Christy Till | Arizona State University (original) (raw)

Papers by Christy Till

Agu Fall Meeting Abstracts, 2007

Recent experimental studies (Grove et al., EPSL 249, 74-89, 2006) determine the solidus for primi... more Recent experimental studies (Grove et al., EPSL 249, 74-89, 2006) determine the solidus for primitive undepleted peridotite at H2O -saturated conditions to 3.2 GPa and reveal the presence of hydrous phases on the solidus above 2 GPa. We present new data from piston cylinder and multi-anvil experiments that extend the peridotite solidus to 5 GPa at H2O -saturated conditions. The H2O -saturated solidus extends from 800°C at 3.2 GPa to 840°C at 5 GPa. Olivine, orthopyroxene, clinopyroxene, garnet ± chlorite ± Ti-clinohumite are stable on the solidus from 3.2 to 5 GPa. The hydrous phase chlorite (12 wt% H2O) is stable on the H2O-saturated solidus from 2 GPa to 3.6 GPa. Above 3.6 GPa, the H2O- saturated solidus and chlorite stability fields diverge. The presence of chlorite on the H2O-saturated solidus has important implications for melting processes at subduction zones and H2O storage in the mantle wedge and subducted lithosphere. Our findings suggest chlorite is stable in the mantle wedge. This provides a new mechanism for transporting hydrous fluids released from the slab in the forearc to the base of the mantle wedge. In addition, chlorite stable within the subducted lithospheric mantle may be an important courier of H2O to melting zones beneath arcs. The location of the breakdown of chlorite and the 10Å phase (both >10 wt% H2O) will control the maximum depth attained by the majority of H2O in the subducted lithosphere. This H2O-out reaction in the slab follows the chlorite-out boundary between 2 - 4.5 GPa and the 10Å-out boundary at pressures greater than 4.5 GPa. For the 10Å phase to transport H2O greater than 4.5 GPa, the mantle lithosphere must remain below 650°C up to 6 GPa, a value colder than those predicted by most thermal models. Therefore, all hydrous minerals with >10 wt% H2O in the subducted lithosphere become unstable at depths of 120 - 150 km in most arcs and elucidate the maximum depth of H2O-saturated melting at subduction zones. Ti-clinohumite (~3 wt% H2O) is stable over a wide P-T range in the subducted lithosphere and may transport small amounts of H2O deep into the mantle.

Newberry Volcano lies ~60km behind the main Cascade arc in central Oregon and at the western term... more Newberry Volcano lies ~60km behind the main Cascade arc in central Oregon and at the western terminus of a trend of volcanic centers that originate from the same location as the eastward trending volcanic series now traced to the Yellowstone hot spot. Alkali basalts of Luna Butte represent one of the primitive end members of the spectrum of lavas erupted from Newberry Volcano and possibly represent a primary mantle melt input into the Newberry system. Phase equilibrium experiments have been carried out on the Luna Butte composition from 0.8 to 1.2 GPa. We find a near-liquidus multiple-saturation with clinopyroxene, olivine, and plagioclase at 1.0 GPa and 1300°C. The liquids produced in these experiments are silica-undersaturated, but appear to be only slightly offset from the predicted multiple-saturation that would include orthopyroxene. Based on the major element composition of the lava, we expect that the Luna butte composition will be multiply saturated with a spinel lherzolite residual assemblage at 1.0 to 1.2 GPa. The lava's composition and its near aphyric character precludes the possibility of significant crustal level fractionation of olivine, and the high-pressure multiple saturation point at 1.0 GPa places the depth of the anhydrous multiple-saturation point right at the seismic Moho beneath Newberry. The phase equilibria and multiple saturation conditions are similar to those of a primitive lava from Jordan Valley volcanic center in eastern Oregon, along the trace of the purported split in the Yellowstone Hot Spot trend. High Sr and Ba contents of these two lavas suggest there may be a fluid component involved in the generation process, which would serve to increase the depth of the multiple-saturation point. This could allow the compositions to saturate with the spinel lherzolite assemblage predicted for the Luna Butte alkali-basalt. However, to produce the silica-rich lavas found elsewhere at Newberry volcano, e.g. high-silica rhyolites, there must be additional magmatic inputs into the sub-volcanic system that may not be present in the eastern High Lava Plains.

