Keith Putirka

Igneous thermometers and barometers based on plagioclase + liquid equilibria: Tests of some existing models and new calibrations

Abstract Although many formulations of plagioclase + liquid equilibria have been calibrated in the last decade, few models specifically address the issue of temperature (T) prediction. Moreover, for those that do, T error is not addressed, greatly limiting their use as geothermometers. Several recent models of plagioclase-liquid equilibria are thus tested for their ability to recover T from their calibration data, and predict T from experiments not used for calibration. The models of Sugawara (2001) and Ghiorso et al. (1995, 2002) outperform earlier calibrations. These models perform reasonably well at T > 1100 °C, though recovery and prediction of T is less precise for hydrous compositions. In addition, these models cannot be integrated with geo-hygrometers, or other mineral-melt thermometers and barometers; the following expression predicts T with up to 40% greater precision: Because these thermometers are pressure (P) sensitive, a temperature-sensitive barometer was also developed: In these models, T is in Kelvins and P is in kbar. Anpl and Abpl are the fractions of anorthite and albite in plagioclase, calculated as cation fractions: An = CaO/(CaO + NaO0.5 + KO0.5) and Ab = NaO0.5/ (CaO+NaO0.5+KO0.5). Terms such as Alliq refer to the anhydrous cation fraction of Al in the liquid; H2O in Equation 1 is in units of wt%. Errors on these models are comparable to those for clinopyroxene thermobarometers: In Equation 1, R = 0.99 and the standard error of estimate (SEE) is 23 K; for Equation 2, R = 0.94 and the SEE is 1.8 kbar. The models successfully recover mean pressures for experimental data that are not used for calibration, and are furthermore able to recover near-1-atm P estimates for volcanic rocks from Kilauea, Hawaii, which are thought to have crystallized at or very near Earths surface.

Excess temperatures at ocean islands: Implications for mantle layering and convection

To test for the prevalence of mantle plumes and the existence of mantle layering, temperatures ( T ) are estimated for 28 oceanic hotspots, using olivine-liquid equilibria ( T ol-liq). There are 27 localities that have T ol-liq hotter than mid-ocean ridges (MOR), by 99–233 °C (average = 146 ± 26 °C), which translates to mantle potential temperatures that exceed those of MOR by 114–290 °C (average = 173 ± 38 °C). Thermally driven mantle plumes are thus common, not rare. Moreover, mantle temperatures at ocean islands are positively correlated with buoyancy flux and 3He/4He. The correlation with buoyancy affirms that oceanic swells are thermal in origin. The positive correlation with 3He/4He is inconsistent with the notion that high 3He/4He and depleted MOR mantle derive from the same layer, but instead shows that high 3He/4He is tied to a lower thermal boundary layer, and thus that the mantle is compositionally layered. Mantle temperatures are negatively correlated with Pb isotope ratios, supporting a model by C. Class and S.L. Goldstein that this deep, high 3He/4He layer may be depleted.

Magma transport at Hawaii: Inferences based on igneous thermobarometry

Pyroxene + liquid equilibrium in Hawaiian lavas occurs at a range of pressures for each volcano. Ranges are systematic and may be related to the stage of development of the magma conduit system. Kilauea, which is in its shield-building phase, yields relatively shallow storage estimates. Loihi and Mauna Kea, which are in the early and late stages of volcano growth, re- spectively, yield deeper storage estimates. Shallowest depth estimates at Loihi and Mauna Kea are similar to estimates of elastic plate thickness, suggesting that the mechanical behavior of the lithosphere, rather than density contrasts at the Moho, regulates magma delivery. Appar- ently, a large increase in fracture energy below the brittle-ductile transition inhibits transport at depth, whereas magma transport by fracture propagation is rapid through the brittle lithosphere. Some shallow depth estimates at Kilauea support the hypothesis that the strength of the unbuttressed southeast flank influences magma storage. Kilauea transport depths cor- relate with an eruption sequence, which illustrates a top-to-bottom emptying of the conduit system. Successively deeper reservoirs at Kilauea were tapped within 300 days, indicating that magma is stored at a range of depths, including in the mantle, rather than at a single level within the lithosphere.

