A Partial Record of Mixing of Mantle Melts Preserved in Icelandic Phenocrysts

The record of mixing of mantle melts in magma chambers has previously been observed in the compositions of olivine-hosted melt inclusions from Borgarhraun, a primitive basalt flow from the Theistareykir volcanic system, northern Iceland. Borgarhraun also contains high Mg-number (85–92) clinopyroxenes, which exist in polycrystalline nodules and as phenocrysts. Coincident major and trace element analyses were made in compositional zones of these clinopyroxenes, and Ce/Yb ratios of the melts in chemical equilibrium with each of the clinopyroxene zones were calculated using carefully selected crystal–melt partition coefficients. These calculations allow direct comparison of clinopyroxene compositions with existing melt inclusion data. The range of Ce/Yb ratios in the crystals and in the equilibrium melts cannot be accounted for by crystallization alone, requiring simultaneous mixing and crystallization of compositionally variable mantle melts. However, the range in Ce/Yb for melts in equilibrium with these high Mg-number clinopyroxenes is smaller than that of melt inclusions hosted by olivines with equivalent Fo contents. Also, the mean composition of the melts from which clinopyroxene grew has significantly lower Ce/Yb than the olivine-hosted melt inclusions. The record of mantle melt variability in clinopyroxenes is thus biased towards more depleted (low Ce/Yb) melt compositions. This bias can be understood if the trace element variation in the Borgarhraun parental melts is coupled to major element variation, as expected from petrological parameterizations of mantle melting. The major element variation influences the phase relationships and controls the appearance of liquidus phases during fractional crystallization in near-Moho magma chambers. Small-degree, deep melts, formed in the presence of garnet, have high Ce/Yb ratios. On cooling, these melts have a longer olivine-only crystallization path than melts derived from the shallow mantle. When these deep-sourced melts eventually become clinopyroxene saturated, they have too low Mg-numbers to crystallize high Mg-number clinopyroxenes such as are found in Borgarhraun. In contrast, shallow, depleted melts saturate in clinopyroxene at high Mg-number. The delayed onset of clinopyroxene crystallization in the enriched melts, coupled with concurrent mixing and crystallization of melts generated at a range of depths in the mantle, can account for the difference in the distribution of the trace element composition of high Mg-number melts saturated in olivine and clinopyroxene. The trace element compositions of high Mg-number clinopyroxenes in Borgarhraun therefore provide only a partial and biased record of the mixing of mantle melts. As well as showing that melt mixing may be preserved in phenocryst compositions, the results illustrate that trace element disequilibrium between crystals and carrier melt can be a consequence of magma mixing, rather than necessitating a xenocrystic origin for the crystals. Furthermore, care must be taken when using clinopyroxene separates from primitive basalts to examine compositional heterogeneity, as they provide a record of the chemical evolution of the magmatic system that is biased towards depleted compositions and therefore incomplete

Hydraulically linked reservoirs simultaneously fed the 1975–1984 Krafla Fires eruptions: Insights from petrochemistry

The 1975–1984 Krafla Fires in northeast Iceland was the first plate-boundary rifting episode to be tracked using seismic and geodetic monitoring. Geophysical observations from this episode have inspired conceptual models of magma transport during plate spreading, but a lack of complementary petrologic insights has hindered a holistic understanding of the events. To address this knowledge gap, we studied the petrochemistry of all nine Krafla Fires basaltic eruptions. Our large dataset of new whole-rock, matrix glass and mineral analyses from samples collected during or shortly after each eruption reveal a clear compositional bimodality in the erupted magmas that persisted across the episode, with evolved quartz tholeiite (MgO = 5.7–6.4 wt.%) erupted inside Krafla caldera, and more primitive (usually olivine-normative) tholeiite (MgO = 6.4–8.7 wt%) erupted north of the caldera margin. Barometric calculations indicate tapping of these magmas from distinct reservoirs: a primitive lower-crustal reservoir at a most probable depth of ∼14–19 km, and a more evolved, shallower reservoir at a most probable depth of ∼7–9 km beneath the caldera. These reservoirs were tapped simultaneously in several of the nine eruptions, and in three events the two magma types mixed near the northern caldera margin. Varying levels of trace element depletion in the deep-sourced primitive melts reflect incomplete mixing of diverse mantle-derived melts at depth; the most enriched of these melts could be parental to evolved inside-caldera magma via fractional crystallization. Clinopyroxene rims on gabbroic nodules from primitive September 1984 lavas record lower crustal pressures, while diffusion models suggest that these rims grew up to within a few months before eruption. Ascent of the primitive magma from the lower crust thus occurred over timescales much shorter than eruptive repose periods, without prolonged stalling at shallow depths. These observations are inconsistent with the view that the eruptions were entirely fed by lateral magma outflow from the shallow reservoir. They instead require some decoupling of the flow paths of the two magma types: the primitive magma either bypassed the sub-caldera reservoir laterally or ascended vertically beneath the northern vents. The two reservoirs nonetheless shared a hydraulic connection and jointly responded to rifting. Comparison of the Krafla Fires with other rifting events and eruptions highlights the complexity and diversity of magma transport during plate boundary rifting events, which is not yet captured by a generalizable model. Integration of petrologic, geochemical and geophysical data is essential to provide a holistic view of future rifting events in Iceland and at other spreading centres.<br/

Periostin is a systemic biomarker of eosinophilic airway inflammation in asthmatic patients

BACKGROUND: Eosinophilic airway inflammation is heterogeneous in asthmatic patients. We recently described a distinct subtype of asthma defined by the expression of genes inducible by T(H)2 cytokines in bronchial epithelium. This gene signature, which includes periostin, is present in approximately half of asthmatic patients and correlates with eosinophilic airway inflammation. However, identification of this subtype depends on invasive airway sampling, and hence noninvasive biomarkers of this phenotype are desirable. OBJECTIVE: We sought to identify systemic biomarkers of eosinophilic airway inflammation in asthmatic patients. METHODS: We measured fraction of exhaled nitric oxide (Feno), peripheral blood eosinophil, periostin, YKL-40, and IgE levels and compared these biomarkers with airway eosinophilia in asthmatic patients. RESULTS: We collected sputum, performed bronchoscopy, and matched peripheral blood samples from 67 asthmatic patients who remained symptomatic despite maximal inhaled corticosteroid treatment (mean FEV(1), 60% of predicted value; mean Asthma Control Questionnaire [ACQ] score, 2.7). Serum periostin levels are significantly increased in asthmatic patients with evidence of eosinophilic airway inflammation relative to those with minimal eosinophilic airway inflammation. A logistic regression model, including sex, age, body mass index, IgE levels, blood eosinophil numbers, Feno levels, and serum periostin levels, in 59 patients with severe asthma showed that, of these indices, the serum periostin level was the single best predictor of airway eosinophilia (P = .007). CONCLUSION: Periostin is a systemic biomarker of airway eosinophilia in asthmatic patients and has potential utility in patient selection for emerging asthma therapeutics targeting T(H)2 inflammation