Kevin Righter

The effect of dissolved water on the oxidation state of iron in natural silicate liquids

Ferric-ferrous ratios have been measured in 22 experiments on three natural compositions equilibrated at known temperature (950°–1100° C) and oxygen fugacity, and at water-saturated conditions over a pressure range from 0.05 to 0.2 GPa. There does not appear to be any reaction between the melt and the capsule material that affects the redox state of the iron in the melt. An empirical expression for the anhydrous behavior of the redox state of iron in each of these compositions has also been determined at 1 bar as a function of temperature and oxygen fugacity. A direct comparison of the hydrous ferric-ferrous values with the calculated anhydrous values shows that the dissolution of water in a per-alkaline rhyolite, andesite, and an augite minette has no effect on the redox state of the iron in these melts. This result parallels the effect of water on sulfide speciation in basaltic melts, and confirms published results on experimental hydrous basalts.

Large-scale mantle metasomatism: a Re–Os perspective

There is a debate on the behavior of Re and Os during mantle metasomatism. Some argue that the mantle can acquire high 187Os/188Os (0.15 to >1.0) either directly from metasomatic events or by the growth of 187Os from 187Re over time. However, any suggestions of subduction-related Os metasomatism producing whole-scale elevation of the mantle to 187Os/188Os values greater than 0.15 need to be supported by comparisons of increased Re and Os concentrations and isotopic ratios, with like and consistent increases in elements (i.e., Ba, B, Rb) and isotopic ratios (i.e., Nd, Pb, Sr) known to be affected by metasomatic processes. All of the samples in the literature, either xenoliths or minerals (sulfides, pyroxene, phlogopite or amphibole) that are thought to represent the products of mantle metasomatism, follow a clear correlation of increasing Re/Os with decreasing Os concentration. This inverse correlation requires enrichment in Os concentrations in the metasomatizing agent by two to four orders of magnitude relative to the original subduction component in order to substantially elevate 187Os/188Os, eliminating sediment or slab melts as an effective metasomatic component. To date there is no evidence to support conclusions that Os metasomatism, either directly or by a two-stage process involving Re metasomatism and 187Os growth over time, will substantially change the 187Os/188Os of the mantle to values >0.15. When other isotopic systems are considered along with Os, in metasomatism-based models, these models are not plausible. Consequently, large-scale mantle melts should reflect near-chondritic values. The recognition of the robustness of the Re–Os system will allow for the discrimination of different mantle and crustal reservoirs involved in magmatic processes.

Pliocene-Quaternary volcanism and faulting at the intersection of the Gulf of California and the Mexican Volcanic Belt

The junction of the western Mexican Volcanic Belt and the Gulf of California represents a superposition of subduction and continental rifting tectonic regimes in the late Cenozoic. Subduction of the oceanic Rivera plate has caused late Cenozoic uplift of western Mexico, forming the Jalisco Block. This paper addresses three questions: Where is the northern boundary of the Jalisco Block; how is the continental fracture system related to that offshore; and what is the spatial distribution and composition of subduction-related lavas? 40 Ar/ 39 Ar dates on Cretaceous to Paleogene silicic ash flows show that the northern boundary of the Jalisco Block may be defined by the abrupt change in basement age from Cretaceous to Miocene. 40 Ar/ 39 Ar and K-Ar dates on faulted lavas from the Nayarit region indicate that extension at the edge of the Jalisco Block has occurred since ca. 4.2 Ma. The least principal stress (σ 3 direction associated with these faults has had two different orientations from 4.2 Ma to the present: Several Pliocene to Holocene (4.20, 1.05, and 0.65 Ma) lava flows, and aligned cinder cones have a N45°W associated least principal stress direction, whereas three Pliocene (3.36, 3.38, and 3.11 Ma) lava flows are cut by faults indicating a north to north-northeast least principal stress direction. The two different stress directions may arise either from structural features in the basement of the arc or from changes in offshore plate boundaries (e.g., 2.5 Ma when the Rivera and Mathematicians plates were locked together). Whole-rock major and trace element analyses of lavas from the coastal Nayarit region reveal three different lava types: an alkali basalt series, basaltic andesite, and andesite. The alkali basalts show FeO and TiO 2 enrichment and have low Ba/Zr and Ba/La ratios, consistent with derivation from an oceanic-island-type mantle. The basaltic andesites and andesites are both alkaline and calc-alkaline, contain hydrous phenocrysts, and have high Sr/Zr and La/Nb ratios, consistent with a subduction-related source. Among these lavas there is a correlation between Sr/Zr, oxygen fugacity, and H 2 O contents, suggesting that an oxidized, hydrous fluid is involved in the genesis of the subduction-related lavas. The distribution of subduction-related volcanic centers in the coastal Nayarit region, and areas southeast within the Jalisco Block, defines front that parallels the Middle America Trench and is consistent with a 45° dip on the subducted Rivera plate.

