Julia E. Hammer

Magmatic processes revealed by textural and compositional trends in Merapi dome lavas

Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases. We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986‐88, 1992‐93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts 1 microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab63An29Or8) surrounded by alkali feldspar rims (Ab53An5Or42), compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock. As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions (PH2 O and temperature) were achieved throughout this extrusive period. However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995. Crystal textures and compositions in the 1992‐93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic. q 2000 Elsevier Science B.V. All rights reserved.

Active hydrothermal systems during the recent uplift of Nanga Parbat, Pakistan Himalaya

During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high-grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near-surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ 18 O and δD depletions in the gneisses and marked oxygen isotopic disequilibriuui between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ 18 O magmatic fluids caused δ 18 O depletions in the gneisses within the migmatite zone. These low δ 18 O fluids were derived from the young (<4 MA), RELATIVELY LOW δ 18 O granites (∼8%) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40 / 39 Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8-3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat-Haramosh Massif

Petrologic indicators of preeruption magma dynamics

New phase-equilibria experiments on dacite from the June 1991 eruption of Mount Pinatubo, Philippines, allow calibration of a geobarometer for matrix glass at pressures of 160–220 MPa. Comparison with natural glass compositions suggests equilibration at lower water fugacity, \(\mathit{f}_{H_{2}O}\) , than inferred from Al-in-hornblende geobarometry. The disparity between geobarometric techniques may be explained by a decrease in \(\mathit{f}_{H_{2}O}\) and consequent crystallization several weeks before eruption—a time period too short for hornblende reequilibration. Reduced \(\mathit{f}_{H_{2}O}\) could have resulted from processes associated with basalt magma injection, including (1) late-stage magma ascent (under H 2 O-saturated conditions) or (2) influx of other volatiles (CO 2 , SO 2 ) into the dacite magma system.

Crystal nucleation in hydrous rhyolite: Experimental data applied to classical theory

Abstract Feldspar nucleation rate data obtained by laboratory decompression of hydrous silicate melt are interpreted in view of the classical theory of nucleation (CNT) and a non-classical variation, the diffuse- interface theory (DIT). The nucleation rate data can be modeled by the CNT formalism only if the interfacial free energy (σ) is allowed to vary as a function of composition. The values thus obtained vary by a factor of four (0.024-0.100 J/m2) and decrease systematically over a sixfold increase in dissolved H2O content (0.8-4.8 wt%). This result is qualitatively consistent with the effects of dissolved H2O on the liquid-vapor interfacial free energy in haplogranite magma (Mangan and Sisson 2000) and the liquid-crystal interfacial free energy in the one-component Li-disilicate system (Davis et al. 1997). The DIT states that the interfacial region between the bulk solid and bulk melt has thermodynamic properties intermediate between these phases, and that σ is defined as the difference between the interfacial enthalpy (Hint) and interfacial entropy (TSint). If the DIT model is correct, the nucleation rate data for feldspar may indicate that: (1) dissolved H2O content controls the spatial distribution of enthalpy and configurational entropy around incipient crystals, and (2) the spatial gradients of these potentials diverge during devolatilization. This study suggests that crystal nucleation studies may yield insights into the structure and thermodynamics of hydrous melts; likewise, experimental studies are important for refining a physical understanding of nucleation phenomena. Our results can be applied to quantitative numerical models of ascent-driven magma crystallization.