Amphibole thermometers and barometers for igneous systems and some implications for eruption mechanisms of felsic magmas at arc volcanoes

Abstract Calcic, igneous amphiboles are of special interest as their compositional diversity and common occurrence provide ample potential to investigate magmatic processes. But not all amphibole-based barometers lead to geologically useful information: recent and new tests reaffirm prior studies (e.g., Erdman et al. 2014), indicating that amphibole barometers are generally unable to distinguish between experiments conducted at 1 atm and at higher pressures, except under highly restrictive conditions. And the fault might not lie with experimental failure. Instead, the problem may relate to an intrinsic sensitivity of amphiboles to temperature (T) and liquid composition, rather than pressure. The exceptional conditions are those identified by Anderson and Smith (1995): current amphibole barometers are more likely to be useful when T < 800 °C and Fe#Amp= FeAmp/ (FeAmp+MgAmp) < 0.65. Experimentally grown and natural calcic amphiboles are here used to investigate amphibole solid solution behavior, and to calibrate new thermometers and tentative amphibole barometers that should be applicable to igneous systems generally. Such analysis reveals that amphiboles are vastly less complex than may be inferred from published catalogs of end-member components. Most amphiboles, for example, consist largely of just three components: pargasite [NaCa2(Fm4Al)Si6Al2], kaersutite [NaCa2(Fm4Ti)Si6Al2O22(OH)], and tremolite + ferro-actinolite [Ca2Fm5Si8O22(OH)2, where Fm = Fe+Mn+Mg]. And nearly all remaining compositionalvariation can be described with just four others: alumino-tschermakite [Ca2(Fm3Al2)Si6Al2O22(OH)2], a Na-K-gedrite-like component [(Na, K)Fm6AlSi6Al2O22(OH)2], a ferri-ferrotschermakite-like component [Ca2(Fm3Fe2)Si6Al2O22(OH)2], and an as yet unrecognized component with 3 to 4 Al atoms per formula unit (apfu), 1 apfu each of Na and Ca, and <6 Si apfu, here termed aluminous kaersutite: NaCaFm4Ti(Fe3+, Al) Si5Al3O23(OH). None of these components, however, are significantly pressure (P) sensitive, leaving the Al-in-amphibole approach, with all its challenges, the best existing hope for an amphibole barometer. A new empirical barometer based on DAlsuccessfully differentiates experimental amphiboles crystallized at 1 to 8 kbar, at least when multiple P estimates, from multiple amphibole compositions, are averaged. Without such averaging however, amphibole barometry is a less certain proposition, providing ±2 kbar precision on individual estimates for calibration data, and ±4 kbar at best for test data; independent checks on P are thus needed. Amphibole compositions, however, provide for very effective thermometers, here based on DTi, DNa, and amphibole compositions alone, with precisions of ±30 °C. These new models, and tests for equilibrium, are collectively applied to Augustine volcano and the 2010 eruption at Merapi. Both localities reveal a significant cooling and crystallization interval (>190–270 °C) at pressures of 0.75 to 2.2 kbar at Augustine and Merapi, respectively, perhaps the likely depths from which pre-eruption magmas are stored. Such considerable intervals of cooling at shallow depths indicate that mafic magma recharge is not a proximal cause of eruption. Rather, eruption triggering is perhaps best explained by the classic “second boiling” concept, where post-recharge cooling and crystallization drive a magmatic system toward vapor saturation and positive buoyancy.

The tectonic significance of high-K2O volcanism in the Sierra Nevada, California

K 2 O contents have long been recognized as a potential indicator of tectonic processes, and in the Sierra Nevada, California, high-K 2 O volcanism has been attributed to lithosphere root delamination. However, new data from the central Sierra suggest a very different control: K 2 O concentrations can be explained by variations in the degree of partial melting in the mantle, where high-K 2 O volcanics are derived from low-degree partial melts of mantle lithosphere. Field evidence in the central Sierra further suggests that the pulse of high-K 2 O volcanism there was synchronous with the development of a pull-apart structure along a series of right-stepping dextral transtensional faults at the onset of Walker Lane transtensional faulting. In our alternative interpretation, high-K, low-degree partial melts were tapped by the inception of transtensional stresses, recording the birth of a plate boundary. We speculate that high-K 2 O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination. A regional southward increase in incompatible element contents and decrease in erupted volumes are also consistent with a model for crustal thickness controls on magmatism. Depth-integrated density models show that dry mafic magmas beneath thick crust have insufficient buoyancy to erupt, but low-degree partial melts carry sufficient volatiles to allow eruption; as with K 2 O, degree of partial melting, not source-region heterogeneity, controls water contents and buoyancy.

