Pyroxenes and olivines from a Galapagos spreading center (GSC) rhyodacite record crystal fractionation and magma mixing
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Abstract:
Phenocrysts and xenocrysts of augite, subcalcic augite, pigeonite, orthopyroxene, and olivine in two glassy rhyodacite dredge samples from 95/sup 0/W on the GSC reflect a complex history of fractional crystallization and magma mixing. The pyroxene compositions can be grouped into clusters reflecting three major sources: a) Fenumber approx. 0.2 from basalt, b) Fenumber approx. 0.5 from rhyodacite, and c) Fenumber approx. 0.6 from rhyolite. Pyroxene Ti/Al ratios of 1:14, 1:7 and 1:3 have Fenumbers suggesting original crystallization from basalt, rhyodacite, and rhyolite melts respectively. These general compositional groups are typical of those produced during fractional crystallization of basalt to rhyolite. At relatively constant Fenumber the augites in any group display a wide spectrum of variation in Wo, Al, and Ti. Basaltic augite core to rim variations exhibit both increases and decreases in Ti at nearly constant Fenumber. A continuous variation in subcalcic augites is present from Fe-augite to Fe-pigeonite. These effects are likely kinetic, perhaps due to rapid cooling rates, but possibly due to supersaturation produced during mixing. Magma mixing may have played an important role in bringing together these contrasting components. The abundance of very-fine-grained basaltic xenoliths and xenocrysts, the glassy rhyolitic inclusions and associated xenocrysts, along with majormore » reverse zoning in Fenumber or major discontinuities in Fenumber in the ferromagnesian phases all point to coexisting melts of radically differing composition.« lessKeywords:
Pigeonite
Phenocryst
Pyroxene
Fractional crystallization (geology)
Igneous differentiation
Xenolith
Magma chamber
Basaltic andesite
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Phenocryst
Melt inclusions
Igneous differentiation
Fractional crystallization (geology)
Incompatible element
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Primitive ankaramite rocks differ from picrite by their richness in clinopyroxene (cpx>ol), their high
CaO/Al2O3 values (>1, wt%) and their high normative di/ol values (>0.7, mol%). Ankaramite rocks are
generally interpreted as a variety of basalt enriched in clinopyroxene crystals yet some studies suggest that
ankararnite rocks crystallized from primitive ankaramitic melts. The origin of ankaramitic melts is of
interest because their compositions differ from the picritic compositions of experimentally produced partial
melts in equilibrium with mantle peridotite, yet many of their of their characteristics are similar to those of
primitive mantle melts, i.e. high Mg# values, high Ni and Cr, and magnesian olivine phenocrysts.
The four primitive ankaramite suites studied in this thesis are from the Ulakan Formation in Bali, and
the Rinjani volcano in Lombok (Sunda arc), and from Merelava and Epi (Vanuatu arc). Primary melt
inclusions in magnesian olivine phenocrysts (Fo>90) from each suite are studied to investigate the early
stages of ankaramite magma evolution. The composition of these melt inclusions after homogenization are
quite unlike picrite and have CaO/Al203 values which range up to —1.7, and are therefore ankaramitic.
Additionally, the CaO/Al203 values of melt inclusions within the same grain are similar, but vary between
phenocrysts.
Critical assessment of the data suggests that the composition of melt inclusions trapped in olivine
can be modified by Fe-Mg re-equilibration with the host, before the magma is erupted. This process might
lower the original FeO* content of the trapped melt by several wt% (Fe-loss). The extent of Fe-loss
depends on (1) the time spent by the host olivine phenocryst in the magma before eruption (residence time)
and (2) the rate at which the re-equilibration occurred. Most of the compositions of homogenized melt
inclusions in this study are affected by Fe-loss and are therefore not directly representative of the original
trapped melt. A recalculation procedure is developed to reconstruct the original composition of the melt at
the moment of trapping. Both, before and after recalculation, the compositions of melt inclusions display
di/ol values >1, thus retaining their ankaramitic affinities. The CaO/Al203 values of melt inclusions are
unaffected by this procedure. The recalculated melt inclusion compositions are also more silica-undersaturated
(ne-, lc- and cs-normative) than the host ankaramite rocks, which range from ne- to hy-normative.
If the melt inclusions are aliquots of the parent melts, these results suggest that the ankaramite
magma formed by the aggregation of strongly-silica-undersaturated primitive ankaramite melts with high
CaO/Al203 and high di/o/ values. A link may therefore exist between these trapped melts, the hypothetical
primary melts and the formation of ankaramitic magmas. This link is explored experimentally at high
pressures, using the composition of a representative melt inclusion in Fosq from the Lombok ankaramite
suite as a starting mix composition. Liquidus and near-liquidus phase relations of this melt, under dry,
hydrous, CO2-H20- and CO2-bearing conditions, and between 1 to 3 GPa pressures, lack orthopyroxene near
the liquidus. Therefore a direct origin is excluded for this composition by partial melting of mantle
peridotite (i.e. not a primary melt). Instead, the trapped melt inclusions may have been derived from even
more mafic primary melts that were generated at higher temperatures and pressures than those prevailing
when the ankaramitic magma aggregated. Two mechanisms are considered for the formation of these primary melts that have high CaO/Al203 values (>1): (a) partial melting of lherzolite at high pressures (>5
GPa) and (b) partial melting of lherzolite at lower pressures (<5 GPa) in the presence of CO2-rich mantle
fluids.