Phosphorus zoning reveals dendritic architecture of olivine

Quantitative spot analyses were acquired with the JEOL JXA-8500F field-emission gun electron probe micro-analyzer of the University of Hawaii – Manoa over two sessions. Olivine was analyzed during the first session with an electron beam tuned at 20 keV, 50 nA and 10 µm probe diameter. The elements were acquired using five analyzing crystals as followed: one LiFH for Cr and Ni, one LiF for Fe and Mn, one PETJ for Ca and Ti, one PETH for P, and two TAPJ crystals for Al, Si and Mg. The standards used were Verma garnet for Mn, San Carlos olivine USNM 111312/444 for Fe, Si, Mg and Ni, sphene glass for Ti, chromite USNM 117075 for Al and Cr, and fluor-apatite USNM 104021 for Ca and P. The on-peak and off-peak counting times were each 20 s for Si, Al and Mg, 30 s for Mn, Ti and Ni, 35 s for Fe, Ca and Cr, and 70 s for P. The off-peak correction method was linear for major elements and exponential for trace elements Ti, Al, Cr and P, using Probe for EPMA software. Detection limits are in the range 0.005 – 0.01 wt%. Precision and accuracy (1) are 0.4 – 2 % for major elements (reported in weight percent oxide), 3 – 5 % for minor elements (Ca, Mn and Ni – reported in µg/g) and 20 – 70 % for trace elements (Al, P, Ti, and Cr – reported in µg/g in Table DR1), based on repeated analyses of San Carlos olivine standard (USNM 111312/444). Matrix glasses and melt inclusions were analysed during the second session with a beam tuned at 15 keV, 15 nA and 10 µm probe diameter. Elements were acquired using one LiFH for Ti and Cr, one LiF for Fe and Mn, one PETH for Ca and K, and two TAPJ crystals for Mg, Al, Na and Si (Na measured first). The standards were Verma garnet for Mn, Kakanui augite USNM 122142 for Al, Fe, Si, Mg and Ca, sphene glass for Ti, chromite USNM 117075 for Cr, Amelia albite for Na, and orthoclase OR-1 for K. The on-peak and off-peak counting times were 30 s for all elements. Detection limits for Cr2O3 and S are 0.02 wt%. Precision is better than 1% for all elements (1), except Na and K (<3%). Accuracy (1) is better than 5 % for all elements.

Vesiculation of high fountaining Hawaiian eruptions: episodes 15 and 16 of 1959 Kīlauea Iki

The 1959 summit eruption of Kīlauea volcano produced the highest recorded Hawaiian fountain in Hawai‘i. Quantitative analysis of closely spaced samples from the final two high-fountaining episodes of the eruption result in a fine-scale textural study of pyroclasts and provide a record of postfragmentation processes. As clast vesicularity increases, the vesicle number density decreases and vesicle morphology shifts from small and round to larger and more irregular. The shift in microtexture corresponds to greater degrees of postfragmentation expansion of clasts with higher vesicularity. We suggest the range of clast morphologies in the deposit is related to thermal zonation within a Hawaiian fountain where the highest vesicularity clasts traveled in the center and lowest traveled along the margins. Vesicle number densities are greatest in the highest fountaining episode and therefore scale with intensity of activity. Major element chemical analyses and fasciculate crystal textures indicate microlite-rich zones within individual clasts are portions of recycled lava lake material that were incorporated into newly vesiculating primary melt.

Heterogeneous nucleation and epitaxial crystal growth of magmatic minerals

Electron backscatter diffraction analysis of dendritic clinopyroxene (cpx) forming in rapidly cooled basalt reveals two features that are unexpected for phases growing from a liquid: (1) helical growth about {010} cpx , the crystallographic b-axis, with incremental rotation (up to 0.4° μm −1 ) within branch segments and large rotational jumps (10°–46°) between closely spaced branch segments, and (2) strong crystallographic preferred orientation (CPO) between cpx and titanomagnetite (timt) decorating branch tips, such that {010} cpx aligns with one of the six symmetrically equivalent {110} timt face poles. More than 80% of timt crystals occur within 5° of the CPO with cpx substrate, as do 22% of timt crystals in contact with euhedral cpx in natural Etna basalt. The probability distribution of distances between an arbitrary unit vector and the nearest {110} timt normal unit vector was found by numerical simulation and indicates that the observed alignment to within 5° occurs with frequency 0.024; thus the CPO occurs 9–33 times more commonly than expected from randomly distributed crystals. The CPO matches previous observations of epitaxial relationship between spinel and host pyroxene during subsolidus exsolution, but has not previously been reported among magmatic cpx and timt. Conspicuous contiguity among phenocryst phases is inferred to result from heterogeneous nucleation facilitated by epitaxy associated with CPO, an inference supported by high-resolution electron microscopy observations of 150 nm timt crystals adhering preferentially to silicate substrates. Epitaxial relationships among phases nucleating from a melt may contribute to fabrics in magnetic properties, dictate the textures of igneous rocks by promoting development of crystal clusters, and even influence magma transport and eruption styles through a control over magma rheology.

