David A. Vanko

Synthetic fluid inclusions: VIII. Vapor-saturated halite solubility in part of the system NaCl-CaCl2-H2O, with application to fluid inclusions from oceanic hydrothermal systems

Abstract Halite solubility along part of the vapor-saturated liquidus in the system NaCl - CaCl 2 - H 2 O has been determined using the synthetic fluid inclusion technique. Data allow the construction of liquidus isotherms for temperatures up to 500°C and bulk compositions containing >60 wt% total salt and as much as 25 wt% CaCl 2 . Combined with previous data for the binary system NaCl - H 2 O and for the ternary system NaCl - CaCl 2 - H 2 O in the low-salinity, low-temperature region, a preliminary ternary phase diagram can be constructed that remains incomplete only in the CaCl 2 -rich region. Results are applied to the interpretation of saline fluid inclusions from quartz veins in oceanic metagabbros, and can be applied to many other natural inclusions containing aqueous solutions with NaCl and CaCl 2 the dominant solutes. Microthermometric measurements at equilibrium of the melting temperature of ice [ Tm (ice)] and of the dissolution temperature of halite [ Tm (halite)] are sufficient to determine the bulk composition of the NaCl - CaCl 2 - H 2 O fluid.

Multistage hydrothermal alteration of gabbroic rocks from the failed Mathematician Ridge

Oxygen isotopic, mineralogical and fluid inclusion data are presented for fresh and hydrothermally altered gabbroic rocks from the failed Mathematician Ridge. The Mathematician Ridge, though initially a fast-spreading ridge presently has a deep central rift valley more typical of slow-spreading rifts. The inner rift walls expose rare plutonic rocks uncomplicated by proximity to deep large-offset fracture zones. Undeformed Mathematician Ridge gabbros preserve most of the original texture and phase compositions and have been only slightly mineralogically or isotopically modified by post-crystallization hydrothermal alteration. Metagabbros have been subjected to a pervasive ductile deformation and recrystallization to foliated rocks composed of aluminous amphibole, low-calcium plagioclase and ilmenite. Oxygen isotope and phase chemical data suggest this event began at temperatures of up to 700°C, water-rock ratios of less than one, and produced modified seawater fluids with isotopic compositions of 3.5–6.0‰. The amphibolites are cross-cut by multiple generation quartz-epidote veins forming a network. Replacement of the amphibole-grade mineralogies by greenschist-grade mineralogies, fluid oxygen isotopic compositions of 2–4‰, and a two-phase separation in the fluid captured in conjugate vapor-fluid inclusions are all associated with the formation of this network. Fluid inclusion data for the quartz indicate initial temperatures of over 500°C. These samples may represent rocks from the 1–2 km deep plumbing system of black-smoker-type venting.

Fluid inclusions in quartz from oceanic hydrothermal specimens and the Bingham, Utah porphyry-Cu deposit: a study with PIXE and SXRF

Abstract Individual natural fluid inclusions in quartz were selected for non-destructive microprobe analysis by synchrotron X-ray fluorescence (SXRF) and proton-induced X-ray emission (PIXE) in order to compare and contrast the compositions of porphyry-type ore forming brines and two types of ocean crustal brines. The inclusions contained brines with high salt concentrations (typically 20–40 wt.% total), and one or more daughter crystals. The X-ray and proton beams produced detectable characteristic X-rays for many elements and allowed the quantification of concentrations for many elements with atomic number Z >14. Fluid inclusions associated with the core of the Bingham, UT, porphyry-Cu(Mo) deposit contain NaCl brines that also contain (element, typical range in ppm): K, 12,000–100,000; Ca, ≤17,000; Mn, ≤8000; Fe, ≤120,000; Cu, ≤8000; Zn, ≤5000; Br, ≤2000. These values are in good agreement with published estimates, and results from SXRF and PIXE experiments are generally consistent. The Bingham fluid compositions closely resemble brines from the porphyry-Mo(Cu) deposit at Questa, NM. Inclusions from two suites of oceanic plutonic rock samples gave PIXE results consistent with published SXRF analyses from other inclusions in the same samples. New inclusion analyses from the Mathematician Ridge locality indicate the NaCl brines also contain, on average (element, ppm): K, 3500; Ca, 35,000; Mn, 4900; Cu, 400; Zn, 200; Br, 1000. New inclusion analyses from the Oceanographer Transform locality indicate NaCl brines that also contain, on average: K, 37,000; Ca, 35,000; Mn, 10,000; Cu, ≤400; Zn, 2200; Br, 1000. These values are calculated on the assumption that Fe in the inclusions is 50,000 ppm. Compared to continental ore forming brines, the oceanic brines have higher Ca and lower K and Cu concentrations.