Hawaiites and related lavas in the Atenguillo graben, western Mexican Volcanic Belt

Hawaiite and mugearite, alkali-olivine basalt, and basaltic andesite have erupted from three shield volcanoes in the Atenguillo graben, Jalisco, Mexico. The youngest shield volcano, La Laja (0.66 Ma), is the focus of this study. Basal pyroclastic surge deposits of basaltic ash, lapilli, and blocks are overlain by seven to eight, 5- to 10-m-thick hawaiite flows. Pillow lavas along a 4-km stretch on the southeast flank of the volcano and lake-bed sediments define a shoreline and lake bottom southward to the town of Atenguillo. Eruption of La Laja apparently dammed the northward-flowing Atenguillo River and formed a lake of as much as 100-m depth. Subsequent fluvial erosion has destroyed the lava dam, exposing the shield succession. Discharge rates for the Atenguillo River and rainfall estimates for the Atenguillo basin both suggest that this lake formed within 20 to 40 yr. Eruption of nearly 10 km 3 of material within 10 to 100 yr indicates an output rate of 0.1 to 1 km 3 /yr, which is larger than any rate calculated for volcanoes in the Mexican Volcanic Belt (MVB), but similar in magnitude to that of Mauna Loa and Kilauea volcanoes (Hawaii). The older volcanoes, El Vigia (2.7 Ma) and Tin Cleto (3.6 Ma), are composed of basaltic-andesite and alkali-olivine basalt, respectively. The composition of these lavas is unusual for a continental-arc setting. Total FeO, TiO 2 , and alkali concentrations are higher than those of basic calc-alkaline lavas from the western MVB. Alkaline earth/light-rare-earth and light-rare-earth/high-field-strength element ratios in these lavas are low relative to those of calc-alkaline lavas but similar to those of oceanic-island basalt values. Such compositional traits coupled with a high magma-output rate are characteristic of oceanic-island volcanoes. Both hydrous, fluid-enriched, sub-arc mantle, and oceanic-island-type mantle components are present beneath western Mexico; contemporaneous subduction and extension of this region of the western MVB allow tapping of both sources and thus the eruption of compositionally diverse lavas.

Volcanism and tectonism in western Mexico: A contrast of style and substance

The Jalisco block of southwestern Mexico records the initial stages of continental rifting superimposed on a convergent continental margin. Pliocene- Quaternary eruptions in north-northwest-trending graben systems within the block produced extraordinarily diverse lava types, including minette, leucitite, absarokite, and andesite. Interspersed with these are several basaltic shield volcanoes that have compositional similarities to lavas from oceanic islands. Evidence of Miocene Basin and Range extension, which is seen around much of the Gulf of California, is found only along the northern margin of the Jalisco block. Large volumes of alkali basalt were also erupted during the late Miocene at several locations along this northern margin. We propose that Pliocene- Quaternary extension and associated volcanism within the confines of the Jalisco block are consequences of complex plate-boundary reorganizations that are causing the block to rift away from the Mexican mainland

High bedrock incision rates in the Atenguillo River valley, Jalisco, Western Mexico

Lava flows from three basaltic shield volcanoes preserve Pliocene–Pleistocene river levels in the Atenguillo River basin, western Mexico. K–Ar dates of these basalt flows, together with present and palaeoriver levels, allow calculation of bedrock incision rates at three points along the length of the Atenguillo River: at Volcan La Laja, dated at 0·65 Ma, incision rates are 25 cm ka−1; at Volcan La Cienega (2·2 Ma) incision rates are 23 cm ka−1; and at Volcan El Vigia (2·7 Ma) incision rates are 23 cm ka−1. These high incision rates, as well as two distinct knickpoints along the profile of the Atenguillo River, are related to a base level c hange at the northern end of the basin. The dynamics of this river basin are controlled by the ongoing process of continental fragmentation associated with the opening of the Gulf of California.

