Megacrysts of “bubbled” kaersutite in the Neogene-Quaternary of Western Syria: evidence of crystallization in a boiled melt/fluid?

<p>The territory of Syria is a classic area of intraplate Neogene-Quaternary plateau basaltic magmatism (Ponikarov et al., 1969; Sharkov, 2000; Lustrino, Sharkov, 2006; Trifonov et al., 2011, etc.). These basalts belong to the Afro-Arabian large igneous province (LIP) (Ernst, 2014), whose origin, according to geophysical data, is related to the ascent of a mantle thermochemical plume that originated at the liquid iron core-silicate mantle boundary of (Hansen et al., 2012).</p><p>The basalt plateaus of Syria have a similar structure and are formed by numerous basaltic flows, as well as scoria and pyroclastic cones, often containing mantle xenoliths. Approximately 80% of them are represented by green spinel lherzolites and harzburgites, and subordinate amount (~20 %) of xenoliths belong to black series (hornblendite, hornblende clinopyroxenites, clinopyroxenites, phlogopitites, etc., as well as megacrysts of kaersutite, clinopyroxene, ilmenite, sanidine, etc.). Some of the kaersutite megacrysts have unusual “bubbled” structure, containing oval cavities up to 3-4 mm in diameter. We believe that these xenoliths are fragments of the upper cooled margin of the mantle plume above the adiabatic melting zone (Sharkov et al., 2017). Thus, they probe substance of mantle plume and bear important information about the processes within its interior.</p><p>As previously shown (Sharkov et al., 2017), the black series rocks were formed from a melt/fluid released fluid during the incongruent ("secondary") melting of the mantle plume head at the final stage of the magmatic system evolution. The crystallization of this fluid-supersaturated melt could be accompanied by its retrograde boiling, which led to the appearance of "bubbled" crystals.</p><p> </p>

Ferrian high sanidine in a lamproite from Cancarix, Spain

Na-poor, Fe-bearing high sanidine from a lamproite near Cancarix (Spain) has 2Vα‖(010) = 37-43° and C2/m, a = 8.598(15), b = 13.050(26), c = 7.209(17) Å, β = 116.00(18)° V = 727(2) Å3. Rims of sanidine crystals against vugs contain up to 60 mole % KFeSi3O8 and up to 10 at.% Si and 6 at.% K above the stoichiometric requirement; otherwise, they have up to 4 mole % □Si4O8 and 3 mole % K2O.Si4O8 in solid solution. Their MgO content may reach 0.46 wt.%. The skeletons of mm sized blocky crystals (Baveno habit) indicate formation under moderate undercooling at temperatures not much above 725°C Feldspar formation was facilitated by a high diffusion rate due to low viscosity in a highly perpotassic melt, supersaturated by pressure release and diopside fractionation, upon extrusion of a huge volume of lava in a crater. After titanian potassium-richterite largely filled the interstices in the sanidine fabric, crystals of dalyite (K2ZrSi6O15) and Fe-rich rims of sanidine and amphibole crystals were formed from an increasingly hydrous, silicic, ferric, and peralkaline residual melt. High rate nonequilibrium crystallisation caused the incorporation of excess SiO2 and K2O in the defect structure of the sanidine. Retrograde boiling initiated the escape of volatiles, causing the quenching, by which the disordered structural state and the nonstoichiometric composition of the sanidine were preserved.

Progressive 18O depletion during CO2 separation from a carbon dioxide-rich hydrothermal fluid: evidence from the Grey River tungsten deposit, Newfoundland

Oxygen isotope data for the Grey River tungsten prospect, Newfoundland, Canada, indicate a progressive depletion in δ18Ofluid during mineralization. Early veins with pegmatitic affinities were deposited at 470 °C and pressures greater than 1 kbar (100 MPa), from a fluid with a δ18O composition of 7.4‰, presumed to be of magmatic origin. Successive vein deposition, at progressively lower temperatures and pressures, culminated in the precipitation of wolframite-bearing veins at a temperature of 300 °C and pressures of 150–320 bar (15–32 MPa), from a low salinity fluid with a δ18O composition in the range 3.2–1.6‰.Low values of δ18O (and δDfluid) are recorded in many vein tungsten deposits and are normally interpreted as reflecting mixing of isotopically light meteoric fluids or formation brines with magmatic fluids. However, fluid-inclusion evidence for the Grey River mineralization indicates that a 40 mol% CO2 loss occurred by immiscibility and retrograde boiling of the hydrothermal fluid between 420 and 300 °C. Such a chemical change would have significantly altered the oxygen isotopic character of the hydrothermal fluid since CO2 fractionates 18O relative to coexisting water by ~10‰ at 400 °C and ~14‰ at 300 °C. Calculations using available CO2–H2O fractionation factors reveal that up to a 7‰ depletion in δ18O of the residual aqueous fluids may occur as a result of the 40 mol% CO2 loss from the hydrothermal fluid.