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| Titel |
The origin and significance of hypersaline magmatic volatiles in giant layered intrusions |
| VerfasserIn |
Jacob Hanley, Erin Adlakha |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250083558
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| Zusammenfassung |
| Fluid and melt inclusions are preserved within pegmatite bodies and cumulus minerals within
mafic-ultramafic layered intrusions that host economic concentrations of the platinum-group
elements (e.g., Bushveld Complex, South Africa; Stillwater Complex, Montana). The
inclusions indicate that the earliest volatile phase to have exsolved from the crystallizing
intrusions was a relatively anhydrous carbonic fluid (CO2-dominated). As crystallization
proceeded, volatiles appear to have become increasingly water-rich and saline, consistent
with the relative saturation limits of carbonic and aqueous fluids in mafic silicate
liquids.
However, the latest stage volatiles in the layered intrusions were unusual halide melts
(only slightly hydrous molten salts) of relatively simply composition (NaCl±KCl, CaCl2)
with salinities in excess of 90 wt% eq. NaCl or CaCl2. These volatiles were trapped at
minimum temperatures of ~750-800oC, near the eutectic temperature for water-saturated
felsic (very late, intercumulate) liquid. Heterogeneous entrapment of late-stage
silicate melt and halide melt provides unambiguous evidence for the coexistence of
both phases. However, experimental constraints on the nature of exsolved volatiles
from mafic silicate liquids suggest that the halide melt phases cannot represent
an exsolved phase from that coexisting silicate liquid, since this would require
unrealistically high (initial) Cl:H2O ratios for the parental silicate liquid (> 9). Analysis of
rhyodacitic silicate melt inclusions that coexist with the halide melt inclusions show
that the coevally-trapped silicate melt had a Cl:H2O ratios of only ~ 0.1 to 0.2.
Similarily, the salt melt phases could not have evolved via the crystallization of hydrous
magmatic minerals (e.g., biotite, apatite) since their abundance in the intrusion are very
low.
The most plausible explanation for the halide melt phases involves the “dehydration” of
an initially lower salinity aqueous fluid. This may have occurred by the reaction of the
aqueous fluid with nominally-anhydrous minerals such as pyroxene, or by the late-stage
alteration of cumulus minerals to hydrous mineral assemblages. Through the use
of conventional hydrothermal experimental techniques, it can be shown that the
reaction of a volumetrically-minor CaCl2-rich aqueous fluid (20 wt% eq. CaCl2) with
the assemblage diopside-enstatite-quartz at near-solidus conditions (700oC, 0.4
kbar) results in the formation of tremolite by the reaction of H2O with the initially
anhydrous mafic mineral assemblage. The resulting salinity of the saline phase, trapped
as synthetic inclusions in quartz, was > 96 wt% eq. CaCl2, consistent with the
water-poor nature of the salt melt inclusions observed in most layered intrusions globally. |
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