Miocene evolution of the western edge of the Nevadaplano in the central and northern Sierra Nevada: palaeocanyons, magmatism, and structure

The Sierra Nevada of California is the longest and tallest mountain range in the co-terminus USA, and has long been regarded as topographically very young ( < 6 Ma); however, recent work has provided evidence that the range is very old (>80 Ma), and represents the western shoulder of a Tibetan-like plateau (the Nevadaplano) that was centred over Nevada. A great deal of effort has been invested in applying modern laboratory and geophysical techniques to understanding the Sierra Nevada, yet some of the most unambiguous constraints on the Sierran landscape evolution are derived from field studies of dated strata preserved in the palaeochannels/palaeocanyons that crossed the range in Cenozoic time. Our work in the Sierra Nevada suggests that neither end-member model is correct for the debate regarding youth vs. antiquity of the range. Many features of the Cenozoic palaeocanyons and palaeochannels reflect the shape of the Cretaceous orogen, but they were also affected by Miocene tectonic and magmatic events. In the central Sierra Nevada, we infer that the inherited Cretaceous landscape was modified by three Miocene tectonic events, each followed by ∼2–5 Myr of subduction-induced magmatism and sedimentation during a period of relative tectonic quiescence. The first event, at about 16 Ma, corresponds to the westward sweep of the Ancestral Cascades arc front into the Sierra Nevada and adjacent western Nevada. We suggest that this caused thermal uplift and extension. The second event, at about 11–10 Ma, records the birth of the ‘future plate boundary’ by transtensional faulting and voluminous high-K volcanism at the western edge of the Walker Lane belt. The third event, at about 8–7 Ma, is associated with renewed range-front faulting in the central Sierra, and rejuvenation and beheading of the palaeocanyons. Volcanic pulses closely followed all three events, and we tentatively infer that footwall uplift of the Sierra Nevada occurred during all three events. By analogy with the ∼11 Ma event, we speculate that high-K volcanic rocks in the southern part of the range mark the inception of yet a fourth pulse of range-front faulting at 3–3.5 Ma, which resulted in a fourth tilting and crestal uplift event. Cenozoic rocks along the western edge of the Nevadaplano record the following variation, from the central to the northern Sierra: decrease in crustal thickness (and presumably palaeoelevation), decrease in palaeorelief and attendant decrease in coarse-grained fluvial- and mass-wasting deposits, and greater degree of encroachment by Walker Lane-related faults beginning at 10–11 Ma. By mapping and dating Cenozoic strata in detail, we show that what is now the Sierra Nevada was, at least in part, shaped by the Miocene structural and magmatic events.

Cross section of a magma conduit system at the margin of the Colorado Plateau

We present crystallization depth vs. temperature estimates for clinopyroxene phenocrysts from the Springerville volcanic field, Arizona. These calculations reveal several intriguing patterns that have considerable implications for magma transport and genesis. First, partial crystallization occurs over a wide depth range (0–60 km), but most partial crystallization occurs between 0 and 30 km. Second, low crystallization temperatures, low-density magmas, and evolved liquid compositions derive exclusively from two depth intervals, 0–12 and 23–30 km. These intervals coincide with a density contrast in the upper crust and a rheology contrast at the base of the middle crust. They also coincide with two highly seismically reflective depth intervals. These relationships indicate that (1) the Moho is not a staging area for volcanic eruptions; (2) density contrasts in the upper crust, and a rheology contrast in the middle crust, control magma transport and liquid evolution; (3) magma conduits are probably magma mush columns, with a preponderance of sills within the 0–12 and 23–30 km intervals; and (4) seismically reflective layers are sills related to Tertiary–Holocene volcanic activity. Moreover, these sills appear to represent the principal sites of magma evolution.

Rates and styles of planetary cooling on Earth, Moon, Mars, and Vesta, using new models for oxygen fugacity, ferric-ferrous ratios, olivine-liquid Fe-Mg exchange, and mantle potential temperature

Abstract Mantle potential temperatures (Tp) provide insights into mantle circulation and tests of whether Earth is the only planet to exhibit thermally bi-modal volcanism—a distinctive signature of modern plate tectonics. Planets that have a stagnant lid, for example, should exhibit volcanism that is uni-modal with Tp, since mantle plumes would have a monopoly on the genesis of volcanism. But new studies of magmatic ferric-ferrous ratios (XFe2O3liq/XFeOliq

Basin and Range volcanism as a passive response to extensional tectonics

X_{{\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}}^{{\text{liq}}}{\text{/}}X_{{\text{FeO}}}^{{\text{liq}}}

Cenozoic volcanism in the Sierra Nevada and Walker Lane, California, and a new model for lithosphere degradation

) (Cottrell and Kelley 2011) and the olivine-liquid Fe-Mg exchange coefficient, KD(Fe-Mg)Ol-liq (or KD) (Matzen et al. 2011) indicate that re-evaluations of Tp are needed. New tests and calibrations are thus presented for oxygen fugacity (fO2), XFe2O3liq/XFeOliq