Geology Reports⌕ Search
Home/Archive/USGS

USGS · 70015089

Metasomatic oxidation of upper mantle periodotite

A. V. McGuire·M.D. Dyar·J. E. Nielson
View original source ↗Export metadata ↓

Abstract

Examination of Fe3+ in metasomatized spinel peridotite xenoliths reveals new information about metasomatic redox processes. Composite xenoliths from Dish Hill, California possess remnants of magmatic dikes which were the sources of the silicate fluids responsible for metasomatism of the peridotite part of the same xenoliths. Mo??ssbauer spectra of mineral separates taken at several distances from the dike remnants provide data on Fe3+ contents of minerals in the metasomatized peridotite. Clinopyroxenes contain 33% of total iron (FeT) as Fe3+ (Fe3+/FeT=0.33); orthopyroxenes contain 0.06-0.09 Fe3+/FeT; spinels contain 0.30-0.40 Fe3+/FeT; olivines contain 0.01-0.06 Fe3+/FeT; and metasomatic amphibole in the peridotite contains 0.85-0.90 Fe3+/FeT. In each mineral, Fe3+ and Fe2+ cations per formula unit (p.f.u.) decrease with distance from the dike, but the Fe3+/FeT ratios of each mineral do not vary. Clinopyroxene, spinel, and olivine Fe3+/FeT ratios are significantly higher than in unmetasomatized spinel peridotites. Metasomatic changes in Fe3+/FeT ratios in each mineral are controlled by the oxygen fugacity of the system, but the mechanism by which each phase accommodates this ratio is affected by crystal chemistry, kinetics, rock mode, fluid composition, fluid/rock ratio, and fluid-mineral partition coefficients. Ratio increases in pyroxene and spinel occur by exchange reactions involving diffusion of Fe3+ into existing mineral grains rather than by oxidation of existing Fe2+ in peridotite mineral grains. The very high Fe3+/FeT ratio in the metasomatic amphibole may be a function of the high Fe3+/FeT of the metasomatic fluid, crystal chemical limitations on the amount of Fe3+ that could be accommodated by the pyroxene, spinel, and olivine of the peridotite, and the ability of the amphibole structure to accommodate large amounts of 3 + valence cations. In the samples studied, metasomatic amphibole accounts for half of the bulk-rock Fe2O3. This suggests that patent metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe2+ and Fe3+ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe3+. With increasing distance from the dike, the ratio of added Fe3+ to added Fe2+ increases. The high Fe3+/FeT of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe3+/added Fe2+ in pyroxenes and spinel suggest that the Fe3+/FeT ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe3+ and C-O-H volatile species were concentrated in the fluid, increasing the fluid Fe3+/FeT. ?? 1991 Springer-Verlag.

Explore related subjects

Contributions to Mineralogy and PetrologyArticle

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

A. V. McGuire, M.D. Dyar, J. E. Nielson. 1991. Metasomatic oxidation of upper mantle periodotite. https://doi.org/10.1007/bf00306483

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Storage timescales and the crystal growth history recorded in the layered reservoir of the Unit 3 Kulanaokuaiki Tephra Member of the Uēkahuna Ash from Kīlauea volcano

The Kulanaokuaiki Tephra Unit 3 (900 C.E.) was a subplinian basaltic scoria eruption, the largest known to have occurred at Kīlauea’s summit. The initiation mechanism and cause(s) of this highly hazardous eruption remain poorly understood, particularly as this style of activity has not been observed in the modern period. We use olivine diffusion timescales from Fe-Mg and lithium zoning to assess magma storage timescales and evaluate evidence for mixing prior to eruption. Of the 200 crystals assessed, 55% are normally zoned for forsterite and 45% are non-zoned. Zoned crystals (> 1 mm) ( n = 96) have average compositions of Fo 89 cores and Fo 81 rims, while small crystals 0.5–1 mm ( n = 14) have Fo 85 cores and Fo 82 rims. Modeled Fo diffusion timescales range from < 1 to 12.6 years (median of 3.0 years). Lithium maps in 32 of the forsterite-zoned crystals reveal a variety of lithium zoning types, most of which we attribute to crystal growth. Many lithium profiles are coupled to phosphorus within the outer ~ 100 μm of the crystal ( n = 17), reflecting growth processes and precluding their use for diffusion studies. Overall, our Unit 3 olivine dataset displays no evidence of magmatic priming shortly prior to eruption, unlike prior observations for the basaltic subplinian eruption of the Keanakāko‘i Tephra Unit E (1650 C.E.). Thus, explosive eruption magmas may be stored for considerably longer than previously thought, and magmatic priming is not a necessary precursor for Kīlauea’s explosive eruptions. We hypothesize that gas-driven filter pressing from crystallization of microlites and macrocrysts in the reservoir slurry contributed to the explosivity of the Unit 3 eruption.

