incompatible with the formation of the observed secondary minerals. The timing of serpentinization, in the Hyblean lower crust, can be deduced from the age of hydrothermal zircons, found in a blackwall-type metasomatite xenolith, which was previously dated back to Early Triassic, by U-Pb analyses.