We present sensitivity analyses on several geothermobarometers used to examine the source of late... more We present sensitivity analyses on several geothermobarometers used to examine the source of lateral variability of temperature in the mantle (Ford et al. 1983, Kinzler and Grove 1992, Putirka et al. 2007, Herzberg and Asimow 2008, Lee et al. 2009). The thermobarometers are used to calculate the mantle potential temperature (Tp) and are based on primitive lava compositions, using a variety of methods to correct the starting composition for melting processes and fractional crystallization. The common thread among these thermobarometers is that they are based on parametrization of petrologic information, which can be mineral and/or glass compositions. When identical inputs of magma composition, pressure, and in some cases an accompanying olivine composition are used, the different thermobarometers yield a range of temperatures that is sometimes greater than the uncertainties quoted for any one model. These differences are examined in an effort to more fully understand the strengths an...

The Sanbagawa subduction-type metamorphic belt in SW Japan represents the deepest exposed portion... more The Sanbagawa subduction-type metamorphic belt in SW Japan represents the deepest exposed portion of a Mesozoic accretionary complex along the Japanese island arc. Located on the island of Shikoku, the Higashi-akaishi peridotite body is the largest ultramafic lens within the Sanbagawa belt and is dominantly composed of dunite, lherzolite and garnet clinopyroxenite, interfingered in one locality with quartz-rich eclogite. Previous work indicates the P-T history of the peridotite includes rapid prograde metamorphism with peak temperatures of 700-810°C and pressures of 2.9-3.8 GPa at approximately 110-120 Ma. Here we present major and trace element and isotopic data for samples within the Higashi-akaishi peridotite body that suggest it records subduction zone melting processes. Ultramafic samples range from 40-52 wt. % SiO2 and 21-45 wt. % MgO with olivine and clinopyroxene Mg#s as high as 0.93 and have trace element concentrations diagnostic of subduction zone processes. The quartz-ri...

We present an experimental investigation of H2O-saturated undepleted peridotite at pressures and ... more We present an experimental investigation of H2O-saturated undepleted peridotite at pressures and temperatures relevant to subduction zone melting processes. Piston cylinder and multi-anvil experiments were conducted from 740-1200°C at 3.2 to 6 GPa to locate the H2O-saturated peridotite solidus and quantify changes in mineral and melt compositions with increased degrees of melting. Abrupt changes in texture, persistent changes in mineral composition (e.g., Mg# and Cr2O3) with increasing temperature, and analyses of both quenched melt and quenched solute, demonstrate that the transition we observe at ˜810°C at 3.2 to 6 GPa is the H2O-saturated solidus rather than a change in the solute composition or content of the vapor phase. Our experiments indicate that melts of H2O-saturated peridotite produced in the base of the mantle wedge evolve from low-alkali quartz tholeiites at low degrees of melting (5 wt.%) to olivine tholeiites at higher degrees of melting (

Data from new experiments where liquid is in equilibrium with olivine + orthopyroxene + high-Ca c... more Data from new experiments where liquid is in equilibrium with olivine + orthopyroxene + high-Ca clinopyroxene + Al-phase (plagioclase, spinel or garnet) have allowed us to recalibrate and update the melting model of Kinzler and Grove (K&G, JGR 97: 6885-6926, 1992) for melting under nominally anhydrous conditions over a larger range of pressure. We use existing literature data along with new experiments on melting of a high K2O primitive high alumina olivine tholeiite (HAOT) from the Oregon High Lava Plains, a high-K olivine leucitite from the Tibetan Plateau and low alkali, high FeO + MgO lunar ultramafic glasses. The new spinel lherzolite model is constrained by 114 experimental data that span a temperature range of 1200 to 1580 oC, a pressure range of 1 to 2.7 GPa and liquid alkali contents of up to 4.5 wt. % K2O and 5 wt. % Na2O. The garnet-lherzolite melting model uses 26 experimental constraints with new experiments containing up to 3.4 wt. % K2O. We use the following dependent...