The natural phenocryst assemblage and composition of the host ankaramite rock are only duplicated
experimentally under hydrous conditions. With increasing CO2 content in the fluid, the compositions of
synthesized clinopyroxene crystals become less calcic and are unlike the natural phenocrysts. Thus the
crystallization and aggregation of the ankaramite magma may have taken place under hydrous conditions at
depths where olivine and clinopyroxene are co-crystallizing as phenocryst phases. In the case of the Lombok
ankaramite suite, these conditions correspond to pressures equivalent to depths of around —35 km below the
base of the arc crust. The aggregated ankararnite magma may mix and continue to crystallize isobarically
and in-situ, thus producing the wide compositional range observed in olivine (—Fo75.91) and clinopyroxene
(—Wo45-47En50-41Fs5-12) phenocrysts, as well as the variable zoning and resorption textures. Reaction and
re-equilibration of this aggregated magma with the sub-arc mantle before eruption, may cause the silicaenrichment
that distinguishes the wholerock ankaramite compositions from the melt inclusions in their
olivine phenocrysts.
Tiny pleonastic daughter spinel crystals with up to 65 wt% Al2O3 and virtually no chromium, occur
within melt inclusions in olivine phenocrysts. These aluminous spinels could form in melt inclusions
provided the trapped melt becomes sufficiently Al-enriched and Cr-depleted, following the fractional
crystallization of olivine on the walls. Daughter aluminous spinels were found in the melt inclusions of
olivine phenocrysts from all four ankaramite suites studied and may therefore, also occur in melt inclusions
of other basaltic rocks. The occurrence of aluminous spinels in melt inclusions is therefore not evidence for
the trapping of contaminant aluminous melts.
This thesis attempts to 1) demonstrate that ankaramitic melts with CaO/Al203 values (>1) and
normative di/ol values >0.7 exist and can be parental to ankaramite rocks; 2) describe the Fe-Mg reequilibration
(Fe-loss) process that affects melt inclusions in olivine phenocrysts before magma eruption;
and 3) re-interpret the formation of aluminous spinel crystals in melt inclusions that have previously been
interpreted as evidence for contaminant aluminous melts in basaltic magma chambers.
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Carbonatite
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The composition and crystallization conditions of the parental melts of avachites were elucidated
by studying melt inclusions in olivine (Fo85.8–90.7) phenocrysts. The melt inclusions captured
during the crys- tallization of primitive magmas subsequently reequilibrated with their host
minerals and became partly recrys- tallized and decrepitated. The diffusion-controlled
reequilibration of the melt inclusions with the olivine occurred at temperatures close to ~1100°e
and was associated with the crystallization of daughter phases: oli- vine, high-Ca pyroxene, and
spinel. The composition of the pyroxene and spinel in the inclusions evolved toward extremely high
Al contents, which is atypical of pyroxene in the rocks and was controlled by plagioclase
absence from the daughter phase assemblage of the inclusions. Magma decompression induced the
partial decrepitation of the melt inclusions, a process that was associated with the escape of
fluid components (ee2 and c2e) and variable amounts of the residual silicate material from the
inclusions. The initial compositions of the melt inclusions, which were reconstructed using
techniques of experimental homogenization and mod- eling, show broad ranges in the contents of
major and trace elements. Compared with the composition of the rocks, the compositions of
inclusions in the olivine Fo > 90% are higher in CaO, Al2O3, and Na2O at lower concentrations of
SiO2. Their geochemical characteristics are identical to those of low-Si ankaramite melts occurring
in many island arcs. The carbonatite metasomatism of the arc mantle, the derivation of nepheline-
normative ankaramite magmas, and the significant crustal contamination of these magmas during their
fraction- ation can be spread more widely than is currently assumed in models for island-arc
petrogenesis. The evolution of the avachite primitive magmas was controlled by the crystallization
of early olivine, high-Ca pyroxene, spinel, and, perhaps, the assimilation of crustal rocks in the
magmatic chambers at different depths (from 5 to
30 km). During two (or more) crystallization stages, olivine–pyroxene cumulates were produced,
remobilized, and transported to the surface by the differentiated hypersthene-normative magmas.
Avachites are hybrid cumu-
lative rocks, which were produced in a long-lived open magmatic system.
