scholarly journals MushPEC: Correcting Post-entrapment Processes Affecting Melt Inclusions Hosted in Olivine Antecrysts

2021 ◽  
Vol 8 ◽  
Author(s):  
Raimundo Brahm ◽  
Georg F. Zellmer ◽  
Takeshi Kuritani ◽  
Daniel Coulthard ◽  
Mitsuhiro Nakagawa ◽  
...  
Keyword(s):  
Melt Inclusions ◽  
Magmatic Evolution ◽  
Original Host ◽  
Varied Temperature ◽  
Correction Scheme ◽  
Homogeneous Composition ◽  
Lines Of Descent ◽  
Melt Composition ◽  
Storage Times ◽  
Diffusive Exchange

Olivine-hosted melt inclusions (MIs) are widely used as a tool to study the early stages of magmatic evolution. There are a series of processes that affect MI compositions after trapping, including post-entrapment crystallization (PEC) of the host mineral at the MI boundaries, exsolution of volatile phases into a “shrinkage bubble” and diffusive exchange between a MI and its host. Classical correction schemes applied to olivine-hosted MIs include PEC correction through addition of olivine back to the melt until it reaches equilibrium with the host composition and “Fe-loss” correction due to Fe-Mg diffusive exchange. These corrections rely on the assumption that the original host composition is preserved. However, for many volcanic samples the crystal cargo is thought to be antecrystic, and the olivine composition may thus have been completely re-equilibrated during long crystal storage times. Here, we develop a novel MI correction scheme that is applicable when the original host crystal composition has not been preserved and the initial MI composition variability can be represented by simple fractional crystallization (FC). The new scheme allows correction of MI compositions in antecrystic hosts with long and varied temperature histories. The correction fits a set of MI compositions to modelled liquid lines of descent generated by FC. A MATLAB® script (called MushPEC) iterates FC simulations using the rhyolite-MELTS algorithm. In addition to obtaining the corrected MI compositions, the application of this methodology provides estimations of magmatic conditions during MI entrapment. A set of MIs hosted in olivine crystals of homogeneous composition (Fo77–78) from a basaltic tephra of Akita-Komagatake volcano was used to test the methodology. The tephra sample shows evidence of re-equilibration of the MIs to a narrow Mg# range equivalent to the carrier melt composition. The correction shows that olivine hosts were stored in the upper crust (c. 125 – 150 MPa) at undersaturated H2O contents of c. 1 – 2 wt% H2O).

Magmatic evolution of the Gaussberg lamproite (Antarctica): volatile content and glass composition

2004 ◽  
Vol 68 (1) ◽  
pp. 83-100 ◽  
Author(s):  
E. Salvioli-Mariani ◽  
L. Toscani ◽  
D. Bersani
Keyword(s):  
High Pressure ◽  
Infrared Spectra ◽  
Melt Inclusions ◽  
Glass Composition ◽  
Relative Increase ◽  
The Other ◽  
Residual Liquid ◽  
Volatile Content ◽  
Magmatic Evolution ◽  
Ultrapotassic Rock

AbstractThe lamproite of Gaussberg is an ultrapotassic rock where leucite, olivine and clinopyroxene microphenocrysts occur in a glass-rich groundmass, containing microliths of leucite, clinopyroxene, apatite, phlogopite and rare K-richterite.Abundant silicate melt inclusions occur in olivine, leucite and, rarely, in clinopyroxene microphenocrysts. Raman investigations on melt inclusions showed the presence of pure CO2 in the shrinkage bubbles. On the other hand, the glass of the groundmass is CO2-poor and contains up to 0.70 wt.% of dissolved H2O, as estimated by infrared spectra. It is inferred that CO2 was released at every stage of evolution of the lamproite magma (CO2-rich shrinkage bubbles), whereas H2O was retained for longer in the liquid. At Gaussberg, CO2 seems to have a major role at relatively high pressure where it favoured the crystallization of H2O-poor microphenocrysts; the uprise of the magma to the surface decreased the solubility of CO2 and caused a relative increase in water activity. As a consequence, phlogopite and K-richterite appeared in the groundmass.The glass composition of both the groundmass and melt inclusions suggests different evolutions for the residual liquids of the investigated samples. Sample G886 shows the typical evolution of a lamproite magma, where the residual liquid evolves toward peralkaline and Na-rich composition and crystallizes K-richterite in the latest stage. Sample G895 derives from mixing/mingling of different batches of magma; effectively glasses from melt inclusions in leucite and clinopyroxene are more alkaline than those found in early crystallized olivine. Leucite and clinopyroxene crystallized early from a relatively more alkaline batch of lamproite magma and, successively, a less alkaline, olivinebearing magma batch assimilated them during its rise to the surface.

Pristine metasomatic melt preserved in mantle rocks of the Bohemian Massif

2021 ◽  
Author(s):  
Alessia Borghini ◽  
Silvio Ferrero ◽  
Patrick J. O'Brien ◽  
Bernd Wunder ◽  
Oscar Laurent
Keyword(s):  
Trace Elements ◽  
Continental Crust ◽  
Bohemian Massif ◽  
Melt Inclusions ◽  
Mafic Rock ◽  
Major Elements ◽  
White Mica ◽  
Garnet Core ◽  
Melt Composition

