The origin of rhyolitic spherulites at Rockhound State Park

The origin of rhyolitic spherulites at Rockhound State Park

Nelia W. DunbarVirginia T. McLemore

New Mexico Bureau of Geology and Mineral ResourcesNew Mexico Tech

Diameter = 14 cm

Schematic cross section of a Rockhound Spherulite

Questions

• What are the spherulites made of?

• What does the visible structure represent?

• How do spherulites form?

• Why are some hollow?

Electron microprobe used to produce:-Backscattered electron images (map of meanatomic number)-X-ray maps to show element distribution-Quantitative chemical analyses of spots as small as 1 micron

Minerals found in spherulites

• Quartz (SiO2)

• Two alkali feldspars (K, Na)[AlSi3O8]

• Magnetite (Fe3O4)

Composition of rhyolitic magma

• SiO2 76.6 wt.%

• Al2O3 12.8 wt.%

• K2O 4.9 wt.%

• Na2O 3.8 wt.%

• FeO 1.2 wt.%• CaO 0.5 wt.%• TiO2 0.1 wt.%

• H2O 0.1 wt.%

Backscattered Electron image of spherulite core (field of view 1.2 mm)

BSE image of interior spherulite and medial “rapid-growth” crystals FOV 2.5 mm

BSE image of transition between “rapid-growth” and banded zone FOV 2.5 mm

Backscattered Electron image of spherulite (field of view 1.2 mm)

Backscattered electron imageSilica X-ray map

200 microns

Backscattered electron imagePotassium X-ray map

200 microns

Backscattered electron imageSodium X-ray map

200 microns

Backscattered electron image Potassium X-ray map

20 microns

Backscattered electron image Sodium X-ray map

20 microns

Sanidine

Anorthoclase

Albite Oligoclase Andesine Labradorite Bytownite Anorthite

Ab An

Or

0 10 202

3

4

5

6

7

K2O

Na2

O

Compositions of morphologically-different feldspar in RHP spherulite

May represent crystallization in 2-feldspar field (T<~660oC)

Schematic cross section of a Rockhound Spherulite

Corecomposed of many small, fine-grained spherulites (quartz and Na-rich alkali feldspar)

Intermediate part.Feathery quench crystalsof quartz and alkali, K-rich feldspar

Outer layered part.Rhythmic intergrowth of quartz and two feldspars

Why are some spherulite hollow?Observations:

• Within a single lava flow, some spherulites may be hollow whereas others are solid. In some cases, there appears to be some stratigraphic control on location of hollow vs. solid spherulites

• In some hollow spherulites, the original solid form appears to have been expanded from within to form the void space.

• Some “solid” spherulites contain many small, finely dispersed void spaces, which appear to be small bubbles.

Why are some spherulite hollow?Speculation:

• Rhyolite magma contains 0.1 wt% H2O at atmospheric pressure, whereas quartz and feldspar are anhydrous. Crystallization would cause water to come out of solution and form bubbles.

• From a simple ideal gas law calculation, at atmospheric pressure, 0.1 wt.% H2O would generate void space equal to 10 times the initial volume of crystallizing melt, ample to create the void space found in spherulites.

• Creation of a void space requires coalescence of this H2O-dominated vapor phase. This may depend on some critical combination of crystal growth rate and pressure at which the spherulite forms.

Conclusions• Spherulites are composed of quartz, feldspar and magnetite

• Spherulites grew at high temperatures from a rhyolitic magma, and the internal structure is controlled by crystallization dynamics.

• Crystals near the core of the spherulite show texture typical of rapid crystal growth

• Diffusion of elements at the crystal-melt interface may be responsible for banding

• By analogy to experimental systems, spherulites may have grown in periods of days to weeks

• The cavities could have been formed by H2O vapor generated during crystallization