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Nepheline
Carbonatite
Fractional crystallization (geology)
Liquidus
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Thermodynamic calculations and models of olivine zoning profiles are used to estimate the crystallization history of a basaltic magma from Cow Lakes, southeast Oregon. The lava is an alkali olivine basalt containing olivine and plagioclase phenocrysts and microphenocrysts. The geometry and range of chemical zoning in the olivine phenocrysts have been delineated by laser interference microscopy and electron microprobe analysis. The olivine phenocrysts are characterized by homogeneous cores and rims that exhibit strong, continuous, normal zoning (ΔFo = 7–19 mol%).Thermodynamic modelling has been used to estimate the magmatic crystallization path of the Cow Lakes basalt on the basis of the phenocryst assemblage and mineral compositions. The calculated crystallization path begins at 1290 °C and 0.5 GPa ([Formula: see text]) with equilibrium crystallization of the olivine to 1265 °C. Plagioclase appears at 1225 °C, followed by clinopyroxene at 1205 °C. Intratelluric crystallization was terminated prior to crystallization of the clinopyroxene, which is seen in the groundmass but not as phenocrysts.The thermodynamic modelling provides a means to numerically simulate the zoning patterns in olivine defined by the laser interference microscopy. Simulated and observed zoning patterns both have compositionally flat cores and strongly zoned rims. The extent of zoning observed in the olivine phenocrysts is, however, approximately twice the predicted extent, and it appears that a significant proportion of olivine phenocrysts crystallized during ascent or upon eruption.
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Fractional crystallization (geology)
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Igneous differentiation
Mineral redox buffer
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Abstract Detailed analysis of cumulate and melt inclusion assemblages in the chassignites provide important constraints on the nature of the melt trapped as inclusions in cumulus olivine (and, by extension, parental magma compositions), the pressures of crystallization, and magmatic volatile contents. These mineral assemblages show strong similarities to the experimental fractionation assemblages that produce the sodic silica‐saturated alkalic lavas on Earth (e.g., Ascension Island, Azores, the Nandewar volcano of Australia). The experimental assemblages were produced from silica‐saturated hawaiite at pressures above 4.3 kbar with dissolved water contents above 0.5 wt%. Such pressures are consistent with Ti:Al ratios of the melt‐inclusion pyroxenes in the Chassigny meteorite. Pyroxene compositions suggest early high crystallization temperatures and thus relatively low initial water and F contents. Feldspars indicate that melt evolution proceeded to rhyolite compositions both within the interstices of the cumulate olivine and within the melt inclusions, even though rhyolitic glass is only found within olivine‐hosted polyphase melt inclusions. The observed rhyolite glass is compositionally similar to the alkali‐rich rhyolite of Ascension Island which is produced experimentally by crystallization of hawaiite. It is proposed that the melt trapped in cumulus olivine of the Chassigny dunite was similar to a terrestrial silica‐saturated hawaiite, while that trapped in olivine of the Northwest Africa (NWA) 2727 dunite was less evolved, perhaps mildly alkalic basalt. Melts similar to terrestrial intra‐plate tholeiite could be parental to the cumulus minerals and evolve upon crystallization at pressures above 4.3 kbar and water contents above ˜0.4 wt% to mildly alkalic basalt, silica‐saturated hawaiite, and alkali‐rich rhyolite. The melt inclusion assemblages are inconsistent with either crystallization of a low‐Al, high‐Fe basalt, or low‐pressure crystallization of a terrestrial‐like tholeiite.
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Trachyte
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We report the occurrence of unusual, high-magnesium (Fo96) olivine phenocrysts in a basaltic lava and an ejected lithic block from the Upper Vancori period (∼13 ka) and the recent activity (2002–2003) of Stromboli volcano, Italy. The samples that contain this distinctive mineral chemistry are a shoshonitic basalt and a basaltic andesite with anomalous bulk-rock chemical characteristics in which the iron is highly oxidized (6–8 wt % Fe2O3 and <1 wt % FeO). In other respects these samples are similar to the majority of Stromboli basalts, characterized by the coexistence of olivine, clinopyroxene, plagioclase and Fe–Ti oxides as phenocrysts, and clinopyroxene, plagioclase and Fe–Ti oxides in the groundmass. In the high-magnesium olivine samples, Fe–Ti oxides (pseudobrookite) typically occur as symplectitic intergrowths with the olivine phenocrysts, indicating simultaneous growth of the two phases. We propose, as a paragenetic model, that the Fo96 olivine phenocrysts crystallized from a highly oxidized basaltic magma in which most of the iron was in the ferric state; hence, only magnesium was available to form olivine. The highly oxidized state of the magma reflects sudden degassing of volatile phases associated with instantaneous, irreversible, transient degassing of the magma chamber; this is postulated to occur during periods of sudden decompression induced by fracturing of the volcanic edifice associated with paroxysmic activity and edifice collapse.
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