<p>Melt inclusions of very unusual nature occur in garnets of eclogites of the Granulitgebirge, Bohemian Massif. This is one of the first direct characterization of a preserved metasomatic melt responsible for the formation of eclogites enclosed in garnet peridotites. The inclusions are micrometric, from glassy to fully crystalized as nanogranitoids and randomly distributed in the garnet core. Nanogranitoids contain kumdykolite/albite, phlogopite, osumilite and kokchetavite with a variable amount of quartz, pyroxene, carbonate and rare white mica. The melt has a granitic composition rather than basaltic or tonalitic/trondhjemitic as would be expected from the partial melting of ultramafic or mafic rocks and it is as well hydrous and peraluminous. The trace elements composition is also unusual for melts in mantle rocks with elements typical of continental crust (Cs, Li, B, Pb and Rb) and subduction zone (Th and U). Similar signatures, i.e. continental crust and subduction, are visible also in the whole rock trace elements in the form of high amounts of LILE and U. The eclogite major elements composition is similar to a Ca- and Fe - rich mafic rock akin more to the crust than to the mantle.</p><p>The peculiar melt composition and the lack of a clear residue of a melting reaction in the eclogites suggest that this melt is external, i.e. metasomatic. It infiltered the peridotites during subduction of the continental crust at mantle depth and aided the transformation of basic layers, already in the peridotite, to eclogite. In addition, similar trace elements patterns to the melt reported here can be found in the so-called durbachite -ultrapotassic melanosyenite present in the high-grade Variscan basement- and in the garnet peridotites and garnet pyroxenites of the T-7 borehole. In both case metasomatism was suggested but the agent was just inferred based on the geochemical signature. All these occurrences suggest that mantle contaminated by melts from deeply subducted continental crust is widespread beneath the Bohemian Massif.</p>

Magmatic evolution of the Campi Flegrei and Procida volcanic fields, Italy, based on interpretation of data from well-constrained melt inclusions

2018 ◽  
Vol 185 ◽  
pp. 325-356 ◽  
Author(s):  
Rosario Esposito ◽  
Kimberly Badescu ◽  
Matthew Steele-MacInnis ◽  
Claudia Cannatelli ◽  
Benedetto De Vivo ◽  
...  
Keyword(s):  
Melt Inclusions ◽  
Campi Flegrei ◽  
Magmatic Evolution ◽  
Volcanic Fields

scholarly journals Effect of redox on Fe–Mg–Mn exchange between olivine and melt and an oxybarometer for basalts

2020 ◽  
Vol 175 (11) ◽  
Author(s):  
Jon Blundy ◽  
Elena Melekhova ◽  
Luca Ziberna ◽  
Madeleine C. S. Humphreys ◽  
Valerio Cerantola ◽  
...  
Keyword(s):  
Melt Inclusions ◽  
Regular Solution Model ◽  
Exchange Coefficient ◽  
Ideal Mixing ◽  
Wide Range ◽  
Effects Of Temperature ◽  
Melt Composition ◽  
Strain Model ◽  
Synchrotron Mössbauer Source ◽  
Lattice Strain Model

Abstract The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg  = (Feol/Fem)·(Mgm/Mgol), with all FeT expressed as Fe2+, is one of the most widely used parameters in petrology. We explore the effect of redox conditions on $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg using experimental, olivine-saturated basaltic glasses with variable H2O (≤ 7 wt%) over a wide range of fO2 (iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe3+ to total Fe (Fe3+/∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe3+/∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H2O. Combining our new data with published experimental data having measured glass Fe3+/∑Fe, we show that for Fo65–98 olivine in equilibrium with basaltic and basaltic andesite melts, $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg decreases linearly with Fe3+/∑Fe with a slope and intercept of 0.3135 ± 0.0011. After accounting for non-ideal mixing of forsterite and fayalite in olivine, using a symmetrical regular solution model, the slope and intercept become 0.3642 ± 0.0011. This is the value at Fo50 olivine; at higher and lower Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe3+/∑Fe in olivine-bearing experimental melts, from which fO2 can be calculated. In contrast to $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg , the Mn–Mg exchange coefficient, $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ Kd Mn - Mg , is relatively constant over a wide range of P–T–fO2 conditions. We present an expression for $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ Kd Mn - Mg that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ Kd Fe T - Mg and $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ Kd Mn - Mg to olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe3+/∑Fe to a precision of ≤ 0.04.

The Stillwater Complex, Montana – Overview and the significance of volatiles

2016 ◽  
Vol 80 (4) ◽  
pp. 585-637 ◽  
Author(s):  
Alan E. Boudreau
Keyword(s):  
High Temperature ◽  
Melt Inclusions ◽  
Indirect Evidence ◽  
Parent Magma ◽  
Stillwater Complex ◽  
South Central ◽  
Cast Doubt ◽  
Volatile Phase ◽  
Lines Of Descent ◽  
Central Montana

AbstractThe geology of the 2.7 Ga Stillwater Complex in South-Central Montana is reviewed with a focus on the role of volatiles in locally modifying both the crystallization sequence of the evolving parent magma and the initially precipitated solid assemblages to favour olivine ± chromite. A secondary origin for these two minerals is particularly probable for the olivine-bearing rocks of the Banded series and, at a minimum, also increasing their modal abundance in the Peridotite zone of the Ultramafic series. Direct evidence for volatiles includes the presence of high-temperature fluid inclusions in pegmatoids and hydrous melt inclusions (now crystallized) in chromite and olivine from both the Ultramafic and the Banded series rocks. Indirect evidence includes the boninitic character of the parent magma, the presence of volatile-bearing minerals including high-temperature carbonates, rock textures, and Cl / F variations in apatite. Mechanisms which favour the formation of olivine (± chromite) over pyroxene include volatile phase boundary shifts induced by added H2O, incongruent melting of pyroxene by hydration of a partly-molten mush, and the near- to sub-solidus replacement of pyroxene by olivine and chromite by silica-undersaturated fluids. These mechanisms cast doubt that magmas with different liquid lines of descent were involved in the crystallization of the Stillwater Complex. A dry Stillwater magma would have been mineralogically and modally much less varied and lacking in high-grade platinum-group element and chromium deposits.

Sign in / Sign up

Export Citation Format

Share Document