17 research outputs found
Phosphorus-rich pyroxene in mantle xenoliths
No full textNumerous recent reports of detailed crystal zoning
patterns and anomalous enrichment in phosphorus (P) have
focused mainly on olivine from various settings [1-4]. P
enrichment and zoning in olivine have been attributed to
rapid crystal growth and development of disequilibrium as
well as to growth from P-rich melts. Here we report the
comparatively novel observation of elevated P in pyroxenes
from glass-bearing veins and pockets in a previously
undescribed xenolith from Cima Volcanic Field-CVF,
California; H.G. Wilshire (sample Ci-1-105) and in newly
collected mantle xenoliths from the Middle Atlas, Morocco.
Analytical techniques included optical microscopy, electron
microprobe and laser-ablation Inductively Coupled Plasma
Mass Spectrometry. We examine whether the P
concentrations in pyroxene (Px), although unusual, are in fact
anomalous compared to the adjacent glass concentration,
consider possible mechanisms for P enrichment, and correlate
the P enrichment in Px with indicators of metasomatism.
The petrogenetic history of each glassy region involves
melt intrusion, reaction with host minerals, cooling
accompanied by crystal growth, quench of glass, and possibly
later modifications. Secondary P-rich pyroxenes (P2O5 ~ 0.6
wt%) in a glassy pocket in the CVF xenolith are
homogeneous and surrounded by P-rich glass. They reflect
fairly slow near-equilibrium pyroxene growth after the melt
temperature became close to the host rock, with P
concentration in the melt buffered by apatite saturation. In the
Moroccan xenoliths, pyroxenes in a glassy vein exhibit
concentric zoning with P2O5 from 0.05 wt% (core) to ca. 0.3
wt% (intermediate) and then from 0.8 wt% (inner rim) to 1.2
wt% (outer rim). We attribute this to an accelerating rate of
crystal growth, with onset of a diffusive boundary layer pileup
effect and excess P incorporation near the pyroxene rim
Phosphorus-rich olivines in a composite xenolith from Morocco: implications for growth processes
No full textPhosphorus(P)-rich zones in olivine may reflect incorporation of P in excess of equilibrium
partitioning during rapid growth (e.g. Milman-Barris et al. 2008). We investigated (by optical
microscopy and electron microprobe) a composite mantle xenolith from the Middle Atlas
Mountains (Morocco) containing two lithologies, wehrlite and harzburgite, in direct contact. The
host alkali basalt (El Messbahi et al. 2015) is present on the margins of the hand sample but not
included in our thin section. Both lithologies display porphyroclastic texture and contain interstitial
devitrified glass. Large primary matrix olivine in both wehrlite and harzburgite has P2O5
concentrations ≤0.09 wt.% and nearly constant composition, Fo90, except for Fe-rich reaction rims
in contact with the interstitial devitrified glass. The P-rich interstitial spaces between these primary
matrix olivines consist of devitrified glass, secondary olivine, clinopyroxene, spinel, and apatite.
The secondary olivine ranges between Fo86-93 and is obviously enriched in P2O5, with
concentrations from 0.36-1.98 wt.%. Whereas matrix clinopyroxene in the wehrlite forms isolated
subhedral to euhedral crystals, the interstitial regions contain elongated and dendritic
clinopyroxene up to 10 μm long as well as replacive clinopyroxene rims on matrix minerals. Spinel
occurs as tiny discrete grains associated with the devitrified glass. Apatite is found only as very
small crystals embedded in devitrified glass.
High-resolution X-ray mapping of P in olivine reveals both alternating P-rich bands parallel to
crystal elongation and patchy zoning. P5+ correlates negatively with Si4+ (R = –0.90) and positively
with Na+ (R = +0.73). Correlation with total divalent cations (Mg2++Mn2++Fe2++Ca2++Ni2+) is weakly
negative (R = –0.44). Although correlation of P5+ and Al3+ is weak (R = -0.42), the combination
P5++Al3+ displays a better anticorrelation with Si4+ (R = –0.92). Overall, the observed correlations
suggest the predominant substitution mechanism is 2 IVSi4+ <=> IVP5+ + IVR3+, with some additional
accommodation by IVSi4+ +VIM2+ <=> IVP5+ + VINa+.
Because no glass was observed, the apparent olivine/melt partition coefficient could not be
directly measured. However, using the maximum P2O5 contents (1.05, 1.18 and 2.31 wt%) measured in glass in melt veins from other xenoliths from the a nearby Moroccan volcanic flow
(Baziotis et al. 2019) and the P-rich olivines from the present study, we infer a DP
ol/melt range
0.85-1.88. The most probable value is greater than unity, despite P being incompatible in olivine
during equilibrium growth. Such an apparent partitioning suggests that olivine crystallization was
rapid enough, ~1-10 K/hour, to develop a P-rich diffusive boundary layer from which the growing
olivine incorporated P in excess of equilibrium partitioning with the bulk melt pocket (Grant &
Kohn, 2013).