Hawaii↗
USGS2024

Shallow storage of the explosive Earthquake Flat Pyroclastics magma body, Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand: Evidence from phase-equilibria experiments

Rhyolitic tuffs range widely in their crystal contents from nearly aphyric to crystal-rich, and their crystal cargoes inform concepts of upper crustal magma reservoirs. The Earthquake Flat pyroclastics (Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand) are 10 km 3 of rhyolitic tuffs with abundant (~ 40 vol.%) plagioclase and quartz, minor biotite, hornblende, and orthopyroxene, and accessory Fe-Ti oxides, apatite, and zircon, set in high-silica rhyolitic glass. Major minerals form large, euhedral phenocrysts and abundant glomerocrysts with few disequilibrium textures excepting some faintly resorbed quartz. Plagioclase phenocrysts have thick rims of nearly constant composition near An 30 , and hornblende is weakly zoned or unzoned. The abundant and texturally complex mineral assemblage contrasts with the nearby (~ 25 km), nearly synchronous, but more voluminous and crystal-moderate rhyolite tuffs from Rotoiti caldera. New H 2 O-saturated phase-equilibria results on the erupted Earthquake Flat melt (glass) determine its co-saturation with the partial phenocryst assemblage of plagioclase, quartz, biotite, and Fe-Ti oxides at: 140 MPa, 755 ºC. These closely approximate the conditions of the pre-eruptive magma body assuming it was saturated with nearly pure H 2 O and at an f O 2 of ~ Ni–NiO. Absence of hornblende and orthopyroxene from the synthesized assemblages may result from those minerals being in a peritectic reaction relation with melt to produce biotite, so they would not grow from the liquid used as starting material. Experimental results on Rotoiti rhyolite (Nicholls et al. 1992) show that the two bodies resided at similar pressures, temperatures, and f O 2 s. Lower crystal abundance of the Rotoiti tuffs may result from slight compositional differences. We interpret that the Earthquake Flat pyroclastics were sourced from the crystal-rich periphery of a mushy reservoir system with the Rotoiti occupying a more melt-rich central location. Uncertain is whether this was a single intrusion zoned continuously in crystallinity, or discrete adjacent intrusions, but our results illustrate and quantify complexities of magma storage across relatively short distances.

North Island, Taupo Volcanic Zone↗
USGS2024

Constraining magma storage conditions of the Toba magmatic system: A plagioclase and amphibole perspective

Silicic magma reservoirs are responsible for producing the largest explosive eruptions in the geologic record. Petrologic and geochronological data provide evidence for these systems spending substantial periods of time (10 4 –10 5 yrs) within the upper crust prior to eruption; however, the long-term thermochemical evolution of these systems is not fully understood, as existing petrologic data make it challenging to quantify the time interval a magmatic system has spent at certain temperatures, or its “thermal history”. Here, we investigate the 74 ka Youngest Toba Tuff (YTT), one of the largest explosive eruptions in the geologic record, to better constrain the long-term thermal evolution of its magmatic system. We combine forward models of Sr diffusion in plagioclase and hornblende, mineral thermometry, and pre-existing trace-element evolution models to quantify the thermochemical evolution of the YTT magmatic system. We find that plagioclase crystals record decades to centuries of storage at temperatures 750 C, while hornblende records up to 6200 years at the same temperatures. Hornblende crystallizes at temperatures around 820 C and adjusting our diffusion modeling to this temperature results in no more than 900 years at initial crystallization conditions. Combined with previous trace-element modeling work, these results indicate that although there was chemical diversity for long durations in the YTT magma system sufficient to produce unique composition eruptive products, the entire system was experiencing a relatively similar thermal history that did not allow for large bodies of eruptible magma to be present for long periods ( 10 2 –10 3 years). Rather, we suggest that magmas within the YTT magmatic system were stored for long durations at thermal conditions where they were uneruptible and only remobilized within a few centuries prior to eruption.

Sumatra↗