Accuracy of timescales retrieved from diffusion modeling in olivine: A 3D perspective

Abstract Diffusion modeling in olivine is a useful tool to resolve the timescales of various magmatic processes. Practical olivine geospeedometry applications employ 1D chemical transects across sections that are randomly sampled from a given 3D crystal population, but the accuracy and precision with which timescales can be retrieved from this procedure are not well constrained. Here, we use numerical 3D diffusion models of Fe-Mg to evaluate and quantify the uncertainties associated with their 1D counterparts. The 3D diffusion models were built using both simple and realistic olivine morphologies, and incorporate diffusion anisotropy as well as different zoning styles. The 3D model crystals were sectioned along ideal or random planes, which were used to perform 1D models and timescale comparisons. Results show that the timescales retrieved from 1D profiles are highly inaccurate and can vary by factors of 0.1-25 if diffusion anisotropy is not taken into account. Even when anisotropy is corrected for, timescales can still vary between 0.2-10 times the true 3D diffusion time due to crystal shape and sectioning effects. Simple grain selection procedures are described to reduce the misfit between calculated and actual diffusion times, and achieve an accuracy and precision of ~5% and ~15-25% relative, respectively. Provided that the grains are carefully selected, about 20 concentration profiles and associated 1D models suffice to achieve this accuracy.

Raman spectroscopy of titanomagnetites: Calibration of the intensity of Raman peaks as a sensitive indicator for their Ti content

Abstract A systematic study of the Raman spectra of the titanomagnetite solid-solution series (Fe3-xTixO4) for x = ~0.0, 0.2, 0.4, and 0.6 has been conducted. The samples showed combinations of five previously predicted Raman peaks at ~190, 310, 460, 540, and 670 cm-1 that correspond to vibrational modes with T2g(1), Eg, T2g(3), T2g(2), and A1g, respectively. The calibration of Raman spectra for titanomagnetite with known values of Ti concentrations reveals a strong dependence of relative intensity for the T2g(2) and T2g(3) modes on Ti concentration. The most prominent feature is the appearance and increase in the relative intensity of a T2g(3) peak above x = ~0.2. On the other hand, the Raman peak for the T2g(2) mode gradually diminishes as Ti increases and nearly disappears at x = ~0.6. Combining the two relative intensities potentially provides a sensitive indicator of Ti content. The technique was applied to study titanomagnetite in grains from Hana Volcanics and melatroctolite from Rhode Island.

Plutonic xenoliths reveal the timing of magma evolution at Hualalai and Mauna Kea, Hawaii

238 U- 230 Th and U-Pb dating of zircons from leucocratic plutonic xenoliths indicating that lava stratigraphy is an incomplete monitor of magmatic evolution within subsurface res- ervoirs. Our results indicate that diorites from Mauna Kea record postshield evolution over tens of thousands of years when the depth of magma storage increased and highly evolved lavas began erupting. In contrast, diorites and syenogabbros from Hualalai record generation of evolved alkalic magma during the shield stage, and growth of a deep composite pluton ABSTRACT Hawaiian volcanoes evolve through stages that have been delimited by the compositions of their erupted lavas. New in situ 238 U- 230 Th and U-Pb dating of single zircons from leucocratic plutonic xenoliths erupted at Mauna Kea and Hualalai volcanoes, Hawaii, reveals that impor- tant episodes of magmatic evolution are not necessarily refl ected in the stratigraphy or com- positions of erupted lavas. Zircons from Mauna Kea diorites form a heterogeneous population with apparently bimodal ages of ca. 125 ka and ca. 65 ka, suggesting fractionation, intrusion, and crystal recycling about the time of transition of postshield volcanism from basaltic to hawaiitic compositions. Hualalai syenogabbros and diorites record extreme fractionation and generation of alkalic magma at 41 ± 9 ka and 261 ± 28 ka, indicating that alkalic magma was generated ~130,000 yr before the shield to postshield transition inferred from lava stra- tigraphy, and that coeval evolution of chemically distinct magma reservoirs at shallow and deep levels may have characterized the shield stage. These episodes at Hualalai did not cause eruption of evolved lavas, indicating that extreme differentiation in Hawaiian volcanoes is not necessarily followed by eruption of highly evolved magma.