Investigating H2O contents in clinopyroxene from explosive versus effusive eruption products from Merapi volcano, Indonesia

<p>The 2010 eruption of Merapi produced pyroclastic deposits and lava flows that are compositionally very similar, raising the question as to the underlying reason of the differences in eruptive styles between the various phases of the 2010 eruptive events. To test whether primary magmatic volatile content is the reason for the different eruption styles, we analyzed magmatic water contents in nominally anhydrous clinopyroxene crystals contained in lava and ash from the 2010 eruptive events. We utilized two analytical approaches: (i) Fourier-transform infrared spectroscopy (FTIR) analysis of fresh clinopyroxene from the ash and lava samples and (ii) FTIR analysis of clinopyroxene both prior to and after experimental re-hydration. By employing calculated partition coefficients, we determined the magmatic water content of the magma from which the various crystals grew. The magmatic water content determined from the unmodified clinopyroxenes from lava samples yield a range of 0.35 wt.% to 2.02 wt.% H<sub>2</sub>O, whereas magmatic water contents determined from untreated clinopyroxene contained in the ash samples range between 0.04 and 3.25 wt.%, with two outliers at 4.62 and 5.19 and wt.%, respectively. In contrast, for the rehydrated crystals the range for lava derived clinopyroxene crystals is between 1.94 and 2.19 wt.% and for ash between 1.74 and 2.66 wt.%, with two crystals at extreme values of 0.85 and 3.20 wt.%. We interpret these results to indicate that crystals from different populations are present in the 2010 eruptive products, with the dominant group reflecting relatively low magmatic H<sub>2</sub>O contents (around 2 wt.%) due to storage in shallow magma reservoirs and pockets at high levels within the Merapi plumbing systems (e.g. top 3 km). The overall higher H<sub>2</sub>O range and the occasionally more extreme values recorded in clinopyroxenes from ash deposits may then represent the presence of a crystal population that last equilibrated at deeper levels and at higher water contents, i.e. these crystals derive from the replenishing magma that activated the shallow portion of the plumbing system during the 2010 events. While this is work in progress, our results so far seem to suggest that the pyroclastic deposits of the 2010 Merapi eruption may contain a higher fraction of clinopyroxene derived from ‘deeper magma’ with higher H<sub>2</sub>O contents then what we have detected in associated lavas.</p>

Importance of Magmatic Water Content and Oxidation State for Porphyry-Style Au Mineralization: An Example from the Giant Beiya Au Deposit, SW China

The Beiya Au deposit is the largest Cenozoic Au deposit in the Jinshajiang-Ailaoshan porphyry metallogenic belt. Numerous studies document that high water content and fO2 are vital factors for the generation of Au mineralization. In this belt, only the Wandongshan and Hongnitang districts are considered to be of economic importance, while the other districts, such as Bailiancun, are barren. So in order to reveal the importance of water content and oxidation state for Beiya porphyry-style Au mineralization, the amphiboles and zircons compositions are used to evaluate the physicochemical conditions (e.g., pressure, temperature, fO2, and water content) of the Wandongshan ore-fertile porphyries and Bailiancun ore-barren porphyries observed in the Beiya Au deposit. The results show that the water content of the Wandongshan parent magma (≤4.11 ± 0.4 wt %) are slightly higher than those of the parent magma at Bailiancun (≤3.91 ± 0.4 wt %), while the emplacement pressure of the Wandongshan parent magma (31.5–68.6 MPa) is much lower than that of the parent magma at Bailiancun (142.3–192.8 MPa), indicating that the Wandongshan magma reached water saturation earlier. In addition, the Wandongshan porphyries crystallized from more oxidized magma (average of ΔFMQ = +3.5) with an average temperature of 778 °C compared to the Bailiancun porphyries (average of ΔFMQ = +1.5) with a mean magmatic temperature of 770 °C. The Ce4+/Ce3+ ratio of zircon in the Wandongshan ore-related intrusions (average Ce4+/Ce3+ of 62.00) is much higher than that of the Bailiancun barren porphyries (average Ce4+/Ce3+of 23.15), which further confirmed Wandongshan ore-related magma is more oxidized than the Bailiancun barren magma. Therefore, melts that are more enriched in water and with a high oxidation state will be more fertile to form an economic porphyry-style Au system.