Calcium-rich brines and other hydrothermal fluids in fluid inclusions from plutonic rocks, Oceanographer Transform, Mid-Atlantic Ridge

Abstract Fluid inclusions in metagabbro, plagiogranite, and epidosite from the Oceanographer Transform, Mid-Atlantic Ridge, were investigated using microthermometric measurements and laser Raman spectroscopy. Inclusions in quartz reveal the presence of a high-salinity (average 40 wt% NaCl + CaCl 2 ), high-temperature (>340°C), calcium-rich (Na:Ca about 1, by weight) fluid in metagabbro and plagiogranite. At room temperature these fluids bear a halite daughter crystal, and may also contain small crystals of calcite, titanite, and hematite. A low-density, vapor-rich fluid bearing a small amount of CO 2 is closely associated with the briney fluids in one sample. All samples contain a population of high-temperature (150 to 400°C) aqueous liquid-dominated fluid inclusions, commonly of secondary origin, having seawaterlike salinity (3 to 8 wt% NaCl equivalent). We conclude that the calcium-rich brines are produced by a combination of two processes: fluid phase separation and segregation of brines from vapor prior to trapping to generate the high salinities, and extensive fluid-mafic rock interaction at low water:rock ratio to promote the calcium enrichment. Later introduction along microfractures of low-salinity, modified seawater at high water:rock ratio is recorded by the secondary fluid inclusions. The chemical complexity and association with CO 2 may indicate that there is a magmatic component to the brines in addition to a seawater source. Differences between the Oceanographer Transform brines and those reported in previous studies of oceanic and ophiolite samples are attributed to variations in the degree of water:rock interaction, the host rock lithology, and the influence of a magmatic fluid component.

Hydrothermal anorthitization of plagioclase within the magmatic/hydrothermal transition at mid-ocean ridges: examples from deep sheeted dikes (Hole 504B, Costa Rica Rift) and a sheeted dike root zone (Oman ophiolite)

Abstract The unique petrological record from the deep sheeted dikes of ODP Hole 504B gives insight into the magmatic/hydrothermal transition and the hydrothermal reaction zone at mid-ocean ridges. A high-temperature peak hydrothermal event at Hole 504B, defined as Stage 1, corresponds to an early stage in the evolution of an axial reaction zone, and perhaps to an initial high-temperature phase of black smoker activity. The temperatures and/or the a Ca / a Na 2 ratios of hydrothermal fluids overlapped and even exceeded those inferred for the reaction zones of many modern black smoker systems. The volume of hydrothermally altered rock at this stage of evolution of the reaction zone was much less than 10%. Stage 1 is manifest in hand specimen by reaction halos adjacent to hornblende veins and by irregular alteration patches. There is no epidote associated with Stage 1. Wallrock reactions involved the replacement of magmatic clinopyroxene by magnesiohornblende and of magmatic plagioclase by hydrothermal plagioclase. These secondary minerals were not in equilibrium. Hydrothermal plagioclase varies in composition from An 54 to An 95 , and has low MgO and FeO Tot . It has distinctive bright cathodoluminescence and a porous appearance. Stage 1 at Hole 504B occurred at pressures between 38 and 85 MPa (0.38 and 0.85 kbar) and temperatures in excess of about 425°C; these P–T conditions closely approach the two-phase field for seawater. Stage 1 is overprinted by retrograde alteration that caused further replacement within halos and patches as well as 10–40% alteration, mostly to chlorite and albite, of all the fresh sheeted dikes that were unaffected during Stage 1. Stage 1 alteration at Hole 504B is unique among oceanic and ophiolitic samples. However, we report one area from the Oman ophiolite that exhibits features similar to Stage 1 at Hole 504B. Early halos around hornblende veins contain secondary hornblende and porous hydrothermal calcic plagioclase (An 55 –An 78 ) with low MgO and FeO Tot . Fluid inclusions in quartz show that some early hydrothermal fluids had 5.6±2.3 wt% NaCl equivalent at temperatures constrained to be above 320° and below 510°C, as well as a saline brine with 31 to 40 wt% equivalent NaCl and similar temperature constraints. Although evidence for Stage 1 alteration of sheeted dikes is far from ubiquitous, it is now identified in an oceanic setting (Hole 504B) and in an ophiolitic example from Oman.

Major-element ratios in synthetic fluid inclusions by synchrotron X-ray fluorescence microprobe☆

Abstract Individual fluid inclusions in quartz were investigated by synchrotron radiation X-ray fluorescence (SXRF) microprobe to develop protocols for major-element microanalysis. Studies were conducted of synthetic fluid inclusions in Brazilian quartz that contain fluids of known NaClCaCl 2 H 2 O and KClH 2 O compositions. Inclusions were irradiated by synchrotron X-rays and the resulting energy-dispersive spectra were corrected for fluorescence yield and absorption effects to obtain Ca/Cl and K/Cl ratios. Results are commonly better than ±30% relative to the nominal ratio. Element ratios obtained by SXRF may be combined with results of heating and freezing studies to characterize non-destructively the bulk major-element composition of individual inclusions. Extension of the analytical protocol to elements of higher atomic number (e.g., trace and transition elements) will result in better precision as absorption effects and peak overlaps become less problematic.

Determination of rare earth elements in fluid inclusions by inductively coupled plasma-mass spectrometry (ICP-MS)

Abstract An inductively coupled plasma-mass spectrometer (ICP-MS) is used to determine rare earth element (REE) abundances in fluid inclusions. Quartz concentrates from the Bingham porphyry Cu-Mo deposit, Utah, USA, and from a quartz vein in brecciated amphibolite dredged from the Mathematician Ridge, an abandoned spreading center in the eastern Pacific, were prepared by a modified crush-leach method. Leachates and solutions from acid digestion of quartz residue from leaching were analyzed by ICP-MS, using conventional sample introduction. The results demonstrate that the crush-leach method, followed by ICP-MS analysis of leachates, is effective for REE determination. Chondrite-normalized REE patterns for the Bingham samples are light rare earth element (LREE) enriched, with a small negative Eu anomaly, and the pattern for the Mathematician Ridge sample is nearly flat, with a positive Eu anomaly.