We develop a new model that simulates melting of variably metasomatized mantle peridotite and exp... more We develop a new model that simulates melting of variably metasomatized mantle peridotite and explore the effects of variations in pressure, temperature, and mantle composition on melt composition. This new model combines the approaches of Kinzler and Grove (1992a; b) and Kinzler (1997) to predict the temperature and major element composition of a broad spectrum of primary basalt types produced under anhydrous conditions at upper mantle pressures. The model can also be used to calculate the temperature and pressure at which primary magmas were produced in the mantle, as well as to model both near-fractional adiabatic decompression and batch melting. Our experimental compilation definitively locates the pressure interval of the plagioclase to spinel transition on the solidus at 1.1 GPa and shows that it is narrow (~0.1 GPa) for melting of natural peridotite compositions. A comparison of our melting models to other mantle thermometers reveals that the choice of fractionated phase asse...

We present major and trace element geochemical data from well-dated Oligocene–Miocene volcanic se... more We present major and trace element geochemical data from well-dated Oligocene–Miocene volcanic sections in southern Sonora, Mexico (28°, 110°) that span the transition from subduction to oblique extension in the proto-Gulf of California. The region of northwest Mexico including Sonora experienced this abrupt change in tectonic setting ca. 15–12.5 Ma. Syn-and post-subduction calc-alkaline volcanic rocks erupted in Sonora all have similar major and trace element signatures indicative of a continental arc setting. Post-subduction rocks in Sonora document only a subtle change in the behavior of high eld strength, large ion lithophile and rare earth elements and a decrease in pre-eruptive water-content. The protracted geochemical changes in Sonora suggest it took at least 4 m.y. to erase the subduction signature from the sub-arc mantle here. Therefore Sonoran geochemistry is not consistent with the predictions of petrotectonic models and contrasts with central and southern Baja where a r...

Geochemistry Geophysics Geosystems

Plagioclase and spinel lherzolite thermometry and barometry are applied to an extensive geochemic... more Plagioclase and spinel lherzolite thermometry and barometry are applied to an extensive geochemical dataset of young (2O contents that are little fractionated near‐primary melts of mantle peridotite (i.e., basalts thought to be products of anhydrous decompression mantle melting). Calculated minimum depths of nominally anhydrous melt extraction are 40–58 km below Oregon's High Lava Plains, 41–51 km below the Modoc Plateau, and 37–60 km below the central and southern Cascades arc. The calculated depths are very close to Moho depths as determined from a number of regional geophysical studies and suggest that the geophysical Moho and lithosphere‐asthenosphere boundary in this region are located in very close proximity to one another (within 5–10 km). The basalts originated at 1185–1383°C and point to a generally warm mantle beneath this area but not one hot enough to obviously require a plume contribution. Our results, combined with a range of other geologic, geophysical, and geoche...

Eos, Transactions American Geophysical Union, 2014

The Encyclopedia of Volcanoes, 2015

Contributions to Mineralogy and Petrology, 2014

Nature, 2010

In their Comment England and Katz 1 suggest that our model 2 contains two flaws and that there ar... more In their Comment England and Katz 1 suggest that our model 2 contains two flaws and that there are additional problems in our thermal models. This Reply points out an important part of our model that England and Katz 1 appear to have missed, addresses their suggestion that there are flaws and discusses whether our thermal models are in error.

Journal of Volcanology and Geothermal Research, 2009

Journal of Geophysical Research, 2012