Redox systematics of martian magmas with implications for magnetite stability

Abstract Magnetite is commonly found at sites on Mars explored by robotic spacecraft, yet is rare in martian meteorites and in experimental studies of martian magma compositions. Iron redox systematics of the high-FeO shergottitic liquids are poorly known, yet have a fundamental control on stability of phases such as magnetite, ilmenite, and pyroxenes. We undertook experiments to constrain the Fe3+/ ΣFe in high-FeO (15-22 wt%) glasses as a function of fO2, melt P2O5, temperature and pressure. We also performed a series of sub-liquidus experiments between 1100 and 1000 °C and FMQ+0.5 to FMQ-1 to define magnetite stability. Run products were analyzed for Fe3+ and Fe2+ by Mössbauer spectroscopy and micro-X-ray absorption near edge structure (micro-XANES) spectroscopy. One bar liquids equilibrated at FMQ-3 to FMQ+3 show a much lower Fe3+/ΣFe than terrestrial basalts at the same conditions. As melt P2O5 contents increase from 0 to 3 wt% (at fixed pressure, temperature, and fO2), Fe3+/ΣFe decreases from 0.07 to 0.05, but this is within error on the measurements. Temperature increases between 1200 and 1500 °C cause little to no variation in Fe3+/ΣFe. Pressure increases from 1 to 4 GPa cause a 0.06 decrease in Fe3+/ΣFe. The trends with pressure and temperature are in agreement with results of previous studies. Combining our new series of data allows derivation of an expression to calculate Fe3+/Fe2+ for high-FeO melts such as martian magmas. ln(XFe3+/XFe2+) = a lnfO2 + b/T + cP/T + dXFeO + eXAl2O3 + fXCaO + gXNa2O + hXK2O + iXP2O5 + j This expression can be used to show that decompressed melts become slightly more oxidized at the surface (compared to 4 GPa). Magnetite stability is suppressed by the lower Fe3+/Fe2+ of the high- FeO melts. Magnetite stability is a function of Fe2O3 and temperature and is stable ~50 °C lower than typical terrestrial basalt. Difficulty in producing magnetite as a liquidus phase in magmatic systems suggests either that many martian basalts are more oxidized than FMQ (but not represented among meteorite collections), that the titano-magnetite only forms upon cooling below ~1000 °C at FMQ, or that the magnetite has a secondary origin.

Modeling siderophile elements during core formation and accretion, and the role of the deep mantle and volatiles

Abstract The last decade has seen general agreement that moderately siderophile elements (MSE) in Earth’s primitive upper mantle (PUM) can be explained by metal-silicate equilibrium at mid-mantle depths in an early Earth magma ocean environment. Despite the agreement, there are some differences in the detailed modeling that has been carried out. This paper will examine siderophile element metal/silicate partitioning with respect to three different topics: (1) an examination of aspects of the modeling that one might suspect leads to differences in outcomes or in comparison between models, but actually are in agreement with experimental data and between models; (2) a discussion of the role of the deep mantle in modeling efforts; and (3) the role and/or fate of volatiles in magma ocean scenarios with an emphasis on where data are lacking.

Redox variations in the inner solar system with new constraints from vanadium XANES in spinels

Abstract Many igneous rocks contain mineral assemblages that are not appropriate for application of common mineral equilibria or oxybarometers to estimate oxygen fugacity. Spinel-structured oxides, common minerals in many igneous rocks, typically contain sufficient V for XANES measurements, allowing use of the correlation between oxygen fugacity and V K pre-edge peak intensity. Here we report V pre-edge peak intensities for a wide range of spinels from source rocks ranging from terrestrial basalt to achondrites to oxidized chondrites. The XANES measurements are used to calculate oxygen fugacity from experimentally produced spinels of known fo2

Redox-driven exsolution of iron-titanium oxides in magnetite in Miller Range (MIL) 03346 nakhlite: Evidence for post crystallization oxidation in the nakhlite cumulate pile?

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