We consider several scenarios for the formation of the interstitial pockets, including partial melting
of the xenolith, intrusion of a metasomatic melt in an event earlier than eruption, and reaction
with the host lava during ascent
On the Color and Genesis of Prase (Green Quartz) and Amethyst from the Island of Serifos, Cyclades, Greece
No full textThe color of quartz and other minerals can be either caused by defects in the crystal structure or by finely dispersed inclusions of other minerals within the crystals. In order to investigate the mineral chemistry and genesis of the famous prase (green quartz) and amethyst association from Serifos Island, Greece, we used electron microprobe analyses and oxygen isotope measurements of quartz. We show that the color of these green quartz crystals is caused by small and acicular amphibole inclusions. Our data also shows that there are two generations of amphibole inclusions within the green quartz crystals, which indicate that the fluid, from which both amphiboles and quartz have crystallized, must have had a change in its chemical composition during the crystallization process. The electron microprobe data also suggests that traces of iron may be responsible for the amethyst coloration. Both quartz varieties are characterized by isotopic compositions that suggest mixing of magmatic and meteoric/marine fluids. The contribution of meteoric fluid is more significant in the final stages and reflects amethyst precipitation under more oxidizing conditions
Replication Data for: Evaporation of Boron from Aluminoborosilicate Melt
No full textKey Points:
- Experiments on B2O3 evaporation from Ca- and Mg-bearing aluminoborosilicate melts were conducted at 1250 and 1350 °C for 60-1020 hours, under varying oxygen fugacities (FMQ-6 to FMQ+1.5) and in air.
- Results indicate that B evaporation increases by an order of magnitude under higher oxygen fugacities compared to reducing conditions, with two possible gas-phase speciation reactions for B2O3 suggested by Gibbs free energy minimization.
- B2O3 evaporation rate constants were calculated from the final glass compositions, showing slower evaporation rates than diffusion rates, indicating that evaporation is the rate-limiting process
Replication Data for: The stability of antigorite in subduction zones revisited: the effect of F on antigorite stability and its breakdown reactions at high pressures and high temperatures, with implications for the geochemical cycles of halogens
No full textWe present new experimental data on the effect of F on the stability of antigorite and its breakdown products at high pressures (2–6 GPa) and high temperatures (570–850 °C). The experiments show that F does not affect the stability of antigorite, but addition of F to the system affects which minerals are formed when antigorite breaks down. In a F-free system and in a system with intermediate F contents (2 wt% F), antigorite breaks down to olivine and orthopyroxene, but in a F-rich system (5 wt% F), antigorite breaks down to other hydrous and F-bearing mineral assemblages which include chlorite, clinohumite and humite-group minerals (HGM). Since the latter mineral phases are stable at higher pressures and temperatures, and contain more F than antigorite, significant amounts of F and potentially other halogens can be retained in the subducting slab and transported deep into the mantle and possibly even into the Earth’s transition zone
Replication Data for: Constraining the presence of amphibole and mica in metasomatized mantle sources through halogen partitioning experiments.
No full text• Fluorine and Cl partition coefficient for amphibole and mica
• Melting model of metasomatized mantle sources
• Halogen geochemical ratios of metasomatized mantle sources
• Identification of hydrous phases in mantle source
Replication Data for: Constraining the presence of amphibole and mica in metasomatized mantle sources through halogen partitioning experiments.
No full text• Fluorine and Cl partition coefficient for amphibole and mica
• Melting model of metasomatized mantle sources
• Halogen geochemical ratios of metasomatized mantle sources
• Identification of hydrous phases in mantle source
Thermal Stability of F-Rich Phlogopite and K-Richterite During Partial Melting of Metasomatized Mantle Peridotite With Implications for Deep Earth Volatile Cycles
No full textISSN:2169-9313ISSN:0148-0227ISSN:2169-9356ISSN:0148-0227ISSN:2169-935
Replication Data for: Constraining the presence of amphibole and mica in metasomatized mantle sources through halogen partitioning experiments.
No full text• Fluorine and Cl partition coefficient for amphibole and mica
• Melting model of metasomatized mantle sources
• Halogen geochemical ratios of metasomatized mantle sources
• Identification of hydrous phases in mantle source
Replication Data for: An Improved Electron Microprobe Method for the Analysis of Halogens in Natural Silicate Glasses
No full textWe present a new analytical method, which allows the simultaneous analysis of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) in geological samples. To account for interferences of Fe on the spectral lines of F, of Al on Br-lines, and of Ca on I-lines, we prepared four new halogen-free calibration glasses. The new method is used to analyze various glass reference materials and crystal-hosted melt inclusions from the Azores. Our results show that our new method allows reliable and reproducible analyses of all four halogens in silicate glasses
